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Checking references for intended status: Informational ---------------------------------------------------------------------------- == Outdated reference: A later version (-30) exists of draft-ietf-6tisch-architecture-29 == Outdated reference: A later version (-09) exists of draft-pthubert-raw-architecture-04 == Outdated reference: A later version (-12) exists of draft-ietf-roll-nsa-extension-09 == Outdated reference: A later version (-13) exists of draft-ietf-bier-te-arch-08 == Outdated reference: A later version (-07) exists of draft-ietf-detnet-ip-over-tsn-03 Summary: 1 error (**), 0 flaws (~~), 7 warnings (==), 1 comment (--). 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: 23 April 2021 Intel 6 X. Vilajosana 7 Universitat Oberta de Catalunya 8 C. Schmitt 9 Research Institute CODE, UniBwM 10 J. Farkas 11 Ericsson 12 20 October 2020 14 Reliable and Available Wireless Technologies 15 draft-ietf-raw-technologies-00 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 availbility. 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 23 April 2021. 41 Copyright Notice 43 Copyright (c) 2020 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 . . . . . . . . . . . . . . . . . . . . . . . . . 3 59 3. On Scheduling . . . . . . . . . . . . . . . . . . . . . . . . 4 60 3.1. Benefits of Scheduling on Wires . . . . . . . . . . . . . 4 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) . . . . . . . . . 10 68 4.3.1. General Characteristics . . . . . . . . . . . . . . . 10 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 . . . . . . . . . . . . . . . 12 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 . . . . . . . . 16 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 . . . . . . . . . . . . . . . . . . . . . . . . . 44 89 7. L-band Digital Aeronautical Communications System . . . . . . 45 90 7.1. Provenance and Documents . . . . . . . . . . . . . . . . 45 91 7.2. General Characteristics . . . . . . . . . . . . . . . . . 46 92 7.3. Applicability to Deterministic Flows . . . . . . . . . . 47 93 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 48 94 9. Security Considerations . . . . . . . . . . . . . . . . . . . 48 95 10. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 48 96 11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 49 97 12. Normative References . . . . . . . . . . . . . . . . . . . . 49 98 13. Informative References . . . . . . . . . . . . . . . . . . . 49 99 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 57 101 1. Introduction 103 When used in math or philosophy, the term "deterministic" generally 104 refers to a perfection where all aspect are understood and 105 predictable. A perfectly Deterministic Network would ensure that 106 every packet reach its destination following a predetermined path 107 along a predefined schedule to be delivered at the exact due time. 108 In a real and imperfect world, a Deterministic Network must highly 109 predictable, which is a combination of reliability and availability. 110 On the one hand the network must be reliable, meaning that it will 111 perform as expected for all packets and in particular that it will 112 always deliver the packet at the destination in due time. On the 113 other hand, the network must be available, meaning that it is 114 resilient to any single outage, whether the cause is a software, a 115 hardware or a transmission issue. 117 RAW (Reliable and Available Wireless) is an effort to provide 118 Deterministic Networking on across a path that include a wireless 119 physical layer. Making Wireless Reliable and Available is even more 120 challenging than it is with wires, due to the numerous causes of loss 121 in transmission that add up to the congestion losses and the delays 122 caused by overbooked shared resources. In order to maintain a 123 similar quality of service along a multihop path that is composed of 124 wired and wireless hops, additional methods that are specific to 125 wireless must be leveraged to combat the sources of loss that are 126 also specific to wireless. 128 Such wireless-specific methods include per-hop retransmissions (HARQ) 129 and P2MP overhearing whereby multiple receivers are scheduled to 130 receive the same transmission, which balances the adverse effects of 131 the transmission losses that are experienced when a radio is used as 132 pure P2P. Those methods are collectively referred to as PAREO 133 functions in the "Reliable and Available Wireless Architecture/ 134 Framework" [I-D.pthubert-raw-architecture]. 136 2. Terminology 138 This specification uses several terms that are uncommon on protocols 139 that ensure bets effort transmissions for stochastics flows, such as 140 found in the traditional Internet and other statistically multiplexed 141 packet networks. 143 ARQ: Automatic Repeat Request, enabling an acknowledged transmission 144 and retries. ARQ is a typical model at Layer-2 on a wireless 145 medium. It is typically avoided end-to-end on deterministic flows 146 because it introduces excessive indetermination in latency, but a 147 limited number of retries within a bounded time may be used over a 148 wireless link and yet respect end-to-end constraints. 150 Available: That is exempt of unscheduled outage, the expectation for 151 a network being that the flow is maintained in the face of any 152 single breakage. 154 FEC: Forward error correction, sending redundant coded data to help 155 the receiver recover transmission errors without the delays 156 incurred with ARQ. 158 HARQ: Hybrid ARQ, a combination of FEC and ARQ. 160 PCE: Path Computation Element. 162 PAREO (functions): the wireless extension of DetNet PREOF. PAREO 163 functions include scheduled ARQ at selected hops, and expect the 164 use of new operations like overhearing where available. 166 Reliable: That consistently performs as expected, the expectation 167 for a network being to always deliver a packet in due time. 169 Track: A DODAG oriented to a destination, and that enables Packet 170 ARQ, Replication, Elimination, and Ordering Functions. 172 3. On Scheduling 174 The operations of a Deterministic Network often rely on precisely 175 applying a tight schedule, in order to avoid collision loss and 176 guarantee the worst-case time of delivery. To achieve this, there 177 must be a shared sense of time throughout the network. The sense of 178 time is usually provided by the lower layer and is not in scope for 179 RAW. 181 3.1. Benefits of Scheduling on Wires 183 A network is reliable when the statistical effects that affect the 184 packet transmission are eliminated. This involves maintaining at all 185 time the amount of critical packets within the physical capabilities 186 of the hardware and that of the radio medium. This is achieved by 187 controlling the use of time-shared resources such as CPUs and 188 buffers, by shaping the flows and by scheduling the time of 189 transmission of the packets that compose the flow at every hop. 191 Equipment failure, such as an access point rebooting, a broken radio 192 adapter, or a permanent obstacle to the transmission, is a secondary 193 source of packet loss. When a breakage occurs, multiple packets are 194 lost in a row before the flows are rerouted or the system may 195 recover. This is not acceptable for critical applications such as 196 related to safety. A typical process control loop will tolerate an 197 occasional packet loss, but a loss of several packets in a row will 198 cause an emergency stop (e.g., after 4 packets lost, within a period 199 of 1 second). 201 Network Availability is obtained by making the transmission resilient 202 against hardware failures and radio transmission losses due to 203 uncontrolled events such as co-channel interferers, multipath fading 204 or moving obstacles. The best results are typically achieved by 205 pseudo randomly cumulating all forms of diversity, in the spatial 206 domain with replication and elimination, in the time domain with ARQ 207 and diverse scheduled transmissions, and in the frequency domain with 208 frequency hopping or channel hopping between frames. 210 3.2. Benefits of Scheduling on Wireless 212 In addition to the benefits listed in Section 3.1, scheduling 213 transmissions provides specific value to the wireless medium. 215 On the one hand, scheduling avoids collisions between scheduled 216 transmissions and can ensure both time and frequency diversity 217 between retries in order to defeat co-channel interference from un- 218 controlled transmitters as well as multipath fading. Transmissions 219 can be scheduled on multiple channels in parallel, which enables to 220 use the full available spectrum while avoiding the hidden terminal 221 problem, e.g., when the next packet in a same flow interferes on a 222 same channel with the previous one that progressed a few hops 223 farther. 225 On the other hand, scheduling optimizes the bandwidth usage: compared 226 to classical Collision Avoidance techniques, there is no blank time 227 related to inter-frame space (IFS) and exponential back-off in 228 scheduled operations. A minimal Clear Channel Assessment may be 229 needed to comply with the local regulations such as ETSI 300-328, but 230 that will not detect a collision when the senders are synchronized. 231 And because scheduling allows a time-sharing operation, there is no 232 limit to the ratio of isolated critical traffic. 234 Finally, scheduling plays a critical role to save energy. In IOT, 235 energy is the foremost concern, and synchronizing sender and listener 236 enables to always maintain them in deep sleep when there is no 237 scheduled transmission. This avoids idle listening and long 238 preambles and enables long sleep periods between traffic and 239 resynchronization, allowing battery-operated nodes to operate in a 240 mesh topology for multiple years. 242 4. IEEE 802.11 244 4.1. Provenance and Documents 246 With an active portfolio of nearly 1,300 standards and projects under 247 development, IEEE is a leading developer of industry standards in a 248 broad range of technologies that drive the functionality, 249 capabilities, and interoperability of products and services, 250 transforming how people live, work, and communicate. 252 The IEEE 802 LAN/MAN Standards Committee (SC) develops and maintains 253 networking standards and recommended practices for local, 254 metropolitan, and other area networks, using an open and accredited 255 process, and advocates them on a global basis. The most widely used 256 standards are for Ethernet, Bridging and Virtual Bridged LANs 257 Wireless LAN, Wireless PAN, Wireless MAN, Wireless Coexistence, Media 258 Independent Handover Services, and Wireless RAN. An individual 259 Working Group provides the focus for each area. Standards produced 260 by the IEEE 802 SC are freely available from the IEEE GET Program 261 after they have been published in PDF for six months. 263 The IEEE 802.11 LAN standards define the underlying MAC and PHY 264 layers for the Wi-Fi technology. Wi-Fi/802.11 is one of the most 265 successful wireless technologies, supporting many application 266 domains. While previous 802.11 generations, such as 802.11n and 267 802.11ac, have focused mainly on improving peak throughput, more 268 recent generations are also considering other performance vectors, 269 such as efficiency enhancements for dense environments in 802.11ax, 270 and latency and support for Time-Sensitive Networking (TSN) 271 capabilities in 802.11be. 273 IEEE 802.11 already supports some 802.1 TSN standards and it is 274 undergoing efforts to support for other 802.1 TSN capabilities 275 required to address the use cases that require time synchronization 276 and timeliness (bounded latency) guarantees with high reliability and 277 availability. The IEEE 802.11 working group has been working in 278 collaboration with the IEEE 802.1 group for several years extending 279 802.1 features over 802.11. As with any wireless media, 802.11 280 imposes new constraints and restrictions to TSN-grade QoS, and 281 tradeoffs between latency and reliability guarantees must be 282 considered as well as managed deployment requirements. An overview 283 of 802.1 TSN capabilities and their extensions to 802.11 are 284 discussed in [Cavalcanti_2019]. 286 Wi-Fi Alliance (WFA) is the worldwide network of companies that 287 drives global Wi-Fi adoption and evolution through thought 288 leadership, spectrum advocacy, and industry-wide collaboration. The 289 WFA work helps ensure that Wi-Fi devices and networks provide users 290 the interoperability, security, and reliability they have come to 291 expect. 293 The following [IEEE Std. 802.11] specifications/certifications are 294 relevant in the context of reliable and available wireless services 295 and support for time-sensitive networking capabilities: 297 Time Synchronization: IEEE802.11-2016 with IEEE802.1AS; WFA TimeSync 298 Certification. 300 Congestion Control: IEEE802.11-2016 Admission Control; WFA Admission 301 Control. 303 Security: WFA Wi-Fi Protected Access, WPA2 and WPA3. 305 Interoperating with IEEE802.1Q bridges: [IEEE Std. 802.11ak]. 307 Stream Reservation Protocol (part of [IEEE Std. 802.1Qat]): AIEEE802 308 .11-2016 310 Scheduled channel access: IEEE802.11ad Enhancements for very high 311 throughput in the 60 GHz band [IEEE Std. 802.11ad]. 313 802.11 Real-Time Applications: Topic Interest Group (TIG) ReportDoc 314 [IEEE_doc_11-18-2009-06]. 316 In addition, major amendments being developed by the IEEE802.11 317 Working Group include capabilities that can be used as the basis for 318 providing more reliable and predictable wireless connectivity and 319 support time-sensitive applications: 321 IEEE 802.11ax D4.0: Enhancements for High Efficiency (HE). [IEEE 322 Std. 802.11ax] 324 IEEE 802.11be Extreme High Throughput (EHT). [IEEE 802.11be WIP] 326 IEE 802.11ay Enhanced throughput for operation in license-exempt 327 bands above 45 GHz. [IEEE Std. 802.11ay] 328 The main 802.11ax and 802.11be capabilities and their relevance to 329 RAW are discussed in the remainder of this document. 331 4.2. 802.11ax High Efficiency (HE) 333 4.2.1. General Characteristics 335 The next generation Wi-Fi (Wi-Fi 6) is based on the IEEE802.11ax 336 amendment [IEEE Std. 802.11ax], which includes new capabilities to 337 increase efficiency, control and reduce latency. Some of the new 338 features include higher order 1024-QAM modulation, support for uplink 339 multi-user MIMO, OFDMA, trigger-based access and Target Wake time 340 (TWT) for enhanced power savings. The OFDMA mode and trigger-based 341 access enable scheduled operation, which is a key capability required 342 to support deterministic latency and reliability for time-sensitive 343 flows. 802.11ax can operate in up to 160 MHz channels and it includes 344 support for operation in the new 6 GHz band, which is expected to be 345 open to unlicensed use by the FCC and other regulatory agencies 346 worldwide. 348 4.2.1.1. Multi-User OFDMA and Trigger-based Scheduled Access 350 802.11ax introduced a new orthogonal frequency-division multiple 351 access (OFDMA) mode in which multiple users can be scheduled across 352 the frequency domain. In this mode, the Access Point (AP) can 353 initiate multi-user (MU) Uplink (UL) transmissions in the same PHY 354 Protocol Data Unit (PPDU) by sending a trigger frame. This 355 centralized scheduling capability gives the AP much more control of 356 the channel, and it can remove contention between devices for uplink 357 transmissions, therefore reducing the randomness caused by CSMA-based 358 access between stations. The AP can also transmit simultaneously to 359 multiple users in the downlink direction by using a Downlink (DL) MU 360 OFDMA PPDU. In order to initiate a contention free Transmission 361 Opportunity (TXOP) using the OFDMA mode, the AP still follows the 362 typical listen before talk procedure to acquire the medium, which 363 ensures interoperability and compliance with unlicensed band access 364 rules. However, 802.11ax also includes a multi-user Enhanced 365 Distributed Channel Access (MU-EDCA) capability, which allows the AP 366 to get higher channel access priority. 368 4.2.1.2. Improved PHY Robustness 370 The 802.11ax PHY can operate with 0.8, 1.6 or 3.2 microsecond guard 371 interval (GI). The larger GI options provide better protection 372 against multipath, which is expected to be a challenge in industrial 373 environments. The possibility to operate with smaller resource units 374 (e.g. 2 MHz) enabled by OFDMA also helps reduce noise power and 375 improve SNR, leading to better packet error rate (PER) performance. 377 802.11ax supports beamforming as in 802.11ac, but introduces UL MU 378 MIMO, which helps improve reliability. The UL MU MIMO capability is 379 also enabled by the trigger based access operation in 802.11ax. 381 4.2.1.3. Support for 6GHz band 383 The 802.11ax specification [IEEE Std. 802.11ax] includes support for 384 operation in the new 6 GHz band. Given the amount of new spectrum 385 available as well as the fact that no legacy 802.11 device (prior 386 802.11ax) will be able to operate in this new band, 802.11ax 387 operation in this new band can be even more efficient. 389 4.2.2. Applicability to deterministic flows 391 TSN capabilities, as defined by the IEEE 802.1 TSN standards, provide 392 the underlying mechanism for supporting deterministic flows in a 393 Local Area Network (LAN). The 802.11 working group has already 394 incorporated support for several TSN capabilities, so that time- 395 sensitive flow can experience precise time synchronization and 396 timeliness when operating over 802.11 links. TSN capabilities 397 supported over 802.11 (which also extends to 802.11ax), include: 399 1. 802.1AS based Time Synchronization (other time synchronization 400 techniques may also be used) 402 2. Interoperating with IEEE802.1Q bridges 404 3. Time-sensitive Traffic Stream identification 406 The exiting 802.11 TSN capabilities listed above, and the 802.11ax 407 OFDMA and scheduled access provide a new set of tools to better 408 server time-sensitive flows. However, it is important to understand 409 the tradeoffs and constraints associated with such capabilities, as 410 well as redundancy and diversity mechanisms that can be used to 411 provide more predictable and reliable performance. 413 4.2.2.1. 802.11 Managed network operation and admission control 415 Time-sensitive applications and TSN standards are expected to operate 416 under a managed network (e.g. industrial/enterprise network). Thus, 417 the Wi-Fi operation must also be carefully managed and integrated 418 with the overall TSN management framework, as defined in the 419 [IEEE8021Qcc] specification. 421 Some of the random-access latency and interference from legacy/ 422 unmanaged devices can be minimized under a centralized management 423 mode as defined in [IEEE8021Qcc], in which admission control 424 procedures are enforced. 426 Existing traffic stream identification, configuration and admission 427 control procedures defined in [IEEE Std. 802.11] QoS mechanism can be 428 re-used. However, given the high degree of determinism required by 429 many time-sensitive applications, additional capabilities to manage 430 interference and legacy devices within tight time-constraints need to 431 be explored. 433 4.2.2.2. Scheduling for bounded latency and diversity 435 As discussed earlier, the [IEEE Std. 802.11ax] OFDMA mode introduces 436 the possibility of assigning different RUs (frequency resources) to 437 users within a PPDU. Several RU sizes are defined in the 438 specification (26, 52, 106, 242, 484, 996 subcarriers). In addition, 439 the AP can also decide on MCS and grouping of users within a given 440 OFMDA PPDU. Such flexibility can be leveraged to support time- 441 sensitive applications with bounded latency, especially in a managed 442 network where stations can be configured to operate under the control 443 of the AP. 445 As shown in [Cavalcanti_2019], it is possible to achieve latencies in 446 the order of 1msec with high reliability in an interference free 447 environment. Obviously, there are latency, reliability and capacity 448 tradeoffs to be considered. For instance, smaller Resource Units 449 (RU)s result in longer transmission durations, which may impact the 450 minimal latency that can be achieved, but the contention latency and 451 randomness elimination due to multi-user transmission is a major 452 benefit of the OFDMA mode. 454 The flexibility to dynamically assign RUs to each transmission also 455 enables the AP to provide frequency diversity, which can help 456 increase reliability. 458 4.3. 802.11be Extreme High Throughput (EHT) 460 4.3.1. General Characteristics 462 The [IEEE 802.11be WIP]is the next major 802.11 amendment (after 463 [IEEE Std. 802.11ax]) for operation in the 2.4, 5 and 6 GHz bands. 464 802.11be is expected to include new PHY and MAC features and it is 465 targeting extremely high throughput (at least 30 Gbps), as well as 466 enhancements to worst case latency and jitter. It is also expected 467 to improve the integration with 802.1 TSN to support time-sensitive 468 applications over Ethernet and Wireless LANs. 470 The 802.11be Task Group started its operation in May 2019, therefore, 471 detailed information about specific features is not yet available. 472 Only high level candidate features have been discussed so far, 473 including: 475 1. 320MHz bandwidth and more efficient utilization of non-contiguous 476 spectrum. 478 2. Multi-band/multi-channel aggregation and operation. 480 3. 16 spatial streams and related MIMO enhancements. 482 4. Multi-Access Point (AP) Coordination. 484 5. Enhanced link adaptation and retransmission protocol, e.g. 485 Hybrid Automatic Repeat Request (HARQ). 487 6. Any required adaptations to regulatory rules for the 6 GHz 488 spectrum. 490 4.3.2. Applicability to deterministic flows 492 The 802.11 Real-Time Applications (RTA) Topic Interest Group (TIG) 493 provided detailed information on use cases, issues and potential 494 solution directions to improve support for time-sensitive 495 applications in 802.11. The RTA TIG report [IEEE_doc_11-18-2009-06] 496 was used as input to the 802.11be project scope. 498 Improvements for worst-case latency, jitter and reliability were the 499 main topics identified in the RTA report, which were motivated by 500 applications in gaming, industrial automation, robotics, etc. The 501 RTA report also highlighted the need to support additional TSN 502 capabilities, such as time-aware (802.1Qbv) shaping and packet 503 replication and elimination as defined in 802.1CB. 505 802.11be is expected to build on and enhance 802.11ax capabilities to 506 improve worst case latency and jitter. Some of the enhancement areas 507 are discussed next. 509 4.3.2.1. Enhanced scheduled operation for bounded latency 511 In addition to the throughput enhancements, 802.11be will leverage 512 the trigger-based scheduled operation enabled by 802.11ax to provide 513 efficient and more predictable medium access. 802.11be is expected to 514 include enhancements to reduce overhead and enable more efficient 515 operation in managed network deployments [IEEE_doc_11-19-0373-00]. 517 4.3.2.2. Multi-AP coordination 519 Multi-AP coordination is one of the main new candidate features in 520 802.11be. It can provide benefits in throughput and capacity and has 521 the potential to address some of the issues that impact worst case 522 latency and reliability. Multi-AP coordination is expected to 523 address the contention due to overlapping Basic Service Sets (OBSS), 524 which is one of the main sources of random latency variations. 525 802.11be can define methods to enable better coordination between 526 APs, for instance, in a managed network scenario, in order to reduce 527 latency due to unmanaged contention. 529 Several multi-AP coordination approaches have been discussed with 530 different levels of complexities and benefits, but specific 531 coordination methods have not yet been defined. 533 4.3.2.3. Multi-band operation 535 802.11be will introduce new features to improve operation over 536 multiple bands and channels. By leveraging multiple bands/channels, 537 802.11be can isolate time-sensitive traffic from network congestion, 538 one of the main causes of large latency variations. In a managed 539 802.11be network, it should be possible to steer traffic to certain 540 bands/channels to isolate time-sensitive traffic from other traffic 541 and help achieve bounded latency. 543 4.4. 802.11ad and 802.11ay (mmWave operation) 545 4.4.1. General Characteristics 547 The IEEE 802.11ad amendment defines PHY and MAC capabilities to 548 enable multi-Gbps throughput in the 60 GHz millimeter wave (mmWave) 549 band. The standard addresses the adverse mmWave signal propagation 550 characteristics and provides directional communication capabilities 551 that take advantage of beamforming to cope with increased 552 attenuation. An overview of the 802.11ad standard can be found in 553 [Nitsche_2015] . 555 The IEEE 802.11ay is currently developing enhancements to the 556 802.11ad standard to enable the next generation mmWave operation 557 targeting 100 Gbps throughput. Some of the main enhancements in 558 802.11ay include MIMO, channel bonding, improved channel access and 559 beamforming training. An overview of the 802.11ay capabilities can 560 be found in [Ghasempour_2017] 562 4.4.2. Applicability to deterministic flows 564 The high data rates achievable with 802.11ad and 802.11ay can 565 significantly reduce latency down to microsecond levels. Limited 566 interference from legacy and other unlicensed devices in 60 GHz is 567 also a benefit. However, directionality and short range typical in 568 mmWave operation impose new challenges such as the overhead required 569 for beam training and blockage issues, which impact both latency and 570 reliability. Therefore, it is important to understand the use case 571 and deployment conditions in order to properly apply and configure 572 802.11ad/ay networks for time sensitive applications. 574 The 802.11ad standard include a scheduled access mode in which 575 stations can be allocated contention-free service periods by a 576 central controller. This scheduling capability is also available in 577 802.11ay, and it is one of the mechanisms that can be used to provide 578 bounded latency to time-sensitive data flows. An analysis of the 579 theoretical latency bounds that can be achieved with 802.11ad service 580 periods is provided in [Cavalcanti_2019]. 582 5. IEEE 802.15.4 584 5.1. Provenance and Documents 586 The IEEE802.15.4 Task Group has been driving the development of low- 587 power low-cost radio technology. The IEEE802.15.4 physical layer has 588 been designed to support demanding low-power scenarios targeting the 589 use of unlicensed bands, both the 2.4 GHz and sub GHz Industrial, 590 Scientific and Medical (ISM) bands. This has imposed requirements in 591 terms of frame size, data rate and bandwidth to achieve reduced 592 collision probability, reduced packet error rate, and acceptable 593 range with limited transmission power. The PHY layer supports frames 594 of up to 127 bytes. The Medium Access Control (MAC) sublayer 595 overhead is in the order of 10-20 bytes, leaving about 100 bytes to 596 the upper layers. IEEE802.15.4 uses spread spectrum modulation such 597 as the Direct Sequence Spread Spectrum (DSSS). 599 The Timeslotted Channel Hopping (TSCH) mode was added to the 2015 600 revision of the IEEE802.15.4 standard [IEEE Std. 802.15.4]. TSCH is 601 targeted at the embedded and industrial world, where reliability, 602 energy consumption and cost drive the application space. 604 At the IETF, the 6TiSCH Working Group (WG) [TiSCH] deals with best 605 effort operation of IPv6 [RFC8200] over TSCH. 6TiSCH has enabled 606 distributed scheduling to exploit the deterministic access 607 capabilities provided by TSCH. The group designed the essential 608 mechanisms to enable the management plane operation while ensuring 609 IPv6 is supported. Yet the charter did not focus to providing a 610 solution to establish end to end Tracks while meeting quality of 611 service requirements. 6TiSCH, through the RFC8480 [RFC8480] defines 612 the 6P protocol which provides a pairwise negotiation mechanism to 613 the control plane operation. The protocol supports agreement on a 614 schedule between neighbors, enabling distributed scheduling. 6P goes 615 hand-in-hand with a Scheduling Function (SF), the policy that decides 616 how to maintain cells and trigger 6P transactions. The Minimal 617 Scheduling Function (MSF) [I-D.ietf-6tisch-msf] is the default SF 618 defined by the 6TiSCH WG; other standardized SFs can be defined in 619 the future. MSF extends the minimal schedule configuration, and is 620 used to add child-parent links according to the traffic load. 622 Time sensitive networking on low power constrained wireless networks 623 have been partially addressed by ISA100.11a [ISA100.11a] and 624 WirelessHART [WirelessHART]. Both technologies involve a central 625 controller that computes redundant paths for industrial process 626 control traffic over a TSCH mesh. Moreover, ISA100.11a introduces 627 IPv6 capabilities with a Link-Local Address for the join process and 628 a global unicast addres for later exchanges, but the IPv6 traffic 629 typically ends at a local application gateway and the full power of 630 IPv6 for end-to-end communication is not enabled. Compared to that 631 state of the art, work at the IETF and in particular at RAW could 632 provide additional techniques such as optimized P2P routing, PAREO 633 functions, and end-to-end secured IPv6/CoAP connectivity. 635 The 6TiSCH architecture [I-D.ietf-6tisch-architecture] identifies 636 different models to schedule resources along so-called Tracks (see 637 Section 5.2.2.2) exploiting the TSCH schedule structure however the 638 focus at 6TiSCH is on best effort traffic and the group was never 639 chartered to produce standard work related to Tracks. 641 Useful References include: 643 1. IEEE Std 802.15.4: "IEEE Std. 802.15.4, Part. 15.4: Wireless 644 Medium Access Control (MAC) and Physical Layer (PHY) 645 Specifications for Low-Rate Wireless Personal Area Networks" 646 [IEEE Std. 802.15.4]. The latest version at the time of this 647 writing is dated year 2015. 649 2. Morell, A. , Vilajosana, X. , Vicario, J. L. and Watteyne, T. 650 (2013), Label switching over IEEE802.15.4e networks. Trans. 651 Emerging Tel. Tech., 24: 458-475. doi:10.1002/ett.2650" 652 [morell13]. 654 3. De Armas, J., Tuset, P., Chang, T., Adelantado, F., Watteyne, T., 655 Vilajosana, X. (2016, September). Determinism through path 656 diversity: Why packet replication makes sense. In 2016 657 International Conference on Intelligent Networking and 658 Collaborative Systems (INCoS) (pp. 150-154). IEEE. [dearmas16]. 660 4. X. Vilajosana, T. Watteyne, M. Vucinic, T. Chang and K. S. 661 J. Pister, "6TiSCH: Industrial Performance for IPv6 Internet-of- 662 Things Networks," in Proceedings of the IEEE, vol. 107, no. 6, 663 pp. 1153-1165, June 2019. [vilajosana19]. 665 5.2. TimeSlotted Channel Hopping 667 5.2.1. General Characteristics 669 As a core technique in IEEE802.15.4, TSCH splits time in multiple 670 time slots that repeat over time. A set of timeslots constructs a 671 Slotframe (see Section 5.2.2.1.4). For each timeslot, a set of 672 available frequencies can be used, resulting in a matrix-like 673 schedule (see Figure 1). 675 timeslot offset 676 | 0 1 2 3 4 | 0 1 2 3 4 | Nodes 677 +------------------------+------------------------+ +-----+ 678 | | | | | | | | | | | | C | 679 CH-1 | EB | | |C->B| | EB | | |C->B| | | | 680 | | | | | | | | | | | +-----+ 681 +-------------------------------------------------+ | 682 | | | | | | | | | | | | 683 CH-2 | | |B->C| |B->A| | |B->C| |B->A| +-----+ 684 | | | | | | | | | | | | B | 685 +-------------------------------------------------+ | | 686 ... +-----+ 687 | 688 +-------------------------------------------------+ | 689 | | | | | | | | | | | +-----+ 690 CH-15| |A->B| | | | |A->B| | | | | A | 691 | | | | | | | | | | | | | 692 +-------------------------------------------------+ +-----+ 693 ch. 694 offset 696 Figure 1: Slotframe example with scheduled cells between nodes A, 697 B and C 699 This schedule represents the possible communications of a node with 700 its neighbors, and is managed by a Scheduling Function such as the 701 Minimal Scheduling Function (MSF) [I-D.ietf-6tisch-msf]. Each cell 702 in the schedule is identified by its slotoffset and channeloffset 703 coordinates. A cell's timeslot offset indicates its position in 704 time, relative to the beginning of the slotframe. A cell's channel 705 offset is an index which maps to a frequency at each iteration of the 706 slotframe. Each packet exchanged between neighbors happens within 707 one cell. The size of a cell is a timeslot duration, between 10 to 708 15 milliseconds. An Absolute Slot Number (ASN) indicates the number 709 of slots elapsed since the network started. It increments at every 710 slot. This is a 5 byte counter that can support networks running for 711 more than 300 years without wrapping (assuming a 10 ms timeslot). 712 Channel hopping provides increased reliability to multi-path fading 713 and external interference. It is handled by TSCH through a channel 714 hopping sequence referred as macHopSeq in the IEEE802.15.4 715 specification. 717 The Time-Frequency Division Multiple Access provided by TSCH enables 718 the orchestration of traffic flows, spreading them in time and 719 frequency, and hence enabling an efficient management of the 720 bandwidth utilization. Such efficient bandwidth utilization can be 721 combined to OFDM modulations also supported by the IEEE802.15.4 722 standard [IEEE Std. 802.15.4] since the 2015 version. 724 In the RAW context, low power reliable networks should address non- 725 critical control scenarios such as Class 2 and monitoring scenarios 726 such as Class 4 defined by the RFC5673 [RFC5673]. As a low power 727 technology targeting industrial scenarios radio transducers provide 728 low data rates (typically between 50kbps to 250kbps) and robust 729 modulations to trade-off performance to reliability. TSCH networks 730 are organized in mesh topologies and connected to a backbone. 731 Latency in the mesh network is mainly influenced by propagation 732 aspects such as interference. ARQ methods and redundancy techniques 733 such as replication and elimination should be studied to provide the 734 needed performance to address deterministic scenarios. 736 5.2.2. Applicability to Deterministic Flows 738 Nodes in a TSCH network are tightly synchronized. This enables to 739 build the slotted structure and ensure efficient utilization of 740 resources thanks to proper scheduling policies. Scheduling is a key 741 to orchestrate the resources that different nodes in a Track or a 742 path are using. Slotframes can be split in resource blocks reserving 743 the needed capacity to certain flows. Periodic and bursty traffic 744 can be handled independently in the schedule, using active and 745 reactive policies and taking advantage of overprovisionned cells to 746 measu reth excursion. Along a Track, resource blocks can be chained 747 so nodes in previous hops transmit their data before the next packet 748 comes. This provides a tight control to latency along a Track. 749 Collision loss is avoided for best effort traffic by 750 overprovisionning resources, giving time to the management plane of 751 the network to dedicate more resources if needed. 753 5.2.2.1. Centralized Path Computation 755 In a controlled environment, a 6TiSCH device usually does not place a 756 request for bandwidth between itself and another device in the 757 network. Rather, an Operation Control System (OCS) invoked through 758 an Human/Machine Interface (HMI) iprovides the Traffic Specification, 759 in particular in terms of latency and reliability, and the end nodes, 760 to a Path Computation element (PCE). With this, the PCE computes a 761 Track between the end nodes and provisions every hop in the Track 762 with per-flow state that describes the per-hop operation for a given 763 packet, the corresponding timeSlots, and the flow identification to 764 recognize which packet is placed in which Track, sort out duplicates, 765 etc. In Figure 2, an example of Operational Control System and HMI 766 is depicted. 768 For a static configuration that serves a certain purpose for a long 769 period of time, it is expected that a node will be provisioned in one 770 shot with a full schedule, which incorporates the aggregation of its 771 behavior for multiple Tracks. The 6TiSCH Architecture expects that 772 the programing of the schedule is done over CoAP as discussed in 773 "6TiSCH Resource Management and Interaction using CoAP" 774 [I-D.ietf-6tisch-coap]. 776 But an Hybrid mode may be required as well whereby a single Track is 777 added, modified, or removed, for instance if it appears that a Track 778 does not perform as expected for, say, Packet Delivery Ratio (PDR). 779 For that case, the expectation is that a protocol that flows along a 780 Track (to be), in a fashion similar to classical Traffic Engineering 781 (TE) [CCAMP], may be used to update the state in the devices. 6TiSCH 782 provides means for a device to negotiate a timeSlot with a neighbor, 783 but in general that flow was not designed and no protocol was 784 selected and it is expected that DetNet will determine the 785 appropriate end-to-end protocols to be used in that case. 787 Stream Management Entity 788 Operational Control System and HMI 790 -+-+-+-+-+-+-+ Northbound -+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- 792 PCE PCE PCE PCE 794 -+-+-+-+-+-+-+ Southbound -+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- 796 --- 6TiSCH------6TiSCH------6TiSCH------6TiSCH-- 797 6TiSCH / Device Device Device Device \ 798 Device- - 6TiSCH 799 \ 6TiSCH 6TiSCH 6TiSCH 6TiSCH / Device 800 ----Device------Device------Device------Device-- 802 Figure 2 804 5.2.2.1.1. Packet Marking and Handling 806 Section "Packet Marking and Handling" of 807 [I-D.ietf-6tisch-architecture] describes the packet tagging and 808 marking that is expected in 6TiSCH networks. 810 5.2.2.1.1.1. Tagging Packets for Flow Identification 812 For packets that are routed by a PCE along a Track, the tuple formed 813 by the IPv6 source address and a local RPLInstanceID is tagged in the 814 packets to identify uniquely the Track and associated transmit bundle 815 of timeSlots. 817 It results that the tagging that is used for a DetNet flow outside 818 the 6TiSCH LLN MUST be swapped into 6TiSCH formats and back as the 819 packet enters and then leaves the 6TiSCH network. 821 Note: The method and format used for encoding the RPLInstanceID at 822 6lo is generalized to all 6TiSCH topological Instances, which 823 includes Tracks. 825 5.2.2.1.1.2. Replication, Retries and Elimination 827 PRE establishes several paths in a network to provide redundancy and 828 parallel transmissions to bound the end-to-end delay. Considering 829 the scenario shown in Figure 3, many different paths are possible for 830 S to reach R. A simple way to benefit from this topology could be to 831 use the two independent paths via nodes A, C, E and via B, D, F. But 832 more complex paths are possible as well. 834 (A) (C) (E) 836 source (S) (R) (destination) 838 (B) (D) (F) 840 Figure 3: A Typical Ladder Shape with Two Parallel Paths Toward 841 the Destination 843 By employing a Packet Replication function, each node forwards a copy 844 of each data packet over two different branches. For instance, in 845 Figure 4, the source node S transmits the data packet to nodes A and 846 B, in two different timeslots within the same TSCH slotframe. 848 ===> (A) => (C) => (E) === 849 // \\// \\// \\ 850 source (S) //\\ //\\ (R) (destination) 851 \\ // \\ // \\ // 852 ===> (B) => (D) => (F) === 854 Figure 4: Packet Replication: S transmits twice the same data 855 packet, to its DP (A) and to its AP (B). 857 By employing Packet Elimination function once a node receives the 858 first copy of a data packet, it discards the subsequent copies. 859 Because the first copy that reaches a node is the one that matters, 860 it is the only copy that will be forwarded upward. 862 Considering that the wireless medium is broadcast by nature, any 863 neighbor of a transmitter may overhear a transmission. By employing 864 the Promiscuous Overhearing function, nodes will have multiple 865 opportunities to receive a given data packet. For instance, in 866 Figure 4, when the source node S transmits the data packet to node A, 867 node B may overhear this transmission. 869 6TiSCH expects elimination and replication of packets along a complex 870 Track, but has no position about how the sequence numbers would be 871 tagged in the packet. 873 As it goes, 6TiSCH expects that timeSlots corresponding to copies of 874 a same packet along a Track are correlated by configuration, and does 875 not need to process the sequence numbers. 877 The semantics of the configuration MUST enable correlated timeSlots 878 to be grouped for transmit (and respectively receive) with 879 a'OR'relations, and then a'AND'relation MUST be configurable between 880 groups. The semantics is that if the transmit (and respectively 881 receive) operation succeeded in one timeSlot in a'OR'group, then all 882 the other timeSLots in the group are ignored. Now, if there are at 883 least two groups, the'AND'relation between the groups indicates that 884 one operation must succeed in each of the groups. 886 On the transmit side, timeSlots provisioned for retries along a same 887 branch of a Track are placed a same'OR'group. The'OR'relation 888 indicates that if a transmission is acknowledged, then further 889 transmissions SHOULD NOT be attempted for timeSlots in that group. 890 There are as many'OR'groups as there are branches of the Track 891 departing from this node. Different'OR'groups are programmed for the 892 purpose of replication, each group corresponding to one branch of the 893 Track. The'AND'relation between the groups indicates that 894 transmission over any of branches MUST be attempted regardless of 895 whether a transmission succeeded in another branch. It is also 896 possible to place cells to different next-hop routers in a 897 same'OR'group. This allows to route along multi-path Tracks, trying 898 one next-hop and then another only if sending to the first fails. 900 On the receive side, all timeSlots are programmed in a same'OR'group. 901 Retries of a same copy as well as converging branches for elimination 902 are converged, meaning that the first successful reception is enough 903 and that all the other timeSlots can be ignored. 905 5.2.2.1.1.3. Differentiated Services Per-Hop-Behavior 907 Additionally, an IP packet that is sent along a Track uses the 908 Differentiated Services Per-Hop-Behavior Group called Deterministic 909 Forwarding, as described in 910 [I-D.svshah-tsvwg-deterministic-forwarding]. 912 5.2.2.1.2. Topology and capabilities 914 6TiSCH nodes are usually IoT devices, characterized by very limited 915 amount of memory, just enough buffers to store one or a few IPv6 916 packets, and limited bandwidth between peers. It results that a node 917 will maintain only a small number of peering information, and will 918 not be able to store many packets waiting to be forwarded. Peers can 919 be identified through MAC or IPv6 addresses. 921 Neighbors can be discovered over the radio using mechanism such as 922 Enhanced Beacons, but, though the neighbor information is available 923 in the 6TiSCH interface data model, 6TiSCH does not describe a 924 protocol to pro-actively push the neighborhood information to a PCE. 925 This protocol should be described and should operate over CoAP. The 926 protocol should be able to carry multiple metrics, in particular the 927 same metrics as used for RPL operations [RFC6551]. 929 The energy that the device consumes in sleep, transmit and receive 930 modes can be evaluated and reported. So can the amount of energy 931 that is stored in the device and the power that it can be scavenged 932 from the environment. The PCE SHOULD be able to compute Tracks that 933 will implement policies on how the energy is consumed, for instance 934 balance between nodes, ensure that the spent energy does not exceeded 935 the scavenged energy over a period of time, etc... 937 5.2.2.1.3. Schedule Management by a PCE 939 6TiSCH supports a mixed model of centralized routes and distributed 940 routes. Centralized routes can for example be computed by a entity 941 such as a PCE [PCE]. Distributed routes are computed by RPL 942 [RFC6550]. 944 Both methods may inject routes in the Routing Tables of the 6TiSCH 945 routers. In either case, each route is associated with a 6TiSCH 946 topology that can be a RPL Instance topology or a Track. The 6TiSCH 947 topology is indexed by a Instance ID, in a format that reuses the 948 RPLInstanceID as defined in RPL. 950 Both RPL and PCE rely on shared sources such as policies to define 951 Global and Local RPLInstanceIDs that can be used by either method. 952 It is possible for centralized and distributed routing to share a 953 same topology. Generally they will operate in different slotFrames, 954 and centralized routes will be used for scheduled traffic and will 955 have precedence over distributed routes in case of conflict between 956 the slotFrames. 958 5.2.2.1.4. SlotFrames and Priorities 960 A slotFrame is the base object that a PCE needs to manipulate to 961 program a schedule into an LLN node. Elaboration on that concept can 962 be fond in section "SlotFrames and Priorities" of 963 [I-D.ietf-6tisch-architecture] 964 IEEE802.15.4 TSCH avoids contention on the medium by formatting time 965 and frequencies in cells of transmission of equal duration. In order 966 to describe that formatting of time and frequencies, the 6TiSCH 967 architecture defines a global concept that is called a Channel 968 Distribution and Usage (CDU) matrix; a CDU matrix is a matrix of 969 cells with an height equal to the number of available channels 970 (indexed by ChannelOffsets) and a width (in timeSlots) that is the 971 period of the network scheduling operation (indexed by slotOffsets) 972 for that CDU matrix. The size of a cell is a timeSlot duration, and 973 values of 10 to 15 milliseconds are typical in 802.15.4 TSCH to 974 accommodate for the transmission of a frame and an acknowledgement, 975 including the security validation on the receive side which may take 976 up to a few milliseconds on some device architecture. 978 The frequency used by a cell in the matrix rotates in a pseudo-random 979 fashion, from an initial position at an epoch time, as the matrix 980 iterates over and over. 982 A CDU matrix is computed by the PCE, but unallocated timeSlots may be 983 used opportunistically by the nodes for classical best effort IP 984 traffic. The PCE has precedence in the allocation in case of a 985 conflict. 987 In a given network, there might be multiple CDU matrices that operate 988 with different width, so they have different durations and represent 989 different periodic operations. It is recommended that all CDU 990 matrices in a 6TiSCH domain operate with the same cell duration and 991 are aligned, so as to reduce the chances of interferences from 992 slotted-aloha operations. The PCE MUST compute the CDU matrices and 993 shared that knowledge with all the nodes. The matrices are used in 994 particular to define slotFrames. 996 A slotFrame is a MAC-level abstraction that is common to all nodes 997 and contains a series of timeSlots of equal length and precedence. 998 It is characterized by a slotFrame_ID, and a slotFrame_size. A 999 slotFrame aligns to a CDU matrix for its parameters, such as number 1000 and duration of timeSlots. 1002 Multiple slotFrames can coexist in a node schedule, i.e., a node can 1003 have multiple activities scheduled in different slotFrames, based on 1004 the precedence of the 6TiSCH topologies. The slotFrames may be 1005 aligned to different CDU matrices and thus have different width. 1006 There is typically one slotFrame for scheduled traffic that has the 1007 highest precedence and one or more slotFrame(s) for RPL traffic. The 1008 timeSlots in the slotFrame are indexed by the SlotOffset; the first 1009 cell is at SlotOffset 0. 1011 The 6TiSCH architecture introduces the concept of chunks 1012 ([I-D.ietf-6tisch-architecture]) to operate such spectrum 1013 distribution for a whole group of cells at a time. The CDU matrix is 1014 formatted into a set of chunks, each of them identified uniquely by a 1015 chunk-ID, see Figure 5. The PCE MUST compute the partitioning of CDU 1016 matrices into chunks and shared that knowledge with all the nodes in 1017 a 6TiSCH network. 1019 +-----+-----+-----+-----+-----+-----+-----+ +-----+ 1020 chan.Off. 0 |chnkA|chnkP|chnk7|chnkO|chnk2|chnkK|chnk1| ... |chnkZ| 1021 +-----+-----+-----+-----+-----+-----+-----+ +-----+ 1022 chan.Off. 1 |chnkB|chnkQ|chnkA|chnkP|chnk3|chnkL|chnk2| ... |chnk1| 1023 +-----+-----+-----+-----+-----+-----+-----+ +-----+ 1024 ... 1025 +-----+-----+-----+-----+-----+-----+-----+ +-----+ 1026 chan.Off. 15 |chnkO|chnk6|chnkN|chnk1|chnkJ|chnkZ|chnkI| ... |chnkG| 1027 +-----+-----+-----+-----+-----+-----+-----+ +-----+ 1028 0 1 2 3 4 5 6 M 1030 Figure 5: CDU matrix Partitioning in Chunks 1032 The appropriation of a chunk can be requested explicitly by the PCE 1033 to any node. After a successful appropriation, the PCE owns the 1034 cells in that chunk, and may use them as hard cells to set up Tracks. 1035 Then again, 6TiSCH did not propose a method for chunk definition and 1036 a protocol for appropriation. This is to be done at RAW. 1038 5.2.2.2. 6TiSCH Tracks 1040 A Track at 6TiSCH is the application to wireless of the concept of a 1041 path in the Detnet architecture [I-D.ietf-detnet-architecture]. A 1042 Track can follow a simple sequence of relay nodes or can be 1043 structured as a more complex Destination Oriented Directed Acyclic 1044 Graph (DODAG) to a unicast destination. Along a Track, 6TiSCH nodes 1045 reserve the resources to enable the efficient transmission of packets 1046 while aiming to optimize certain properties such as reliability and 1047 ensure small jitter or bounded latency. The Track structure enables 1048 Layer-2 forwarding schemes, reducing the overhead of taking routing 1049 decisions at the Layer-3. 1051 Serial Tracks can be understood as the concatenation of cells or 1052 bundles along a routing path from a source towards a destination. 1053 The serial Track concept is analogous to the circuit concept where 1054 resources are chained through the multi-hop topology. For example, A 1055 bundle of Tx Cells in a particular node is paired to a bundle of Rx 1056 Cells in the next hop node following a routing path. 1058 Whereas scheduling ensures reliable delivery in bounded time along 1059 any Track, high availability requires the application of PAREO 1060 functions along a more complex DODAG Track structure. A DODAG has 1061 forking and joining nodes where the concepts such as Replication and 1062 Elimination can be exploited. Spatial redundancy increases the 1063 oveall energy consumption in the network but improves significantly 1064 the availability of the network as well as the packet delivery ratio. 1065 A Track may also branch off and rejoin, for the purpose of the so- 1066 called Packet Replication and Elimination (PRE), over non congruent 1067 branches. PRE may be used to complement layer-2 Automatic Repeat 1068 reQuest (ARQ) and receiver-end Ordering to form the PAREO functions. 1069 PAREO functions enable to meet industrial expectations in PDR within 1070 bounded delivery time over a Track that includes wireless links, even 1071 when the Track extends beyond the 6TiSCH network. 1073 +-----+ 1074 | IoT | 1075 | G/W | 1076 +-----+ 1077 ^ <---- Elimination 1078 | | 1079 Track branch | | 1080 +-------+ +--------+ Subnet Backbone 1081 | | 1082 +--|--+ +--|--+ 1083 | | | Backbone | | | Backbone 1084 o | | | router | | | router 1085 +--/--+ +--|--+ 1086 o / o o---o----/ o 1087 o o---o--/ o o o o o 1088 o \ / o o LLN o 1089 o v <---- Replication 1090 o 1092 Figure 6: End-to-End deterministic Track 1094 In the example above (see Figure 6), a Track is laid out from a field 1095 device in a 6TiSCH network to an IoT gateway that is located on a 1096 IEEE802.1 TSN backbone. 1098 The Replication function in the field device sends a copy of each 1099 packet over two different branches, and a PCE schedules each hop of 1100 both branches so that the two copies arrive in due time at the 1101 gateway. In case of a loss on one branch, hopefully the other copy 1102 of the packet still makes it in due time. If two copies make it to 1103 the IoT gateway, the Elimination function in the gateway ignores the 1104 extra packet and presents only one copy to upper layers. 1106 At each 6TiSCH hop along the Track, the PCE may schedule more than 1107 one timeSlot for a packet, so as to support Layer-2 retries (ARQ). 1108 It is also possible that the field device only uses the second branch 1109 if sending over the first branch fails. 1111 In current deployments, a TSCH Track does not necessarily support PRE 1112 but is systematically multi-path. This means that a Track is 1113 scheduled so as to ensure that each hop has at least two forwarding 1114 solutions, and the forwarding decision is to try the preferred one 1115 and use the other in case of Layer-2 transmission failure as detected 1116 by ARQ. 1118 Methods to implement complex Tracks are described in 1119 [I-D.papadopoulos-paw-pre-reqs] and complemented by extensions to the 1120 RPL routing protocol in [I-D.ietf-roll-nsa-extension] for best effort 1121 traffic, but a centralized routing technique such as promoted in 1122 DetNet is still missing. 1124 5.2.2.2.1. Track Scheduling Protocol 1126 Section "Schedule Management Mechanisms" of the 6TiSCH architecture 1127 describes 4 paradigms to manage the TSCH schedule of the LLN nodes: 1128 Static Scheduling, neighbor-to-neighbor Scheduling, remote monitoring 1129 and scheduling management, and Hop-by-hop scheduling. The Track 1130 operation for DetNet corresponds to a remote monitoring and 1131 scheduling management by a PCE. 1133 Early work at 6TiSCH on a data model and a protocol to program the 1134 schedule in the 6TiSCH device was never concluded as the group 1135 focussed on best effort traffic. This work would be revived by RAW: 1137 The 6top interface document [RFC8480] (to be reopened at RAW) was 1138 intended to specify the generic data model that can be used to 1139 monitor and manage resources of the 6top sublayer. Abstract 1140 methods were suggested for use by a management entity in the 1141 device. The data model also enables remote control operations on 1142 the 6top sublayer. 1144 [I-D.ietf-6tisch-coap] (to be reopened at RAW) was intended to 1145 define a mapping of the 6top set of commands, which is described 1146 in RFC 8480, to CoAP resources. This allows an entity to interact 1147 with the 6top layer of a node that is multiple hops away in a 1148 RESTful fashion. 1150 [I-D.ietf-6tisch-coap] also defined a basic set CoAP resources and 1151 associated RESTful access methods (GET/PUT/POST/DELETE). The 1152 payload (body) of the CoAP messages is encoded using the CBOR 1153 format. The PCE commands are expected to be issued directly as 1154 CoAP requests or to be mapped back and forth into CoAP by a 1155 gateway function at the edge of the 6TiSCH network. For instance, 1156 it is possible that a mapping entity on the backbone transforms a 1157 non-CoAP protocol such as PCEP into the RESTful interfaces that 1158 the 6TiSCH devices support. 1160 5.2.2.2.2. Track Forwarding 1162 By forwarding, this specification means the per-packet operation that 1163 allows to deliver a packet to a next hop or an upper layer in this 1164 node. Forwarding is based on pre-existing state that was installed 1165 as a result of the routing computation of a Track by a PCE. The 1166 6TiSCH architecture supports three different forwarding model, G-MPLS 1167 Track Forwarding (TF), 6LoWPAN Fragment Forwarding (FF) and IPv6 1168 Forwarding (6F) which is the classical IP operation 1169 [I-D.ietf-6tisch-architecture]. The DetNet case relates to the Track 1170 Forwarding operation under the control of a PCE. 1172 A Track is a unidirectional path between a source and a destination. 1173 In a Track cell, the normal operation of IEEE802.15.4 Automatic 1174 Repeat-reQuest (ARQ) usually happens, though the acknowledgment may 1175 be omitted in some cases, for instance if there is no scheduled cell 1176 for a retry. 1178 Track Forwarding is the simplest and fastest. A bundle of cells set 1179 to receive (RX-cells) is uniquely paired to a bundle of cells that 1180 are set to transmit (TX-cells), representing a layer-2 forwarding 1181 state that can be used regardless of the network layer protocol. 1182 This model can effectively be seen as a Generalized Multi-protocol 1183 Label Switching (G-MPLS) operation in that the information used to 1184 switch a frame is not an explicit label, but rather related to other 1185 properties of the way the packet was received, a particular cell in 1186 the case of 6TiSCH. As a result, as long as the TSCH MAC (and 1187 Layer-2 security) accepts a frame, that frame can be switched 1188 regardless of the protocol, whether this is an IPv6 packet, a 6LoWPAN 1189 fragment, or a frame from an alternate protocol such as WirelessHART 1190 or ISA100.11a. 1192 A data frame that is forwarded along a Track normally has a 1193 destination MAC address that is set to broadcast - or a multicast 1194 address depending on MAC support. This way, the MAC layer in the 1195 intermediate nodes accepts the incoming frame and 6top switches it 1196 without incurring a change in the MAC header. In the case of 1197 IEEE802.15.4, this means effectively broadcast, so that along the 1198 Track the short address for the destination of the frame is set to 1199 0xFFFF. 1201 A Track is thus formed end-to-end as a succession of paired bundles, 1202 a receive bundle from the previous hop and a transmit bundle to the 1203 next hop along the Track, and a cell in such a bundle belongs to at 1204 most one Track. For a given iteration of the device schedule, the 1205 effective channel of the cell is obtained by adding a pseudo-random 1206 number to the channelOffset of the cell, which results in a rotation 1207 of the frequency that used for transmission. The bundles may be 1208 computed so as to accommodate both variable rates and 1209 retransmissions, so they might not be fully used at a given iteration 1210 of the schedule. The 6TiSCH architecture provides additional means 1211 to avoid waste of cells as well as overflows in the transmit bundle, 1212 as follows: 1214 In one hand, a TX-cell that is not needed for the current iteration 1215 may be reused opportunistically on a per-hop basis for routed 1216 packets. When all of the frame that were received for a given Track 1217 are effectively transmitted, any available TX-cell for that Track can 1218 be reused for upper layer traffic for which the next-hop router 1219 matches the next hop along the Track. In that case, the cell that is 1220 being used is effectively a TX-cell from the Track, but the short 1221 address for the destination is that of the next-hop router. It 1222 results that a frame that is received in a RX-cell of a Track with a 1223 destination MAC address set to this node as opposed to broadcast must 1224 be extracted from the Track and delivered to the upper layer (a frame 1225 with an unrecognized MAC address is dropped at the lower MAC layer 1226 and thus is not received at the 6top sublayer). 1228 On the other hand, it might happen that there are not enough TX-cells 1229 in the transmit bundle to accommodate the Track traffic, for instance 1230 if more retransmissions are needed than provisioned. In that case, 1231 the frame can be placed for transmission in the bundle that is used 1232 for layer-3 traffic towards the next hop along the Track as long as 1233 it can be routed by the upper layer, that is, typically, if the frame 1234 transports an IPv6 packet. The MAC address should be set to the 1235 next-hop MAC address to avoid confusion. It results that a frame 1236 that is received over a layer-3 bundle may be in fact associated to a 1237 Track. In a classical IP link such as an Ethernet, off-Track traffic 1238 is typically in excess over reservation to be routed along the non- 1239 reserved path based on its QoS setting. But with 6TiSCH, since the 1240 use of the layer-3 bundle may be due to transmission failures, it 1241 makes sense for the receiver to recognize a frame that should be re- 1242 Tracked, and to place it back on the appropriate bundle if possible. 1243 A frame should be re-Tracked if the Per-Hop-Behavior group indicated 1244 in the Differentiated Services Field in the IPv6 header is set to 1245 Deterministic Forwarding, as discussed in Section 5.2.2.1.1. A frame 1246 is re-Tracked by scheduling it for transmission over the transmit 1247 bundle associated to the Track, with the destination MAC address set 1248 to broadcast. 1250 There are 2 modes for a Track, transport mode and tunnel mode. 1252 5.2.2.2.2.1. Transport Mode 1254 In transport mode, the Protocol Data Unit (PDU) is associated with 1255 flow-dependant meta-data that refers uniquely to the Track, so the 1256 6top sublayer can place the frame in the appropriate cell without 1257 ambiguity. In the case of IPv6 traffic, this flow identification is 1258 transported in the Flow Label of the IPv6 header. Associated with 1259 the source IPv6 address, the Flow Label forms a globally unique 1260 identifier for that particular Track that is validated at egress 1261 before restoring the destination MAC address (DMAC) and punting to 1262 the upper layer. 1264 | ^ 1265 +--------------+ | | 1266 | IPv6 | | | 1267 +--------------+ | | 1268 | 6LoWPAN HC | | | 1269 +--------------+ ingress egress 1270 | 6top | sets +----+ +----+ restores 1271 +--------------+ dmac to | | | | dmac to 1272 | TSCH MAC | brdcst | | | | self 1273 +--------------+ | | | | | | 1274 | LLN PHY | +-------+ +--...-----+ +-------+ 1275 +--------------+ 1277 Figure 7: Track Forwarding, Transport Mode 1279 5.2.2.2.2.2. Tunnel Mode 1281 In tunnel mode, the frames originate from an arbitrary protocol over 1282 a compatible MAC that may or may not be synchronized with the 6TiSCH 1283 network. An example of this would be a router with a dual radio that 1284 is capable of receiving and sending WirelessHART or ISA100.11a frames 1285 with the second radio, by presenting itself as an Access Point or a 1286 Backbone Router, respectively. 1288 In that mode, some entity (e.g. PCE) can coordinate with a 1289 WirelessHART Network Manager or an ISA100.11a System Manager to 1290 specify the flows that are to be transported transparently over the 1291 Track. 1293 +--------------+ 1294 | IPv6 | 1295 +--------------+ 1296 | 6LoWPAN HC | 1297 +--------------+ set restore 1298 | 6top | +dmac+ +dmac+ 1299 +--------------+ to|brdcst to|nexthop 1300 | TSCH MAC | | | | | 1301 +--------------+ | | | | 1302 | LLN PHY | +-------+ +--...-----+ +-------+ 1303 +--------------+ | ingress egress | 1304 | | 1305 +--------------+ | | 1306 | LLN PHY | | | 1307 +--------------+ | | 1308 | TSCH MAC | | | 1309 +--------------+ | dmac = | dmac = 1310 |ISA100/WiHART | | nexthop v nexthop 1311 +--------------+ 1313 Figure 8: Track Forwarding, Tunnel Mode 1315 In that case, the flow information that identifies the Track at the 1316 ingress 6TiSCH router is derived from the RX-cell. The dmac is set 1317 to this node but the flow information indicates that the frame must 1318 be tunneled over a particular Track so the frame is not passed to the 1319 upper layer. Instead, the dmac is forced to broadcast and the frame 1320 is passed to the 6top sublayer for switching. 1322 At the egress 6TiSCH router, the reverse operation occurs. Based on 1323 metadata associated to the Track, the frame is passed to the 1324 appropriate link layer with the destination MAC restored. 1326 5.2.2.2.2.3. Tunnel Metadata 1328 Metadata coming with the Track configuration is expected to provide 1329 the destination MAC address of the egress endpoint as well as the 1330 tunnel mode and specific data depending on the mode, for instance a 1331 service access point for frame delivery at egress. If the tunnel 1332 egress point does not have a MAC address that matches the 1333 configuration, the Track installation fails. 1335 In transport mode, if the final layer-3 destination is the tunnel 1336 termination, then it is possible that the IPv6 address of the 1337 destination is compressed at the 6LoWPAN sublayer based on the MAC 1338 address. It is thus mandatory at the ingress point to validate that 1339 the MAC address that was used at the 6LoWPAN sublayer for compression 1340 matches that of the tunnel egress point. For that reason, the node 1341 that injects a packet on a Track checks that the destination is 1342 effectively that of the tunnel egress point before it overwrites it 1343 to broadcast. The 6top sublayer at the tunnel egress point reverts 1344 that operation to the MAC address obtained from the tunnel metadata. 1346 5.2.2.2.2.4. OAM 1348 An Overview of Operations, Administration, and Maintenance (OAM) 1349 Tools [RFC7276] provides an overwiew of the existing tooling for OAM 1350 [RFC6291]. Tracks are complex paths and new tooling is necessary to 1351 manage them, with respect to load control, timing, and the Packet 1352 Replication and Elimination Functions (PREF). 1354 An example of such tooling can be found in the context of BIER 1355 [RFC8279] and more specifically BIER Traffic Engineering 1356 [I-D.ietf-bier-te-arch] (BIER-TE): 1357 [I-D.thubert-bier-replication-elimination] leverages BIER-TE to 1358 control the process of PREF, and to provide traceability of these 1359 operations, in the deterministic dataplane, along a complex Track. 1360 For the 6TiSCH type of constrained environment, 1361 [I-D.thubert-6lo-bier-dispatch] enables an efficient encoding of the 1362 BIER bitmap within the 6LoRH framework. 1364 6. 5G 1366 6.1. Provenance and Documents 1368 The 3rd Generation Partnership Project (3GPP) incorporates many 1369 companies whose business is related to cellular network operation as 1370 well as network equipment and device manufacturing. All generations 1371 of 3GPP technologies provide scheduled wireless segments, primarily 1372 in licensed spectrum which is beneficial for reliability and 1373 availability. 1375 In 2016, the 3GPP started to design New Radio (NR) technology 1376 belonging to the fifth generation (5G) of cellular networks. NR has 1377 been designed from the beginning to not only address enhanced Mobile 1378 Broadband (eMBB) services for consumer devices such as smart phones 1379 or tablets but is also tailored for future Internet of Things (IoT) 1380 communication and connected cyber-physical systems. In addition to 1381 eMBB, requirement categories have been defined on Massive Machine- 1382 Type Communication (M-MTC) for a large number of connected devices/ 1383 sensors, and Ultra-Reliable Low-Latency Communication (URLLC) for 1384 connected control systems and critical communication as illustrated 1385 in Figure 9. It is the URLLC capabilities that make 5G a great 1386 candidate for reliable low-latency communication. With these three 1387 corner stones, NR is a complete solution supporting the connectivity 1388 needs of consumers, enterprises, and public sector for both wide area 1389 and local area, e.g. indoor deployments. A general overview of NR 1390 can be found in [TS38300]. 1392 enhanced 1393 Mobile Broadband 1394 ^ 1395 / \ 1396 / \ 1397 / \ 1398 / \ 1399 / 5G \ 1400 / \ 1401 / \ 1402 / \ 1403 +-----------------+ 1404 Massive Ultra-Reliable 1405 Machine-Type Low-Latency 1406 Communication Communication 1408 Figure 9: 5G Application Areas 1410 As a result of releasing the first NR specification in 2018 (Release 1411 15), it has been proven by many companies that NR is a URLLC-capable 1412 technology and can deliver data packets at 10^-5 packet error rate 1413 within 1ms latency budget [TR37910]. Those evaluations were 1414 consolidated and forwarded to ITU to be included in the [IMT2020] 1415 work. 1417 In order to understand communication requirements for automation in 1418 vertical domains, 3GPP studied different use cases [TR22804] and 1419 released technical specification with reliability, availability and 1420 latency demands for a variety of applications [TS22104]. 1422 As an evolution of NR, multiple studies have been conducted in scope 1423 of 3GPP Release 16 including the following two, focusing on radio 1424 aspects: 1426 1. Study on physical layer enhancements for NR ultra-reliable and 1427 low latency communication (URLLC) [TR38824]. 1429 2. Study on NR industrial Internet of Things (I-IoT) [TR38825]. 1431 In addition, several enhancements have been done on system 1432 architecture level which are reflected in System architecture for the 1433 5G System (5GS) [TS23501]. 1435 6.2. General Characteristics 1437 The 5G Radio Access Network (5G RAN) with its NR interface includes 1438 several features to achieve Quality of Service (QoS), such as a 1439 guaranteeably low latency or tolerable packet error rates for 1440 selected data flows. Determinism is achieved by centralized 1441 admission control and scheduling of the wireless frequency resources, 1442 which are typically licensed frequency bands assigned to a network 1443 operator. 1445 NR enables short transmission slots in a radio subframe, which 1446 benefits low-latency applications. NR also introduces mini-slots, 1447 where prioritized transmissions can be started without waiting for 1448 slot boundaries, further reducing latency. As part of giving 1449 priority and faster radio access to URLLC traffic, NR introduces 1450 preemption where URLLC data transmission can preempt ongoing non- 1451 URLLC transmissions. Additionally, NR applies very fast processing, 1452 enabling retransmissions even within short latency bounds. 1454 NR defines extra-robust transmission modes for increased reliability 1455 both for data and control radio channels. Reliability is further 1456 improved by various techniques, such as multi-antenna transmission, 1457 the use of multiple frequency carriers in parallel and packet 1458 duplication over independent radio links. NR also provides full 1459 mobility support, which is an important reliability aspect not only 1460 for devices that are moving, but also for devices located in a 1461 changing environment. 1463 Network slicing is seen as one of the key features for 5G, allowing 1464 vertical industries to take advantage of 5G networks and services. 1465 Network slicing is about transforming a Public Land Mobile Network 1466 (PLMN) from a single network to a network where logical partitions 1467 are created, with appropriate network isolation, resources, optimized 1468 topology and specific configuration to serve various service 1469 requirements. An operator can configure and manage the mobile 1470 network to support various types of services enabled by 5G, for 1471 example eMBB and URLLC, depending on the different customers' needs. 1473 Exposure of capabilities of 5G Systems to the network or applications 1474 outside the 3GPP domain have been added to Release 16 [TS23501]. Via 1475 exposure interfaces, applications can access 5G capabilities, e.g., 1476 communication service monitoring and network maintenance. 1478 For several generations of mobile networks, 3GPP has considered how 1479 the communication system should work on a global scale with billions 1480 of users, taking into account resilience aspects, privacy regulation, 1481 protection of data, encryption, access and core network security, as 1482 well as interconnect. Security requirements evolve as demands on 1483 trustworthiness increase. For example, this has led to the 1484 introduction of enhanced privacy protection features in 5G. 5G also 1485 employs strong security algorithms, encryption of traffic, protection 1486 of signaling and protection of interfaces. 1488 One particular strength of mobile networks is the authentication, 1489 based on well-proven algorithms and tightly coupled with a global 1490 identity management infrastructure. Since 3G, there is also mutual 1491 authentication, allowing the network to authenticate the device and 1492 the device to authenticate the network. Another strength is secure 1493 solutions for storage and distribution of keys fulfilling regulatory 1494 requirements and allowing international roaming. When connecting to 1495 5G, the user meets the entire communication system, where security is 1496 the result of standardization, product security, deployment, 1497 operations and management as well as incident handling capabilities. 1498 The mobile networks approach the entirety in a rather coordinated 1499 fashion which is beneficial for security. 1501 6.3. Deployment and Spectrum 1503 The 5G system allows deployment in a vast spectrum range, addressing 1504 use-cases in both wide-area as well as local networks. Furthermore, 1505 5G can be configured for public and non-public access. 1507 When it comes to spectrum, NR allows combining the merits of many 1508 frequency bands, such as the high bandwidths in millimeter Waves 1509 (mmW) for extreme capacity locally, as well as the broad coverage 1510 when using mid- and low frequency bands to address wide-area 1511 scenarios. URLLC is achievable in all these bands. Spectrum can be 1512 either licensed, which means that the license holder is the only 1513 authorized user of that spectrum range, or unlicensed, which means 1514 that anyone who wants to use the spectrum can do so. 1516 A prerequisite for critical communication is performance 1517 predictability, which can be achieved by the full control of the 1518 access to the spectrum, which 5G provides. Licensed spectrum 1519 guarantees control over spectrum usage by the system, making it a 1520 preferable option for critical communication. However, unlicensed 1521 spectrum can provide an additional resource for scaling non-critical 1522 communications. While NR is initially developed for usage of 1523 licensed spectrum, the functionality to access also unlicensed 1524 spectrum was introduced in 3GPP Release 16. 1526 Licensed spectrum dedicated to mobile communications has been 1527 allocated to mobile service providers, i.e. issued as longer-term 1528 licenses by national administrations around the world. These 1529 licenses have often been associated with coverage requirements and 1530 issued across whole countries, or in large regions. Besides this, 1531 configured as a non-public network (NPN) deployment, 5G can provide 1532 network services also to a non-operator defined organization and its 1533 premises such as a factory deployment. By this isolation, quality of 1534 service requirements, as well as security requirements can be 1535 achieved. An integration with a public network, if required, is also 1536 possible. The non-public (local) network can thus be interconnected 1537 with a public network, allowing devices to roam between the networks. 1539 In an alternative model, some countries are now in the process of 1540 allocating parts of the 5G spectrum for local use to industries. 1541 These non-service providers then have a choice of applying for a 1542 local license themselves and operating their own network or 1543 cooperating with a public network operator or service provider. 1545 6.4. Applicability to Deterministic Flows 1547 6.4.1. System Architecture 1549 The 5G system [TS23501] consists of the User Equipment (UE) at the 1550 terminal side, and the Radio Access Network (RAN) with the gNB as 1551 radio base station node, as well as the Core Network (CN). The core 1552 network is based on a service-based architecture with the central 1553 functions: Access and Mobility Management Function (AMF), Session 1554 Management Function (SMF) and User Plane Function (UPF) as 1555 illustrated in Figure 10. 1557 The gNB's main responsibility is the radio resource management, 1558 including admission control and scheduling, mobility control and 1559 radio measurement handling. The AMF handles the UE's connection 1560 status and security, while the SMF controls the UE's data sessions. 1561 The UPF handles the user plane traffic. 1563 The SMF can instantiate various Packet Data Unit (PDU) sessions for 1564 the UE, each associated with a set of QoS flows, i.e., with different 1565 QoS profiles. Segregation of those sessions is also possible, e.g., 1566 resource isolation in the RAN and in the CN can be defined (slicing). 1568 +----+ +---+ +---+ +---+ +---+ +---+ 1569 |NSSF| |NEF| |NRF| |PCF| |UDM| |AF | 1570 +--+-+ +-+-+ +-+-+ +-+-+ +-+-+ +-+-+ 1571 | | | | | | 1572 Nnssf| Nnef| Nnrf| Npcf| Nudm| Naf| 1573 | | | | | | 1574 ---+------+-+-----+-+------------+--+-----+-+--- 1575 | | | | 1576 Nausf| Nausf| Nsmf| | 1577 | | | | 1578 +--+-+ +-+-+ +-+-+ +-+-+ 1579 |AUSF| |AMF| |SMF| |SCP| 1580 +----+ +++-+ +-+-+ +---+ 1581 / | | 1582 / | | 1583 / | | 1584 N1 N2 N4 1585 / | | 1586 / | | 1587 / | | 1588 +--+-+ +--+--+ +--+---+ +----+ 1589 | UE +---+(R)AN+--N3--+ UPF +--N6--+ DN | 1590 +----+ +-----+ ++----++ +----+ 1591 | | 1592 +-N9-+ 1594 Figure 10: 5G System Architecture 1596 To allow UE mobility across cells/gNBs, handover mechanisms are 1597 supported in NR. For an established connection, i.e., connected mode 1598 mobility, a gNB can configure a UE to report measurements of received 1599 signal strength and quality of its own and neighbouring cells, 1600 periodically or event-based. Based on these measurement reports, the 1601 gNB decides to handover a UE to another target cell/gNB. Before 1602 triggering the handover, it is hand-shaked with the target gNB based 1603 on network signalling. A handover command is then sent to the UE and 1604 the UE switches its connection to the target cell/gNB. The Packet 1605 Data Convergence Protocol (PDCP) of the UE can be configured to avoid 1606 data loss in this procedure, i.e., handle retransmissions if needed. 1607 Data forwarding is possible between source and target gNB as well. 1608 To improve the mobility performance further, i.e., to avoid 1609 connection failures, e.g., due to too-late handovers, the mechanism 1610 of conditional handover is introduced in Release 16 specifications. 1612 Therein a conditional handover command, defining a triggering point, 1613 can be sent to the UE before UE enters a handover situation. A 1614 further improvement introduced in Release 16 is the Dual Active 1615 Protocol Stack (DAPS), where the UE maintains the connection to the 1616 source cell while connecting to the target cell. This way, potential 1617 interruptions in packet delivery can be avoided entirely. 1619 6.4.2. Overview of The Radio Protocol Stack 1621 The protocol architecture for NR consists of the L1 Physical layer 1622 (PHY) and as part of the L2, the sublayers of Medium Access Control 1623 (MAC), Radio Link Control (RLC), Packet Data Convergence Protocol 1624 (PDCP), as well as the Service Data Adaption Protocol (SDAP). 1626 The PHY layer handles signal processing related actions, such as 1627 encoding/decoding of data and control bits, modulation, antenna 1628 precoding and mapping. 1630 The MAC sub-layer handles multiplexing and priority handling of 1631 logical channels (associated with QoS flows) to transport blocks for 1632 PHY transmission, as well as scheduling information reporting and 1633 error correction through Hybrid Automated Repeat Request (HARQ). 1635 The RLC sublayer handles sequence numbering of higher layer packets, 1636 retransmissions through Automated Repeat Request (ARQ), if 1637 configured, as well as segmentation and reassembly and duplicate 1638 detection. 1640 The PDCP sublayer consists of functionalities for ciphering/ 1641 deciphering, integrity protection/verification, re-ordering and in- 1642 order delivery, duplication and duplicate handling for higher layer 1643 packets, and acts as the anchor protocol to support handovers. 1645 The SDAP sublayer provides services to map QoS flows, as established 1646 by the 5G core network, to data radio bearers (associated with 1647 logical channels), as used in the 5G RAN. 1649 Additionally, in RAN, the Radio Resource Control (RRC) protocol, 1650 handles the access control and configuration signalling for the 1651 aforementioned protocol layers. RRC messages are considered L3 and 1652 thus transmitted also via those radio protocol layers. 1654 To provide low latency and high reliability for one transmission 1655 link, i.e., to transport data (or control signaling) of one radio 1656 bearer via one carrier, several features have been introduced on the 1657 user plane protocols for PHY and L2, as explained in the following. 1659 6.4.3. Radio (PHY) 1661 NR is designed with native support of antenna arrays utilizing 1662 benefits from beamforming, transmissions over multiple MIMO layers 1663 and advanced receiver algorithms allowing effective interference 1664 cancellation. Those antenna techniques are the basis for high signal 1665 quality and effectiveness of spectral usage. Spatial diversity with 1666 up to 4 MIMO layers in UL and up to 8 MIMO layers in DL is supported. 1667 Together with spatial-domain multiplexing, antenna arrays can focus 1668 power in desired direction to form beams. NR supports beam 1669 management mechanisms to find the best suitable beam for UE initially 1670 and when it is moving. In addition, gNBs can coordinate their 1671 respective DL and UL transmissions over the backhaul network keeping 1672 interference reasonably low, and even make transmissions or 1673 receptions from multiple points (multi-TRP). Multi-TRP can be used 1674 for repetition of data packet in time, in frequency or over multiple 1675 MIMO layers which can improve reliability even further. 1677 Any downlink transmission to a UE starts from resource allocation 1678 signaling over the Physical Downlink Control Channel (PDCCH). If it 1679 is successfully received, the UE will know about the scheduled 1680 transmission and may receive data over the Physical Downlink Shared 1681 Channel (PDSCH). If retransmission is required according to the HARQ 1682 scheme, a signaling of negative acknowledgement (NACK) on the 1683 Physical Uplink Control Channel (PUCCH) is involved and PDCCH 1684 together with PDSCH transmissions (possibly with additional 1685 redundancy bits) are transmitted and soft-combined with previously 1686 received bits. Otherwise, if no valid control signaling for 1687 scheduling data is received, nothing is transmitted on PUCCH 1688 (discontinuous transmission - DTX),and the base station upon 1689 detecting DTX will retransmit the initial data. 1691 An uplink transmission normally starts from a Scheduling Request (SR) 1692 - a signaling message from the UE to the base station sent via PUCCH. 1693 Once the scheduler is informed about buffer data in UE, e.g., by SR, 1694 the UE transmits a data packet on the Physical Uplink Shared Channel 1695 (PUSCH). Pre-scheduling not relying on SR is also possible (see 1696 following section). 1698 Since transmission of data packets require usage of control and data 1699 channels, there are several methods to maintain the needed 1700 reliability. NR uses Low Density Parity Check (LDPC) codes for data 1701 channels, Polar codes for PDCCH, as well as orthogonal sequences and 1702 Polar codes for PUCCH. For ultra-reliability of data channels, very 1703 robust (low spectral efficiency) Modulation and Coding Scheme (MCS) 1704 tables are introduced containing very low (down to 1/20) LDPC code 1705 rates using BPSK or QPSK. Also, PDCCH and PUCCH channels support 1706 multiple code rates including very low ones for the channel 1707 robustness. 1709 A connected UE reports downlink (DL) quality to gNB by sending 1710 Channel State Information (CSI) reports via PUCCH while uplink (UL) 1711 quality is measured directly at gNB. For both uplink and downlink, 1712 gNB selects the desired MCS number and signals it to the UE by 1713 Downlink Control Information (DCI) via PDCCH channel. For URLLC 1714 services, the UE can assist the gNB by advising that MCS targeting 1715 10^-5 Block Error Rate (BLER) are used. Robust link adaptation 1716 algorithms can maintain the needed level of reliability considering a 1717 given latency bound. 1719 Low latency on the physical layer is provided by short transmission 1720 duration which is possible by using high Subcarrier Spacing (SCS) and 1721 the allocation of only one or a few Orthogonal Frequency Division 1722 Multiplexing (OFDM) symbols. For example, the shortest latency for 1723 the worst case in DL can be 0.23ms and in UL can be 0.24ms according 1724 to (section 5.7.1 in [TR37910]). Moreover, if the initial 1725 transmission has failed, HARQ feedback can quickly be provided and an 1726 HARQ retransmission is scheduled. 1728 Dynamic multiplexing of data associated with different services is 1729 highly desirable for efficient use of system resources and to 1730 maximize system capacity. Assignment of resources for eMBB is 1731 usually done with regular (longer) transmission slots, which can lead 1732 to blocking of low latency services. To overcome the blocking, eMBB 1733 resources can be pre-empted and re-assigned to URLLC services. In 1734 this way, spectrally efficient assignments for eMBB can be ensured 1735 while providing flexibility required to ensure a bounded latency for 1736 URLLC services. In downlink, the gNB can notify the eMBB UE about 1737 pre-emption after it has happened, while in uplink there are two pre- 1738 emption mechanisms: special signaling to cancel eMBB transmission and 1739 URLLC dynamic power boost to suppress eMBB transmission. 1741 6.4.4. Scheduling and QoS (MAC) 1743 One integral part of the 5G system is the Quality of Service (QoS) 1744 framework [TS23501]. QoS flows are setup by the 5G system for 1745 certain IP or Ethernet packet flows, so that packets of each flow 1746 receive the same forwarding treatment, i.e., in scheduling and 1747 admission control. QoS flows can for example be associated with 1748 different priority level, packet delay budgets and tolerable packet 1749 error rates. Since radio resources are centrally scheduled in NR, 1750 the admission control function can ensure that only those QoS flows 1751 are admitted for which QoS targets can be reached. 1753 NR transmissions in both UL and DL are scheduled by the gNB 1754 [TS38300]. This ensures radio resource efficiency, fairness in 1755 resource usage of the users and enables differentiated treatment of 1756 the data flows of the users according to the QoS targets of the 1757 flows. Those QoS flows are handled as data radio bearers or logical 1758 channels in NR RAN scheduling. 1760 The gNB can dynamically assign DL and UL radio resources to users, 1761 indicating the resources as DL assignments or UL grants via control 1762 channel to the UE. Radio resources are defined as blocks of OFDM 1763 symbols in spectral domain and time domain. Different lengths are 1764 supported in time domain, i.e., (multiple) slot or mini-slot lengths. 1765 Resources of multiple frequency carriers can be aggregated and 1766 jointly scheduled to the UE. 1768 Scheduling decisions are based, e.g., on channel quality measured on 1769 reference signals and reported by the UE (cf. periodical CSI reports 1770 for DL channel quality). The transmission reliability can be chosen 1771 in the scheduling algorithm, i.e., by link adaptation where an 1772 appropriate transmission format (e.g., robustness of modulation and 1773 coding scheme, controlled UL power) is selected for the radio channel 1774 condition of the UE. Retransmissions, based on HARQ feedback, are 1775 also controlled by the scheduler. If needed to avoid HARQ round-trip 1776 time delays, repeated transmissions can be also scheduled beforehand, 1777 to the cost of reduced spectral efficiency. 1779 In dynamic DL scheduling, transmission can be initiated immediately 1780 when DL data becomes available in the gNB. However, for dynamic UL 1781 scheduling, when data becomes available but no UL resources are 1782 available yet, the UE indicates the need for UL resources to the gNB 1783 via a (single bit) scheduling request message in the UL control 1784 channel. When thereupon UL resources are scheduled to the UE, the UE 1785 can transmit its data and may include a buffer status report, 1786 indicating the exact amount of data per logical channel still left to 1787 be sent. More UL resources may be scheduled accordingly. To avoid 1788 the latency introduced in the scheduling request loop, UL radio 1789 resources can also be pre-scheduled. 1791 In particular for periodical traffic patterns, the pre-scheduling can 1792 rely on the scheduling features DL Semi-Persistent Scheduling (SPS) 1793 and UL Configured Grant (CG). With these features, periodically 1794 recurring resources can be assigned in DL and UL. Multiple parallels 1795 of those configurations are supported, in order to serve multiple 1796 parallel traffic flows of the same UE. 1798 To support QoS enforcement in the case of mixed traffic with 1799 different QoS requirements, several features have recently been 1800 introduced. This way, e.g., different periodical critical QoS flows 1801 can be served together with best effort transmissions, by the same 1802 UE. Among others, these features (partly Release 16) are: 1) UL 1803 logical channel transmission restrictions allowing to map logical 1804 channels of certain QoS only to intended UL resources of a certain 1805 frequency carrier, slot-length, or CG configuration, and 2) intra-UE 1806 pre-emption, allowing critical UL transmissions to pre-empt non- 1807 critical transmissions. 1809 When multiple frequency carriers are aggregated, duplicate parallel 1810 transmissions can be employed (beside repeated transmissions on one 1811 carrier). This is possible in the Carrier Aggregation (CA) 1812 architecture where those carriers originate from the same gNB, or in 1813 the Dual Connectivity (DC) architecture where the carriers originate 1814 from different gNBs, i.e., the UE is connected to two gNBs in this 1815 case. In both cases, transmission reliability is improved by this 1816 means of providing frequency diversity. 1818 In addition to licensed spectrum, a 5G system can also utilize 1819 unlicensed spectrum to offload non-critical traffic. This version of 1820 NR is called NR-U, part of 3GPP Release 16. The central scheduling 1821 approach applies also for unlicensed radio resources, but in addition 1822 also the mandatory channel access mechanisms for unlicensed spectrum, 1823 e.g., Listen Before Talk (LBT) are supported in NR-U. This way, by 1824 using NR, operators have and can control access to both licensed and 1825 unlicensed frequency resources. 1827 6.4.5. Time-Sensitive Networking (TSN) Integration 1829 The main objective of Time-Sensitive Networking (TSN) is to provide 1830 guaranteed data delivery within a guaranteed time window, i.e., 1831 bounded low latency. IEEE 802.1 TSN [IEEE802.1TSN] is a set of open 1832 standards that provide features to enable deterministic communication 1833 on standard IEEE 802.3 Ethernet [IEEE802.3]. TSN standards can be 1834 seen as a toolbox for traffic shaping, resource management, time 1835 synchronization, and reliability. 1837 A TSN stream is a data flow between one end station (Talker) to 1838 another end station (Listener). In the centralized configuration 1839 model, TSN bridges are configured by the Central Network Controller 1840 (CNC) [IEEE802.1Qcc] to provide deterministic connectivity for the 1841 TSN stream through the network. Time-based traffic shaping provided 1842 by Scheduled Traffic [IEEE802.1Qbv] may be used to achieve bounded 1843 low latency. The TSN tool for time synchronization is the 1844 generalized Precision Time Protocol (gPTP) [IEEE802.1AS]), which 1845 provides reliable time synchronization that can be used by end 1846 stations and by other TSN tools, e.g., Scheduled Traffic 1847 [IEEE802.1Qbv]. High availability, as a result of ultra-reliability, 1848 is provided for data flows by the Frame Replication and Elimination 1849 for Reliability (FRER) [IEEE802.1CB] mechanism. 1851 3GPP Release 16 includes integration of 5G with TSN, i.e., specifies 1852 functions for the 5G System (5GS) to deliver TSN streams such that 1853 the meet their QoS requirements. A key aspect of the integration is 1854 the 5GS appears from the rest of the network as a set of TSN bridges, 1855 in particular, one virtual bridge per User Plane Function (UPF) on 1856 the user plane. The 5GS includes TSN Translator (TT) functionality 1857 for the adaptation of the 5GS to the TSN bridged network and for 1858 hiding the 5GS internal procedures. The 5GS provides the following 1859 components: 1861 1. interface to TSN controller, as per [IEEE802.1Qcc] for the fully 1862 centralized configuration model 1864 2. time synchronization via reception and transmission of gPTP PDUs 1865 [IEEE802.1AS] 1867 3. low latency, hence, can be integrated with Scheduled Traffic 1868 [IEEE802.1Qbv] 1870 4. reliability, hence, can be integrated with FRER [IEEE802.1CB] 1871 Figure 10 shows an illustration of 5G-TSN integration where an 1872 industrial controller (Ind Ctrlr) is connected to industrial Input/ 1873 Output devices (I/O dev) via 5G. The 5GS can directly transport 1874 Ethernet frames since Release 15, thus, end-to-end Ethernet 1875 connectivity is provided. The 5GS implements the required interfaces 1876 towards the TSN controller functions such as the CNC, thus adapts to 1877 the settings of the TSN network. A 5G user plane virtual bridge 1878 interconnects TSN bridges or connect end stations, e.g., I/O devices 1879 to the network. Note that the introduction of 5G brings flexibility 1880 in various aspects, e.g., more flexible network topology because a 1881 wireless hop can replace several wireline hops thus significantly 1882 reduce the number of hops end-to-end. [ETR5GTSN] dives more into the 1883 integration of 5G with TSN. 1885 +------------------------------+ 1886 | 5G System | 1887 | +---+| 1888 | +-+ +-+ +-+ +-+ +-+ |TSN|| 1889 | | | | | | | | | | | |AF |......+ 1890 | +++ +++ +++ +++ +++ +-+-+| . 1891 | | | | | | | | . 1892 | -+---+---++--+-+-+--+-+- | . 1893 | | | | | | +--+--+ 1894 | +++ +++ +++ +++ | | TSN | 1895 | | | | | | | | | | |Ctrlr+.......+ 1896 | +++ +++ +++ +++ | +--+--+ . 1897 | | . . 1898 | | . . 1899 | +..........................+ | . . 1900 | . Virtual Bridge . | . . 1901 +---+ | . +--+--+ +---+ +---+--+ . | +--+---+ . 1902 |I/O+----------------+DS|UE+---+RAN+-+UPF|NW+------+ TSN +----+ . 1903 |dev| | . |TT| | | | | |TT| . | |bridge| | . 1904 +---+ | . +--+--+ +---+ +---+--+ . | +------+ | . 1905 | +..........................+ | . +-+-+-+ 1906 | | . | Ind | 1907 | +..........................+ | . |Ctrlr| 1908 | . Virtual Bridge . | . +-+---+ 1909 +---+ +------+ | . +--+--+ +---+ +---+--+ . | +--+---+ | 1910 |I/O+--+ TSN +------+DS|UE+---+RAN+-+UPF|NW+------+ TSN +----+ 1911 |dev| |bridge| | . |TT| | | | | |TT| . | |bridge| 1912 +---+ +------+ | . +--+--+ +---+ +---+--+ . | +------+ 1913 | +..........................+ | 1914 +------------------------------+ 1916 <----------------- end-to-end Ethernet -------------------> 1918 Figure 11: 5G - TSN Integration 1920 NR supports accurate reference time synchronization in 1us accuracy 1921 level. Since NR is a scheduled system, an NR UE and a gNB are 1922 tightly synchronized to their OFDM symbol structures. A 5G internal 1923 reference time can be provided to the UE via broadcast or unicast 1924 signaling, associating a known OFDM symbol to this reference clock. 1925 The 5G internal reference time can be shared within the 5G network, 1926 i.e., radio and core network components. For the interworking with 1927 gPTP for multiple time domains, the 5GS acts as a virtual gPTP time- 1928 aware system and supports the forwarding of gPTP time synchronization 1929 information between end stations and bridges through the 5G user 1930 plane TTs. These account for the residence time of the 5GS in the 1931 time synchronization procedure. One special option is when the 5GS 1932 internal reference time in not only used within the 5GS, but also to 1933 the rest of the devices in the deployment, including connected TSN 1934 bridges and end stations. 1936 Redundancy architectures were specified in order to provide 1937 reliability against any kind of failure on the radio link or nodes in 1938 the RAN and the core network, Redundant user plane paths can be 1939 provided based on the dual connectivity architecture, where the UE 1940 sets up two PDU sessions towards the same data network, and the 5G 1941 system makes the paths of the two PDU sessions independent as 1942 illustrated in Figure 13. There are two PDU sessions involved in the 1943 solution: the first spans from the UE via gNB1 to UPF1, acting as the 1944 first PDU session anchor, while the second spans from the UE via gNB2 1945 to UPF2, acting as second the PDU session anchor. The independent 1946 paths may continue beyond the 3GPP network. Redundancy Handling 1947 Functions (RHFs) are deployed outside of the 5GS, i.e., in Host A 1948 (the device) and in Host B (the network). RHF can implement 1949 replication and elimination functions as per [IEEE802.1CB] or the 1950 Packet Replication, Elimination, and Ordering Functions (PREOF) of 1951 IETF Deterministic Networking (DetNet) [RFC8655]. 1953 +........+ 1954 . Device . +------+ +------+ +------+ 1955 . . + gNB1 +--N3--+ UPF1 |--N6--+ | 1956 . ./+------+ +------+ | | 1957 . +----+ / | | 1958 . | |/. | | 1959 . | UE + . | DN | 1960 . | |\. | | 1961 . +----+ \ | | 1962 . .\+------+ +------+ | | 1963 +........+ + gNB2 +--N3--+ UPF2 |--N6--+ | 1964 +------+ +------+ +------+ 1966 Figure 12: Reliability with Single UE 1968 An alternative solution is that multiple UEs per device are used for 1969 user plane redundancy as illustrated in Figure 13. Each UE sets up a 1970 PDU session. The 5GS ensures that those PDU sessions of the 1971 different UEs are handled independently internal to the 5GS. There 1972 is no single point of failure in this solution, which also includes 1973 RHF outside of the 5G system, e.g., as per FRER or as PREOF 1974 specifications. 1976 +.........+ 1977 . Device . 1978 . . 1979 . +----+ . +------+ +------+ +------+ 1980 . | UE +-----+ gNB1 +--N3--+ UPF1 |--N6--+ | 1981 . +----+ . +------+ +------+ | | 1982 . . | DN | 1983 . +----+ . +------+ +------+ | | 1984 . | UE +-----+ gNB2 +--N3--+ UPF2 |--N6--+ | 1985 . +----+ . +------+ +------+ +------+ 1986 . . 1987 +.........+ 1989 Figure 13: Reliability with Dual UE 1991 Note that the abstraction provided by the RHF and the location of the 1992 RHF being outside of the 5G system make 5G equally supporting 1993 integration for reliability both with FRER of TSN and PREOF of DetNet 1994 as they both rely on the same concept. 1996 Note also that TSN is the primary subnetwork technology for DetNet. 1997 Thus, the DetNet over TSN work, e.g., [I-D.ietf-detnet-ip-over-tsn], 1998 can be leveraged via the TSN support built in 5G. 2000 6.5. Summary 2002 5G technology enables deterministic communication. Based on the 2003 centralized admission control and the scheduling of the wireless 2004 resources, licensed or unlicensed, quality of service such as latency 2005 and reliability can be guaranteed. 5G contains several features to 2006 achieve ultra-reliable and low latency performance, e.g., support for 2007 different OFDM numerologies and slot-durations, as well as fast 2008 processing capabilities and redundancy techniques that lead to 2009 achievable latency numbers of below 1ms with reliability guarantees 2010 up to 99.999%. 2012 5G also includes features to support Industrial IoT use cases, e.g., 2013 via the integration of 5G with TSN. This includes 5G capabilities 2014 for each TSN component, latency, resource management, time 2015 synchronization, and reliability. Furthermore, 5G support for TSN 2016 can be leveraged when 5G is used as subnet technology for DetNet, in 2017 combination with or instead of TSN, which is the primary subnet for 2018 DetNet. In addition, the support for integration with TSN 2019 reliability was added to 5G by making DetNet reliability also 2020 applicable, thus making 5G DetNet ready. Moreover, providing IP 2021 service is native to 5G. 2023 Overall, 5G provides scheduled wireless segments with high 2024 reliability and availability. In addition, 5G includes capabilities 2025 for integration to IP networks. 2027 7. L-band Digital Aeronautical Communications System 2029 One of the main pillars of the modern Air Traffic Management (ATM) 2030 system is the existence of a communication infrastructure that 2031 enables efficient aircraft guidance and safe separation in all phases 2032 of flight. Although current systems are technically mature, they are 2033 suffering from the VHF band's increasing saturation in high-density 2034 areas and the limitations posed by analogue radio. Therefore, 2035 aviation globally and the European Union (EU) in particular, strives 2036 for a sustainable modernization of the aeronautical communication 2037 infrastructure. 2039 In the long-term, ATM communication shall transition from analogue 2040 VHF voice and VDL2 communication to more spectrum efficient digital 2041 data communication. The European ATM Master Plan foresees this 2042 transition to be realized for terrestrial communications by the 2043 development and implementation of the L-band Digital Aeronautical 2044 Communications System (LDACS). LDACS shall enable IPv6 based air- 2045 ground communication related to the safety and regularity of the 2046 flight. The particular challenge is that no new frequencies can be 2047 made available for terrestrial aeronautical communication. It was 2048 thus necessary to develop procedures to enable the operation of LDACS 2049 in parallel with other services in the same frequency band. 2051 7.1. Provenance and Documents 2053 The development of LDACS has already made substantial progress in the 2054 Single European Sky ATM Research (SESAR) framework, and is currently 2055 being continued in the follow-up program, SESAR2020 [RIH18]. A key 2056 objective of the SESAR activities is to develop, implement and 2057 validate a modern aeronautical data link able to evolve with aviation 2058 needs over long-term. To this end, an LDACS specification has been 2059 produced [GRA19] and is continuously updated; transmitter 2060 demonstrators were developed to test the spectrum compatibility of 2061 LDACS with legacy systems operating in the L-band [SAJ14]; and the 2062 overall system performance was analyzed by computer simulations, 2063 indicating that LDACS can fulfil the identified requirements [GRA11]. 2065 LDACS standardization within the framework of the International Civil 2066 Aviation Organization (ICAO) started in December 2016. The ICAO 2067 standardization group has produced an initial Standards and 2068 Recommended Practices (SARPs) document [ICAO18]. The SARPs document 2069 defines the general characteristics of LDACS. The ICAO 2070 standardization group plans to produce an ICAO technical manual - the 2071 ICAO equivalent to a technical standard - within the next years. 2072 Generally, the group is open to input from all sources and develops 2073 LDACS in the open. 2075 Up to now the LDACS standardization has been focused on the 2076 development of the physical layer and the data link layer, only 2077 recently have higher layers come into the focus of the LDACS 2078 development activities. There is currently no "IPv6 over LDACS" 2079 specification; however, SESAR2020 has started the testing of 2080 IPv6-based LDACS testbeds. The IPv6 architecture for the 2081 aeronautical telecommunication network is called the Future 2082 Communications Infrastructure (FCI). FCI shall support quality of 2083 service, diversity, and mobility under the umbrella of the "multi- 2084 link concept". This work is conducted by ICAO working group WG-I. 2086 In addition to standardization activities several industrial LDACS 2087 prototypes have been built. One set of LDACS prototypes has been 2088 evaluated in flight trials confirming the theoretical results 2089 predicting the system performance [GRA18][SCH19]. 2091 7.2. General Characteristics 2093 LDACS will become one of several wireless access networks connecting 2094 aircraft to the Aeronautical Telecommunications Network (ATN). The 2095 LDACS access network contains several ground stations, each of them 2096 providing one LDACS radio cell. The LDACS air interface is a 2097 cellular data link with a star-topology connecting aircraft to 2098 ground-stations with a full duplex radio link. Each ground-station 2099 is the centralized instance controlling all air-ground communications 2100 within its radio cell. A ground-station supports up to 512 aircraft. 2102 The LDACS air interface protocol stack defines two layers, the 2103 physical layer and the data link layer. 2105 The physical layer provides the means to transfer data over the radio 2106 channel. The LDACS ground-station supports bi-directional links to 2107 multiple aircraft under its control. The forward link direction (FL; 2108 ground-to-air) and the reverse link direction (RL; air-to-ground) are 2109 separated by frequency division duplex. Forward link and reverse 2110 link use a 500 kHz channel each. The ground-station transmits a 2111 continuous stream of OFDM symbols on the forward link. In the 2112 reverse link different aircraft are separated in time and frequency 2113 using a combination of Orthogonal Frequency-Division Multiple-Access 2114 (OFDMA) and Time-Division Multiple-Access (TDMA). Aircraft thus 2115 transmit discontinuously on the reverse link with radio bursts sent 2116 in precisely defined transmission opportunities allocated by the 2117 ground-station. LDACS does not support beam-forming or Multiple 2118 Input Multiple Output (MIMO). 2120 The data-link layer provides the necessary protocols to facilitate 2121 concurrent and reliable data transfer for multiple users. The LDACS 2122 data link layer is organized in two sub-layers: The medium access 2123 sub-layer and the logical link control sub-layer. The medium access 2124 sub-layer manages the organization of transmission opportunities in 2125 slots of time and frequency. The logical link control sub-layer 2126 provides acknowledged point-to-point logical channels between the 2127 aircraft and the ground-station using an automatic repeat request 2128 protocol. LDACS supports also unacknowledged point-to-point channels 2129 and ground-to-air broadcast. 2131 The user data rate of LDACS is 315 kbit/s to 1428 kbit/s on the 2132 forward link, and 294 kbit/s to 1390 kbit/s on the reverse link, 2133 depending on coding and modulation. Due to strong interference from 2134 legacy systems in the L-band, the most robust coding and modulation 2135 should be expected for initial deployment i.e. 315/294 kbit/s on the 2136 forward/reverse link, respectively. 2138 Since LDACS has been mainly designed for air traffic management 2139 communication it supports mutual entity authentication, integrity and 2140 confidentiality capabilities of user data messages and some control 2141 channel protection capabilities [MAE19]. 2143 7.3. Applicability to Deterministic Flows 2145 LDACS has been designed with applications related to the safety and 2146 regularity of the flight in mind. It has therefore been designed as 2147 a deterministic wireless data link (as far as possible). 2149 LDACS medium access is always under the control of the ground-station 2150 of a radio cell. Any medium access for the transmission of user data 2151 has to be requested with a resource request message stating the 2152 requested amount of resources and class of service. The ground- 2153 station performs resource scheduling on the basis of these requests 2154 and grants resources with resource allocation messages. Resource 2155 request and allocation messages are exchanged over dedicated 2156 contention-free control channels. 2158 LDACS has two mechanisms to request resources from the scheduler in 2159 the ground-station. Resources can either be requested "on demand" 2160 with a given class of service. On the forward link, this is done 2161 locally in the ground-station, on the reverse link a dedicated 2162 contention-free control channel is used (Dedicated Control Channel 2163 (DCCH); roughly 83 bit every 60 ms). A resource allocation is always 2164 announced in the control channel of the forward link (Common Control 2165 Channel (CCCH); variable sized). Due to the spacing of the reverse 2166 link control channels of every 60 ms, a medium access delay in the 2167 same order of magnitude is to be expected. 2169 Resources can also be requested "permanently". The permanent 2170 resource request mechanism supports requesting recurring resources in 2171 given time intervals. A permanent resource request has to be 2172 canceled by the user (or by the ground-station, which is always in 2173 control). User data transmissions over LDACS are therefore always 2174 scheduled by the ground-station, while control data uses statically 2175 (i.e. at net entry) allocated recurring resources (DCCH and CCCH). 2176 The current specification documents specify no scheduling algorithm. 2177 However performance evaluations so far have used strict priority 2178 scheduling and round robin for equal priorities for simplicity. In 2179 the current prototype implementations LDACS classes of service are 2180 thus realized as priorities of medium access and not as flows. Note 2181 that this can starve out low priority flows. However, this is not 2182 seen as a big problem since safety related message always go first in 2183 any case. Scheduling of reverse link resources is done in physical 2184 Protocol Data Units (PDU) of 112 bit (or larger if more aggressive 2185 coding and modulation is used). Scheduling on the forward link is 2186 done Byte-wise since the forward link is transmitted continuously by 2187 the ground-station. 2189 In order to support diversity, LDACS supports handovers to other 2190 ground-stations on different channels. Handovers may be initiated by 2191 the aircraft (break-before-make) or by the ground-station (make- 2192 before-break) if it is connected to an alternative ground-station via 2193 the same ground-station controller. Beyond this, FCI diversity shall 2194 be implemented by the multi-link concept. 2196 8. IANA Considerations 2198 This specification does not require IANA action. 2200 9. Security Considerations 2202 Most RAW technologies integrate some authentication or encryption 2203 mechanisms that were defined outside the IETF. 2205 10. Contributors 2207 Georgios Z. Papadopoulos: Contributed to the TSCH section. 2209 Nils Mäurer: Contributed to the LDACS section. 2211 Thomas Gräupl: Contributed to the LDACS section. 2213 Janos Farkas, Torsten Dudda, Alexey Shapin, and Sara Sandberg: Contr 2214 ibuted to the 5G section. 2216 11. Acknowledgments 2218 Many thanks to the participants of the RAW WG where a lot of the work 2219 discussed here happened. 2221 12. Normative References 2223 [RFC8480] Wang, Q., Ed., Vilajosana, X., and T. Watteyne, "6TiSCH 2224 Operation Sublayer (6top) Protocol (6P)", RFC 8480, 2225 DOI 10.17487/RFC8480, November 2018, 2226 . 2228 [RFC8200] Deering, S. and R. Hinden, "Internet Protocol, Version 6 2229 (IPv6) Specification", STD 86, RFC 8200, 2230 DOI 10.17487/RFC8200, July 2017, 2231 . 2233 [RFC5673] Pister, K., Ed., Thubert, P., Ed., Dwars, S., and T. 2234 Phinney, "Industrial Routing Requirements in Low-Power and 2235 Lossy Networks", RFC 5673, DOI 10.17487/RFC5673, October 2236 2009, . 2238 [I-D.ietf-detnet-architecture] 2239 Finn, N., Thubert, P., Varga, B., and J. Farkas, 2240 "Deterministic Networking Architecture", Work in Progress, 2241 Internet-Draft, draft-ietf-detnet-architecture-13, 6 May 2242 2019, . 2245 [I-D.ietf-6tisch-architecture] 2246 Thubert, P., "An Architecture for IPv6 over the TSCH mode 2247 of IEEE 802.15.4", Work in Progress, Internet-Draft, 2248 draft-ietf-6tisch-architecture-29, 27 August 2020, 2249 . 2252 13. Informative References 2254 [RFC6550] Winter, T., Ed., Thubert, P., Ed., Brandt, A., Hui, J., 2255 Kelsey, R., Levis, P., Pister, K., Struik, R., Vasseur, 2256 JP., and R. Alexander, "RPL: IPv6 Routing Protocol for 2257 Low-Power and Lossy Networks", RFC 6550, 2258 DOI 10.17487/RFC6550, March 2012, 2259 . 2261 [RFC6551] Vasseur, JP., Ed., Kim, M., Ed., Pister, K., Dejean, N., 2262 and D. Barthel, "Routing Metrics Used for Path Calculation 2263 in Low-Power and Lossy Networks", RFC 6551, 2264 DOI 10.17487/RFC6551, March 2012, 2265 . 2267 [RFC6291] Andersson, L., van Helvoort, H., Bonica, R., Romascanu, 2268 D., and S. Mansfield, "Guidelines for the Use of the "OAM" 2269 Acronym in the IETF", BCP 161, RFC 6291, 2270 DOI 10.17487/RFC6291, June 2011, 2271 . 2273 [RFC7276] Mizrahi, T., Sprecher, N., Bellagamba, E., and Y. 2274 Weingarten, "An Overview of Operations, Administration, 2275 and Maintenance (OAM) Tools", RFC 7276, 2276 DOI 10.17487/RFC7276, June 2014, 2277 . 2279 [RFC8279] Wijnands, IJ., Ed., Rosen, E., Ed., Dolganow, A., 2280 Przygienda, T., and S. Aldrin, "Multicast Using Bit Index 2281 Explicit Replication (BIER)", RFC 8279, 2282 DOI 10.17487/RFC8279, November 2017, 2283 . 2285 [I-D.ietf-6tisch-msf] 2286 Chang, T., Vucinic, M., Vilajosana, X., Duquennoy, S., and 2287 D. Dujovne, "6TiSCH Minimal Scheduling Function (MSF)", 2288 Work in Progress, Internet-Draft, draft-ietf-6tisch-msf- 2289 18, 12 September 2020, 2290 . 2292 [I-D.pthubert-raw-architecture] 2293 Thubert, P., Papadopoulos, G., and R. Buddenberg, 2294 "Reliable and Available Wireless Architecture/Framework", 2295 Work in Progress, Internet-Draft, draft-pthubert-raw- 2296 architecture-04, 6 July 2020, 2297 . 2300 [I-D.ietf-roll-nsa-extension] 2301 Koutsiamanis, R., Papadopoulos, G., Montavont, N., and P. 2302 Thubert, "Common Ancestor Objective Function and Parent 2303 Set DAG Metric Container Extension", Work in Progress, 2304 Internet-Draft, draft-ietf-roll-nsa-extension-09, 26 2305 September 2020, . 2308 [I-D.papadopoulos-paw-pre-reqs] 2309 Papadopoulos, G., Koutsiamanis, R., Montavont, N., and P. 2310 Thubert, "Exploiting Packet Replication and Elimination in 2311 Complex Tracks in LLNs", Work in Progress, Internet-Draft, 2312 draft-papadopoulos-paw-pre-reqs-01, 25 March 2019, 2313 . 2316 [I-D.thubert-bier-replication-elimination] 2317 Thubert, P., Eckert, T., Brodard, Z., and H. Jiang, "BIER- 2318 TE extensions for Packet Replication and Elimination 2319 Function (PREF) and OAM", Work in Progress, Internet- 2320 Draft, draft-thubert-bier-replication-elimination-03, 3 2321 March 2018, . 2324 [I-D.thubert-6lo-bier-dispatch] 2325 Thubert, P., Brodard, Z., Jiang, H., and G. Texier, "A 2326 6loRH for BitStrings", Work in Progress, Internet-Draft, 2327 draft-thubert-6lo-bier-dispatch-06, 28 January 2019, 2328 . 2331 [I-D.ietf-bier-te-arch] 2332 Eckert, T., Cauchie, G., and M. Menth, "Tree Engineering 2333 for Bit Index Explicit Replication (BIER-TE)", Work in 2334 Progress, Internet-Draft, draft-ietf-bier-te-arch-08, 13 2335 July 2020, 2336 . 2338 [I-D.ietf-6tisch-coap] 2339 Sudhaakar, R. and P. Zand, "6TiSCH Resource Management and 2340 Interaction using CoAP", Work in Progress, Internet-Draft, 2341 draft-ietf-6tisch-coap-03, 9 March 2015, 2342 . 2344 [I-D.svshah-tsvwg-deterministic-forwarding] 2345 Shah, S. and P. Thubert, "Deterministic Forwarding PHB", 2346 Work in Progress, Internet-Draft, draft-svshah-tsvwg- 2347 deterministic-forwarding-04, 30 August 2015, 2348 . 2351 [IEEE Std. 802.15.4] 2352 IEEE standard for Information Technology, "IEEE Std. 2353 802.15.4, Part. 15.4: Wireless Medium Access Control (MAC) 2354 and Physical Layer (PHY) Specifications for Low-Rate 2355 Wireless Personal Area Networks". 2357 [IEEE Std. 802.11] 2358 "IEEE Standard 802.11 - IEEE Standard for Information 2359 Technology - Telecommunications and information exchange 2360 between systems Local and metropolitan area networks - 2361 Specific requirements - Part 11: Wireless LAN Medium 2362 Access Control (MAC) and Physical Layer (PHY) 2363 Specifications.". 2365 [IEEE Std. 802.11ak] 2366 "802.11ak: Enhancements for Transit Links Within Bridged 2367 Networks", 2017. 2369 [IEEE Std. 802.11ax] 2370 "802.11ax D4.0: Enhancements for High Efficiency WLAN". 2372 [IEEE Std. 802.11ay] 2373 "802.11ay: Enhanced throughput for operation in license- 2374 exempt bands above 45 GHz". 2376 [IEEE Std. 802.11ad] 2377 "802.11ad: Enhancements for very high throughput in the 60 2378 GHz band". 2380 [IEEE 802.11be WIP] 2381 "802.11be: Extreme High Throughput". 2383 [IEEE Std. 802.1Qat] 2384 "802.1Qat: Stream Reservation Protocol". 2386 [IEEE8021Qcc] 2387 "802.1Qcc: IEEE Standard for Local and Metropolitan Area 2388 Networks--Bridges and Bridged Networks -- Amendment 31: 2389 Stream Reservation Protocol (SRP) Enhancements and 2390 Performance Improvements". 2392 [Cavalcanti_2019] 2393 Dave Cavalcanti et al., "Extending Time Distribution and 2394 Timeliness Capabilities over the Air to Enable Future 2395 Wireless Industrial Automation Systems, the Proceedings of 2396 IEEE", June 2019. 2398 [Nitsche_2015] 2399 Thomas Nitsche et al., "IEEE 802.11ad: directional 60 GHz 2400 communication for multi-Gigabit-per-second Wi-Fi", 2401 December 2014. 2403 [Ghasempour_2017] 2404 Yasaman Ghasempour et al., "802.11ay: Next-Generation 60 2405 GHz Communications for 100 Gb/s Wi-Fi", December 2017. 2407 [IEEE_doc_11-18-2009-06] 2408 "802.11 Real-Time Applications (RTA) Topic Interest Group 2409 (TIG) Report", November 2018. 2411 [IEEE_doc_11-19-0373-00] 2412 Kevin Stanton et Al., "Time-Sensitive Applications Support 2413 in EHT", March 2019. 2415 [morell13] Antoni Morell et al., "Label switching over IEEE802.15.4e 2416 networks", April 2013. 2418 [dearmas16] 2419 Jesica de Armas et al., "Determinism through path 2420 diversity: Why packet replication makes sense", September 2421 2016. 2423 [vilajosana19] 2424 Xavier Vilajosana et al., "6TiSCH: Industrial Performance 2425 for IPv6 Internet-of-Things Networks", June 2019. 2427 [ISA100.11a] 2428 ISA/IEC, "ISA100.11a, Wireless Systems for Automation, 2429 also IEC 62734", 2011, . 2433 [WirelessHART] 2434 www.hartcomm.org, "Industrial Communication Networks - 2435 Wireless Communication Network and Communication Profiles 2436 - WirelessHART - IEC 62591", 2010. 2438 [PCE] IETF, "Path Computation Element", 2439 . 2441 [CCAMP] IETF, "Common Control and Measurement Plane", 2442 . 2444 [TiSCH] IETF, "IPv6 over the TSCH mode over 802.15.4", 2445 . 2447 [RIH18] Rihacek, C., Haindl, B., Fantappie, P., Pierattelli, S., 2448 Gräupl, T., Schnell, M., and N. Fistas, "L-band Digital 2449 Aeronautical Communications System (LDACS) Activities in 2450 SESAR2020", Proceedings of the Integrated Communications 2451 Navigation and Surveillance Conference (ICNS) Herndon, VA, 2452 USA, April 2018. 2454 [GRA19] Gräupl, T., Rihacek, C., and B. Haindl, "LDACS A/G 2455 Specification", SESAR2020 PJ14-02-01 D3.3.010, February 2456 2019. 2458 [SAJ14] Sajatovic, M., Günzel, H., and S. Müller, "WA04 D22 Test 2459 Report for Assessing LDACS1 Transmitter Impact upon DME/ 2460 TACAN Receivers", April 2014. 2462 [GRA11] Gräupl, T. and M. Ehammer, "L-DACS1 Data Link Layer 2463 Evolution of ATN/IPS", Proceedings of the 30th IEEE/AIAA 2464 Digital Avionics Systems Conference (DASC) Seattle, WA, 2465 USA, October 2011. 2467 [ICAO18] International Civil Aviation Organization (ICAO), "L-Band 2468 Digital Aeronautical Communication System (LDACS)", 2469 International Standards and Recommended Practices Annex 10 2470 - Aeronautical Telecommunications, Vol. III - 2471 Communication Systems, July 2018. 2473 [GRA18] al., T. G. E., "L-band Digital Aeronautical Communications 2474 System (LDACS) flight trials in the national German 2475 project MICONAV", Proceedings of the Integrated 2476 Communications, Navigation, Surveillance Conference 2477 (ICNS) Herndon, VA, USA, April 2018. 2479 [SCH19] Schnell, M., "DLR tests digital communications 2480 technologies combined with additional navigation functions 2481 for the first time", 3 March 2019, 2482 . 2485 [MAE19] Mäurer, N. and C. 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Farkas, 2530 "Deterministic Networking Architecture", RFC 8655, 2531 DOI 10.17487/RFC8655, October 2019, 2532 . 2534 [I-D.ietf-detnet-ip-over-tsn] 2535 Varga, B., Farkas, J., Malis, A., and S. 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Sachs, "5G-TSN 2578 integration meets networking requirements for industrial 2579 automation", Ericsson Technology Review, Volume 9, No 7, 2580 August 2019, . 2584 Authors' Addresses 2586 Pascal Thubert (editor) 2587 Cisco Systems, Inc 2588 Building D 2589 45 Allee des Ormes - BP1200 2590 06254 MOUGINS - Sophia Antipolis 2591 France 2593 Phone: +33 497 23 26 34 2594 Email: pthubert@cisco.com 2596 Dave Cavalcanti 2597 Intel Corporation 2598 2111 NE 25th Ave 2599 Hillsboro, OR, 97124 2600 United States of America 2602 Phone: 503 712 5566 2603 Email: dave.cavalcanti@intel.com 2605 Xavier Vilajosana 2606 Universitat Oberta de Catalunya 2607 156 Rambla Poblenou 2608 08018 Barcelona Catalonia 2609 Spain 2611 Email: xvilajosana@uoc.edu 2613 Corinna Schmitt 2614 Research Institute CODE, UniBwM 2615 Werner-Heisenberg-Weg 28 2616 85577 Neubiberg 2617 Germany 2619 Email: corinna.schmitt@unibw.de 2620 Janos Farkas 2621 Ericsson 2622 Budapest 2623 Magyar tudosok korutja 11 2624 1117 2625 Hungary 2627 Email: janos.farkas@ericsson.com