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Checking references for intended status: Informational ---------------------------------------------------------------------------- == Outdated reference: A later version (-04) exists of draft-bernardos-raw-use-cases-03 Summary: 0 errors (**), 0 flaws (~~), 2 warnings (==), 1 comment (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 RAW N. Maeurer, Ed. 3 Internet-Draft T. Graeupl, Ed. 4 Intended status: Informational German Aerospace Center (DLR) 5 Expires: 5 December 2020 C. Schmitt, Ed. 6 Research Institute CODE, UniBwM 7 3 June 2020 9 L-band Digital Aeronautical Communications System (LDACS) 10 draft-maeurer-raw-ldacs-03 12 Abstract 14 This document provides an overview of the architecture of the L-band 15 Digital Aeronautical Communications System (LDACS), which provides a 16 secure, scalable and spectrum efficient terrestrial data link for 17 civil aviation. LDACS is a scheduled, reliable multi-application 18 cellular broadband system with support for IPv6. 20 Status of This Memo 22 This Internet-Draft is submitted in full conformance with the 23 provisions of BCP 78 and BCP 79. 25 Internet-Drafts are working documents of the Internet Engineering 26 Task Force (IETF). Note that other groups may also distribute 27 working documents as Internet-Drafts. The list of current Internet- 28 Drafts is at https://datatracker.ietf.org/drafts/current/. 30 Internet-Drafts are draft documents valid for a maximum of six months 31 and may be updated, replaced, or obsoleted by other documents at any 32 time. It is inappropriate to use Internet-Drafts as reference 33 material or to cite them other than as "work in progress." 35 This Internet-Draft will expire on 5 December 2020. 37 Copyright Notice 39 Copyright (c) 2020 IETF Trust and the persons identified as the 40 document authors. All rights reserved. 42 This document is subject to BCP 78 and the IETF Trust's Legal 43 Provisions Relating to IETF Documents (https://trustee.ietf.org/ 44 license-info) in effect on the date of publication of this document. 45 Please review these documents carefully, as they describe your rights 46 and restrictions with respect to this document. Code Components 47 extracted from this document must include Simplified BSD License text 48 as described in Section 4.e of the Trust Legal Provisions and are 49 provided without warranty as described in the Simplified BSD License. 51 Table of Contents 53 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 54 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 55 3. Motivation and Use Cases . . . . . . . . . . . . . . . . . . 4 56 3.1. Voice Communications Today . . . . . . . . . . . . . . . 5 57 3.2. Data Communications Today . . . . . . . . . . . . . . . . 5 58 4. Provenance and Documents . . . . . . . . . . . . . . . . . . 6 59 5. Applicability . . . . . . . . . . . . . . . . . . . . . . . . 7 60 5.1. Advances Beyond the State-of-the-Art . . . . . . . . . . 7 61 5.1.1. Priorities . . . . . . . . . . . . . . . . . . . . . 7 62 5.1.2. Security . . . . . . . . . . . . . . . . . . . . . . 8 63 5.1.3. High Data Rates . . . . . . . . . . . . . . . . . . . 8 64 5.2. Application . . . . . . . . . . . . . . . . . . . . . . . 8 65 5.3. Multilink Technology . . . . . . . . . . . . . . . . . . 8 66 5.4. Air-to-Air Extension for LDACS . . . . . . . . . . . . . 9 67 5.5. GBAS via LDACS for Secure, Automated Landings . . . . . . 9 68 5.6. LDACS Navigation . . . . . . . . . . . . . . . . . . . . 10 69 6. Characteristics of LDACS . . . . . . . . . . . . . . . . . . 10 70 6.1. LDACS Sub-Network . . . . . . . . . . . . . . . . . . . . 11 71 6.2. Topology . . . . . . . . . . . . . . . . . . . . . . . . 11 72 6.3. LDACS Physical Layer . . . . . . . . . . . . . . . . . . 12 73 6.4. LDACS Data Link Layer . . . . . . . . . . . . . . . . . . 12 74 6.5. LDACS Data Rates . . . . . . . . . . . . . . . . . . . . 12 75 6.6. Reliability and Availability . . . . . . . . . . . . . . 13 76 6.6.1. LDACS Medium Access . . . . . . . . . . . . . . . . . 13 77 6.6.2. LDACS Mobility . . . . . . . . . . . . . . . . . . . 14 78 6.6.3. LDACS Incremental Deployment . . . . . . . . . . . . 14 79 7. Protocol Stack . . . . . . . . . . . . . . . . . . . . . . . 14 80 7.1. Medium Access Control (MAC) Entity Services . . . . . . . 15 81 7.2. Data Link Service (DLS) Entity Services . . . . . . . . . 17 82 7.3. Voice Interface (VI) Services . . . . . . . . . . . . . . 18 83 7.4. LDACS Management Entity (LME) Services . . . . . . . . . 18 84 7.5. Sub-Network Protocol (SNP) Services . . . . . . . . . . . 18 85 8. Security Considerations . . . . . . . . . . . . . . . . . . . 18 86 9. Privacy Considerations . . . . . . . . . . . . . . . . . . . 19 87 10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 19 88 11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 19 89 12. Normative References . . . . . . . . . . . . . . . . . . . . 20 90 13. Informative References . . . . . . . . . . . . . . . . . . . 20 91 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 21 93 1. Introduction 95 One of the main pillars of the modern Air Traffic Management (ATM) 96 system is the existence of a communication infrastructure that 97 enables efficient aircraft control and safe separation in all phases 98 of flight. Current systems are technically mature but suffering from 99 the VHF band's increasing saturation in high-density areas and the 100 limitations posed by analogue radio communications. Therefore, 101 aviation globally and the European Union (EU) in particular, strives 102 for a sustainable modernization of the aeronautical communication 103 infrastructure. 105 In the long-term, ATM communication shall transition from analogue 106 VHF voice and VDL2 communication to more spectrum efficient digital 107 data communication. The European ATM Master Plan foresees this 108 transition to be realized for terrestrial communications by the 109 development (and potential implementation) of the L-band Digital 110 Aeronautical Communications System (LDACS). LDACS shall enable IPv6 111 based air- ground communication related to the aviation safety and 112 regularity of flight. The particular challenge is that no additional 113 spectrum can be made available for terrestrial aeronautical 114 communication. It was thus necessary to develop co-existence 115 mechanism/procedures to enable the interference free operation of 116 LDACS in parallel with other aeronautical services/systems in the 117 same frequency band. 119 2. Terminology 121 The following terms are used in the context of RAW in this document: 123 A2A Air-to-Air 124 LDACS A2A LDACS Air-to-Air 125 AeroMACS Aeronautical Mobile Airport Communication System 126 A2G Air-to-Ground 127 AM(R)S Aeronautical Mobile (Route) Service 128 ANSP Air traffic Network Service Provider 129 AOC Aeronautical Operational Control 130 AS Aircraft Station 131 ATC Air-Traffic Control 132 ATM Air-Traffic Management 133 ATN Aeronautical Telecommunication Network 134 ATS Air Traffic Service 135 CCCH Common Control Channel 136 DCCH Dedicated Control Channel 137 DCH Data Channel 138 DLL Data Link Layer 139 DLS Data Link Service 140 DME Distance Measuring Equipment 141 DSB-AM Double Side-Band Amplitude Modulation 142 FAA Federal Aviation Administration 143 FCI Future Communication Infrastructure 144 FDD Frequency Division Duplex 145 FL Forward Link 146 GANP Global Air Navigation Plan 147 GNSS Global Navigation Satellite System 148 GS Ground Station 149 GSC Ground-Station Controller 150 G2A Ground-to-Air 151 HF High Frequency 152 ICAO International Civil Aviation Organization 153 kbit/s kilobit per second 154 LDACS L-band Digital Aeronautical Communications System 155 LLC Logical Link Layer 156 LME LDACS Management Entity 157 MAC Medium Access Layer 158 MF Multi Frame 159 OFDM Orthogonal Frequency-Division Multiplexing 160 OFDMA Orthogonal Frequency-Division Multiplexing Access 161 PDU Protocol Data Units 162 PHY Physical Layer 163 QoS Quality of Service 164 RL Reverse Link 165 SARPs Standards And Recommended Practices 166 SESAR Single European Sky ATM Research 167 SF Super-Frame 168 SNP Sub-Network Protocol 169 SSB-AM Single Side-Band Amplitude Modulation 170 TBO Trajectory-Based Operations 171 TDM Time Division Multiplexing 172 TDMA Time-Division Multiplexing-Access 173 VDL2 VHF Data Link mode 2 174 VHF Very High Frequency 175 VI Voice Interface 177 3. Motivation and Use Cases 179 Aircraft are currently connected to Air-Traffic Control (ATC) and 180 Airline Operational Control (AOC) via voice and data communications 181 systems through all phases of a flight. Within the airport terminal, 182 connectivity is focused on high bandwidth communications, while 183 during en-route high reliability, robustness, and range is the main 184 focus. Voice communications may use the same or different equipment 185 as data communications systems. In the following the main 186 differences between voice and data communications capabilities are 187 summarized. The assumed use cases for LDACS completes the list of 188 use cases stated in [RAW-USE-CASES] and the list of reliable and 189 available wireless technologies presented in [RAW-TECHNOS]. 191 3.1. Voice Communications Today 193 Voice links are used for Air-to-Ground (A2G) and Air-to-Air (A2A) 194 communications. The communication equipment is either ground-based 195 working in the High Frequency (HF) or Very High Frequency (VHF) 196 frequency band or satellite-based. All VHF and HF voice 197 communications is operated via open broadcast channels without any 198 authentication, encryption or other protective measures. The use of 199 well-proven communication procedures via broadcast channels helps to 200 enhance the safety of communications by taking into account that 201 other users may encounter communication problems and may be 202 supported, if required. The main voice communications media is still 203 the analogue VHF Double Side-Band Amplitude Modulation (DSB-AM) 204 communications technique, supplemented by HF Single Side-Band 205 Amplitude Modulation (SSB-AM) and satellite communications for remote 206 and oceanic areas. DSB-AM has been in use since 1948, works reliably 207 and safely, and uses low-cost communication equipment. These are the 208 main reasons why VHF DSB-AM communications is still in use, and it is 209 likely that this technology will remain in service for many more 210 years. This however results in current operational limitations and 211 becomes impediments in deploying new Air-Traffic Management (ATM) 212 applications, such as flight-centric operation with Point-to-Point 213 communications. 215 3.2. Data Communications Today 217 Like for voice, data communications into the cockpit is currently 218 provided by ground-based equipment operating either on HF or VHF 219 radio bands or by legacy satellite systems. All these communication 220 systems are using narrowband radio channels with a data throughput 221 capacity of some kilobits per second. While the aircraft is on 222 ground some additional communications systems are available, like 223 Aeronautical Mobile Airport Communication System (AeroMACS; as of now 224 not widely used) or public cellular networks, operating in the 225 Airport (APT) domain and able to deliver broadband communication 226 capability. 228 The data communication networks used for the transmission of data 229 relating to the safety and regularity of the flight must be strictly 230 isolated from those providing entertainment services to passengers. 231 This leads to a situation that the flight crews are supported by 232 narrowband services during flight while passengers have access to 233 inflight broadband services. The current HF and VHF data links 234 cannot provide broadband services now or in the future, due to the 235 lack of available spectrum. This technical shortcoming is becoming a 236 limitation to enhanced ATM operations, such as Trajectory-Based 237 Operations (TBO) and 4D trajectory negotiations. 239 Satellite-based communications are currently under investigation and 240 enhanced capabilities are under development which will be able to 241 provide inflight broadband services and communications supporting the 242 safety and regularity of flight. In parallel, the ground-based 243 broadband data link technology LDACS is being standardized by ICAO 244 and has recently shown its maturity during flight tests [SCH191]. 245 The LDACS technology is scalable, secure and spectrum efficient and 246 provides significant advantages to the users and service providers. 247 It is expected that both - satellite systems and LDACS - will be 248 deployed to support the future aeronautical communication needs as 249 envisaged by the ICAO Global Air Navigation Plan (GANP). 251 4. Provenance and Documents 253 The development of LDACS has already made substantial progress in the 254 Single European Sky ATM Research (SESAR) framework, and is currently 255 being continued in the follow-up program, SESAR2020 [RIH18]. A key 256 objective of the SESAR activities is to develop, implement and 257 validate a modern aeronautical data link able to evolve with aviation 258 needs over long-term. To this end, an LDACS specification has been 259 produced [GRA19] and is continuously updated; transmitter 260 demonstrators were developed to test the spectrum compatibility of 261 LDACS with legacy systems operating in the L-band [SAJ14]; and the 262 overall system performance was analyzed by computer simulations, 263 indicating that LDACS can fulfil the identified requirements [GRA11]. 265 LDACS standardization within the framework of the ICAO started in 266 December 2016. The ICAO standardization group has produced an 267 initial Standards and Recommended Practices (SARPs) document 268 [ICAO18]. The SARPs document defines the general characteristics of 269 LDACS. The ICAO standardization group plans to produce an ICAO 270 technical manual - the ICAO equivalent to a technical standard - 271 within the next years. Generally, the group is open to input from 272 all sources and develops LDACS in the open. 274 Up to now the LDACS standardization has been focused on the 275 development of the physical layer and the data link layer, only 276 recently have higher layers come into the focus of the LDACS 277 development activities. There is currently no "IPv6 over LDACS" 278 specification publicly available; however, SESAR2020 has started the 279 testing of IPv6-based LDACS testbeds. 281 The IPv6 architecture for the aeronautical telecommunication network 282 is called the Future Communications Infrastructure (FCI). FCI shall 283 support quality of service, diversity, and mobility under the 284 umbrella of the "multi-link concept". This work is conducted by ICAO 285 Communication Panel working group WG-I. 287 In addition to standardization activities several industrial LDACS 288 prototypes have been built. One set of LDACS prototypes has been 289 evaluated in flight trials confirming the theoretical results 290 predicting the system performance [GRA18] [SCH191]. 292 5. Applicability 294 LDACS is a multi-application cellular broadband system capable of 295 simultaneously providing various kinds of Air Traffic Services 296 (including ATS-B3) and Aeronautical Operational Control (AOC) 297 communications services from deployed Ground Stations (GS). The 298 LDACS A2G sub-system physical layer and data link layer are optimized 299 for data link communications, but the system also supports air-ground 300 voice communications. 302 LDACS supports communication in all airspaces (airport, TMA, and en- 303 route), and on the airport surface. The physical LDACS cell coverage 304 is effectively de-coupled from the operational coverage required for 305 a particular service. This is new in aeronautical communications. 306 Services requiring wide-area coverage can be installed at several 307 adjacent LDACS cells. The handover between the involved LDACS cells 308 is seamless, automatic, and transparent to the user. Therefore, the 309 LDACS A2G communications concept enables the aeronautical 310 communication infrastructure to support future dynamic airspace 311 management concepts. 313 5.1. Advances Beyond the State-of-the-Art 315 LDACS offers several capabilities that are not provided in 316 contemporarily deployed aeronautical communication systems. 318 5.1.1. Priorities 320 LDACS is able to manage services priorities, an important feature not 321 available in some of current data link deployments. Thus, LDACS 322 guarantees bandwidth, low latency, and high continuity of service for 323 safety critical ATS applications while simultaneously accommodating 324 less safety-critical AOC services. 326 5.1.2. Security 328 LDACS is a secure data link with built-in security mechanisms. It 329 enables secure data communications for ATS and AOC services, 330 including secured private communications for aircraft operators and 331 ANSPs (Air Navigation Service Providers). This includes concepts for 332 key and trust management, mutual authenticated key exchange 333 protocols, key derivation measures, user and control message-in- 334 transit confidentiality and authenticity protection, secure logging 335 and availability and robustness measures [MAE18], [MAE191], [MAE192]. 337 5.1.3. High Data Rates 339 The user data rate of LDACS is 315 kbit/s to 1428 kbit/s on the 340 forward link, and 294 kbit/s to 1390 kbit/s on the reverse link, 341 depending on coding and modulation. This is 50 times the amount 342 terrestrial digital aeronautical communications systems such as VDLm2 343 provide [SCH191]. 345 5.2. Application 347 LDACS shall be used by several aeronautical applications ranging from 348 enhanced communication protocol stacks (multi-homed mobile IPv6 349 networks in the aircraft; ad-hoc networks between aircraft) to 350 classical communication applications (sending GBAS correction data) 351 and integration with other service domains (using the communication 352 signal for navigation). 354 5.3. Multilink Technology 356 It is expected that LDACS together with upgraded satellite-based 357 communications systems will be deployed within the Future 358 Communication Infrastructure (FCI) and constitute the main components 359 of the multilink concept within the FCI. 361 Both technologies, LDACS and satellite systems, have their specific 362 benefits and technical capabilities which complement each other. 363 Especially, satellite systems are well-suited for large coverage 364 areas with less dense air traffic, e.g. oceanic regions. LDACS is 365 well-suited for dense air traffic areas, e.g. continental areas or 366 hot-spots around airports and terminal airspace. In addition, both 367 technologies offer comparable data link capacity and, thus, are well- 368 suited for redundancy, mutual back-up, or load balancing. 370 Technically the FCI multilink concept shall be realized by multi- 371 homed mobile IPv6 networks. The related protocol stack is currently 372 under development by ICAO and SESAR. 374 5.4. Air-to-Air Extension for LDACS 376 Direct Air-to-Air (A2A) communication between aircraft in terms of 377 ad-hoc data networks is currently considered a research topic since 378 there is no immediate operational need for it, although several 379 possible use cases are discussed (wake vortex and trajectory 380 negotiation). It should also be noted that currently deployed analog 381 VHF voice radios support direct voice communication between aircraft, 382 making a similar use case for digital voice plausible. 384 There are some challenges for the design of the LDACS A2A mode. 385 First, the scarcity of free spectrum in the L-band, where LDACS 386 operates, significantly limits the design freedom with respect to the 387 radiated power, suitable frequency allocations, and usable spectrum 388 bandwidth. Second, in contrast to the LDACS A2G, the LDACS A2A must 389 be able to operate without any external support, given that it must 390 also support Aircraft-to-Aircraft communications in oceanic, remote, 391 and polar (ORP) regions, and in autonomous operation areas, where 392 support from satellites or ground infrastructure might not be 393 available. 395 Consequently, the LDACS A2A mode must provide means for the aircraft 396 to establish and organize a communications ad-hoc network without any 397 external support. Such a network entails numerous additional 398 challenges for the design, primarily in the medium-access control and 399 the network routing. To enable the new services and operational 400 concepts, the LDACS A2A mode shall support broadcast communications, 401 for concepts such as self-separation and wake vortex prediction, and 402 Point-to-Point communications to allow aircraft to negotiate 403 trajectories, resolve conflicts, and use other aircraft as relays to 404 enable communications beyond radio line-of-sight [BELL19]. 406 5.5. GBAS via LDACS for Secure, Automated Landings 408 The Global Navigation Satellite System (GNSS) based Ground Based 409 Augmentation System (GBAS) is used to improve the accuracy of GNSS to 410 allow GNSS based instrument landings. This is realized by sending 411 GNSS correction data (e.g., compensating ionospheric errors in the 412 GNSS signal) to the airborne GNSS receiver via a separate data link. 413 Currently the VDB data link is used. VDB is a narrow-band single- 414 purpose datalink without advanced security only used to transmit GBAS 415 correction data. 417 With GBAS evolving to GAST-D, allowing for safe and secure automatic 418 CAT III landings for civil aircraft, it will have to be extended in 419 multiple ways. VDB provides no cyber-security comparable to modern 420 wireless networks. The VDB datalink will not be sufficient in 421 bandwidth for GAST-D GBAS, as it lacks the necessary capacity to 422 transmit additional corrections and parameters. VDB siting is also 423 very difficult, as it requires Line of Sight (LoS) to work properly, 424 which is difficult especially in the aircraft-on-the-apron situation. 425 Fourthly, VDB has too little range for long-range approach 426 calculations, forcing aircraft to wait for landing approach 427 trajectories until when they are very close to the airport. A 428 possible solution is the transition from the VDB datalink to LDACS 429 for GBAS. 431 5.6. LDACS Navigation 433 Beyond communication radio signals can always also be used for 434 navigation. LDACS takes this into account. 436 For future aeronautical navigation, ICAO recommends the further 437 development of Global Navigation Satellite System (GNSS) based 438 technologies as primary means for navigation. However, the drawback 439 of GNSS is its inherent single point of failure - the satellite. Due 440 to the large separation between navigational satellites and aircraft, 441 the received power of GNSS signals on the ground is very low. As a 442 result, GNSS disruptions might occasionally occur due to 443 unintentional interference, or even intentional jamming. Yet the 444 navigation services must be available with sufficient performance for 445 all phases of flight. Therefore, during GNSS outages, or blockages, 446 an alternative solution is needed. This is commonly referred to as 447 Alternative Positioning, Navigation, and Timing (APNT). 449 One of such APNT solution consists of integrating the navigation 450 functionality into LDACS, referred to as LDACS-NAV. The ground 451 infrastructure for APNT is deployed through the implementation of 452 LDACS ground stations and the navigation capability comes "for free". 454 LDACS navigation has already been demonstrated in practice in a 455 flight measurement campaign [SCH191]. 457 6. Characteristics of LDACS 459 LDACS will become one of several wireless access networks connecting 460 aircraft to both Aeronautical Telecommunications Network (ATN, IPS as 461 well as OSI) and ACARS/FANS networks [FAN19]. 463 6.1. LDACS Sub-Network 465 An LDACS sub-network contains an Access Router (AR), a Ground-Station 466 Controller (GSC), and several Ground-Stations (GS), each of them 467 providing one LDACS radio cell serving up to 512 aircraft stations 468 (AS). User plane interconnection to the ATN is facilitated by the 469 Access Router (AR) peering with an Air-to-Ground Router (A2G Router) 470 connected to the ATN. It is up to implementer's choice to keep 471 Access Router and Air-Ground Router functions separated, or to merge 472 them. The internal control plane of an LDACS sub-network is managed 473 by the Ground-Station Controller (GSC). An LDACS sub-network is 474 illustrated in Figure 1. 476 wireless user 477 link plane 478 A--------------G-------------Access---A2G-----ATN 479 S..............S Router Router 480 . control . | 481 . plane . | 482 . . | 483 GSC..............| 484 . | 485 . | 486 GS---------------+ 488 Figure 1: LDACS sub-network with two GSs and one AS 490 The LDACS wireless link protocol stack defines two layers, the 491 physical layer and the data link layer. 493 6.2. Topology 495 LDACS operating in A2G mode is a cellular point-to-multipoint system. 496 The A2G mode assumes a star-topology in each cell where Airborne 497 Stations (AS) belonging to aircraft within a certain volume of space 498 (the LDACS cell) is connected to the controlling GS. The LDACS GS is 499 a centralized instance that controls LDACS A2G communications within 500 its cell. The LDACS GS can simultaneously support multiple bi- 501 directional communications to the ASs under its control. LDACS 502 ground stations themselves are connected to a ground station 503 controller (GSC) controlling the LDACS sub-network. 505 Prior to utilizing the system an AS has to register at the 506 controlling GS to establish dedicated logical channels for user and 507 control data. Control channels have statically allocated resources, 508 while user channels have dynamically assigned resources according to 509 the current demand. Logical channels exist only between the GS and 510 the AS. 512 The LDACS wireless link protocol stack defines two layers, the 513 physical layer and the data link layer. 515 6.3. LDACS Physical Layer 517 The physical layer provides the means to transfer data over the radio 518 channel. The LDACS GS supports bi-directional links to multiple 519 aircraft under its control. The forward link direction (FL; G2A) and 520 the reverse link direction (RL; A2G) are separated by frequency 521 division duplex. Forward link and reverse link use a 500 kHz channel 522 each. The ground-station transmits a continuous stream of Orthogonal 523 Frequency-Division Multiplexing (OFDM) symbols on the forward link. 524 In the reverse link different aircraft are separated in time and 525 frequency using a combination of Orthogonal Frequency-Division 526 Multiple-Access (OFDMA) and Time-Division Multiple-Access (TDMA). 527 Aircraft thus transmit discontinuously on the reverse link with radio 528 bursts sent in precisely defined transmission opportunities allocated 529 by the ground-station. 531 6.4. LDACS Data Link Layer 533 The data-link layer provides the necessary protocols to facilitate 534 concurrent and reliable data transfer for multiple users. The LDACS 535 data link layer is organized in two sub-layers: The medium access 536 sub-layer and the logical link control sub-layer. The medium access 537 sub-layer manages the organization of transmission opportunities in 538 slots of time and frequency. The logical link control sub-layer 539 provides acknowledged Point-to-Point logical channels between the 540 aircraft and the ground-station using an automatic repeat request 541 protocol. LDACS supports also unacknowledged Point-to-Point channels 542 and ground-to-air broadcast. 544 6.5. LDACS Data Rates 546 The user data rate of LDACS is 315 kbit/s to 1428 kbit/s on the 547 forward link, and 294 kbit/s to 1390 kbit/s on the reverse link, 548 depending on coding and modulation. 550 6.6. Reliability and Availability 552 LDACS has been designed with applications related to the safety and 553 regularity of flight in mind. It has therefore been designed as a 554 deterministic wireless data link (as far as possible). 556 6.6.1. LDACS Medium Access 558 LDACS medium access is always under the control of the ground-station 559 of a radio cell. Any medium access for the transmission of user data 560 has to be requested with a resource request message stating the 561 requested amount of resources and class of service. The ground- 562 station performs resource scheduling on the basis of these requests 563 and grants resources with resource allocation messages. Resource 564 request and allocation messages are exchanged over dedicated 565 contention-free control channels. 567 LDACS has two mechanisms to request resources from the scheduler in 568 the ground-station. 570 Resources can either be requested "on demand" with a given class of 571 service. On the forward link, this is done locally in the ground- 572 station, on the reverse link a dedicated contention-free control 573 channel is used called Dedicated Control Channel (DCCH; roughly 83 574 bit every 60 ms). A resource allocation is always announced in the 575 control channel of the forward link (Common Control Channel (CCCH); 576 variable sized). Due to the spacing of the reverse link control 577 channels every 60 ms, a medium access delay in the same order of 578 magnitude is to be expected. 580 Resources can also be requested "permanently". The permanent 581 resource request mechanism supports requesting recurring resources in 582 given time intervals. A permanent resource request has to be 583 canceled by the user (or by the ground-station, which is always in 584 control). 586 User data transmissions over LDACS are therefore always scheduled by 587 the ground-station, while control data uses statically (i.e. at cell 588 entry) allocated recurring resources (DCCH and CCCH). The current 589 specification documents specify no scheduling algorithm. However 590 performance evaluations so far have used strict priority scheduling 591 and round robin for equal priorities for simplicity. In the current 592 prototype implementations LDACS classes of service are thus realized 593 as priorities of medium access and not as flows. Note that this can 594 starve out low priority flows. However, this is not seen as a big 595 problem since safety related message always go first in any case. 596 Scheduling of reverse link resources is done in physical Protocol 597 Data Units (PDU) of 112 bit (or larger if more aggressive coding and 598 modulation is used). Scheduling on the forward link is done Byte- 599 wise since the forward link is transmitted continuously by the 600 ground-station. 602 The LDACS data link layer protocol running on top of the medium 603 access sub-layer uses ARQ to provide reliable data transmission. 605 6.6.2. LDACS Mobility 607 The LDACS mobility service manages in the GSC and LME cell entry, 608 cell exit and handover between cells. 610 LDACS supports internal handovers to different RF channels. 611 Handovers may be initiated by the aircraft (break-before-make) or by 612 the ground- station (make-before-break). Make-before-break handovers 613 are only supported for ground-stations connected to the same ground- 614 station controller. 616 External handovers between non-connected LDACS deployments or 617 different aeronautical data links shall be handled by the FCI multi- 618 link concept. 620 6.6.3. LDACS Incremental Deployment 622 The LDACS data link provides enhanced capabilities to the future IPv6 623 based ATN enabling it to better support user needs and new 624 applications. The deployment scalability of LDACS allows its 625 implementation to start in areas where most needed to improve 626 immediately the performance of already fielded infrastructure. Later 627 the deployment is extended based on operational demand. 629 7. Protocol Stack 631 The protocol stack of LDACS is implemented in the AS, GS, and GSC: It 632 consists of the Physical Layer (PHY) with five major functional 633 blocks above it. Four are placed in the Data Link Layer (DLL) of the 634 AS and GS: (1) Medium Access Layer (MAC), (2) Voice Interface (VI), 635 (3) Data Link Service (DLS), (4) LDACS Management Entity (LME). The 636 last entity resides within the sub-network layer: Sub-Network 637 Protocol (SNP). The LDACS network is externally connected to voice 638 units, radio control units, and the ATN network layer. 640 Figure 2 shows the protocol stack of LDACS as implemented in the AS 641 and GS. 643 IPv6 network layer 644 | 645 | 646 +------------------+ +----+ 647 | SNP |--| | sub-network 648 | | | | layer 649 +------------------+ | | 650 | | LME| 651 +------------------+ | | 652 | DLS | | | logical link 653 | | | | control layer 654 +------------------+ +----+ 655 | | 656 DCH DCCH/CCCH 657 | RACH/BCCH 658 | | 659 +--------------------------+ 660 | MAC | medium access 661 | | layer 662 +--------------------------+ 663 | 664 +--------------------------+ 665 | PHY | physical layer 666 +--------------------------+ 667 | 668 | 669 ((*)) 670 FL/RL radio channels 671 separated by FDD 673 Figure 2: LDACS protocol stack in AS and GS 675 7.1. Medium Access Control (MAC) Entity Services 677 The MAC time framing service provides the frame structure necessary 678 to realize slot-based Time Division Multiplex (TDM) access on the 679 physical link. It provides the functions for the synchronization of 680 the MAC framing structure and the PHY layer framing. The MAC time 681 framing provides a dedicated time slot for each logical channel. 683 The MAC sub-layer offers access to the physical channel to its 684 service users. Channel access is provided through transparent 685 logical channels. The MAC sub-layer maps logical channels onto the 686 appropriate slots and manages the access to these channels. Logical 687 channels are used as interface between the MAC and LLC sub-layers. 689 The LDACS framing structure for FL and RL is based on Super-Frames 690 (SF) of 240 ms duration. Each SF corresponds to 2000 OFDM symbols. 691 The FL and RL SF boundaries are aligned in time (from the view of the 692 GS). 694 In the FL, an SF contains a Broadcast Frame of duration 6.72 ms (56 695 OFDM symbols) for the Broadcast Control Channel (BCCH), and four 696 Multi-Frames (MF), each of duration 58.32 ms (486 OFDM symbols). 698 In the RL, each SF starts with a Random Access (RA) slot of length 699 6.72 ms with two opportunities for sending reverse link random access 700 frames for the Random Access Channel (RACH), followed by four MFs. 701 These MFs have the same fixed duration of 58.32 ms as in the FL, but 702 a different internal structure 704 Figure 3 and Figure 4 illustrates the LDACS frame structure. 706 ^ 707 | +------+------------+------------+------------+------------+ 708 | FL | BCCH | MF | MF | MF | MF | 709 F +------+------------+------------+------------+------------+ 710 r <---------------- Super-Frame (SF) - 240ms ----------------> 711 e 712 q +------+------------+------------+------------+------------+ 713 u RL | RACH | MF | MF | MF | MF | 714 e +------+------------+------------+------------+------------+ 715 n <---------------- Super-Frame (SF) - 240ms ----------------> 716 c 717 y 718 | 719 ----------------------------- Time ------------------------------> 720 | 722 Figure 3: LDACS super-frame structure 724 ^ 725 | +-------------+------+-------------+ 726 | FL | DCH | CCCH | DCH | 727 F +-------------+------+-------------+ 728 r <---- Multi-Frame (MF) - 58.32ms --> 729 e 730 q +------+---------------------------+ 731 u RL | DCCH | DCH | 732 e +------+---------------------------+ 733 n <---- Multi-Frame (MF) - 58.32ms --> 734 c 735 y 736 | 737 ----------------------------- Time ------------------------------> 738 | 740 Figure 4: LDACS multi-frame (MF) structure 742 7.2. Data Link Service (DLS) Entity Services 744 The DLS provides acknowledged and unacknowledged (including broadcast 745 and packet mode voice) bi-directional exchange of user data. If user 746 data is transmitted using the acknowledged data link service, the 747 sending DLS entity will wait for an acknowledgement from the 748 receiver. If no acknowledgement is received within a specified time 749 frame, the sender may automatically try to retransmit its data. 750 However, after a certain number of failed retries, the sender will 751 suspend further retransmission attempts and inform its client of the 752 failure. 754 The data link service uses the logical channels provided by the MAC: 756 1. A ground-stations announces its existence and access parameters 757 in the Broadcast Channel (BC). 758 2. The Random Access Channel (RA) enables AS to request access to an 759 LDACS cell. 760 3. In the Forward Link (FL) the Common Control Channel (CCCH) is 761 used by the GS to grant access to data channel resources. 762 4. The reverse direction is covered by the Reverse Link (RL), where 763 aircraft-stations need to request resources before sending. This 764 happens via the Dedicated Common Control Channel (DCCH). 765 5. User data itself is communicated in the Data Channel (DCH) on the 766 FL and RL. 768 7.3. Voice Interface (VI) Services 770 The VI provides support for virtual voice circuits. Voice circuits 771 may either be set-up permanently by the GS (e.g., to emulate voice 772 party line) or may be created on demand. The creation and selection 773 of voice circuits is performed in the LME. The VI provides only the 774 transmission services. 776 7.4. LDACS Management Entity (LME) Services 778 The mobility management service in the LME provides support for 779 registration and de-registration (cell entry and cell exit), scanning 780 RF channels of neighboring cells and handover between cells. In 781 addition, it manages the addressing of aircraft/ ASs within cells. 782 It is controlled by the network management service in the GSC. 784 The resource management service provides link maintenance (power, 785 frequency and time adjustments), support for adaptive coding and 786 modulation (ACM), and resource allocation. 788 7.5. Sub-Network Protocol (SNP) Services 790 The data link service provides functions required for the transfer of 791 user plane data and control plane data over the LDACS sub-network. 793 The security service provides functions for secure communication over 794 the LDACS sub-network. Note that the SNP security service applies 795 cryptographic measures as configured by the ground station 796 controller. 798 8. Security Considerations 800 Aviation will require secure exchanges of data and voice messages for 801 managing the air-traffic flow safely through the airspaces all over 802 the world. The main communication method for ATC today is still an 803 open analogue voice broadcast within the aeronautical VHF band. 804 Currently, the information security is purely procedural based by 805 using well-trained personnel and proven communications procedures. 806 This communication method has been in service since 1948. Future 807 digital communications waveforms will need additional embedded 808 security features to fulfill modern information security requirements 809 like authentication and integrity. These security features require 810 sufficient bandwidth which is beyond the capabilities of a VHF 811 narrowband communications system. For voice and data communications, 812 sufficient data throughput capability is needed to support the 813 security functions while not degrading performance. LDACS is a 814 mature data link technology with sufficient bandwidth to support 815 security. 817 Security considerations for LDACS are defined by the official ICAO 818 SARPS [ICAO18]: 820 1. LDACS shall provide a capability to protect the availability and 821 continuity of the system. 822 2. LDACS shall provide a capability including cryptographic 823 mechanisms to protect the integrity of messages in transit. 824 3. LDACS shall provide a capability to ensure the authenticity of 825 messages in transit. 826 4. LDACS should provide a capability for nonrepudiation of origin 827 for messages in transit. 828 5. LDACS should provide a capability to protect the confidentiality 829 of messages in transit. 830 6. LDACS shall provide an authentication capability. 831 7. LDACS shall provide a capability to authorize the permitted 832 actions of users of the system and to deny actions that are not 833 explicitly authorized. 834 8. If LDACS provides interfaces to multiple domains, LDACS shall 835 provide capability to prevent the propagation of intrusions within 836 LDACS domains and towards external domains. 838 The cybersecurity architecture of LDACS [ICAO18], [MAE18] and its 839 extensions [MAE191], [MAE192] regard all of the aforementioned 840 requirements, since LDACS has been mainly designed for air traffic 841 management communication. Thus it supports mutual entity 842 authentication, integrity and confidentiality capabilities of user 843 data messages and some control channel protection capabilities 844 [MAE192]. 846 9. Privacy Considerations 848 LDACS provides a Quality of Service (QoS), and the generic 849 considerations for such mechanisms apply. 851 10. IANA Considerations 853 This memo includes no request to IANA. 855 11. Acknowledgements 857 Thanks to all contributors to the development of LDACS and ICAO PT-T. 859 Thanks to Klaus-Peter Hauf, Bart Van Den Einden, and Pierluigi 860 Fantappie for further input to this draft. 862 Thanks to SBA Research Vienna for fruitful discussions on 863 aeronautical communications concerning security incentives for 864 industry and potential economic spillovers. 866 12. Normative References 868 13. Informative References 870 [MAE191] Maeurer, N., Graeupl, T., and C. Schmitt, "Evaluation of 871 the LDACS Cybersecurity Implementation", IEEE 38th Digital 872 Avionics Systems Conference (DACS), pp. 1-10, San Diego, 873 CA, USA , 2019. 875 [MAE192] Maeurer, N. and C. Schmitt, "Towards Successful 876 Realization of the LDACS Cybersecurity Architecture: An 877 Updated Datalink Security Threat- and Risk Analysis", IEEE 878 Integrated Communications, Navigation and Surveillance 879 Conference (ICNS), pp. 1-13, Herndon, VA, USA , 2019. 881 [GRA19] Graeupl, T., Rihacek, C., and B. Haindl, "LDACS A/G 882 Specification", SESAR2020 PJ14-02-01 D3.3.030 , 2019. 884 [FAN19] Pierattelli, S., Fantappie, P., Tamalet, S., van den 885 Einden, B., Rihacek, C., and T. Graeupl, "LDACS Deployment 886 Options and Recommendations", SESAR2020 PJ14-02-01 887 D3.4.020 , 2019. 889 [MAE18] Maeurer, N. and A. Bilzhause, "A Cybersecurity 890 Architecture for the L-band Digital Aeronautical 891 Communications System (LDACS)", IEEE 37th Digital Avionics 892 Systems Conference (DASC), pp. 1-10, London, UK , 2017. 894 [GRA11] Graeupl, T. and M. Ehammer, "L-DACS1 Data Link Layer 895 Evolution of ATN/IPS", 30th IEEE/AIAA Digital Avionics 896 Systems Conference (DASC), pp. 1-28, Seattle, WA, USA , 897 2011. 899 [GRA18] Graeupl, T., Schneckenburger, N., Jost, T., Schnell, M., 900 Filip, A., Bellido-Manganell, M.A., Mielke, D.M., Maeurer, 901 N., Kumar, R., Osechas, O., and G. Battista, "L-band 902 Digital Aeronautical Communications System (LDACS) flight 903 trials in the national German project MICONAV", Integrated 904 Communications, Navigation, Surveillance Conference 905 (ICNS), pp. 1-7, Herndon, VA, USA , 2018. 907 [SCH191] Schnell, M., "DLR Tests Digital Communications 908 Technologies Combined with Additional Navigation Functions 909 for the First Time", 2019. 911 [ICAO18] International Civil Aviation Organization (ICAO), "L-Band 912 Digital Aeronautical Communication System (LDACS)", 913 International Standards and Recommended Practices Annex 10 914 - Aeronautical Telecommunications, Vol. III - 915 Communication Systems , 2018. 917 [SAJ14] Haindl, B., Meser, J., Sajatovic, M., Mueller, S., 918 Arthaber, H., Faseth, T., and M. Zaisberger, "LDACS1 919 Conformance and Compatibility Assessment", IEEE/AIAA 33rd 920 Digital Avionics Systems Conference (DASC), pp. 1-11, 921 Colorado Springs, CO, USA , 2014. 923 [RIH18] Rihacek, C., Haindl, B., Fantappie, P., Pierattelli, S., 924 Graeupl, T., Schnell, M., and N. Fistas, "L-band Digital 925 Aeronautical Communications System (LDACS) Activities in 926 SESAR2020", Integrated Communications Navigation and 927 Surveillance Conference (ICNS), pp. 1-8, Herndon, VA, 928 USA , 2018. 930 [BELL19] Bellido-Manganell, M. A. and M. Schnell, "Towards Modern 931 Air-to-Air Communications: the LDACS A2A Mode", IEEE/AIAA 932 38th Digital Avionics Systems Conference (DASC), pp. 1-10, 933 San Diego, CA, USA , 2019. 935 [RAW-TECHNOS] 936 Thubert, P., Cavalcanti, D., Vilajosana, X., Schmitt, C., 937 and J. Farkas, "Reliable and Available Wireless 938 Technologies", Work in Progress, Internet-Draft, draft- 939 thubert-raw-technologies-05, 18 May 2020, 940 . 943 [RAW-USE-CASES] 944 Papadopoulos, G., Thubert, P., Theoleyre, F., and C. 945 Bernardos, "RAW use cases", Work in Progress, Internet- 946 Draft, draft-bernardos-raw-use-cases-03, 8 March 2020, 947 . 950 Authors' Addresses 952 Nils Maeurer (editor) 953 German Aerospace Center (DLR) 954 Muenchner Strasse 20 955 82234 Wessling 956 Germany 958 Email: Nils.Maeurer@dlr.de 959 Thomas Graeupl (editor) 960 German Aerospace Center (DLR) 961 Muenchner Strasse 20 962 82234 Wessling 963 Germany 965 Email: Thomas.Graeupl@dlr.de 967 Corinna Schmitt (editor) 968 Research Institute CODE, UniBwM 969 Werner-Heisenberg-Weg 28 970 85577 Neubiberg 971 Germany 973 Email: corinna.schmitt@unibw.de