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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 RAW P. Thubert, Ed. 3 Internet-Draft Cisco Systems 4 Intended status: Informational D. Cavalcanti 5 Expires: 9 December 2021 Intel 6 X. Vilajosana 7 Universitat Oberta de Catalunya 8 C. Schmitt 9 Research Institute CODE, UniBwM 10 J. Farkas 11 Ericsson 12 7 June 2021 14 Reliable and Available Wireless Technologies 15 draft-ietf-raw-technologies-02 17 Abstract 19 This document presents a series of recent technologies that are 20 capable of time synchronization and scheduling of transmission, 21 making them suitable to carry time-sensitive flows with high 22 reliability and availability. 24 Status of This Memo 26 This Internet-Draft is submitted in full conformance with the 27 provisions of BCP 78 and BCP 79. 29 Internet-Drafts are working documents of the Internet Engineering 30 Task Force (IETF). Note that other groups may also distribute 31 working documents as Internet-Drafts. The list of current Internet- 32 Drafts is at https://datatracker.ietf.org/drafts/current/. 34 Internet-Drafts are draft documents valid for a maximum of six months 35 and may be updated, replaced, or obsoleted by other documents at any 36 time. It is inappropriate to use Internet-Drafts as reference 37 material or to cite them other than as "work in progress." 39 This Internet-Draft will expire on 9 December 2021. 41 Copyright Notice 43 Copyright (c) 2021 IETF Trust and the persons identified as the 44 document authors. All rights reserved. 46 This document is subject to BCP 78 and the IETF Trust's Legal 47 Provisions Relating to IETF Documents (https://trustee.ietf.org/ 48 license-info) in effect on the date of publication of this document. 49 Please review these documents carefully, as they describe your rights 50 and restrictions with respect to this document. Code Components 51 extracted from this document must include Simplified BSD License text 52 as described in Section 4.e of the Trust Legal Provisions and are 53 provided without warranty as described in the Simplified BSD License. 55 Table of Contents 57 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 58 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4 59 3. On Scheduling . . . . . . . . . . . . . . . . . . . . . . . . 4 60 3.1. Benefits of Scheduling on Wires . . . . . . . . . . . . . 5 61 3.2. Benefits of Scheduling on Wireless . . . . . . . . . . . 5 62 4. IEEE 802.11 . . . . . . . . . . . . . . . . . . . . . . . . . 6 63 4.1. Provenance and Documents . . . . . . . . . . . . . . . . 6 64 4.2. 802.11ax High Efficiency (HE) . . . . . . . . . . . . . . 8 65 4.2.1. General Characteristics . . . . . . . . . . . . . . . 8 66 4.2.2. Applicability to deterministic flows . . . . . . . . 9 67 4.3. 802.11be Extreme High Throughput (EHT) . . . . . . . . . 11 68 4.3.1. General Characteristics . . . . . . . . . . . . . . . 11 69 4.3.2. Applicability to deterministic flows . . . . . . . . 11 70 4.4. 802.11ad and 802.11ay (mmWave operation) . . . . . . . . 12 71 4.4.1. General Characteristics . . . . . . . . . . . . . . . 13 72 4.4.2. Applicability to deterministic flows . . . . . . . . 13 73 5. IEEE 802.15.4 . . . . . . . . . . . . . . . . . . . . . . . . 13 74 5.1. Provenance and Documents . . . . . . . . . . . . . . . . 13 75 5.2. TimeSlotted Channel Hopping . . . . . . . . . . . . . . . 15 76 5.2.1. General Characteristics . . . . . . . . . . . . . . . 15 77 5.2.2. Applicability to Deterministic Flows . . . . . . . . 17 78 6. 5G . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 79 6.1. Provenance and Documents . . . . . . . . . . . . . . . . 30 80 6.2. General Characteristics . . . . . . . . . . . . . . . . . 32 81 6.3. Deployment and Spectrum . . . . . . . . . . . . . . . . . 33 82 6.4. Applicability to Deterministic Flows . . . . . . . . . . 34 83 6.4.1. System Architecture . . . . . . . . . . . . . . . . . 34 84 6.4.2. Overview of The Radio Protocol Stack . . . . . . . . 36 85 6.4.3. Radio (PHY) . . . . . . . . . . . . . . . . . . . . . 37 86 6.4.4. Scheduling and QoS (MAC) . . . . . . . . . . . . . . 39 87 6.4.5. Time-Sensitive Networking (TSN) Integration . . . . . 41 88 6.5. Summary . . . . . . . . . . . . . . . . . . . . . . . . . 45 89 7. L-band Digital Aeronautical Communications System . . . . . . 46 90 7.1. Provenance and Documents . . . . . . . . . . . . . . . . 47 91 7.2. General Characteristics . . . . . . . . . . . . . . . . . 48 92 7.3. Deployment and Spectrum . . . . . . . . . . . . . . . . . 49 93 7.4. Applicability to Deterministic Flows . . . . . . . . . . 49 94 7.4.1. System Architecture . . . . . . . . . . . . . . . . . 50 95 7.4.2. Overview of The Radio Protocol Stack . . . . . . . . 50 96 7.4.3. Radio (PHY) . . . . . . . . . . . . . . . . . . . . . 51 97 7.4.4. Scheduling, Frame Structure and QoS (MAC) . . . . . . 52 98 7.5. Summary . . . . . . . . . . . . . . . . . . . . . . . . . 54 99 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 55 100 9. Security Considerations . . . . . . . . . . . . . . . . . . . 55 101 10. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 55 102 11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 55 103 12. Normative References . . . . . . . . . . . . . . . . . . . . 55 104 13. Informative References . . . . . . . . . . . . . . . . . . . 56 105 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 64 107 1. Introduction 109 When used in math or philosophy, the term "deterministic" generally 110 refers to a perfection where all aspect are understood and 111 predictable. A perfectly Deterministic Network would ensure that 112 every packet reach its destination following a predetermined path 113 along a predefined schedule to be delivered at the exact due time. 114 In a real and imperfect world, a Deterministic Network must highly 115 predictable, which is a combination of reliability and availability. 116 On the one hand the network must be reliable, meaning that it will 117 perform as expected for all packets and in particular that it will 118 always deliver the packet at the destination in due time. On the 119 other hand, the network must be available, meaning that it is 120 resilient to any single outage, whether the cause is a software, a 121 hardware or a transmission issue. 123 RAW (Reliable and Available Wireless) is an effort to provide 124 Deterministic Networking on across a path that include a wireless 125 physical layer. Making Wireless Reliable and Available is even more 126 challenging than it is with wires, due to the numerous causes of loss 127 in transmission that add up to the congestion losses and the delays 128 caused by overbooked shared resources. In order to maintain a 129 similar quality of service along a multihop path that is composed of 130 wired and wireless hops, additional methods that are specific to 131 wireless must be leveraged to combat the sources of loss that are 132 also specific to wireless. 134 Such wireless-specific methods include per-hop retransmissions (HARQ) 135 and P2MP overhearing whereby multiple receivers are scheduled to 136 receive the same transmission, which balances the adverse effects of 137 the transmission losses that are experienced when a radio is used as 138 pure P2P. Those methods are collectively referred to as PAREO 139 functions in the "Reliable and Available Wireless Architecture/ 140 Framework" [I-D.pthubert-raw-architecture]. 142 2. Terminology 144 This specification uses several terms that are uncommon on protocols 145 that ensure bets effort transmissions for stochastics flows, such as 146 found in the traditional Internet and other statistically multiplexed 147 packet networks. 149 ARQ: Automatic Repeat Request, enabling an acknowledged transmission 150 and retries. ARQ is a typical model at Layer-2 on a wireless 151 medium. It is typically avoided end-to-end on deterministic flows 152 because it introduces excessive indetermination in latency, but a 153 limited number of retries within a bounded time may be used over a 154 wireless link and yet respect end-to-end constraints. 156 Available: That is exempt of unscheduled outage, the expectation for 157 a network being that the flow is maintained in the face of any 158 single breakage. 160 Deterministic Networking We refer to section 2 of [RFC8557] for this 161 term. 163 FEC: Forward error correction, sending redundant coded data to help 164 the receiver recover transmission errors without the delays 165 incurred with ARQ. 167 HARQ: Hybrid ARQ, a combination of FEC and ARQ. 169 PCE: Path Computation Element. 171 PAREO (functions): the wireless extension of DetNet PREOF. PAREO 172 functions include scheduled ARQ at selected hops, and expect the 173 use of new operations like overhearing where available. 175 Reliable: That consistently performs as expected, the expectation 176 for a network being to always deliver a packet in due time. 178 Track: A DODAG oriented to a destination, and that enables Packet 179 ARQ, Replication, Elimination, and Ordering Functions. 181 3. On Scheduling 183 The operations of a Deterministic Network often rely on precisely 184 applying a tight schedule, in order to avoid collision loss and 185 guarantee the worst-case time of delivery. To achieve this, there 186 must be a shared sense of time throughout the network. The sense of 187 time is usually provided by the lower layer and is not in scope for 188 RAW. 190 3.1. Benefits of Scheduling on Wires 192 A network is reliable when the statistical effects that affect the 193 packet transmission are eliminated. This involves maintaining at all 194 time the amount of critical packets within the physical capabilities 195 of the hardware and that of the radio medium. This is achieved by 196 controlling the use of time-shared resources such as CPUs and 197 buffers, by shaping the flows and by scheduling the time of 198 transmission of the packets that compose the flow at every hop. 200 Equipment failure, such as an access point rebooting, a broken radio 201 adapter, or a permanent obstacle to the transmission, is a secondary 202 source of packet loss. When a breakage occurs, multiple packets are 203 lost in a row before the flows are rerouted or the system may 204 recover. This is not acceptable for critical applications such as 205 related to safety. A typical process control loop will tolerate an 206 occasional packet loss, but a loss of several packets in a row will 207 cause an emergency stop (e.g., after 4 packets lost, within a period 208 of 1 second). 210 Network Availability is obtained by making the transmission resilient 211 against hardware failures and radio transmission losses due to 212 uncontrolled events such as co-channel interferers, multipath fading 213 or moving obstacles. The best results are typically achieved by 214 pseudo randomly cumulating all forms of diversity, in the spatial 215 domain with replication and elimination, in the time domain with ARQ 216 and diverse scheduled transmissions, and in the frequency domain with 217 frequency hopping or channel hopping between frames. 219 3.2. Benefits of Scheduling on Wireless 221 In addition to the benefits listed in Section 3.1, scheduling 222 transmissions provides specific value to the wireless medium. 224 On the one hand, scheduling avoids collisions between scheduled 225 transmissions and can ensure both time and frequency diversity 226 between retries in order to defeat co-channel interference from un- 227 controlled transmitters as well as multipath fading. Transmissions 228 can be scheduled on multiple channels in parallel, which enables to 229 use the full available spectrum while avoiding the hidden terminal 230 problem, e.g., when the next packet in a same flow interferes on a 231 same channel with the previous one that progressed a few hops 232 farther. 234 On the other hand, scheduling optimizes the bandwidth usage: compared 235 to classical Collision Avoidance techniques, there is no blank time 236 related to inter-frame space (IFS) and exponential back-off in 237 scheduled operations. A minimal Clear Channel Assessment may be 238 needed to comply with the local regulations such as ETSI 300-328, but 239 that will not detect a collision when the senders are synchronized. 240 And because scheduling allows a time-sharing operation, there is no 241 limit to the ratio of isolated critical traffic. 243 Finally, scheduling plays a critical role to save energy. In IOT, 244 energy is the foremost concern, and synchronizing sender and listener 245 enables to always maintain them in deep sleep when there is no 246 scheduled transmission. This avoids idle listening and long 247 preambles and enables long sleep periods between traffic and 248 resynchronization, allowing battery-operated nodes to operate in a 249 mesh topology for multiple years. 251 4. IEEE 802.11 253 4.1. Provenance and Documents 255 With an active portfolio of nearly 1,300 standards and projects under 256 development, IEEE is a leading developer of industry standards in a 257 broad range of technologies that drive the functionality, 258 capabilities, and interoperability of products and services, 259 transforming how people live, work, and communicate. 261 The IEEE 802 LAN/MAN Standards Committee (SC) develops and maintains 262 networking standards and recommended practices for local, 263 metropolitan, and other area networks, using an open and accredited 264 process, and advocates them on a global basis. The most widely used 265 standards are for Ethernet, Bridging and Virtual Bridged LANs 266 Wireless LAN, Wireless PAN, Wireless MAN, Wireless Coexistence, Media 267 Independent Handover Services, and Wireless RAN. An individual 268 Working Group provides the focus for each area. Standards produced 269 by the IEEE 802 SC are freely available from the IEEE GET Program 270 after they have been published in PDF for six months. 272 The IEEE 802.11 LAN standards define the underlying MAC and PHY 273 layers for the Wi-Fi technology. Wi-Fi/802.11 is one of the most 274 successful wireless technologies, supporting many application 275 domains. While previous 802.11 generations, such as 802.11n and 276 802.11ac, have focused mainly on improving peak throughput, more 277 recent generations are also considering other performance vectors, 278 such as efficiency enhancements for dense environments in 802.11ax, 279 and latency and support for Time-Sensitive Networking (TSN) 280 capabilities in 802.11be. 282 IEEE 802.11 already supports some 802.1 TSN standards and it is 283 undergoing efforts to support for other 802.1 TSN capabilities 284 required to address the use cases that require time synchronization 285 and timeliness (bounded latency) guarantees with high reliability and 286 availability. The IEEE 802.11 working group has been working in 287 collaboration with the IEEE 802.1 group for several years extending 288 802.1 features over 802.11. As with any wireless media, 802.11 289 imposes new constraints and restrictions to TSN-grade QoS, and 290 tradeoffs between latency and reliability guarantees must be 291 considered as well as managed deployment requirements. An overview 292 of 802.1 TSN capabilities and their extensions to 802.11 are 293 discussed in [Cavalcanti_2019]. 295 Wi-Fi Alliance (WFA) is the worldwide network of companies that 296 drives global Wi-Fi adoption and evolution through thought 297 leadership, spectrum advocacy, and industry-wide collaboration. The 298 WFA work helps ensure that Wi-Fi devices and networks provide users 299 the interoperability, security, and reliability they have come to 300 expect. 302 The following [IEEE Std. 802.11] specifications/certifications are 303 relevant in the context of reliable and available wireless services 304 and support for time-sensitive networking capabilities: 306 Time Synchronization: IEEE802.11-2016 with IEEE802.1AS; WFA TimeSync 307 Certification. 309 Congestion Control: IEEE802.11-2016 Admission Control; WFA Admission 310 Control. 312 Security: WFA Wi-Fi Protected Access, WPA2 and WPA3. 314 Interoperating with IEEE802.1Q bridges: [IEEE Std. 802.11ak]. 316 Stream Reservation Protocol (part of [IEEE Std. 802.1Qat]): AIEEE802 317 .11-2016 319 Scheduled channel access: IEEE802.11ad Enhancements for very high 320 throughput in the 60 GHz band [IEEE Std. 802.11ad]. 322 802.11 Real-Time Applications: Topic Interest Group (TIG) ReportDoc 323 [IEEE_doc_11-18-2009-06]. 325 In addition, major amendments being developed by the IEEE802.11 326 Working Group include capabilities that can be used as the basis for 327 providing more reliable and predictable wireless connectivity and 328 support time-sensitive applications: 330 IEEE 802.11ax D4.0: Enhancements for High Efficiency (HE). [IEEE 331 Std. 802.11ax] 333 IEEE 802.11be Extreme High Throughput (EHT). [IEEE 802.11be WIP] 335 IEE 802.11ay Enhanced throughput for operation in license-exempt 336 bands above 45 GHz. [IEEE Std. 802.11ay] 338 The main 802.11ax and 802.11be capabilities and their relevance to 339 RAW are discussed in the remainder of this document. 341 4.2. 802.11ax High Efficiency (HE) 343 4.2.1. General Characteristics 345 The next generation Wi-Fi (Wi-Fi 6) is based on the IEEE802.11ax 346 amendment [IEEE Std. 802.11ax], which includes new capabilities to 347 increase efficiency, control and reduce latency. Some of the new 348 features include higher order 1024-QAM modulation, support for uplink 349 multi-user MIMO, OFDMA, trigger-based access and Target Wake time 350 (TWT) for enhanced power savings. The OFDMA mode and trigger-based 351 access enable scheduled operation, which is a key capability required 352 to support deterministic latency and reliability for time-sensitive 353 flows. 802.11ax can operate in up to 160 MHz channels and it includes 354 support for operation in the new 6 GHz band, which is expected to be 355 open to unlicensed use by the FCC and other regulatory agencies 356 worldwide. 358 4.2.1.1. Multi-User OFDMA and Trigger-based Scheduled Access 360 802.11ax introduced a new orthogonal frequency-division multiple 361 access (OFDMA) mode in which multiple users can be scheduled across 362 the frequency domain. In this mode, the Access Point (AP) can 363 initiate multi-user (MU) Uplink (UL) transmissions in the same PHY 364 Protocol Data Unit (PPDU) by sending a trigger frame. This 365 centralized scheduling capability gives the AP much more control of 366 the channel, and it can remove contention between devices for uplink 367 transmissions, therefore reducing the randomness caused by CSMA-based 368 access between stations. The AP can also transmit simultaneously to 369 multiple users in the downlink direction by using a Downlink (DL) MU 370 OFDMA PPDU. In order to initiate a contention free Transmission 371 Opportunity (TXOP) using the OFDMA mode, the AP still follows the 372 typical listen before talk procedure to acquire the medium, which 373 ensures interoperability and compliance with unlicensed band access 374 rules. However, 802.11ax also includes a multi-user Enhanced 375 Distributed Channel Access (MU-EDCA) capability, which allows the AP 376 to get higher channel access priority. 378 4.2.1.2. Improved PHY Robustness 380 The 802.11ax PHY can operate with 0.8, 1.6 or 3.2 microsecond guard 381 interval (GI). The larger GI options provide better protection 382 against multipath, which is expected to be a challenge in industrial 383 environments. The possibility to operate with smaller resource units 384 (e.g. 2 MHz) enabled by OFDMA also helps reduce noise power and 385 improve SNR, leading to better packet error rate (PER) performance. 387 802.11ax supports beamforming as in 802.11ac, but introduces UL MU 388 MIMO, which helps improve reliability. The UL MU MIMO capability is 389 also enabled by the trigger based access operation in 802.11ax. 391 4.2.1.3. Support for 6GHz band 393 The 802.11ax specification [IEEE Std. 802.11ax] includes support for 394 operation in the new 6 GHz band. Given the amount of new spectrum 395 available as well as the fact that no legacy 802.11 device (prior 396 802.11ax) will be able to operate in this new band, 802.11ax 397 operation in this new band can be even more efficient. 399 4.2.2. Applicability to deterministic flows 401 TSN capabilities, as defined by the IEEE 802.1 TSN standards, provide 402 the underlying mechanism for supporting deterministic flows in a 403 Local Area Network (LAN). The 802.11 working group has already 404 incorporated support for several TSN capabilities, so that time- 405 sensitive flow can experience precise time synchronization and 406 timeliness when operating over 802.11 links. TSN capabilities 407 supported over 802.11 (which also extends to 802.11ax), include: 409 1. 802.1AS based Time Synchronization (other time synchronization 410 techniques may also be used) 412 2. Interoperating with IEEE802.1Q bridges 414 3. Time-sensitive Traffic Stream identification 416 The exiting 802.11 TSN capabilities listed above, and the 802.11ax 417 OFDMA and scheduled access provide a new set of tools to better 418 server time-sensitive flows. However, it is important to understand 419 the tradeoffs and constraints associated with such capabilities, as 420 well as redundancy and diversity mechanisms that can be used to 421 provide more predictable and reliable performance. 423 4.2.2.1. 802.11 Managed network operation and admission control 425 Time-sensitive applications and TSN standards are expected to operate 426 under a managed network (e.g. industrial/enterprise network). Thus, 427 the Wi-Fi operation must also be carefully managed and integrated 428 with the overall TSN management framework, as defined in the 429 [IEEE8021Qcc] specification. 431 Some of the random-access latency and interference from legacy/ 432 unmanaged devices can be minimized under a centralized management 433 mode as defined in [IEEE8021Qcc], in which admission control 434 procedures are enforced. 436 Existing traffic stream identification, configuration and admission 437 control procedures defined in [IEEE Std. 802.11] QoS mechanism can be 438 re-used. However, given the high degree of determinism required by 439 many time-sensitive applications, additional capabilities to manage 440 interference and legacy devices within tight time-constraints need to 441 be explored. 443 4.2.2.2. Scheduling for bounded latency and diversity 445 As discussed earlier, the [IEEE Std. 802.11ax] OFDMA mode introduces 446 the possibility of assigning different RUs (frequency resources) to 447 users within a PPDU. Several RU sizes are defined in the 448 specification (26, 52, 106, 242, 484, 996 subcarriers). In addition, 449 the AP can also decide on MCS and grouping of users within a given 450 OFMDA PPDU. Such flexibility can be leveraged to support time- 451 sensitive applications with bounded latency, especially in a managed 452 network where stations can be configured to operate under the control 453 of the AP. 455 As shown in [Cavalcanti_2019], it is possible to achieve latencies in 456 the order of 1msec with high reliability in an interference free 457 environment. Obviously, there are latency, reliability and capacity 458 tradeoffs to be considered. For instance, smaller Resource Units 459 (RU)s result in longer transmission durations, which may impact the 460 minimal latency that can be achieved, but the contention latency and 461 randomness elimination due to multi-user transmission is a major 462 benefit of the OFDMA mode. 464 The flexibility to dynamically assign RUs to each transmission also 465 enables the AP to provide frequency diversity, which can help 466 increase reliability. 468 4.3. 802.11be Extreme High Throughput (EHT) 470 4.3.1. General Characteristics 472 The [IEEE 802.11be WIP]is the next major 802.11 amendment (after 473 [IEEE Std. 802.11ax]) for operation in the 2.4, 5 and 6 GHz bands. 474 802.11be is expected to include new PHY and MAC features and it is 475 targeting extremely high throughput (at least 30 Gbps), as well as 476 enhancements to worst case latency and jitter. It is also expected 477 to improve the integration with 802.1 TSN to support time-sensitive 478 applications over Ethernet and Wireless LANs. 480 The 802.11be Task Group started its operation in May 2019, therefore, 481 detailed information about specific features is not yet available. 482 Only high level candidate features have been discussed so far, 483 including: 485 1. 320MHz bandwidth and more efficient utilization of non-contiguous 486 spectrum. 488 2. Multi-band/multi-channel aggregation and operation. 490 3. 16 spatial streams and related MIMO enhancements. 492 4. Multi-Access Point (AP) Coordination. 494 5. Enhanced link adaptation and retransmission protocol, e.g. 495 Hybrid Automatic Repeat Request (HARQ). 497 6. Any required adaptations to regulatory rules for the 6 GHz 498 spectrum. 500 4.3.2. Applicability to deterministic flows 502 The 802.11 Real-Time Applications (RTA) Topic Interest Group (TIG) 503 provided detailed information on use cases, issues and potential 504 solution directions to improve support for time-sensitive 505 applications in 802.11. The RTA TIG report [IEEE_doc_11-18-2009-06] 506 was used as input to the 802.11be project scope. 508 Improvements for worst-case latency, jitter and reliability were the 509 main topics identified in the RTA report, which were motivated by 510 applications in gaming, industrial automation, robotics, etc. The 511 RTA report also highlighted the need to support additional TSN 512 capabilities, such as time-aware (802.1Qbv) shaping and packet 513 replication and elimination as defined in 802.1CB. 515 802.11be is expected to build on and enhance 802.11ax capabilities to 516 improve worst case latency and jitter. Some of the enhancement areas 517 are discussed next. 519 4.3.2.1. Enhanced scheduled operation for bounded latency 521 In addition to the throughput enhancements, 802.11be will leverage 522 the trigger-based scheduled operation enabled by 802.11ax to provide 523 efficient and more predictable medium access. 802.11be is expected to 524 include enhancements to reduce overhead and enable more efficient 525 operation in managed network deployments [IEEE_doc_11-19-0373-00]. 527 4.3.2.2. Multi-AP coordination 529 Multi-AP coordination is one of the main new candidate features in 530 802.11be. It can provide benefits in throughput and capacity and has 531 the potential to address some of the issues that impact worst case 532 latency and reliability. Multi-AP coordination is expected to 533 address the contention due to overlapping Basic Service Sets (OBSS), 534 which is one of the main sources of random latency variations. 535 802.11be can define methods to enable better coordination between 536 APs, for instance, in a managed network scenario, in order to reduce 537 latency due to unmanaged contention. 539 Several multi-AP coordination approaches have been discussed with 540 different levels of complexities and benefits, but specific 541 coordination methods have not yet been defined. 543 4.3.2.3. Multi-band operation 545 802.11be will introduce new features to improve operation over 546 multiple bands and channels. By leveraging multiple bands/channels, 547 802.11be can isolate time-sensitive traffic from network congestion, 548 one of the main causes of large latency variations. In a managed 549 802.11be network, it should be possible to steer traffic to certain 550 bands/channels to isolate time-sensitive traffic from other traffic 551 and help achieve bounded latency. 553 4.4. 802.11ad and 802.11ay (mmWave operation) 554 4.4.1. General Characteristics 556 The IEEE 802.11ad amendment defines PHY and MAC capabilities to 557 enable multi-Gbps throughput in the 60 GHz millimeter wave (mmWave) 558 band. The standard addresses the adverse mmWave signal propagation 559 characteristics and provides directional communication capabilities 560 that take advantage of beamforming to cope with increased 561 attenuation. An overview of the 802.11ad standard can be found in 562 [Nitsche_2015] . 564 The IEEE 802.11ay is currently developing enhancements to the 565 802.11ad standard to enable the next generation mmWave operation 566 targeting 100 Gbps throughput. Some of the main enhancements in 567 802.11ay include MIMO, channel bonding, improved channel access and 568 beamforming training. An overview of the 802.11ay capabilities can 569 be found in [Ghasempour_2017] 571 4.4.2. Applicability to deterministic flows 573 The high data rates achievable with 802.11ad and 802.11ay can 574 significantly reduce latency down to microsecond levels. Limited 575 interference from legacy and other unlicensed devices in 60 GHz is 576 also a benefit. However, directionality and short range typical in 577 mmWave operation impose new challenges such as the overhead required 578 for beam training and blockage issues, which impact both latency and 579 reliability. Therefore, it is important to understand the use case 580 and deployment conditions in order to properly apply and configure 581 802.11ad/ay networks for time sensitive applications. 583 The 802.11ad standard include a scheduled access mode in which 584 stations can be allocated contention-free service periods by a 585 central controller. This scheduling capability is also available in 586 802.11ay, and it is one of the mechanisms that can be used to provide 587 bounded latency to time-sensitive data flows. An analysis of the 588 theoretical latency bounds that can be achieved with 802.11ad service 589 periods is provided in [Cavalcanti_2019]. 591 5. IEEE 802.15.4 593 5.1. Provenance and Documents 595 The IEEE802.15.4 Task Group has been driving the development of low- 596 power low-cost radio technology. The IEEE802.15.4 physical layer has 597 been designed to support demanding low-power scenarios targeting the 598 use of unlicensed bands, both the 2.4 GHz and sub GHz Industrial, 599 Scientific and Medical (ISM) bands. This has imposed requirements in 600 terms of frame size, data rate and bandwidth to achieve reduced 601 collision probability, reduced packet error rate, and acceptable 602 range with limited transmission power. The PHY layer supports frames 603 of up to 127 bytes. The Medium Access Control (MAC) sublayer 604 overhead is in the order of 10-20 bytes, leaving about 100 bytes to 605 the upper layers. IEEE802.15.4 uses spread spectrum modulation such 606 as the Direct Sequence Spread Spectrum (DSSS). 608 The Timeslotted Channel Hopping (TSCH) mode was added to the 2015 609 revision of the IEEE802.15.4 standard [IEEE Std. 802.15.4]. TSCH is 610 targeted at the embedded and industrial world, where reliability, 611 energy consumption and cost drive the application space. 613 At the IETF, the 6TiSCH Working Group (WG) [TiSCH] deals with best 614 effort operation of IPv6 [RFC8200] over TSCH. 6TiSCH has enabled 615 distributed scheduling to exploit the deterministic access 616 capabilities provided by TSCH. The group designed the essential 617 mechanisms to enable the management plane operation while ensuring 618 IPv6 is supported. Yet the charter did not focus to providing a 619 solution to establish end to end Tracks while meeting quality of 620 service requirements. 6TiSCH, through the RFC8480 [RFC8480] defines 621 the 6P protocol which provides a pairwise negotiation mechanism to 622 the control plane operation. The protocol supports agreement on a 623 schedule between neighbors, enabling distributed scheduling. 6P goes 624 hand-in-hand with a Scheduling Function (SF), the policy that decides 625 how to maintain cells and trigger 6P transactions. The Minimal 626 Scheduling Function (MSF) [RFC9033] is the default SF defined by the 627 6TiSCH WG; other standardized SFs can be defined in the future. MSF 628 extends the minimal schedule configuration, and is used to add child- 629 parent links according to the traffic load. 631 Time sensitive networking on low power constrained wireless networks 632 have been partially addressed by ISA100.11a [ISA100.11a] and 633 WirelessHART [WirelessHART]. Both technologies involve a central 634 controller that computes redundant paths for industrial process 635 control traffic over a TSCH mesh. Moreover, ISA100.11a introduces 636 IPv6 capabilities with a Link-Local Address for the join process and 637 a global unicast addres for later exchanges, but the IPv6 traffic 638 typically ends at a local application gateway and the full power of 639 IPv6 for end-to-end communication is not enabled. Compared to that 640 state of the art, work at the IETF and in particular at RAW could 641 provide additional techniques such as optimized P2P routing, PAREO 642 functions, and end-to-end secured IPv6/CoAP connectivity. 644 The 6TiSCH architecture [RFC9030] identifies different models to 645 schedule resources along so-called Tracks (see Section 5.2.2.2) 646 exploiting the TSCH schedule structure however the focus at 6TiSCH is 647 on best effort traffic and the group was never chartered to produce 648 standard work related to Tracks. 650 Useful References include: 652 1. IEEE Std 802.15.4: "IEEE Std. 802.15.4, Part. 15.4: Wireless 653 Medium Access Control (MAC) and Physical Layer (PHY) 654 Specifications for Low-Rate Wireless Personal Area Networks" 655 [IEEE Std. 802.15.4]. The latest version at the time of this 656 writing is dated year 2015. 658 2. Morell, A. , Vilajosana, X. , Vicario, J. L. and Watteyne, T. 659 (2013), Label switching over IEEE802.15.4e networks. Trans. 660 Emerging Tel. Tech., 24: 458-475. doi:10.1002/ett.2650" 661 [morell13]. 663 3. De Armas, J., Tuset, P., Chang, T., Adelantado, F., Watteyne, T., 664 Vilajosana, X. (2016, September). Determinism through path 665 diversity: Why packet replication makes sense. In 2016 666 International Conference on Intelligent Networking and 667 Collaborative Systems (INCoS) (pp. 150-154). IEEE. [dearmas16]. 669 4. X. Vilajosana, T. Watteyne, M. Vucinic, T. Chang and K. S. 670 J. Pister, "6TiSCH: Industrial Performance for IPv6 Internet-of- 671 Things Networks," in Proceedings of the IEEE, vol. 107, no. 6, 672 pp. 1153-1165, June 2019. [vilajosana19]. 674 5.2. TimeSlotted Channel Hopping 676 5.2.1. General Characteristics 678 As a core technique in IEEE802.15.4, TSCH splits time in multiple 679 time slots that repeat over time. A set of timeslots constructs a 680 Slotframe (see Section 5.2.2.1.4). For each timeslot, a set of 681 available frequencies can be used, resulting in a matrix-like 682 schedule (see Figure 1). 684 timeslot offset 685 | 0 1 2 3 4 | 0 1 2 3 4 | Nodes 686 +------------------------+------------------------+ +-----+ 687 | | | | | | | | | | | | C | 688 CH-1 | EB | | |C->B| | EB | | |C->B| | | | 689 | | | | | | | | | | | +-----+ 690 +-------------------------------------------------+ | 691 | | | | | | | | | | | | 692 CH-2 | | |B->C| |B->A| | |B->C| |B->A| +-----+ 693 | | | | | | | | | | | | B | 694 +-------------------------------------------------+ | | 695 ... +-----+ 696 | 697 +-------------------------------------------------+ | 698 | | | | | | | | | | | +-----+ 699 CH-15| |A->B| | | | |A->B| | | | | A | 700 | | | | | | | | | | | | | 701 +-------------------------------------------------+ +-----+ 702 ch. 703 offset 705 Figure 1: Slotframe example with scheduled cells between nodes A, 706 B and C 708 This schedule represents the possible communications of a node with 709 its neighbors, and is managed by a Scheduling Function such as the 710 Minimal Scheduling Function (MSF) [RFC9033]. Each cell in the 711 schedule is identified by its slotoffset and channeloffset 712 coordinates. A cell's timeslot offset indicates its position in 713 time, relative to the beginning of the slotframe. A cell's channel 714 offset is an index which maps to a frequency at each iteration of the 715 slotframe. Each packet exchanged between neighbors happens within 716 one cell. The size of a cell is a timeslot duration, between 10 to 717 15 milliseconds. An Absolute Slot Number (ASN) indicates the number 718 of slots elapsed since the network started. It increments at every 719 slot. This is a 5 byte counter that can support networks running for 720 more than 300 years without wrapping (assuming a 10 ms timeslot). 721 Channel hopping provides increased reliability to multi-path fading 722 and external interference. It is handled by TSCH through a channel 723 hopping sequence referred as macHopSeq in the IEEE802.15.4 724 specification. 726 The Time-Frequency Division Multiple Access provided by TSCH enables 727 the orchestration of traffic flows, spreading them in time and 728 frequency, and hence enabling an efficient management of the 729 bandwidth utilization. Such efficient bandwidth utilization can be 730 combined to OFDM modulations also supported by the IEEE802.15.4 731 standard [IEEE Std. 802.15.4] since the 2015 version. 733 TSCH networks operate in ISM bands in which the spectrum is shared by 734 different coexisting technologies. Regulations such as FCC, ETSI and 735 ARIB impose duty cycle regulations to limit the use of the bands but 736 yet interference may constraint the probability to deliver a packet. 737 Part of these reliability challenges are addressed at the MAC 738 introducing redundancy and diversity, thanks to channel hopping, 739 scheduling and ARQ policies. Yet, the MAC layer operates with a 740 1-hop vision, being limited to local actions to mitigate 741 underperforming links. 743 In the RAW context, low power reliable networks should address non- 744 critical control scenarios such as Class 2 and monitoring scenarios 745 such as Class 4 defined by the RFC5673 [RFC5673]. As a low power 746 technology targeting industrial scenarios radio transducers provide 747 low data rates (typically between 50kbps to 250kbps) and robust 748 modulations to trade-off performance to reliability. TSCH networks 749 are organized in mesh topologies and connected to a backbone. 750 Latency in the mesh network is mainly influenced by propagation 751 aspects such as interference. ARQ methods and redundancy techniques 752 such as replication and elimination should be studied to provide the 753 needed performance to address deterministic scenarios. 755 5.2.2. Applicability to Deterministic Flows 757 Nodes in a TSCH network are tightly synchronized. This enables to 758 build the slotted structure and ensure efficient utilization of 759 resources thanks to proper scheduling policies. Scheduling is a key 760 to orchestrate the resources that different nodes in a Track or a 761 path are using. Slotframes can be split in resource blocks reserving 762 the needed capacity to certain flows. Periodic and bursty traffic 763 can be handled independently in the schedule, using active and 764 reactive policies and taking advantage of overprovisionned cells to 765 measu reth excursion. Along a Track, resource blocks can be chained 766 so nodes in previous hops transmit their data before the next packet 767 comes. This provides a tight control to latency along a Track. 768 Collision loss is avoided for best effort traffic by 769 overprovisionning resources, giving time to the management plane of 770 the network to dedicate more resources if needed. 772 5.2.2.1. Centralized Path Computation 774 In a controlled environment, a 6TiSCH device usually does not place a 775 request for bandwidth between itself and another device in the 776 network. Rather, an Operation Control System (OCS) invoked through 777 an Human/Machine Interface (HMI) iprovides the Traffic Specification, 778 in particular in terms of latency and reliability, and the end nodes, 779 to a Path Computation element (PCE). With this, the PCE computes a 780 Track between the end nodes and provisions every hop in the Track 781 with per-flow state that describes the per-hop operation for a given 782 packet, the corresponding timeSlots, and the flow identification to 783 recognize which packet is placed in which Track, sort out duplicates, 784 etc. In Figure 2, an example of Operational Control System and HMI 785 is depicted. 787 For a static configuration that serves a certain purpose for a long 788 period of time, it is expected that a node will be provisioned in one 789 shot with a full schedule, which incorporates the aggregation of its 790 behavior for multiple Tracks. The 6TiSCH Architecture expects that 791 the programing of the schedule is done over CoAP as discussed in 792 "6TiSCH Resource Management and Interaction using CoAP" 793 [I-D.ietf-6tisch-coap]. 795 But an Hybrid mode may be required as well whereby a single Track is 796 added, modified, or removed, for instance if it appears that a Track 797 does not perform as expected for, say, Packet Delivery Ratio (PDR). 798 For that case, the expectation is that a protocol that flows along a 799 Track (to be), in a fashion similar to classical Traffic Engineering 800 (TE) [CCAMP], may be used to update the state in the devices. 6TiSCH 801 provides means for a device to negotiate a timeSlot with a neighbor, 802 but in general that flow was not designed and no protocol was 803 selected and it is expected that DetNet will determine the 804 appropriate end-to-end protocols to be used in that case. 806 Stream Management Entity 808 Operational Control System and HMI 810 -+-+-+-+-+-+-+ Northbound -+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- 812 PCE PCE PCE PCE 814 -+-+-+-+-+-+-+ Southbound -+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- 816 --- 6TiSCH------6TiSCH------6TiSCH------6TiSCH-- 817 6TiSCH / Device Device Device Device \ 818 Device- - 6TiSCH 819 \ 6TiSCH 6TiSCH 6TiSCH 6TiSCH / Device 820 ----Device------Device------Device------Device-- 822 Figure 2 824 5.2.2.1.1. Packet Marking and Handling 826 Section "Packet Marking and Handling" of [RFC9030] describes the 827 packet tagging and marking that is expected in 6TiSCH networks. 829 5.2.2.1.1.1. Tagging Packets for Flow Identification 831 For packets that are routed by a PCE along a Track, the tuple formed 832 by the IPv6 source address and a local RPLInstanceID is tagged in the 833 packets to identify uniquely the Track and associated transmit bundle 834 of timeSlots. 836 It results that the tagging that is used for a DetNet flow outside 837 the 6TiSCH LLN MUST be swapped into 6TiSCH formats and back as the 838 packet enters and then leaves the 6TiSCH network. 840 Note: The method and format used for encoding the RPLInstanceID at 841 6lo is generalized to all 6TiSCH topological Instances, which 842 includes Tracks. 844 5.2.2.1.1.2. Replication, Retries and Elimination 846 PRE establishes several paths in a network to provide redundancy and 847 parallel transmissions to bound the end-to-end delay. Considering 848 the scenario shown in Figure 3, many different paths are possible for 849 S to reach R. A simple way to benefit from this topology could be to 850 use the two independent paths via nodes A, C, E and via B, D, F. But 851 more complex paths are possible as well. 853 (A) (C) (E) 855 source (S) (R) (destination) 857 (B) (D) (F) 859 Figure 3: A Typical Ladder Shape with Two Parallel Paths Toward 860 the Destination 862 By employing a Packet Replication function, each node forwards a copy 863 of each data packet over two different branches. For instance, in 864 Figure 4, the source node S transmits the data packet to nodes A and 865 B, in two different timeslots within the same TSCH slotframe. 867 ===> (A) => (C) => (E) === 868 // \\// \\// \\ 869 source (S) //\\ //\\ (R) (destination) 870 \\ // \\ // \\ // 871 ===> (B) => (D) => (F) === 873 Figure 4: Packet Replication: S transmits twice the same data 874 packet, to its DP (A) and to its AP (B). 876 By employing Packet Elimination function once a node receives the 877 first copy of a data packet, it discards the subsequent copies. 878 Because the first copy that reaches a node is the one that matters, 879 it is the only copy that will be forwarded upward. 881 Considering that the wireless medium is broadcast by nature, any 882 neighbor of a transmitter may overhear a transmission. By employing 883 the Promiscuous Overhearing function, nodes will have multiple 884 opportunities to receive a given data packet. For instance, in 885 Figure 4, when the source node S transmits the data packet to node A, 886 node B may overhear this transmission. 888 6TiSCH expects elimination and replication of packets along a complex 889 Track, but has no position about how the sequence numbers would be 890 tagged in the packet. 892 As it goes, 6TiSCH expects that timeSlots corresponding to copies of 893 a same packet along a Track are correlated by configuration, and does 894 not need to process the sequence numbers. 896 The semantics of the configuration MUST enable correlated timeSlots 897 to be grouped for transmit (and respectively receive) with 898 a'OR'relations, and then a'AND'relation MUST be configurable between 899 groups. The semantics is that if the transmit (and respectively 900 receive) operation succeeded in one timeSlot in a'OR'group, then all 901 the other timeSLots in the group are ignored. Now, if there are at 902 least two groups, the'AND'relation between the groups indicates that 903 one operation must succeed in each of the groups. 905 On the transmit side, timeSlots provisioned for retries along a same 906 branch of a Track are placed a same'OR'group. The'OR'relation 907 indicates that if a transmission is acknowledged, then further 908 transmissions SHOULD NOT be attempted for timeSlots in that group. 909 There are as many'OR'groups as there are branches of the Track 910 departing from this node. Different'OR'groups are programmed for the 911 purpose of replication, each group corresponding to one branch of the 912 Track. The'AND'relation between the groups indicates that 913 transmission over any of branches MUST be attempted regardless of 914 whether a transmission succeeded in another branch. It is also 915 possible to place cells to different next-hop routers in a 916 same'OR'group. This allows to route along multi-path Tracks, trying 917 one next-hop and then another only if sending to the first fails. 919 On the receive side, all timeSlots are programmed in a same'OR'group. 920 Retries of a same copy as well as converging branches for elimination 921 are converged, meaning that the first successful reception is enough 922 and that all the other timeSlots can be ignored. 924 5.2.2.1.1.3. Differentiated Services Per-Hop-Behavior 926 Additionally, an IP packet that is sent along a Track uses the 927 Differentiated Services Per-Hop-Behavior Group called Deterministic 928 Forwarding, as described in 929 [I-D.svshah-tsvwg-deterministic-forwarding]. 931 5.2.2.1.2. Topology and capabilities 933 6TiSCH nodes are usually IoT devices, characterized by very limited 934 amount of memory, just enough buffers to store one or a few IPv6 935 packets, and limited bandwidth between peers. It results that a node 936 will maintain only a small number of peering information, and will 937 not be able to store many packets waiting to be forwarded. Peers can 938 be identified through MAC or IPv6 addresses. 940 Neighbors can be discovered over the radio using mechanism such as 941 Enhanced Beacons, but, though the neighbor information is available 942 in the 6TiSCH interface data model, 6TiSCH does not describe a 943 protocol to pro-actively push the neighborhood information to a PCE. 944 This protocol should be described and should operate over CoAP. The 945 protocol should be able to carry multiple metrics, in particular the 946 same metrics as used for RPL operations [RFC6551]. 948 The energy that the device consumes in sleep, transmit and receive 949 modes can be evaluated and reported. So can the amount of energy 950 that is stored in the device and the power that it can be scavenged 951 from the environment. The PCE SHOULD be able to compute Tracks that 952 will implement policies on how the energy is consumed, for instance 953 balance between nodes, ensure that the spent energy does not exceeded 954 the scavenged energy over a period of time, etc... 956 5.2.2.1.3. Schedule Management by a PCE 958 6TiSCH supports a mixed model of centralized routes and distributed 959 routes. Centralized routes can for example be computed by a entity 960 such as a PCE [PCE]. Distributed routes are computed by RPL 961 [RFC6550]. 963 Both methods may inject routes in the Routing Tables of the 6TiSCH 964 routers. In either case, each route is associated with a 6TiSCH 965 topology that can be a RPL Instance topology or a Track. The 6TiSCH 966 topology is indexed by a Instance ID, in a format that reuses the 967 RPLInstanceID as defined in RPL. 969 Both RPL and PCE rely on shared sources such as policies to define 970 Global and Local RPLInstanceIDs that can be used by either method. 971 It is possible for centralized and distributed routing to share a 972 same topology. Generally they will operate in different slotFrames, 973 and centralized routes will be used for scheduled traffic and will 974 have precedence over distributed routes in case of conflict between 975 the slotFrames. 977 5.2.2.1.4. SlotFrames and Priorities 979 A slotFrame is the base object that a PCE needs to manipulate to 980 program a schedule into an LLN node. Elaboration on that concept can 981 be fond in section "SlotFrames and Priorities" of [RFC9030] 983 IEEE802.15.4 TSCH avoids contention on the medium by formatting time 984 and frequencies in cells of transmission of equal duration. In order 985 to describe that formatting of time and frequencies, the 6TiSCH 986 architecture defines a global concept that is called a Channel 987 Distribution and Usage (CDU) matrix; a CDU matrix is a matrix of 988 cells with an height equal to the number of available channels 989 (indexed by ChannelOffsets) and a width (in timeSlots) that is the 990 period of the network scheduling operation (indexed by slotOffsets) 991 for that CDU matrix. The size of a cell is a timeSlot duration, and 992 values of 10 to 15 milliseconds are typical in 802.15.4 TSCH to 993 accommodate for the transmission of a frame and an acknowledgement, 994 including the security validation on the receive side which may take 995 up to a few milliseconds on some device architecture. 997 The frequency used by a cell in the matrix rotates in a pseudo-random 998 fashion, from an initial position at an epoch time, as the matrix 999 iterates over and over. 1001 A CDU matrix is computed by the PCE, but unallocated timeSlots may be 1002 used opportunistically by the nodes for classical best effort IP 1003 traffic. The PCE has precedence in the allocation in case of a 1004 conflict. 1006 In a given network, there might be multiple CDU matrices that operate 1007 with different width, so they have different durations and represent 1008 different periodic operations. It is recommended that all CDU 1009 matrices in a 6TiSCH domain operate with the same cell duration and 1010 are aligned, so as to reduce the chances of interferences from 1011 slotted-aloha operations. The PCE MUST compute the CDU matrices and 1012 shared that knowledge with all the nodes. The matrices are used in 1013 particular to define slotFrames. 1015 A slotFrame is a MAC-level abstraction that is common to all nodes 1016 and contains a series of timeSlots of equal length and precedence. 1017 It is characterized by a slotFrame_ID, and a slotFrame_size. A 1018 slotFrame aligns to a CDU matrix for its parameters, such as number 1019 and duration of timeSlots. 1021 Multiple slotFrames can coexist in a node schedule, i.e., a node can 1022 have multiple activities scheduled in different slotFrames, based on 1023 the precedence of the 6TiSCH topologies. The slotFrames may be 1024 aligned to different CDU matrices and thus have different width. 1025 There is typically one slotFrame for scheduled traffic that has the 1026 highest precedence and one or more slotFrame(s) for RPL traffic. The 1027 timeSlots in the slotFrame are indexed by the SlotOffset; the first 1028 cell is at SlotOffset 0. 1030 The 6TiSCH architecture introduces the concept of chunks ([RFC9030]) 1031 to operate such spectrum distribution for a whole group of cells at a 1032 time. The CDU matrix is formatted into a set of chunks, each of them 1033 identified uniquely by a chunk-ID, see Figure 5. The PCE MUST 1034 compute the partitioning of CDU matrices into chunks and shared that 1035 knowledge with all the nodes in a 6TiSCH network. 1037 +-----+-----+-----+-----+-----+-----+-----+ +-----+ 1038 chan.Off. 0 |chnkA|chnkP|chnk7|chnkO|chnk2|chnkK|chnk1| ... |chnkZ| 1039 +-----+-----+-----+-----+-----+-----+-----+ +-----+ 1040 chan.Off. 1 |chnkB|chnkQ|chnkA|chnkP|chnk3|chnkL|chnk2| ... |chnk1| 1041 +-----+-----+-----+-----+-----+-----+-----+ +-----+ 1042 ... 1043 +-----+-----+-----+-----+-----+-----+-----+ +-----+ 1044 chan.Off. 15 |chnkO|chnk6|chnkN|chnk1|chnkJ|chnkZ|chnkI| ... |chnkG| 1045 +-----+-----+-----+-----+-----+-----+-----+ +-----+ 1046 0 1 2 3 4 5 6 M 1048 Figure 5: CDU matrix Partitioning in Chunks 1050 The appropriation of a chunk can be requested explicitly by the PCE 1051 to any node. After a successful appropriation, the PCE owns the 1052 cells in that chunk, and may use them as hard cells to set up Tracks. 1053 Then again, 6TiSCH did not propose a method for chunk definition and 1054 a protocol for appropriation. This is to be done at RAW. 1056 5.2.2.2. 6TiSCH Tracks 1058 A Track at 6TiSCH is the application to wireless of the concept of a 1059 path in the "Detnet architecture" [RFC8655]. A Track can follow a 1060 simple sequence of relay nodes or can be structured as a more complex 1061 Destination Oriented Directed Acyclic Graph (DODAG) to a unicast 1062 destination. Along a Track, 6TiSCH nodes reserve the resources to 1063 enable the efficient transmission of packets while aiming to optimize 1064 certain properties such as reliability and ensure small jitter or 1065 bounded latency. The Track structure enables Layer-2 forwarding 1066 schemes, reducing the overhead of taking routing decisions at the 1067 Layer-3. 1069 Serial Tracks can be understood as the concatenation of cells or 1070 bundles along a routing path from a source towards a destination. 1071 The serial Track concept is analogous to the circuit concept where 1072 resources are chained through the multi-hop topology. For example, A 1073 bundle of Tx Cells in a particular node is paired to a bundle of Rx 1074 Cells in the next hop node following a routing path. 1076 Whereas scheduling ensures reliable delivery in bounded time along 1077 any Track, high availability requires the application of PAREO 1078 functions along a more complex DODAG Track structure. A DODAG has 1079 forking and joining nodes where the concepts such as Replication and 1080 Elimination can be exploited. Spatial redundancy increases the 1081 oveall energy consumption in the network but improves significantly 1082 the availability of the network as well as the packet delivery ratio. 1083 A Track may also branch off and rejoin, for the purpose of the so- 1084 called Packet Replication and Elimination (PRE), over non congruent 1085 branches. PRE may be used to complement layer-2 Automatic Repeat 1086 reQuest (ARQ) and receiver-end Ordering to form the PAREO functions. 1087 PAREO functions enable to meet industrial expectations in PDR within 1088 bounded delivery time over a Track that includes wireless links, even 1089 when the Track extends beyond the 6TiSCH network. 1091 +-----+ 1092 | IoT | 1093 | G/W | 1094 +-----+ 1095 ^ <---- Elimination 1096 | | 1097 Track branch | | 1098 +-------+ +--------+ Subnet Backbone 1099 | | 1100 +--|--+ +--|--+ 1101 | | | Backbone | | | Backbone 1102 o | | | router | | | router 1103 +--/--+ +--|--+ 1104 o / o o---o----/ o 1105 o o---o--/ o o o o o 1106 o \ / o o LLN o 1107 o v <---- Replication 1108 o 1110 Figure 6: End-to-End deterministic Track 1112 In the example above (see Figure 6), a Track is laid out from a field 1113 device in a 6TiSCH network to an IoT gateway that is located on a 1114 IEEE802.1 TSN backbone. 1116 The Replication function in the field device sends a copy of each 1117 packet over two different branches, and a PCE schedules each hop of 1118 both branches so that the two copies arrive in due time at the 1119 gateway. In case of a loss on one branch, hopefully the other copy 1120 of the packet still makes it in due time. If two copies make it to 1121 the IoT gateway, the Elimination function in the gateway ignores the 1122 extra packet and presents only one copy to upper layers. 1124 At each 6TiSCH hop along the Track, the PCE may schedule more than 1125 one timeSlot for a packet, so as to support Layer-2 retries (ARQ). 1126 It is also possible that the field device only uses the second branch 1127 if sending over the first branch fails. 1129 In current deployments, a TSCH Track does not necessarily support PRE 1130 but is systematically multi-path. This means that a Track is 1131 scheduled so as to ensure that each hop has at least two forwarding 1132 solutions, and the forwarding decision is to try the preferred one 1133 and use the other in case of Layer-2 transmission failure as detected 1134 by ARQ. 1136 Methods to implement complex Tracks are described in 1137 [I-D.papadopoulos-paw-pre-reqs] and complemented by extensions to the 1138 RPL routing protocol in [I-D.ietf-roll-nsa-extension] for best effort 1139 traffic, but a centralized routing technique such as promoted in 1140 DetNet is still missing. 1142 5.2.2.2.1. Track Scheduling Protocol 1144 Section "Schedule Management Mechanisms" of the 6TiSCH architecture 1145 describes 4 paradigms to manage the TSCH schedule of the LLN nodes: 1146 Static Scheduling, neighbor-to-neighbor Scheduling, remote monitoring 1147 and scheduling management, and Hop-by-hop scheduling. The Track 1148 operation for DetNet corresponds to a remote monitoring and 1149 scheduling management by a PCE. 1151 Early work at 6TiSCH on a data model and a protocol to program the 1152 schedule in the 6TiSCH device was never concluded as the group 1153 focussed on best effort traffic. This work would be revived by RAW: 1155 The 6top interface document [RFC8480] (to be reopened at RAW) was 1156 intended to specify the generic data model that can be used to 1157 monitor and manage resources of the 6top sublayer. Abstract 1158 methods were suggested for use by a management entity in the 1159 device. The data model also enables remote control operations on 1160 the 6top sublayer. 1162 [I-D.ietf-6tisch-coap] (to be reopened at RAW) was intended to 1163 define a mapping of the 6top set of commands, which is described 1164 in RFC 8480, to CoAP resources. This allows an entity to interact 1165 with the 6top layer of a node that is multiple hops away in a 1166 RESTful fashion. 1168 [I-D.ietf-6tisch-coap] also defined a basic set CoAP resources and 1169 associated RESTful access methods (GET/PUT/POST/DELETE). The 1170 payload (body) of the CoAP messages is encoded using the CBOR 1171 format. The PCE commands are expected to be issued directly as 1172 CoAP requests or to be mapped back and forth into CoAP by a 1173 gateway function at the edge of the 6TiSCH network. For instance, 1174 it is possible that a mapping entity on the backbone transforms a 1175 non-CoAP protocol such as PCEP into the RESTful interfaces that 1176 the 6TiSCH devices support. 1178 5.2.2.2.2. Track Forwarding 1180 By forwarding, this specification means the per-packet operation that 1181 allows to deliver a packet to a next hop or an upper layer in this 1182 node. Forwarding is based on pre-existing state that was installed 1183 as a result of the routing computation of a Track by a PCE. The 1184 6TiSCH architecture supports three different forwarding model, G-MPLS 1185 Track Forwarding (TF), 6LoWPAN Fragment Forwarding (FF) and IPv6 1186 Forwarding (6F) which is the classical IP operation [RFC9030]. The 1187 DetNet case relates to the Track Forwarding operation under the 1188 control of a PCE. 1190 A Track is a unidirectional path between a source and a destination. 1191 In a Track cell, the normal operation of IEEE802.15.4 Automatic 1192 Repeat-reQuest (ARQ) usually happens, though the acknowledgment may 1193 be omitted in some cases, for instance if there is no scheduled cell 1194 for a retry. 1196 Track Forwarding is the simplest and fastest. A bundle of cells set 1197 to receive (RX-cells) is uniquely paired to a bundle of cells that 1198 are set to transmit (TX-cells), representing a layer-2 forwarding 1199 state that can be used regardless of the network layer protocol. 1200 This model can effectively be seen as a Generalized Multi-protocol 1201 Label Switching (G-MPLS) operation in that the information used to 1202 switch a frame is not an explicit label, but rather related to other 1203 properties of the way the packet was received, a particular cell in 1204 the case of 6TiSCH. As a result, as long as the TSCH MAC (and 1205 Layer-2 security) accepts a frame, that frame can be switched 1206 regardless of the protocol, whether this is an IPv6 packet, a 6LoWPAN 1207 fragment, or a frame from an alternate protocol such as WirelessHART 1208 or ISA100.11a. 1210 A data frame that is forwarded along a Track normally has a 1211 destination MAC address that is set to broadcast - or a multicast 1212 address depending on MAC support. This way, the MAC layer in the 1213 intermediate nodes accepts the incoming frame and 6top switches it 1214 without incurring a change in the MAC header. In the case of 1215 IEEE802.15.4, this means effectively broadcast, so that along the 1216 Track the short address for the destination of the frame is set to 1217 0xFFFF. 1219 A Track is thus formed end-to-end as a succession of paired bundles, 1220 a receive bundle from the previous hop and a transmit bundle to the 1221 next hop along the Track, and a cell in such a bundle belongs to at 1222 most one Track. For a given iteration of the device schedule, the 1223 effective channel of the cell is obtained by adding a pseudo-random 1224 number to the channelOffset of the cell, which results in a rotation 1225 of the frequency that used for transmission. The bundles may be 1226 computed so as to accommodate both variable rates and 1227 retransmissions, so they might not be fully used at a given iteration 1228 of the schedule. The 6TiSCH architecture provides additional means 1229 to avoid waste of cells as well as overflows in the transmit bundle, 1230 as follows: 1232 In one hand, a TX-cell that is not needed for the current iteration 1233 may be reused opportunistically on a per-hop basis for routed 1234 packets. When all of the frame that were received for a given Track 1235 are effectively transmitted, any available TX-cell for that Track can 1236 be reused for upper layer traffic for which the next-hop router 1237 matches the next hop along the Track. In that case, the cell that is 1238 being used is effectively a TX-cell from the Track, but the short 1239 address for the destination is that of the next-hop router. It 1240 results that a frame that is received in a RX-cell of a Track with a 1241 destination MAC address set to this node as opposed to broadcast must 1242 be extracted from the Track and delivered to the upper layer (a frame 1243 with an unrecognized MAC address is dropped at the lower MAC layer 1244 and thus is not received at the 6top sublayer). 1246 On the other hand, it might happen that there are not enough TX-cells 1247 in the transmit bundle to accommodate the Track traffic, for instance 1248 if more retransmissions are needed than provisioned. In that case, 1249 the frame can be placed for transmission in the bundle that is used 1250 for layer-3 traffic towards the next hop along the Track as long as 1251 it can be routed by the upper layer, that is, typically, if the frame 1252 transports an IPv6 packet. The MAC address should be set to the 1253 next-hop MAC address to avoid confusion. It results that a frame 1254 that is received over a layer-3 bundle may be in fact associated to a 1255 Track. In a classical IP link such as an Ethernet, off-Track traffic 1256 is typically in excess over reservation to be routed along the non- 1257 reserved path based on its QoS setting. But with 6TiSCH, since the 1258 use of the layer-3 bundle may be due to transmission failures, it 1259 makes sense for the receiver to recognize a frame that should be re- 1260 Tracked, and to place it back on the appropriate bundle if possible. 1261 A frame should be re-Tracked if the Per-Hop-Behavior group indicated 1262 in the Differentiated Services Field in the IPv6 header is set to 1263 Deterministic Forwarding, as discussed in Section 5.2.2.1.1. A frame 1264 is re-Tracked by scheduling it for transmission over the transmit 1265 bundle associated to the Track, with the destination MAC address set 1266 to broadcast. 1268 There are 2 modes for a Track, transport mode and tunnel mode. 1270 5.2.2.2.2.1. Transport Mode 1272 In transport mode, the Protocol Data Unit (PDU) is associated with 1273 flow-dependant meta-data that refers uniquely to the Track, so the 1274 6top sublayer can place the frame in the appropriate cell without 1275 ambiguity. In the case of IPv6 traffic, this flow identification is 1276 transported in the Flow Label of the IPv6 header. Associated with 1277 the source IPv6 address, the Flow Label forms a globally unique 1278 identifier for that particular Track that is validated at egress 1279 before restoring the destination MAC address (DMAC) and punting to 1280 the upper layer. 1282 | ^ 1283 +--------------+ | | 1284 | IPv6 | | | 1285 +--------------+ | | 1286 | 6LoWPAN HC | | | 1287 +--------------+ ingress egress 1288 | 6top | sets +----+ +----+ restores 1289 +--------------+ dmac to | | | | dmac to 1290 | TSCH MAC | brdcst | | | | self 1291 +--------------+ | | | | | | 1292 | LLN PHY | +-------+ +--...-----+ +-------+ 1293 +--------------+ 1295 Figure 7: Track Forwarding, Transport Mode 1297 5.2.2.2.2.2. Tunnel Mode 1299 In tunnel mode, the frames originate from an arbitrary protocol over 1300 a compatible MAC that may or may not be synchronized with the 6TiSCH 1301 network. An example of this would be a router with a dual radio that 1302 is capable of receiving and sending WirelessHART or ISA100.11a frames 1303 with the second radio, by presenting itself as an Access Point or a 1304 Backbone Router, respectively. 1306 In that mode, some entity (e.g. PCE) can coordinate with a 1307 WirelessHART Network Manager or an ISA100.11a System Manager to 1308 specify the flows that are to be transported transparently over the 1309 Track. 1311 +--------------+ 1312 | IPv6 | 1313 +--------------+ 1314 | 6LoWPAN HC | 1315 +--------------+ set restore 1316 | 6top | +dmac+ +dmac+ 1317 +--------------+ to|brdcst to|nexthop 1318 | TSCH MAC | | | | | 1319 +--------------+ | | | | 1320 | LLN PHY | +-------+ +--...-----+ +-------+ 1321 +--------------+ | ingress egress | 1322 | | 1323 +--------------+ | | 1324 | LLN PHY | | | 1325 +--------------+ | | 1326 | TSCH MAC | | | 1327 +--------------+ | dmac = | dmac = 1328 |ISA100/WiHART | | nexthop v nexthop 1329 +--------------+ 1331 Figure 8: Track Forwarding, Tunnel Mode 1333 In that case, the flow information that identifies the Track at the 1334 ingress 6TiSCH router is derived from the RX-cell. The dmac is set 1335 to this node but the flow information indicates that the frame must 1336 be tunneled over a particular Track so the frame is not passed to the 1337 upper layer. Instead, the dmac is forced to broadcast and the frame 1338 is passed to the 6top sublayer for switching. 1340 At the egress 6TiSCH router, the reverse operation occurs. Based on 1341 metadata associated to the Track, the frame is passed to the 1342 appropriate link layer with the destination MAC restored. 1344 5.2.2.2.2.3. Tunnel Metadata 1346 Metadata coming with the Track configuration is expected to provide 1347 the destination MAC address of the egress endpoint as well as the 1348 tunnel mode and specific data depending on the mode, for instance a 1349 service access point for frame delivery at egress. If the tunnel 1350 egress point does not have a MAC address that matches the 1351 configuration, the Track installation fails. 1353 In transport mode, if the final layer-3 destination is the tunnel 1354 termination, then it is possible that the IPv6 address of the 1355 destination is compressed at the 6LoWPAN sublayer based on the MAC 1356 address. It is thus mandatory at the ingress point to validate that 1357 the MAC address that was used at the 6LoWPAN sublayer for compression 1358 matches that of the tunnel egress point. For that reason, the node 1359 that injects a packet on a Track checks that the destination is 1360 effectively that of the tunnel egress point before it overwrites it 1361 to broadcast. The 6top sublayer at the tunnel egress point reverts 1362 that operation to the MAC address obtained from the tunnel metadata. 1364 5.2.2.2.2.4. OAM 1366 An Overview of Operations, Administration, and Maintenance (OAM) 1367 Tools [RFC7276] provides an overwiew of the existing tooling for OAM 1368 [RFC6291]. Tracks are complex paths and new tooling is necessary to 1369 manage them, with respect to load control, timing, and the Packet 1370 Replication and Elimination Functions (PREF). 1372 An example of such tooling can be found in the context of BIER 1373 [RFC8279] and more specifically BIER Traffic Engineering 1374 [I-D.ietf-bier-te-arch] (BIER-TE): 1375 [I-D.thubert-bier-replication-elimination] leverages BIER-TE to 1376 control the process of PREF, and to provide traceability of these 1377 operations, in the deterministic dataplane, along a complex Track. 1378 For the 6TiSCH type of constrained environment, 1379 [I-D.thubert-6lo-bier-dispatch] enables an efficient encoding of the 1380 BIER bitmap within the 6LoRH framework. 1382 6. 5G 1384 6.1. Provenance and Documents 1386 The 3rd Generation Partnership Project (3GPP) incorporates many 1387 companies whose business is related to cellular network operation as 1388 well as network equipment and device manufacturing. All generations 1389 of 3GPP technologies provide scheduled wireless segments, primarily 1390 in licensed spectrum which is beneficial for reliability and 1391 availability. 1393 In 2016, the 3GPP started to design New Radio (NR) technology 1394 belonging to the fifth generation (5G) of cellular networks. NR has 1395 been designed from the beginning to not only address enhanced Mobile 1396 Broadband (eMBB) services for consumer devices such as smart phones 1397 or tablets but is also tailored for future Internet of Things (IoT) 1398 communication and connected cyber-physical systems. In addition to 1399 eMBB, requirement categories have been defined on Massive Machine- 1400 Type Communication (M-MTC) for a large number of connected devices/ 1401 sensors, and Ultra-Reliable Low-Latency Communication (URLLC) for 1402 connected control systems and critical communication as illustrated 1403 in Figure 9. It is the URLLC capabilities that make 5G a great 1404 candidate for reliable low-latency communication. With these three 1405 corner stones, NR is a complete solution supporting the connectivity 1406 needs of consumers, enterprises, and public sector for both wide area 1407 and local area, e.g. indoor deployments. A general overview of NR 1408 can be found in [TS38300]. 1410 enhanced 1411 Mobile Broadband 1412 ^ 1413 / \ 1414 / \ 1415 / \ 1416 / \ 1417 / 5G \ 1418 / \ 1419 / \ 1420 / \ 1421 +-----------------+ 1422 Massive Ultra-Reliable 1423 Machine-Type Low-Latency 1424 Communication Communication 1426 Figure 9: 5G Application Areas 1428 As a result of releasing the first NR specification in 2018 (Release 1429 15), it has been proven by many companies that NR is a URLLC-capable 1430 technology and can deliver data packets at 10^-5 packet error rate 1431 within 1ms latency budget [TR37910]. Those evaluations were 1432 consolidated and forwarded to ITU to be included in the [IMT2020] 1433 work. 1435 In order to understand communication requirements for automation in 1436 vertical domains, 3GPP studied different use cases [TR22804] and 1437 released technical specification with reliability, availability and 1438 latency demands for a variety of applications [TS22104]. 1440 As an evolution of NR, multiple studies have been conducted in scope 1441 of 3GPP Release 16 including the following two, focusing on radio 1442 aspects: 1444 1. Study on physical layer enhancements for NR ultra-reliable and 1445 low latency communication (URLLC) [TR38824]. 1447 2. Study on NR industrial Internet of Things (I-IoT) [TR38825]. 1449 In addition, several enhancements have been done on system 1450 architecture level which are reflected in System architecture for the 1451 5G System (5GS) [TS23501]. 1453 6.2. General Characteristics 1455 The 5G Radio Access Network (5G RAN) with its NR interface includes 1456 several features to achieve Quality of Service (QoS), such as a 1457 guaranteeably low latency or tolerable packet error rates for 1458 selected data flows. Determinism is achieved by centralized 1459 admission control and scheduling of the wireless frequency resources, 1460 which are typically licensed frequency bands assigned to a network 1461 operator. 1463 NR enables short transmission slots in a radio subframe, which 1464 benefits low-latency applications. NR also introduces mini-slots, 1465 where prioritized transmissions can be started without waiting for 1466 slot boundaries, further reducing latency. As part of giving 1467 priority and faster radio access to URLLC traffic, NR introduces 1468 preemption where URLLC data transmission can preempt ongoing non- 1469 URLLC transmissions. Additionally, NR applies very fast processing, 1470 enabling retransmissions even within short latency bounds. 1472 NR defines extra-robust transmission modes for increased reliability 1473 both for data and control radio channels. Reliability is further 1474 improved by various techniques, such as multi-antenna transmission, 1475 the use of multiple frequency carriers in parallel and packet 1476 duplication over independent radio links. NR also provides full 1477 mobility support, which is an important reliability aspect not only 1478 for devices that are moving, but also for devices located in a 1479 changing environment. 1481 Network slicing is seen as one of the key features for 5G, allowing 1482 vertical industries to take advantage of 5G networks and services. 1483 Network slicing is about transforming a Public Land Mobile Network 1484 (PLMN) from a single network to a network where logical partitions 1485 are created, with appropriate network isolation, resources, optimized 1486 topology and specific configuration to serve various service 1487 requirements. An operator can configure and manage the mobile 1488 network to support various types of services enabled by 5G, for 1489 example eMBB and URLLC, depending on the different customers' needs. 1491 Exposure of capabilities of 5G Systems to the network or applications 1492 outside the 3GPP domain have been added to Release 16 [TS23501]. Via 1493 exposure interfaces, applications can access 5G capabilities, e.g., 1494 communication service monitoring and network maintenance. 1496 For several generations of mobile networks, 3GPP has considered how 1497 the communication system should work on a global scale with billions 1498 of users, taking into account resilience aspects, privacy regulation, 1499 protection of data, encryption, access and core network security, as 1500 well as interconnect. Security requirements evolve as demands on 1501 trustworthiness increase. For example, this has led to the 1502 introduction of enhanced privacy protection features in 5G. 5G also 1503 employs strong security algorithms, encryption of traffic, protection 1504 of signaling and protection of interfaces. 1506 One particular strength of mobile networks is the authentication, 1507 based on well-proven algorithms and tightly coupled with a global 1508 identity management infrastructure. Since 3G, there is also mutual 1509 authentication, allowing the network to authenticate the device and 1510 the device to authenticate the network. Another strength is secure 1511 solutions for storage and distribution of keys fulfilling regulatory 1512 requirements and allowing international roaming. When connecting to 1513 5G, the user meets the entire communication system, where security is 1514 the result of standardization, product security, deployment, 1515 operations and management as well as incident handling capabilities. 1516 The mobile networks approach the entirety in a rather coordinated 1517 fashion which is beneficial for security. 1519 6.3. Deployment and Spectrum 1521 The 5G system allows deployment in a vast spectrum range, addressing 1522 use-cases in both wide-area as well as local networks. Furthermore, 1523 5G can be configured for public and non-public access. 1525 When it comes to spectrum, NR allows combining the merits of many 1526 frequency bands, such as the high bandwidths in millimeter Waves 1527 (mmW) for extreme capacity locally, as well as the broad coverage 1528 when using mid- and low frequency bands to address wide-area 1529 scenarios. URLLC is achievable in all these bands. Spectrum can be 1530 either licensed, which means that the license holder is the only 1531 authorized user of that spectrum range, or unlicensed, which means 1532 that anyone who wants to use the spectrum can do so. 1534 A prerequisite for critical communication is performance 1535 predictability, which can be achieved by the full control of the 1536 access to the spectrum, which 5G provides. Licensed spectrum 1537 guarantees control over spectrum usage by the system, making it a 1538 preferable option for critical communication. However, unlicensed 1539 spectrum can provide an additional resource for scaling non-critical 1540 communications. While NR is initially developed for usage of 1541 licensed spectrum, the functionality to access also unlicensed 1542 spectrum was introduced in 3GPP Release 16. 1544 Licensed spectrum dedicated to mobile communications has been 1545 allocated to mobile service providers, i.e. issued as longer-term 1546 licenses by national administrations around the world. These 1547 licenses have often been associated with coverage requirements and 1548 issued across whole countries, or in large regions. Besides this, 1549 configured as a non-public network (NPN) deployment, 5G can provide 1550 network services also to a non-operator defined organization and its 1551 premises such as a factory deployment. By this isolation, quality of 1552 service requirements, as well as security requirements can be 1553 achieved. An integration with a public network, if required, is also 1554 possible. The non-public (local) network can thus be interconnected 1555 with a public network, allowing devices to roam between the networks. 1557 In an alternative model, some countries are now in the process of 1558 allocating parts of the 5G spectrum for local use to industries. 1559 These non-service providers then have a choice of applying for a 1560 local license themselves and operating their own network or 1561 cooperating with a public network operator or service provider. 1563 6.4. Applicability to Deterministic Flows 1565 6.4.1. System Architecture 1567 The 5G system [TS23501] consists of the User Equipment (UE) at the 1568 terminal side, and the Radio Access Network (RAN) with the gNB as 1569 radio base station node, as well as the Core Network (CN). The core 1570 network is based on a service-based architecture with the central 1571 functions: Access and Mobility Management Function (AMF), Session 1572 Management Function (SMF) and User Plane Function (UPF) as 1573 illustrated in Figure 10. 1575 The gNB's main responsibility is the radio resource management, 1576 including admission control and scheduling, mobility control and 1577 radio measurement handling. The AMF handles the UE's connection 1578 status and security, while the SMF controls the UE's data sessions. 1579 The UPF handles the user plane traffic. 1581 The SMF can instantiate various Packet Data Unit (PDU) sessions for 1582 the UE, each associated with a set of QoS flows, i.e., with different 1583 QoS profiles. Segregation of those sessions is also possible, e.g., 1584 resource isolation in the RAN and in the CN can be defined (slicing). 1586 +----+ +---+ +---+ +---+ +---+ +---+ 1587 |NSSF| |NEF| |NRF| |PCF| |UDM| |AF | 1588 +--+-+ +-+-+ +-+-+ +-+-+ +-+-+ +-+-+ 1589 | | | | | | 1590 Nnssf| Nnef| Nnrf| Npcf| Nudm| Naf| 1591 | | | | | | 1592 ---+------+-+-----+-+------------+--+-----+-+--- 1593 | | | | 1594 Nausf| Nausf| Nsmf| | 1595 | | | | 1596 +--+-+ +-+-+ +-+-+ +-+-+ 1597 |AUSF| |AMF| |SMF| |SCP| 1598 +----+ +++-+ +-+-+ +---+ 1599 / | | 1600 / | | 1601 / | | 1602 N1 N2 N4 1603 / | | 1604 / | | 1605 / | | 1606 +--+-+ +--+--+ +--+---+ +----+ 1607 | UE +---+(R)AN+--N3--+ UPF +--N6--+ DN | 1608 +----+ +-----+ ++----++ +----+ 1609 | | 1610 +-N9-+ 1612 Figure 10: 5G System Architecture 1614 To allow UE mobility across cells/gNBs, handover mechanisms are 1615 supported in NR. For an established connection, i.e., connected mode 1616 mobility, a gNB can configure a UE to report measurements of received 1617 signal strength and quality of its own and neighbouring cells, 1618 periodically or event-based. Based on these measurement reports, the 1619 gNB decides to handover a UE to another target cell/gNB. Before 1620 triggering the handover, it is hand-shaked with the target gNB based 1621 on network signalling. A handover command is then sent to the UE and 1622 the UE switches its connection to the target cell/gNB. The Packet 1623 Data Convergence Protocol (PDCP) of the UE can be configured to avoid 1624 data loss in this procedure, i.e., handle retransmissions if needed. 1625 Data forwarding is possible between source and target gNB as well. 1626 To improve the mobility performance further, i.e., to avoid 1627 connection failures, e.g., due to too-late handovers, the mechanism 1628 of conditional handover is introduced in Release 16 specifications. 1629 Therein a conditional handover command, defining a triggering point, 1630 can be sent to the UE before UE enters a handover situation. A 1631 further improvement introduced in Release 16 is the Dual Active 1632 Protocol Stack (DAPS), where the UE maintains the connection to the 1633 source cell while connecting to the target cell. This way, potential 1634 interruptions in packet delivery can be avoided entirely. 1636 6.4.2. Overview of The Radio Protocol Stack 1638 The protocol architecture for NR consists of the L1 Physical layer 1639 (PHY) and as part of the L2, the sublayers of Medium Access Control 1640 (MAC), Radio Link Control (RLC), Packet Data Convergence Protocol 1641 (PDCP), as well as the Service Data Adaption Protocol (SDAP). 1643 The PHY layer handles signal processing related actions, such as 1644 encoding/decoding of data and control bits, modulation, antenna 1645 precoding and mapping. 1647 The MAC sub-layer handles multiplexing and priority handling of 1648 logical channels (associated with QoS flows) to transport blocks for 1649 PHY transmission, as well as scheduling information reporting and 1650 error correction through Hybrid Automated Repeat Request (HARQ). 1652 The RLC sublayer handles sequence numbering of higher layer packets, 1653 retransmissions through Automated Repeat Request (ARQ), if 1654 configured, as well as segmentation and reassembly and duplicate 1655 detection. 1657 The PDCP sublayer consists of functionalities for ciphering/ 1658 deciphering, integrity protection/verification, re-ordering and in- 1659 order delivery, duplication and duplicate handling for higher layer 1660 packets, and acts as the anchor protocol to support handovers. 1662 The SDAP sublayer provides services to map QoS flows, as established 1663 by the 5G core network, to data radio bearers (associated with 1664 logical channels), as used in the 5G RAN. 1666 Additionally, in RAN, the Radio Resource Control (RRC) protocol, 1667 handles the access control and configuration signalling for the 1668 aforementioned protocol layers. RRC messages are considered L3 and 1669 thus transmitted also via those radio protocol layers. 1671 To provide low latency and high reliability for one transmission 1672 link, i.e., to transport data (or control signaling) of one radio 1673 bearer via one carrier, several features have been introduced on the 1674 user plane protocols for PHY and L2, as explained in the following. 1676 6.4.3. Radio (PHY) 1678 NR is designed with native support of antenna arrays utilizing 1679 benefits from beamforming, transmissions over multiple MIMO layers 1680 and advanced receiver algorithms allowing effective interference 1681 cancellation. Those antenna techniques are the basis for high signal 1682 quality and effectiveness of spectral usage. Spatial diversity with 1683 up to 4 MIMO layers in UL and up to 8 MIMO layers in DL is supported. 1684 Together with spatial-domain multiplexing, antenna arrays can focus 1685 power in desired direction to form beams. NR supports beam 1686 management mechanisms to find the best suitable beam for UE initially 1687 and when it is moving. In addition, gNBs can coordinate their 1688 respective DL and UL transmissions over the backhaul network keeping 1689 interference reasonably low, and even make transmissions or 1690 receptions from multiple points (multi-TRP). Multi-TRP can be used 1691 for repetition of data packet in time, in frequency or over multiple 1692 MIMO layers which can improve reliability even further. 1694 Any downlink transmission to a UE starts from resource allocation 1695 signaling over the Physical Downlink Control Channel (PDCCH). If it 1696 is successfully received, the UE will know about the scheduled 1697 transmission and may receive data over the Physical Downlink Shared 1698 Channel (PDSCH). If retransmission is required according to the HARQ 1699 scheme, a signaling of negative acknowledgement (NACK) on the 1700 Physical Uplink Control Channel (PUCCH) is involved and PDCCH 1701 together with PDSCH transmissions (possibly with additional 1702 redundancy bits) are transmitted and soft-combined with previously 1703 received bits. Otherwise, if no valid control signaling for 1704 scheduling data is received, nothing is transmitted on PUCCH 1705 (discontinuous transmission - DTX),and the base station upon 1706 detecting DTX will retransmit the initial data. 1708 An uplink transmission normally starts from a Scheduling Request (SR) 1709 - a signaling message from the UE to the base station sent via PUCCH. 1710 Once the scheduler is informed about buffer data in UE, e.g., by SR, 1711 the UE transmits a data packet on the Physical Uplink Shared Channel 1712 (PUSCH). Pre-scheduling not relying on SR is also possible (see 1713 following section). 1715 Since transmission of data packets require usage of control and data 1716 channels, there are several methods to maintain the needed 1717 reliability. NR uses Low Density Parity Check (LDPC) codes for data 1718 channels, Polar codes for PDCCH, as well as orthogonal sequences and 1719 Polar codes for PUCCH. For ultra-reliability of data channels, very 1720 robust (low spectral efficiency) Modulation and Coding Scheme (MCS) 1721 tables are introduced containing very low (down to 1/20) LDPC code 1722 rates using BPSK or QPSK. Also, PDCCH and PUCCH channels support 1723 multiple code rates including very low ones for the channel 1724 robustness. 1726 A connected UE reports downlink (DL) quality to gNB by sending 1727 Channel State Information (CSI) reports via PUCCH while uplink (UL) 1728 quality is measured directly at gNB. For both uplink and downlink, 1729 gNB selects the desired MCS number and signals it to the UE by 1730 Downlink Control Information (DCI) via PDCCH channel. For URLLC 1731 services, the UE can assist the gNB by advising that MCS targeting 1732 10^-5 Block Error Rate (BLER) are used. Robust link adaptation 1733 algorithms can maintain the needed level of reliability considering a 1734 given latency bound. 1736 Low latency on the physical layer is provided by short transmission 1737 duration which is possible by using high Subcarrier Spacing (SCS) and 1738 the allocation of only one or a few Orthogonal Frequency Division 1739 Multiplexing (OFDM) symbols. For example, the shortest latency for 1740 the worst case in DL can be 0.23ms and in UL can be 0.24ms according 1741 to (section 5.7.1 in [TR37910]). Moreover, if the initial 1742 transmission has failed, HARQ feedback can quickly be provided and an 1743 HARQ retransmission is scheduled. 1745 Dynamic multiplexing of data associated with different services is 1746 highly desirable for efficient use of system resources and to 1747 maximize system capacity. Assignment of resources for eMBB is 1748 usually done with regular (longer) transmission slots, which can lead 1749 to blocking of low latency services. To overcome the blocking, eMBB 1750 resources can be pre-empted and re-assigned to URLLC services. In 1751 this way, spectrally efficient assignments for eMBB can be ensured 1752 while providing flexibility required to ensure a bounded latency for 1753 URLLC services. In downlink, the gNB can notify the eMBB UE about 1754 pre-emption after it has happened, while in uplink there are two pre- 1755 emption mechanisms: special signaling to cancel eMBB transmission and 1756 URLLC dynamic power boost to suppress eMBB transmission. 1758 6.4.4. Scheduling and QoS (MAC) 1760 One integral part of the 5G system is the Quality of Service (QoS) 1761 framework [TS23501]. QoS flows are setup by the 5G system for 1762 certain IP or Ethernet packet flows, so that packets of each flow 1763 receive the same forwarding treatment, i.e., in scheduling and 1764 admission control. QoS flows can for example be associated with 1765 different priority level, packet delay budgets and tolerable packet 1766 error rates. Since radio resources are centrally scheduled in NR, 1767 the admission control function can ensure that only those QoS flows 1768 are admitted for which QoS targets can be reached. 1770 NR transmissions in both UL and DL are scheduled by the gNB 1771 [TS38300]. This ensures radio resource efficiency, fairness in 1772 resource usage of the users and enables differentiated treatment of 1773 the data flows of the users according to the QoS targets of the 1774 flows. Those QoS flows are handled as data radio bearers or logical 1775 channels in NR RAN scheduling. 1777 The gNB can dynamically assign DL and UL radio resources to users, 1778 indicating the resources as DL assignments or UL grants via control 1779 channel to the UE. Radio resources are defined as blocks of OFDM 1780 symbols in spectral domain and time domain. Different lengths are 1781 supported in time domain, i.e., (multiple) slot or mini-slot lengths. 1782 Resources of multiple frequency carriers can be aggregated and 1783 jointly scheduled to the UE. 1785 Scheduling decisions are based, e.g., on channel quality measured on 1786 reference signals and reported by the UE (cf. periodical CSI reports 1787 for DL channel quality). The transmission reliability can be chosen 1788 in the scheduling algorithm, i.e., by link adaptation where an 1789 appropriate transmission format (e.g., robustness of modulation and 1790 coding scheme, controlled UL power) is selected for the radio channel 1791 condition of the UE. Retransmissions, based on HARQ feedback, are 1792 also controlled by the scheduler. If needed to avoid HARQ round-trip 1793 time delays, repeated transmissions can be also scheduled beforehand, 1794 to the cost of reduced spectral efficiency. 1796 In dynamic DL scheduling, transmission can be initiated immediately 1797 when DL data becomes available in the gNB. However, for dynamic UL 1798 scheduling, when data becomes available but no UL resources are 1799 available yet, the UE indicates the need for UL resources to the gNB 1800 via a (single bit) scheduling request message in the UL control 1801 channel. When thereupon UL resources are scheduled to the UE, the UE 1802 can transmit its data and may include a buffer status report, 1803 indicating the exact amount of data per logical channel still left to 1804 be sent. More UL resources may be scheduled accordingly. To avoid 1805 the latency introduced in the scheduling request loop, UL radio 1806 resources can also be pre-scheduled. 1808 In particular for periodical traffic patterns, the pre-scheduling can 1809 rely on the scheduling features DL Semi-Persistent Scheduling (SPS) 1810 and UL Configured Grant (CG). With these features, periodically 1811 recurring resources can be assigned in DL and UL. Multiple parallels 1812 of those configurations are supported, in order to serve multiple 1813 parallel traffic flows of the same UE. 1815 To support QoS enforcement in the case of mixed traffic with 1816 different QoS requirements, several features have recently been 1817 introduced. This way, e.g., different periodical critical QoS flows 1818 can be served together with best effort transmissions, by the same 1819 UE. Among others, these features (partly Release 16) are: 1) UL 1820 logical channel transmission restrictions allowing to map logical 1821 channels of certain QoS only to intended UL resources of a certain 1822 frequency carrier, slot-length, or CG configuration, and 2) intra-UE 1823 pre-emption, allowing critical UL transmissions to pre-empt non- 1824 critical transmissions. 1826 When multiple frequency carriers are aggregated, duplicate parallel 1827 transmissions can be employed (beside repeated transmissions on one 1828 carrier). This is possible in the Carrier Aggregation (CA) 1829 architecture where those carriers originate from the same gNB, or in 1830 the Dual Connectivity (DC) architecture where the carriers originate 1831 from different gNBs, i.e., the UE is connected to two gNBs in this 1832 case. In both cases, transmission reliability is improved by this 1833 means of providing frequency diversity. 1835 In addition to licensed spectrum, a 5G system can also utilize 1836 unlicensed spectrum to offload non-critical traffic. This version of 1837 NR is called NR-U, part of 3GPP Release 16. The central scheduling 1838 approach applies also for unlicensed radio resources, but in addition 1839 also the mandatory channel access mechanisms for unlicensed spectrum, 1840 e.g., Listen Before Talk (LBT) are supported in NR-U. This way, by 1841 using NR, operators have and can control access to both licensed and 1842 unlicensed frequency resources. 1844 6.4.5. Time-Sensitive Networking (TSN) Integration 1846 The main objective of Time-Sensitive Networking (TSN) is to provide 1847 guaranteed data delivery within a guaranteed time window, i.e., 1848 bounded low latency. IEEE 802.1 TSN [IEEE802.1TSN] is a set of open 1849 standards that provide features to enable deterministic communication 1850 on standard IEEE 802.3 Ethernet [IEEE802.3]. TSN standards can be 1851 seen as a toolbox for traffic shaping, resource management, time 1852 synchronization, and reliability. 1854 A TSN stream is a data flow between one end station (Talker) to 1855 another end station (Listener). In the centralized configuration 1856 model, TSN bridges are configured by the Central Network Controller 1857 (CNC) [IEEE802.1Qcc] to provide deterministic connectivity for the 1858 TSN stream through the network. Time-based traffic shaping provided 1859 by Scheduled Traffic [IEEE802.1Qbv] may be used to achieve bounded 1860 low latency. The TSN tool for time synchronization is the 1861 generalized Precision Time Protocol (gPTP) [IEEE802.1AS]), which 1862 provides reliable time synchronization that can be used by end 1863 stations and by other TSN tools, e.g., Scheduled Traffic 1864 [IEEE802.1Qbv]. High availability, as a result of ultra-reliability, 1865 is provided for data flows by the Frame Replication and Elimination 1866 for Reliability (FRER) [IEEE802.1CB] mechanism. 1868 3GPP Release 16 includes integration of 5G with TSN, i.e., specifies 1869 functions for the 5G System (5GS) to deliver TSN streams such that 1870 the meet their QoS requirements. A key aspect of the integration is 1871 the 5GS appears from the rest of the network as a set of TSN bridges, 1872 in particular, one virtual bridge per User Plane Function (UPF) on 1873 the user plane. The 5GS includes TSN Translator (TT) functionality 1874 for the adaptation of the 5GS to the TSN bridged network and for 1875 hiding the 5GS internal procedures. The 5GS provides the following 1876 components: 1878 1. interface to TSN controller, as per [IEEE802.1Qcc] for the fully 1879 centralized configuration model 1881 2. time synchronization via reception and transmission of gPTP PDUs 1882 [IEEE802.1AS] 1884 3. low latency, hence, can be integrated with Scheduled Traffic 1885 [IEEE802.1Qbv] 1887 4. reliability, hence, can be integrated with FRER [IEEE802.1CB] 1889 Figure 10 shows an illustration of 5G-TSN integration where an 1890 industrial controller (Ind Ctrlr) is connected to industrial Input/ 1891 Output devices (I/O dev) via 5G. The 5GS can directly transport 1892 Ethernet frames since Release 15, thus, end-to-end Ethernet 1893 connectivity is provided. The 5GS implements the required interfaces 1894 towards the TSN controller functions such as the CNC, thus adapts to 1895 the settings of the TSN network. A 5G user plane virtual bridge 1896 interconnects TSN bridges or connect end stations, e.g., I/O devices 1897 to the network. Note that the introduction of 5G brings flexibility 1898 in various aspects, e.g., more flexible network topology because a 1899 wireless hop can replace several wireline hops thus significantly 1900 reduce the number of hops end-to-end. [ETR5GTSN] dives more into the 1901 integration of 5G with TSN. 1903 +------------------------------+ 1904 | 5G System | 1905 | +---+| 1906 | +-+ +-+ +-+ +-+ +-+ |TSN|| 1907 | | | | | | | | | | | |AF |......+ 1908 | +++ +++ +++ +++ +++ +-+-+| . 1909 | | | | | | | | . 1910 | -+---+---++--+-+-+--+-+- | . 1911 | | | | | | +--+--+ 1912 | +++ +++ +++ +++ | | TSN | 1913 | | | | | | | | | | |Ctrlr+.......+ 1914 | +++ +++ +++ +++ | +--+--+ . 1915 | | . . 1916 | | . . 1917 | +..........................+ | . . 1918 | . Virtual Bridge . | . . 1919 +---+ | . +--+--+ +---+ +---+--+ . | +--+---+ . 1920 |I/O+----------------+DS|UE+---+RAN+-+UPF|NW+------+ TSN +----+ . 1921 |dev| | . |TT| | | | | |TT| . | |bridge| | . 1922 +---+ | . +--+--+ +---+ +---+--+ . | +------+ | . 1923 | +..........................+ | . +-+-+-+ 1924 | | . | Ind | 1925 | +..........................+ | . |Ctrlr| 1926 | . Virtual Bridge . | . +-+---+ 1927 +---+ +------+ | . +--+--+ +---+ +---+--+ . | +--+---+ | 1928 |I/O+--+ TSN +------+DS|UE+---+RAN+-+UPF|NW+------+ TSN +----+ 1929 |dev| |bridge| | . |TT| | | | | |TT| . | |bridge| 1930 +---+ +------+ | . +--+--+ +---+ +---+--+ . | +------+ 1931 | +..........................+ | 1932 +------------------------------+ 1934 <----------------- end-to-end Ethernet -------------------> 1936 Figure 11: 5G - TSN Integration 1938 NR supports accurate reference time synchronization in 1us accuracy 1939 level. Since NR is a scheduled system, an NR UE and a gNB are 1940 tightly synchronized to their OFDM symbol structures. A 5G internal 1941 reference time can be provided to the UE via broadcast or unicast 1942 signaling, associating a known OFDM symbol to this reference clock. 1943 The 5G internal reference time can be shared within the 5G network, 1944 i.e., radio and core network components. For the interworking with 1945 gPTP for multiple time domains, the 5GS acts as a virtual gPTP time- 1946 aware system and supports the forwarding of gPTP time synchronization 1947 information between end stations and bridges through the 5G user 1948 plane TTs. These account for the residence time of the 5GS in the 1949 time synchronization procedure. One special option is when the 5GS 1950 internal reference time in not only used within the 5GS, but also to 1951 the rest of the devices in the deployment, including connected TSN 1952 bridges and end stations. 1954 Redundancy architectures were specified in order to provide 1955 reliability against any kind of failure on the radio link or nodes in 1956 the RAN and the core network, Redundant user plane paths can be 1957 provided based on the dual connectivity architecture, where the UE 1958 sets up two PDU sessions towards the same data network, and the 5G 1959 system makes the paths of the two PDU sessions independent as 1960 illustrated in Figure 13. There are two PDU sessions involved in the 1961 solution: the first spans from the UE via gNB1 to UPF1, acting as the 1962 first PDU session anchor, while the second spans from the UE via gNB2 1963 to UPF2, acting as second the PDU session anchor. The independent 1964 paths may continue beyond the 3GPP network. Redundancy Handling 1965 Functions (RHFs) are deployed outside of the 5GS, i.e., in Host A 1966 (the device) and in Host B (the network). RHF can implement 1967 replication and elimination functions as per [IEEE802.1CB] or the 1968 Packet Replication, Elimination, and Ordering Functions (PREOF) of 1969 IETF Deterministic Networking (DetNet) [RFC8655]. 1971 +........+ 1972 . Device . +------+ +------+ +------+ 1973 . . + gNB1 +--N3--+ UPF1 |--N6--+ | 1974 . ./+------+ +------+ | | 1975 . +----+ / | | 1976 . | |/. | | 1977 . | UE + . | DN | 1978 . | |\. | | 1979 . +----+ \ | | 1980 . .\+------+ +------+ | | 1981 +........+ + gNB2 +--N3--+ UPF2 |--N6--+ | 1982 +------+ +------+ +------+ 1984 Figure 12: Reliability with Single UE 1986 An alternative solution is that multiple UEs per device are used for 1987 user plane redundancy as illustrated in Figure 13. Each UE sets up a 1988 PDU session. The 5GS ensures that those PDU sessions of the 1989 different UEs are handled independently internal to the 5GS. There 1990 is no single point of failure in this solution, which also includes 1991 RHF outside of the 5G system, e.g., as per FRER or as PREOF 1992 specifications. 1994 +.........+ 1995 . Device . 1996 . . 1997 . +----+ . +------+ +------+ +------+ 1998 . | UE +-----+ gNB1 +--N3--+ UPF1 |--N6--+ | 1999 . +----+ . +------+ +------+ | | 2000 . . | DN | 2001 . +----+ . +------+ +------+ | | 2002 . | UE +-----+ gNB2 +--N3--+ UPF2 |--N6--+ | 2003 . +----+ . +------+ +------+ +------+ 2004 . . 2005 +.........+ 2007 Figure 13: Reliability with Dual UE 2009 Note that the abstraction provided by the RHF and the location of the 2010 RHF being outside of the 5G system make 5G equally supporting 2011 integration for reliability both with FRER of TSN and PREOF of DetNet 2012 as they both rely on the same concept. 2014 Note also that TSN is the primary subnetwork technology for DetNet. 2015 Thus, the DetNet over TSN work, e.g., [I-D.ietf-detnet-ip-over-tsn], 2016 can be leveraged via the TSN support built in 5G. 2018 6.5. Summary 2020 5G technology enables deterministic communication. Based on the 2021 centralized admission control and the scheduling of the wireless 2022 resources, licensed or unlicensed, quality of service such as latency 2023 and reliability can be guaranteed. 5G contains several features to 2024 achieve ultra-reliable and low latency performance, e.g., support for 2025 different OFDM numerologies and slot-durations, as well as fast 2026 processing capabilities and redundancy techniques that lead to 2027 achievable latency numbers of below 1ms with reliability guarantees 2028 up to 99.999%. 2030 5G also includes features to support Industrial IoT use cases, e.g., 2031 via the integration of 5G with TSN. This includes 5G capabilities 2032 for each TSN component, latency, resource management, time 2033 synchronization, and reliability. Furthermore, 5G support for TSN 2034 can be leveraged when 5G is used as subnet technology for DetNet, in 2035 combination with or instead of TSN, which is the primary subnet for 2036 DetNet. In addition, the support for integration with TSN 2037 reliability was added to 5G by making DetNet reliability also 2038 applicable, thus making 5G DetNet ready. Moreover, providing IP 2039 service is native to 5G. 2041 Overall, 5G provides scheduled wireless segments with high 2042 reliability and availability. In addition, 5G includes capabilities 2043 for integration to IP networks. 2045 7. L-band Digital Aeronautical Communications System 2047 One of the main pillars of the modern Air Traffic Management (ATM) 2048 system is the existence of a communication infrastructure that 2049 enables efficient aircraft guidance and safe separation in all phases 2050 of flight. Although current systems are technically mature, they are 2051 suffering from the VHF band's increasing saturation in high-density 2052 areas and the limitations posed by analogue radio. Therefore, 2053 aviation globally and the European Union (EU) in particular, strives 2054 for a sustainable modernization of the aeronautical communication 2055 infrastructure. 2057 In the long-term, ATM communication shall transition from analogue 2058 VHF voice and VDL2 communication to more spectrum efficient digital 2059 data communication. The European ATM Master Plan foresees this 2060 transition to be realized for terrestrial communications by the 2061 development and implementation of the L-band Digital Aeronautical 2062 Communications System (LDACS). LDACS shall enable IPv6 based air- 2063 ground communication related to the safety and regularity of the 2064 flight. The particular challenge is that no new frequencies can be 2065 made available for terrestrial aeronautical communication. It was 2066 thus necessary to develop procedures to enable the operation of LDACS 2067 in parallel with other services in the same frequency band. 2069 7.1. Provenance and Documents 2071 The development of LDACS has already made substantial progress in the 2072 Single European Sky ATM Research (SESAR) framework, and is currently 2073 being continued in the follow-up program, SESAR2020 [RIH18]. A key 2074 objective of the SESAR activities is to develop, implement and 2075 validate a modern aeronautical data link able to evolve with aviation 2076 needs over long-term. To this end, an LDACS specification has been 2077 produced [GRA19] and is continuously updated; transmitter 2078 demonstrators were developed to test the spectrum compatibility of 2079 LDACS with legacy systems operating in the L-band [SAJ14]; and the 2080 overall system performance was analyzed by computer simulations, 2081 indicating that LDACS can fulfill the identified requirements 2082 [GRA11]. 2084 LDACS standardization within the framework of the International Civil 2085 Aviation Organization (ICAO) started in December 2016. The ICAO 2086 standardization group has produced an initial Standards and 2087 Recommended Practices (SARPs) document [ICAO18]. The SARPs document 2088 defines the general characteristics of LDACS. The ICAO 2089 standardization group plans to produce an ICAO technical manual - the 2090 ICAO equivalent to a technical standard - within the next years. 2091 Generally, the group is open to input from all sources and develops 2092 LDACS in the open. 2094 Up to now the LDACS standardization has been focused on the 2095 development of the physical layer and the data link layer, only 2096 recently have higher layers come into the focus of the LDACS 2097 development activities. There is currently no "IPv6 over LDACS" 2098 specification; however, SESAR2020 has started the testing of 2099 IPv6-based LDACS testbeds. The IPv6 architecture for the 2100 aeronautical telecommunication network is called the Future 2101 Communications Infrastructure (FCI). FCI shall support quality of 2102 service, diversity, and mobility under the umbrella of the "multi- 2103 link concept". This work is conducted by ICAO working group WG-I. 2105 In addition to standardization activities several industrial LDACS 2106 prototypes have been built. One set of LDACS prototypes has been 2107 evaluated in flight trials confirming the theoretical results 2108 predicting the system performance [GRA18][SCH19]. 2110 7.2. General Characteristics 2112 LDACS will become one of several wireless access networks connecting 2113 aircraft to the Aeronautical Telecommunications Network (ATN). The 2114 LDACS access network contains several ground stations, each of them 2115 providing one LDACS radio cell. The LDACS air interface is a 2116 cellular data link with a star-topology connecting aircraft to 2117 ground-stations with a full duplex radio link. Each ground-station 2118 is the centralized instance controlling all air-ground communications 2119 within its radio cell. 2121 The user data rate of LDACS is 315 kbit/s to 1428 kbit/s on the 2122 forward link, and 294 kbit/s to 1390 kbit/s on the reverse link, 2123 depending on coding and modulation. Due to strong interference from 2124 legacy systems in the L-band, the most robust coding and modulation 2125 SHOULD be expected for initial deployment i.e. 315/294 kbit/s on the 2126 forward/reverse link, respectively. 2128 In addition to the communications capability, LDACS also offers a 2129 navigation capability. Ranging data, similar to DME (Distance 2130 Measuring Equipment), is extracted from the LDACS communication links 2131 between aircraft and LDACS ground stations. This results in LDACS 2132 providing an APNT (Alternative Position, Navigation and Timing) 2133 capability to supplement the existing on-board GNSS (Global 2134 Navigation Satellite System) without the need for additional 2135 bandwidth. Operationally, there will be no difference for pilots 2136 whether the navigation data are provided by LDACS or DME. This 2137 capability was flight tested and proven during the MICONAV flight 2138 trials in 2019 [BAT19]. 2140 In previous works and during the MICONAV flight campaign in 2019, it 2141 was also shown, that LDACS can be used for surveillance capability. 2142 Filip et al. [FIL19] shown passive radar capabilities of LDACS and 2143 Automatic Dependence Surveillance - Contract (ADS-C) was demonstrated 2144 via LDACS during the flight campaign 2019 [SCH19]. 2146 Since LDACS has been mainly designed for air traffic management 2147 communication it supports mutual entity authentication, integrity and 2148 confidentiality capabilities of user data messages and some control 2149 channel protection capabilities [MAE18], [MAE191], [MAE192], [MAE20]. 2151 Overall this makes LDACS the world's first truly integrated CNS 2152 system and is the worldwide most mature, secure, terrestrial long- 2153 range CNS technology for civil aviation. 2155 7.3. Deployment and Spectrum 2157 LDACS has its origin in merging parts of the B-VHF [BRA06], B-AMC 2158 [SCH08], TIA-902 (P34) [HAI09], and WiMAX IEEE 802.16e technologies 2159 [EHA11]. In 2007 the spectrum for LDACS was allocated at the World 2160 Radio Conference (WRC). 2162 It was decided to allocate the spectrum next to Distance Measuring 2163 Equipment (DME), resulting in an in-lay approach between the DME 2164 channels for LDAC [SCH14]. 2166 LDACS is currently being standardized by ICAO and several roll-out 2167 strategies are discussed: 2169 The LDACS data link provides enhanced capabilities to existing 2170 Aeronautical communications infrastructure enabling them to better 2171 support user needs and new applications. The deployment scalability 2172 of LDACS allows its implementation to start in areas where most 2173 needed to Improve immediately the performance of already fielded 2174 infrastructure. Later the deployment is extended based on 2175 operational demand. An attractive scenario for upgrading the 2176 existing VHF communication systems by adding an additional LDACS data 2177 link is described below. 2179 When considering the current VDL Mode 2 infrastructure and user base, 2180 a very attractive win-win situation comes about, when the 2181 technological advantages of LDACS are combined with the existing VDL 2182 mode 2 infrastructure. LDACS provides at least 50 time more capacity 2183 than VDL Mode 2 and is a natural enhancement to the existing VDL Mode 2184 2 business model. The advantage of this approach is that the VDL 2185 Mode 2 infrastructure can be fully reused. Beyond that, it opens the 2186 way for further enhancements which can increase business efficiency 2187 and minimize investment risk. [ICAO19] 2189 7.4. Applicability to Deterministic Flows 2191 As LDACS is a ground-based digital communications system for flight 2192 guidance and communications related to safety and regularity of 2193 flight, time-bounded deterministic arrival times for safety critical 2194 messages are a key feature for its successful deployment and roll- 2195 out. 2197 7.4.1. System Architecture 2199 Up to 512 Aircraft Station (AS) communicate to an LDACS Ground 2200 Station (GS) in the Reverse Link (RL). GS communicate to AS in the 2201 Forward Link (FL). Via an Access-Router (AC-R) GSs connect the LDACS 2202 sub-network to the global Aeronautical Telecommunications Network 2203 (ATN) to which the corresponding Air Traffic Services (ATS) and 2204 Aeronautical Operational Control (AOC) end systems are attached. 2206 7.4.2. Overview of The Radio Protocol Stack 2208 The protocol stack of LDACS is implemented in the AS and GS: It 2209 consists of the Physical Layer (PHY) with five major functional 2210 blocks above it. Four are placed in the Data Link Layer (DLL) of the 2211 AS and GS: (1) Medium Access Layer (MAC), (2) Voice Interface (VI), 2212 (3) Data Link Service (DLS), and (4) LDACS Management Entity (LME). 2213 The last entity resides within the Sub-Network Layer: Sub-Network 2214 Protocol (SNP). The LDACS network is externally connected to voice 2215 units, radio control units, and the ATN Network Layer. 2217 Figure 14 shows the protocol stack of LDACS as implemented in the AS 2218 and GS. 2220 IPv6 Network Layer 2221 | 2222 | 2223 +------------------+ +----+ 2224 | SNP |--| | Sub-Network 2225 | | | | Layer 2226 +------------------+ | | 2227 | | LME| 2228 +------------------+ | | 2229 | DLS | | | Logical Link 2230 | | | | Control Layer 2231 +------------------+ +----+ 2232 | | 2233 DCH DCCH/CCCH 2234 | RACH/BCCH 2235 | | 2236 +--------------------------+ 2237 | MAC | Medium Access 2238 | | Layer 2239 +--------------------------+ 2240 | 2241 +--------------------------+ 2242 | PHY | Physical Layer 2243 +--------------------------+ 2244 | 2245 | 2246 ((*)) 2247 FL/RL radio channels 2248 separated by 2249 Frequency Division Duplex 2251 Figure 14: LDACS protocol stack in AS and GS 2253 7.4.3. Radio (PHY) 2255 The physical layer provides the means to transfer data over the radio 2256 channel. The LDACS ground-station supports bi-directional links to 2257 multiple aircraft under its control. The forward link direction (FL; 2258 ground-to-air) and the reverse link direction (RL; air-to-ground) are 2259 separated by frequency division duplex. Forward link and reverse 2260 link use a 500 kHz channel each. The ground-station transmits a 2261 continuous stream of OFDM symbols on the forward link. In the 2262 reverse link different aircraft are separated in time and frequency 2263 using a combination of Orthogonal Frequency-Division Multiple-Access 2264 (OFDMA) and Time-Division Multiple-Access (TDMA). Aircraft thus 2265 transmit discontinuously on the reverse link with radio bursts sent 2266 in precisely defined transmission opportunities allocated by the 2267 ground-station. The most important service on the PHY layer of LDACS 2268 is the PHY time framing service, which indicates that the PHY layer 2269 is ready to transmit in a given slot and to indicate PHY layer 2270 framing and timing to the MAC time framing service. LDACS does not 2271 support beam-forming or Multiple Input Multiple Output (MIMO). 2273 7.4.4. Scheduling, Frame Structure and QoS (MAC) 2275 The data-link layer provides the necessary protocols to facilitate 2276 concurrent and reliable data transfer for multiple users. The LDACS 2277 data link layer is organized in two sub-layers: The medium access 2278 sub-layer and the logical link control sub-layer. The medium access 2279 sub-layer manages the organization of transmission opportunities in 2280 slots of time and frequency. The logical link control sub-layer 2281 provides acknowledged point-to-point logical channels between the 2282 aircraft and the ground-station using an automatic repeat request 2283 protocol. LDACS supports also unacknowledged point-to-point channels 2284 and ground-to-air broadcast. Before going more into depth about the 2285 LDACS medium access, the frame structure of LDACS is introduced: 2287 The LDACS framing structure for FL and RL is based on Super-Frames 2288 (SF) of 240 ms duration. Each SF corresponds to 2000 OFDM symbols. 2289 The FL and RL SF boundaries are aligned in time (from the view of the 2290 GS). 2292 In the FL, an SF contains a Broadcast Frame of duration 6.72 ms (56 2293 OFDM symbols) for the Broadcast Control Channel (BCCH), and four 2294 Multi-Frames (MF), each of duration 58.32 ms (486 OFDM symbols). 2296 In the RL, each SF starts with a Random Access (RA) slot of length 2297 6.72 ms with two opportunities for sending RL random access frames 2298 for the Random Access Channel (RACH), followed by four MFs. These 2299 MFs have the same fixed duration of 58.32 ms as in the FL, but a 2300 different internal structure 2302 Figure 15 and Figure 16 illustrate the LDACS frame structure. 2304 ^ 2305 | +------+------------+------------+------------+------------+ 2306 | FL | BCCH | MF | MF | MF | MF | 2307 F +------+------------+------------+------------+------------+ 2308 r <---------------- Super-Frame (SF) - 240ms ----------------> 2309 e 2310 q +------+------------+------------+------------+------------+ 2311 u RL | RACH | MF | MF | MF | MF | 2312 e +------+------------+------------+------------+------------+ 2313 n <---------------- Super-Frame (SF) - 240ms ----------------> 2314 c 2315 y 2316 | 2317 ----------------------------- Time ------------------------------> 2318 | 2320 Figure 15: SF structure for LDACS 2322 ^ 2323 | +-------------+------+-------------+ 2324 | FL | DCH | CCCH | DCH | 2325 F +-------------+------+-------------+ 2326 r <---- Multi-Frame (MF) - 58.32ms --> 2327 e 2328 q +------+---------------------------+ 2329 u RL | DCCH | DCH | 2330 e +------+---------------------------+ 2331 n <---- Multi-Frame (MF) - 58.32ms --> 2332 c 2333 y 2334 | 2335 -------------------- Time ------------------> 2336 | 2338 Figure 16: MF structure for LDACS 2340 This fixed frame structure allows for a reliable and dependable 2341 transmission of data. Next, the LDACS medium access layer is 2342 introduced: 2344 LDACS medium access is always under the control of the ground-station 2345 of a radio cell. Any medium access for the transmission of user data 2346 has to be requested with a resource request message stating the 2347 requested amount of resources and class of service. The ground- 2348 station performs resource scheduling on the basis of these requests 2349 and grants resources with resource allocation messages. Resource 2350 request and allocation messages are exchanged over dedicated 2351 contention-free control channels. 2353 LDACS has two mechanisms to request resources from the scheduler in 2354 the ground-station. Resources can either be requested "on demand" 2355 with a given class of service. On the forward link, this is done 2356 locally in the ground-station, on the reverse link a dedicated 2357 contention-free control channel is used (Dedicated Control Channel 2358 (DCCH); roughly 83 bit every 60 ms). A resource allocation is always 2359 announced in the control channel of the forward link (Common Control 2360 Channel (CCCH); variable sized). Due to the spacing of the reverse 2361 link control channels of every 60 ms, a medium access delay in the 2362 same order of magnitude is to be expected. 2364 Resources can also be requested "permanently". The permanent 2365 resource request mechanism supports requesting recurring resources in 2366 given time intervals. A permanent resource request has to be 2367 canceled by the user (or by the ground-station, which is always in 2368 control). User data transmissions over LDACS are therefore always 2369 scheduled by the ground-station, while control data uses statically 2370 (i.e. at net entry) allocated recurring resources (DCCH and CCCH). 2371 The current specification documents specify no scheduling algorithm. 2372 However performance evaluations so far have used strict priority 2373 scheduling and round robin for equal priorities for simplicity. In 2374 the current prototype implementations LDACS classes of service are 2375 thus realized as priorities of medium access and not as flows. Note 2376 that this can starve out low priority flows. However, this is not 2377 seen as a big problem since safety related message always go first in 2378 any case. Scheduling of reverse link resources is done in physical 2379 Protocol Data Units (PDU) of 112 bit (or larger if more aggressive 2380 coding and modulation is used). Scheduling on the forward link is 2381 done Byte-wise since the forward link is transmitted continuously by 2382 the ground-station. 2384 In order to support diversity, LDACS supports handovers to other 2385 ground-stations on different channels. Handovers may be initiated by 2386 the aircraft (break-before-make) or by the ground-station (make- 2387 before-break). Beyond this, FCI diversity shall be implemented by 2388 the multi-link concept. 2390 7.5. Summary 2392 LDACS has been designed with applications related to the safety and 2393 regularity of the flight in mind. It has therefore been designed as 2394 a deterministic wireless data link (as far as possible). 2396 It is a secure, scalable and spectrum efficient data link with 2397 embedded navigation capability and thus, is the first truly 2398 integrated CNS system recognized by ICAO. During flight tests the 2399 LDACS capabilities have been successfully demonstrated. A viable 2400 roll-out scenario has been developed which allows gradual 2401 introduction of LDACS with immediate use and revenues. Finally, ICAO 2402 is developing LDACS standards to pave the way for a successful roll- 2403 out in the near future. 2405 8. IANA Considerations 2407 This specification does not require IANA action. 2409 9. Security Considerations 2411 Most RAW technologies integrate some authentication or encryption 2412 mechanisms that were defined outside the IETF. 2414 10. Contributors 2416 Georgios Z. Papadopoulos: Contributed to the TSCH section. 2418 Nils Mäurer: Contributed to the LDACS section. 2420 Thomas Gräupl: Contributed to the LDACS section. 2422 Janos Farkas, Torsten Dudda, Alexey Shapin, and Sara Sandberg: Contr 2423 ibuted to the 5G section. 2425 11. Acknowledgments 2427 Many thanks to the participants of the RAW WG where a lot of the work 2428 discussed here happened. 2430 12. Normative References 2432 [RFC8480] Wang, Q., Ed., Vilajosana, X., and T. Watteyne, "6TiSCH 2433 Operation Sublayer (6top) Protocol (6P)", RFC 8480, 2434 DOI 10.17487/RFC8480, November 2018, 2435 . 2437 [RFC8200] Deering, S. and R. Hinden, "Internet Protocol, Version 6 2438 (IPv6) Specification", STD 86, RFC 8200, 2439 DOI 10.17487/RFC8200, July 2017, 2440 . 2442 [RFC5673] Pister, K., Ed., Thubert, P., Ed., Dwars, S., and T. 2443 Phinney, "Industrial Routing Requirements in Low-Power and 2444 Lossy Networks", RFC 5673, DOI 10.17487/RFC5673, October 2445 2009, . 2447 [RFC8557] Finn, N. and P. Thubert, "Deterministic Networking Problem 2448 Statement", RFC 8557, DOI 10.17487/RFC8557, May 2019, 2449 . 2451 [RFC8655] Finn, N., Thubert, P., Varga, B., and J. Farkas, 2452 "Deterministic Networking Architecture", RFC 8655, 2453 DOI 10.17487/RFC8655, October 2019, 2454 . 2456 [RFC9030] Thubert, P., Ed., "An Architecture for IPv6 over the Time- 2457 Slotted Channel Hopping Mode of IEEE 802.15.4 (6TiSCH)", 2458 RFC 9030, DOI 10.17487/RFC9030, May 2021, 2459 . 2461 [RFC9033] Chang, T., Ed., Vucinic, M., Vilajosana, X., Duquennoy, 2462 S., and D. Dujovne, "6TiSCH Minimal Scheduling Function 2463 (MSF)", RFC 9033, DOI 10.17487/RFC9033, May 2021, 2464 . 2466 13. Informative References 2468 [RFC6550] Winter, T., Ed., Thubert, P., Ed., Brandt, A., Hui, J., 2469 Kelsey, R., Levis, P., Pister, K., Struik, R., Vasseur, 2470 JP., and R. Alexander, "RPL: IPv6 Routing Protocol for 2471 Low-Power and Lossy Networks", RFC 6550, 2472 DOI 10.17487/RFC6550, March 2012, 2473 . 2475 [RFC6551] Vasseur, JP., Ed., Kim, M., Ed., Pister, K., Dejean, N., 2476 and D. Barthel, "Routing Metrics Used for Path Calculation 2477 in Low-Power and Lossy Networks", RFC 6551, 2478 DOI 10.17487/RFC6551, March 2012, 2479 . 2481 [RFC6291] Andersson, L., van Helvoort, H., Bonica, R., Romascanu, 2482 D., and S. Mansfield, "Guidelines for the Use of the "OAM" 2483 Acronym in the IETF", BCP 161, RFC 6291, 2484 DOI 10.17487/RFC6291, June 2011, 2485 . 2487 [RFC7276] Mizrahi, T., Sprecher, N., Bellagamba, E., and Y. 2488 Weingarten, "An Overview of Operations, Administration, 2489 and Maintenance (OAM) Tools", RFC 7276, 2490 DOI 10.17487/RFC7276, June 2014, 2491 . 2493 [RFC8279] Wijnands, IJ., Ed., Rosen, E., Ed., Dolganow, A., 2494 Przygienda, T., and S. Aldrin, "Multicast Using Bit Index 2495 Explicit Replication (BIER)", RFC 8279, 2496 DOI 10.17487/RFC8279, November 2017, 2497 . 2499 [I-D.pthubert-raw-architecture] 2500 Thubert, P., Papadopoulos, G. Z., and R. Buddenberg, 2501 "Reliable and Available Wireless Architecture/Framework", 2502 Work in Progress, Internet-Draft, draft-pthubert-raw- 2503 architecture-05, 15 November 2020, 2504 . 2507 [I-D.ietf-roll-nsa-extension] 2508 Koutsiamanis, R., Papadopoulos, G., Montavont, N., and P. 2509 Thubert, "Common Ancestor Objective Function and Parent 2510 Set DAG Metric Container Extension", Work in Progress, 2511 Internet-Draft, draft-ietf-roll-nsa-extension-10, 29 2512 October 2020, . 2515 [I-D.papadopoulos-paw-pre-reqs] 2516 Papadopoulos, G., Koutsiamanis, R., Montavont, N., and P. 2517 Thubert, "Exploiting Packet Replication and Elimination in 2518 Complex Tracks in LLNs", Work in Progress, Internet-Draft, 2519 draft-papadopoulos-paw-pre-reqs-01, 25 March 2019, 2520 . 2523 [I-D.thubert-bier-replication-elimination] 2524 Thubert, P., Eckert, T., Brodard, Z., and H. Jiang, "BIER- 2525 TE extensions for Packet Replication and Elimination 2526 Function (PREF) and OAM", Work in Progress, Internet- 2527 Draft, draft-thubert-bier-replication-elimination-03, 3 2528 March 2018, . 2531 [I-D.thubert-6lo-bier-dispatch] 2532 Thubert, P., Brodard, Z., Jiang, H., and G. Texier, "A 2533 6loRH for BitStrings", Work in Progress, Internet-Draft, 2534 draft-thubert-6lo-bier-dispatch-06, 28 January 2019, 2535 . 2538 [I-D.ietf-bier-te-arch] 2539 Eckert, T., Cauchie, G., and M. Menth, "Tree Engineering 2540 for Bit Index Explicit Replication (BIER-TE)", Work in 2541 Progress, Internet-Draft, draft-ietf-bier-te-arch-09, 30 2542 October 2020, 2543 . 2545 [I-D.ietf-6tisch-coap] 2546 Sudhaakar, R. S. and P. Zand, "6TiSCH Resource Management 2547 and Interaction using CoAP", Work in Progress, Internet- 2548 Draft, draft-ietf-6tisch-coap-03, 9 March 2015, 2549 . 2551 [I-D.svshah-tsvwg-deterministic-forwarding] 2552 Shah, S. and P. Thubert, "Deterministic Forwarding PHB", 2553 Work in Progress, Internet-Draft, draft-svshah-tsvwg- 2554 deterministic-forwarding-04, 30 August 2015, 2555 . 2558 [IEEE Std. 802.15.4] 2559 IEEE standard for Information Technology, "IEEE Std. 2560 802.15.4, Part. 15.4: Wireless Medium Access Control (MAC) 2561 and Physical Layer (PHY) Specifications for Low-Rate 2562 Wireless Personal Area Networks". 2564 [IEEE Std. 802.11] 2565 "IEEE Standard 802.11 - IEEE Standard for Information 2566 Technology - Telecommunications and information exchange 2567 between systems Local and metropolitan area networks - 2568 Specific requirements - Part 11: Wireless LAN Medium 2569 Access Control (MAC) and Physical Layer (PHY) 2570 Specifications.". 2572 [IEEE Std. 802.11ak] 2573 "802.11ak: Enhancements for Transit Links Within Bridged 2574 Networks", 2017. 2576 [IEEE Std. 802.11ax] 2577 "802.11ax D4.0: Enhancements for High Efficiency WLAN". 2579 [IEEE Std. 802.11ay] 2580 "802.11ay: Enhanced throughput for operation in license- 2581 exempt bands above 45 GHz". 2583 [IEEE Std. 802.11ad] 2584 "802.11ad: Enhancements for very high throughput in the 60 2585 GHz band". 2587 [IEEE 802.11be WIP] 2588 "802.11be: Extreme High Throughput". 2590 [IEEE Std. 802.1Qat] 2591 "802.1Qat: Stream Reservation Protocol". 2593 [IEEE8021Qcc] 2594 "802.1Qcc: IEEE Standard for Local and Metropolitan Area 2595 Networks--Bridges and Bridged Networks -- Amendment 31: 2596 Stream Reservation Protocol (SRP) Enhancements and 2597 Performance Improvements". 2599 [Cavalcanti_2019] 2600 Dave Cavalcanti et al., "Extending Time Distribution and 2601 Timeliness Capabilities over the Air to Enable Future 2602 Wireless Industrial Automation Systems, the Proceedings of 2603 IEEE", June 2019. 2605 [Nitsche_2015] 2606 Thomas Nitsche et al., "IEEE 802.11ad: directional 60 GHz 2607 communication for multi-Gigabit-per-second Wi-Fi", 2608 December 2014. 2610 [Ghasempour_2017] 2611 Yasaman Ghasempour et al., "802.11ay: Next-Generation 60 2612 GHz Communications for 100 Gb/s Wi-Fi", December 2017. 2614 [IEEE_doc_11-18-2009-06] 2615 "802.11 Real-Time Applications (RTA) Topic Interest Group 2616 (TIG) Report", November 2018. 2618 [IEEE_doc_11-19-0373-00] 2619 Kevin Stanton et Al., "Time-Sensitive Applications Support 2620 in EHT", March 2019. 2622 [morell13] Antoni Morell et al., "Label switching over IEEE802.15.4e 2623 networks", April 2013. 2625 [dearmas16] 2626 Jesica de Armas et al., "Determinism through path 2627 diversity: Why packet replication makes sense", September 2628 2016. 2630 [vilajosana19] 2631 Xavier Vilajosana et al., "6TiSCH: Industrial Performance 2632 for IPv6 Internet-of-Things Networks", June 2019. 2634 [ISA100.11a] 2635 ISA/IEC, "ISA100.11a, Wireless Systems for Automation, 2636 also IEC 62734", 2011, . 2640 [WirelessHART] 2641 www.hartcomm.org, "Industrial Communication Networks - 2642 Wireless Communication Network and Communication Profiles 2643 - WirelessHART - IEC 62591", 2010. 2645 [PCE] IETF, "Path Computation Element", 2646 . 2648 [CCAMP] IETF, "Common Control and Measurement Plane", 2649 . 2651 [TiSCH] IETF, "IPv6 over the TSCH mode over 802.15.4", 2652 . 2654 [RIH18] Rihacek, C., Haindl, B., Fantappie, P., Pierattelli, S., 2655 Gräupl, T., Schnell, M., and N. Fistas, "L-band Digital 2656 Aeronautical Communications System (LDACS) Activities in 2657 SESAR2020", Proceedings of the Integrated Communications 2658 Navigation and Surveillance Conference (ICNS) Herndon, VA, 2659 USA, April 2018. 2661 [GRA19] Gräupl, T., Rihacek, C., and B. Haindl, "LDACS A/G 2662 Specification", SESAR2020 PJ14-02-01 D3.3.010, February 2663 2019. 2665 [SAJ14] Sajatovic, M., Günzel, H., and S. Müller, "WA04 D22 Test 2666 Report for Assessing LDACS1 Transmitter Impact upon DME/ 2667 TACAN Receivers", April 2014. 2669 [GRA11] Gräupl, T. and M. Ehammer, "L-DACS1 Data Link Layer 2670 Evolution of ATN/IPS", Proceedings of the 30th IEEE/AIAA 2671 Digital Avionics Systems Conference (DASC) Seattle, WA, 2672 USA, October 2011. 2674 [ICAO18] International Civil Aviation Organization (ICAO), "L-Band 2675 Digital Aeronautical Communication System (LDACS)", 2676 International Standards and Recommended Practices Annex 10 2677 - Aeronautical Telecommunications, Vol. III - 2678 Communication Systems, July 2018. 2680 [GRA18] al., T. G. E., "L-band Digital Aeronautical Communications 2681 System (LDACS) flight trials in the national German 2682 project MICONAV", Proceedings of the Integrated 2683 Communications, Navigation, Surveillance Conference 2684 (ICNS) Herndon, VA, USA, April 2018. 2686 [SCH19] Schnell, M., "DLR tests digital communications 2687 technologies combined with additional navigation functions 2688 for the first time", 3 March 2019, 2689 . 2692 [TR37910] "3GPP TR 37.910, Study on self evaluation towards IMT-2020 2693 submission", 2694 . 2697 [TR38824] "3GPP TR 38.824, Study on physical layer enhancements for 2698 NR ultra-reliable and low latency case (URLLC)", 2699 . 2702 [TR38825] "3GPP TR 38.825, Study on NR industrial Internet of Things 2703 (IoT)", 2704 . 2707 [TS22104] "3GPP TS 22.104, Service requirements for cyber-physical 2708 control applications in vertical domains", 2709 . 2712 [TR22804] "3GPP TR 22.804, Study on Communication for Automation in 2713 Vertical domains (CAV)", 2714 . 2717 [TS23501] "3GPP TS 23.501, System architecture for the 5G System 2718 (5GS)", 2719 . 2722 [TS38300] "3GPP TS 38.300, NR Overall description", 2723 . 2726 [IMT2020] "ITU towards IMT for 2020 and beyond", 2727 . 2730 [I-D.ietf-detnet-ip-over-tsn] 2731 Varga, B., Farkas, J., Malis, A. G., and S. Bryant, 2732 "DetNet Data Plane: IP over IEEE 802.1 Time Sensitive 2733 Networking (TSN)", Work in Progress, Internet-Draft, 2734 draft-ietf-detnet-ip-over-tsn-07, 19 February 2021, 2735 . 2738 [IEEE802.1TSN] 2739 IEEE 802.1, "Time-Sensitive Networking (TSN) Task Group", 2740 . 2742 [IEEE802.1AS] 2743 IEEE, "IEEE Standard for Local and metropolitan area 2744 networks -- Timing and Synchronization for Time-Sensitive 2745 Applications", IEEE 802.1AS-2020, 2746 . 2749 [IEEE802.1CB] 2750 IEEE, "IEEE Standard for Local and metropolitan area 2751 networks -- Frame Replication and Elimination for 2752 Reliability", DOI 10.1109/IEEESTD.2017.8091139, IEEE 2753 802.1CB-2017, 2754 . 2756 [IEEE802.1Qbv] 2757 IEEE, "IEEE Standard for Local and metropolitan area 2758 networks -- Bridges and Bridged Networks -- Amendment 25: 2759 Enhancements for Scheduled Traffic", IEEE 802.1Qbv-2015, 2760 . 2762 [IEEE802.1Qcc] 2763 IEEE, "IEEE Standard for Local and metropolitan area 2764 networks -- Bridges and Bridged Networks -- Amendment 31: 2765 Stream Reservation Protocol (SRP) Enhancements and 2766 Performance Improvements", IEEE 802.1Qcc-2018, 2767 . 2769 [IEEE802.3] 2770 IEEE, "IEEE Standard for Ethernet", IEEE 802.3-2018, 2771 . 2773 [ETR5GTSN] Farkas, J., Varga, B., Miklos, G., and J. Sachs, "5G-TSN 2774 integration meets networking requirements for industrial 2775 automation", Ericsson Technology Review, Volume 9, No 7, 2776 August 2019, . 2780 [MAE18] Maeurer, N. and A. Bilzhause, "A Cybersecurity 2781 Architecture for the L-band Digital Aeronautical 2782 Communications System (LDACS)", IEEE 37th Digital Avionics 2783 Systems Conference (DASC), pp. 1-10, London, UK , 2017. 2785 [MAE191] Maeurer, N. and C. Schmitt, "Towards Successful 2786 Realization of the LDACS Cybersecurity Architecture: An 2787 Updated Datalink Security Threat- and Risk Analysis", IEEE 2788 Integrated Communications, Navigation and Surveillance 2789 Conference (ICNS), pp. 1-13, Herndon, VA, USA , 2019. 2791 [ICAO19] International Civil Aviation Organization (ICAO), "TLDACS 2792 White Paper–A Roll-out Scenario", Working Paper 2793 COMMUNICATIONS PANEL-DATA COMMUNICATIONS INFRASTRUCTURE 2794 WORKING GROUP, Montreal, Canada , October 2019. 2796 [MAE192] Maeurer, N., Graeupl, T., and C. Schmitt, "Evaluation of 2797 the LDACS Cybersecurity Implementation", IEEE 38th Digital 2798 Avionics Systems Conference (DACS), pp. 1-10, San Diego, 2799 CA, USA , September 2019. 2801 [MAE20] Maeurer, N., Graeupl, T., and C. Schmitt, "Comparing 2802 Different Diffie-Hellman Key Exchange Flavors for LDACS", 2803 IEEE 39th Digital Avionics Systems Conference (DACS), pp. 2804 1-10, San Diego, CA, USA , October 2019. 2806 [FIL19] Filip-Dhaubhadel, A. and D. Shutin, "LDACS- Based Non- 2807 Cooperative Surveillance Multistatic Radar Design and 2808 Detection Coverage Assessment", IEEE 38th Digital Avionics 2809 Systems Conference (DACS), pp. 1-10, San Diego, CA, USA , 2810 September 2019. 2812 [BAT19] Battista, G., Osechas, O., Narayanan, S., Crespillo, O.G., 2813 Gerbeth, D., Maeurer, N., Mielke, D., and T. Graeupl, 2814 "Real-Time Demonstration of Integrated Communication and 2815 Navigation Services Using LDACS", IEEE Integrated 2816 Communications, Navigation and Surveillance Conference 2817 (ICNS), pp. 1-12, Herndon, VA, USA , 2019. 2819 [BRA06] Brandes, S., Schnell, M., Rokitansky, C.H., Ehammer, M., 2820 Graeupl, T., Steendam, H., Guenach, M., Rihacek, C., and 2821 B. Haindl, "B-VHF -Selected Simulation Results and Final 2822 Assessment", IEEE 25th Digital Avionics Systems Conference 2823 (DACS), pp. 1-12, New York, NY, USA , September 2019. 2825 [SCH08] Schnell, M., Brandes, S., Gligorevic, S., Rokitansky, 2826 C.H., Ehammer, M., Graeupl, T., Rihacek, C., and M. 2827 Sajatovic, "B-AMC - Broadband Aeronautical Multi-carrier 2828 Communications", IEEE 8th Integrated Communications, 2829 Navigation and Surveillance Conference (ICNS), pp. 1-13, 2830 New York, NY, USA , April 2008. 2832 [HAI09] Haindl, B., Rihacek, C., Sajatovic, M., Phillips, B., 2833 Budinger, J., Schnell, M., Kamiano, D., and W. Wilson, 2834 "Improvement of L-DACS1 Design by Combining B-AMC with P34 2835 and WiMAX Technologies", IEEE 9th Integrated 2836 Communications, Navigation and Surveillance Conference 2837 (ICNS), pp. 1-8, New York, NY, USA , May 2009. 2839 [EHA11] Ehammer, M. and T. Graeupl, "AeroMACS - An Airport 2840 Communications System", IEEE 30th Digital Avionics Systems 2841 Conference (DACS), pp. 1-16, New York, NY, USA , September 2842 2011. 2844 [SCH14] Schnell, M., Epple, U., Shutin, D., and N. 2845 Schneckenburger, "LDACS: Future Aeronautical 2846 Communications for Air- Traffic Management", IEEE 2847 Communications Magazine, 52(5), 104-110 , 2017. 2849 Authors' Addresses 2851 Pascal Thubert (editor) 2852 Cisco Systems, Inc 2853 Building D 2854 45 Allee des Ormes - BP1200 2855 06254 MOUGINS - Sophia Antipolis 2856 France 2858 Phone: +33 497 23 26 34 2859 Email: pthubert@cisco.com 2860 Dave Cavalcanti 2861 Intel Corporation 2862 2111 NE 25th Ave 2863 Hillsboro, OR, 97124 2864 United States of America 2866 Phone: 503 712 5566 2867 Email: dave.cavalcanti@intel.com 2869 Xavier Vilajosana 2870 Universitat Oberta de Catalunya 2871 156 Rambla Poblenou 2872 08018 Barcelona Catalonia 2873 Spain 2875 Email: xvilajosana@uoc.edu 2877 Corinna Schmitt 2878 Research Institute CODE, UniBwM 2879 Werner-Heisenberg-Weg 39 2880 85577 Neubiberg 2881 Germany 2883 Email: corinna.schmitt@unibw.de 2885 Janos Farkas 2886 Ericsson 2887 Budapest 2888 Magyar tudosok korutja 11 2889 1117 2890 Hungary 2892 Email: janos.farkas@ericsson.com