idnits 2.17.1 draft-maeurer-raw-ldacs-04.txt: Checking boilerplate required by RFC 5378 and the IETF Trust (see https://trustee.ietf.org/license-info): ---------------------------------------------------------------------------- No issues found here. Checking nits according to https://www.ietf.org/id-info/1id-guidelines.txt: ---------------------------------------------------------------------------- No issues found here. Checking nits according to https://www.ietf.org/id-info/checklist : ---------------------------------------------------------------------------- No issues found here. Miscellaneous warnings: ---------------------------------------------------------------------------- == The copyright year in the IETF Trust and authors Copyright Line does not match the current year -- The document date (2 July 2020) is 1394 days in the past. Is this intentional? 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for intended status: Informational ---------------------------------------------------------------------------- == Outdated reference: A later version (-04) exists of draft-bernardos-raw-use-cases-03 Summary: 0 errors (**), 0 flaws (~~), 2 warnings (==), 1 comment (--).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: 3 January 2021 C. Schmitt, Ed. 6 Research Institute CODE, UniBwM 7 2 July 2020 9 L-band Digital Aeronautical Communications System (LDACS) 10 draft-maeurer-raw-ldacs-04 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. LDACS shall provide 19 a data link for IP network-based aircraft guidance. High reliability 20 and availability for IP connectivity over LDACS are therefore 21 essential. 23 Status of This Memo 25 This Internet-Draft is submitted in full conformance with the 26 provisions of BCP 78 and BCP 79. 28 Internet-Drafts are working documents of the Internet Engineering 29 Task Force (IETF). Note that other groups may also distribute 30 working documents as Internet-Drafts. The list of current Internet- 31 Drafts is at https://datatracker.ietf.org/drafts/current/. 33 Internet-Drafts are draft documents valid for a maximum of six months 34 and may be updated, replaced, or obsoleted by other documents at any 35 time. It is inappropriate to use Internet-Drafts as reference 36 material or to cite them other than as "work in progress." 38 This Internet-Draft will expire on 3 January 2021. 40 Copyright Notice 42 Copyright (c) 2020 IETF Trust and the persons identified as the 43 document authors. All rights reserved. 45 This document is subject to BCP 78 and the IETF Trust's Legal 46 Provisions Relating to IETF Documents (https://trustee.ietf.org/ 47 license-info) in effect on the date of publication of this document. 48 Please review these documents carefully, as they describe your rights 49 and restrictions with respect to this document. Code Components 50 extracted from this document must include Simplified BSD License text 51 as described in Section 4.e of the Trust Legal Provisions and are 52 provided without warranty as described in the Simplified BSD License. 54 Table of Contents 56 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 57 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 58 3. Motivation and Use Cases . . . . . . . . . . . . . . . . . . 4 59 3.1. Voice Communications Today . . . . . . . . . . . . . . . 5 60 3.2. Data Communications Today . . . . . . . . . . . . . . . . 5 61 4. Provenance and Documents . . . . . . . . . . . . . . . . . . 6 62 5. Applicability . . . . . . . . . . . . . . . . . . . . . . . . 7 63 5.1. Advances Beyond the State-of-the-Art . . . . . . . . . . 7 64 5.1.1. Priorities . . . . . . . . . . . . . . . . . . . . . 7 65 5.1.2. Security . . . . . . . . . . . . . . . . . . . . . . 8 66 5.1.3. High Data Rates . . . . . . . . . . . . . . . . . . . 8 67 5.2. Application . . . . . . . . . . . . . . . . . . . . . . . 8 68 5.2.1. Air-to-Ground Multilink . . . . . . . . . . . . . . . 8 69 5.2.2. Air-to-Air Extension for LDACS . . . . . . . . . . . 9 70 5.2.3. Flight Guidance . . . . . . . . . . . . . . . . . . . 9 71 5.2.4. Business Communication of Airlines . . . . . . . . . 10 72 5.2.5. LDACS Navigation . . . . . . . . . . . . . . . . . . 10 73 6. Characteristics of LDACS . . . . . . . . . . . . . . . . . . 11 74 6.1. LDACS Sub-Network . . . . . . . . . . . . . . . . . . . . 11 75 6.2. Topology . . . . . . . . . . . . . . . . . . . . . . . . 12 76 6.3. LDACS Physical Layer . . . . . . . . . . . . . . . . . . 12 77 6.4. LDACS Data Link Layer . . . . . . . . . . . . . . . . . . 12 78 6.5. LDACS Mobility . . . . . . . . . . . . . . . . . . . . . 13 79 7. Reliability and Availability . . . . . . . . . . . . . . . . 13 80 8. Protocol Stack . . . . . . . . . . . . . . . . . . . . . . . 15 81 8.1. Medium Access Control (MAC) Entity Services . . . . . . . 16 82 8.2. Data Link Service (DLS) Entity Services . . . . . . . . . 18 83 8.3. Voice Interface (VI) Services . . . . . . . . . . . . . . 19 84 8.4. LDACS Management Entity (LME) Services . . . . . . . . . 19 85 8.5. Sub-Network Protocol (SNP) Services . . . . . . . . . . . 19 86 9. Security Considerations . . . . . . . . . . . . . . . . . . . 19 87 10. Privacy Considerations . . . . . . . . . . . . . . . . . . . 20 88 11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20 89 12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 20 90 13. Normative References . . . . . . . . . . . . . . . . . . . . 21 91 14. Informative References . . . . . . . . . . . . . . . . . . . 21 92 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 23 94 1. Introduction 96 One of the main pillars of the modern Air Traffic Management (ATM) 97 system is the existence of a communication infrastructure that 98 enables efficient aircraft control and safe separation in all phases 99 of flight. Current systems are technically mature but suffering from 100 the VHF band's increasing saturation in high-density areas and the 101 limitations posed by analogue radio communications. Therefore, 102 aviation globally and the European Union (EU) in particular, strives 103 for a sustainable modernization of the aeronautical communication 104 infrastructure. 106 In the long-term, ATM communication shall transition from analogue 107 VHF voice and VDL2 communication to more spectrum efficient digital 108 data communication. The European ATM Master Plan foresees this 109 transition to be realized for terrestrial communications by the 110 development (and potential implementation) of the L-band Digital 111 Aeronautical Communications System (LDACS). LDACS shall enable IPv6 112 based air- ground communication related to the aviation safety and 113 regularity of flight. The particular challenge is that no additional 114 spectrum can be made available for terrestrial aeronautical 115 communication. It was thus necessary to develop co-existence 116 mechanism/procedures to enable the interference free operation of 117 LDACS in parallel with other aeronautical services/systems in the 118 same frequency band. 120 Since LDACS shall be used for aircraft guidance, high reliability and 121 availability for IP connectivity over LDACS are essential. 123 2. Terminology 125 The following terms are used in the context of RAW in this document: 127 A2A Air-to-Air 128 LDACS A2A LDACS Air-to-Air 129 AeroMACS Aeronautical Mobile Airport Communication System 130 A2G Air-to-Ground 131 AM(R)S Aeronautical Mobile (Route) Service 132 ANSP Air traffic Network Service Provider 133 AOC Aeronautical Operational Control 134 AS Aircraft Station 135 ATC Air-Traffic Control 136 ATM Air-Traffic Management 137 ATN Aeronautical Telecommunication Network 138 ATS Air Traffic Service 139 CCCH Common Control Channel 140 DCCH Dedicated Control Channel 141 DCH Data Channel 142 DLL Data Link Layer 143 DLS Data Link Service 144 DME Distance Measuring Equipment 145 DSB-AM Double Side-Band Amplitude Modulation 146 FAA Federal Aviation Administration 147 FCI Future Communication Infrastructure 148 FDD Frequency Division Duplex 149 FL Forward Link 150 GANP Global Air Navigation Plan 151 GNSS Global Navigation Satellite System 152 GS Ground Station 153 GSC Ground-Station Controller 154 G2A Ground-to-Air 155 HF High Frequency 156 ICAO International Civil Aviation Organization 157 kbit/s kilobit per second 158 LDACS L-band Digital Aeronautical Communications System 159 LLC Logical Link Layer 160 LME LDACS Management Entity 161 MAC Medium Access Layer 162 MF Multi Frame 163 OFDM Orthogonal Frequency-Division Multiplexing 164 OFDMA Orthogonal Frequency-Division Multiplexing Access 165 PDU Protocol Data Units 166 PHY Physical Layer 167 QoS Quality of Service 168 RL Reverse Link 169 SARPs Standards And Recommended Practices 170 SESAR Single European Sky ATM Research 171 SF Super-Frame 172 SNP Sub-Network Protocol 173 SSB-AM Single Side-Band Amplitude Modulation 174 TBO Trajectory-Based Operations 175 TDM Time Division Multiplexing 176 TDMA Time-Division Multiplexing-Access 177 VDL2 VHF Data Link mode 2 178 VHF Very High Frequency 179 VI Voice Interface 181 3. Motivation and Use Cases 183 Aircraft are currently connected to Air-Traffic Control (ATC) and 184 Airline Operational Control (AOC) via voice and data communications 185 systems through all phases of a flight. Within the airport terminal, 186 connectivity is focused on high bandwidth communications, while 187 during en-route high reliability, robustness, and range is the main 188 focus. Voice communications may use the same or different equipment 189 as data communications systems. In the following the main 190 differences between voice and data communications capabilities are 191 summarized. The assumed use cases for LDACS completes the list of 192 use cases stated in [RAW-USE-CASES] and the list of reliable and 193 available wireless technologies presented in [RAW-TECHNOS]. 195 3.1. Voice Communications Today 197 Voice links are used for Air-to-Ground (A2G) and Air-to-Air (A2A) 198 communications. The communication equipment is either ground-based 199 working in the High Frequency (HF) or Very High Frequency (VHF) 200 frequency band or satellite-based. All VHF and HF voice 201 communications is operated via open broadcast channels without 202 authentication, encryption or other protective measures. The use of 203 well-proven communication procedures via broadcast channels helps to 204 enhance the safety of communications by taking into account that 205 other users may encounter communication problems and may be 206 supported, if required. The main voice communications media is still 207 the analogue VHF Double Side-Band Amplitude Modulation (DSB-AM) 208 communications technique, supplemented by HF Single Side-Band 209 Amplitude Modulation (SSB-AM) and satellite communications for remote 210 and oceanic areas. DSB-AM has been in use since 1948, works reliably 211 and safely, and uses low-cost communication equipment. These are the 212 main reasons why VHF DSB-AM communications is still in use, and it is 213 likely that this technology will remain in service for many more 214 years. This however results in current operational limitations and 215 impediments in deploying new Air-Traffic Management (ATM) 216 applications, such as flight-centric operation with Point-to-Point 217 communications. 219 3.2. Data Communications Today 221 Like for voice, data communications into the cockpit is currently 222 provided by ground-based equipment operating either on HF or VHF 223 radio bands or by legacy satellite systems. All these communication 224 systems are using narrowband radio channels with a data throughput 225 capacity in order of kilobits per second. While the aircraft is on 226 ground some additional communications systems are available, like 227 Aeronautical Mobile Airport Communication System (AeroMACS; as of now 228 not widely used) or public cellular networks, operating in the 229 Airport (APT) domain and able to deliver broadband communication 230 capability. 232 The data communication networks used for the transmission of data 233 relating to the safety and regularity of the flight must be strictly 234 isolated from those providing entertainment services to passengers. 235 This leads to a situation that the flight crews are supported by 236 narrowband services during flight while passengers have access to 237 inflight broadband services. The current HF and VHF data links 238 cannot provide broadband services now or in the future, due to the 239 lack of available spectrum. This technical shortcoming is becoming a 240 limitation to enhanced ATM operations, such as Trajectory-Based 241 Operations (TBO) and 4D trajectory negotiations. 243 Satellite-based communications are currently under investigation and 244 enhanced capabilities are under development which will be able to 245 provide inflight broadband services and communications supporting the 246 safety and regularity of flight. In parallel, the ground-based 247 broadband data link technology LDACS is being standardized by ICAO 248 and has recently shown its maturity during flight tests [SCH191]. 249 The LDACS technology is scalable, secure and spectrum efficient and 250 provides significant advantages to the users and service providers. 251 It is expected that both - satellite systems and LDACS - will be 252 deployed to support the future aeronautical communication needs as 253 envisaged by the ICAO Global Air Navigation Plan (GANP). 255 4. Provenance and Documents 257 The development of LDACS has already made substantial progress in the 258 Single European Sky ATM Research (SESAR) framework, and is currently 259 being continued in the follow-up program, SESAR2020 [RIH18]. A key 260 objective of the SESAR activities is to develop, implement and 261 validate a modern aeronautical data link able to evolve with aviation 262 needs over long-term. To this end, an LDACS specification has been 263 produced [GRA19] and is continuously updated; transmitter 264 demonstrators were developed to test the spectrum compatibility of 265 LDACS with legacy systems operating in the L-band [SAJ14]; and the 266 overall system performance was analyzed by computer simulations, 267 indicating that LDACS can fulfil the identified requirements [GRA11]. 269 LDACS standardization within the framework of the ICAO started in 270 December 2016. The ICAO standardization group has produced an 271 initial Standards and Recommended Practices (SARPs) document 272 [ICAO18]. The SARPs document defines the general characteristics of 273 LDACS. The ICAO standardization group plans to produce an ICAO 274 technical manual - the ICAO equivalent to a technical standard - 275 within the next years. Generally, the group is open to input from 276 all sources and develops LDACS in the open. 278 Up to now LDACS standardization has been focused on the development 279 of the physical layer and the data link layer, only recently have 280 higher layers come into the focus of the LDACS development 281 activities. There is currently no "IPv6 over LDACS" specification 282 publicly available; however, SESAR2020 has started the testing of 283 IPv6-based LDACS testbeds. 285 The IPv6 architecture for the aeronautical telecommunication network 286 is called the Future Communications Infrastructure (FCI). FCI shall 287 support quality of service, diversity, and mobility under the 288 umbrella of the "multi-link concept". This work is conducted by ICAO 289 Communication Panel working group WG-I. 291 In addition to standardization activities several industrial LDACS 292 prototypes have been built. One set of LDACS prototypes has been 293 evaluated in flight trials confirming the theoretical results 294 predicting the system performance [GRA18] [SCH191]. 296 5. Applicability 298 LDACS is a multi-application cellular broadband system capable of 299 simultaneously providing various kinds of Air Traffic Services 300 (including ATS-B3) and Aeronautical Operational Control (AOC) 301 communications services from deployed Ground Stations (GS). The 302 LDACS A2G sub-system physical layer and data link layer are optimized 303 for data link communications, but the system also supports digital 304 air-ground voice communications. 306 LDACS supports communication in all airspaces (airport, terminal 307 maneuvering area, and en-route), and on the airport surface. The 308 physical LDACS cell coverage is effectively de-coupled from the 309 operational coverage required for a particular service. This is new 310 in aeronautical communications. Services requiring wide-area 311 coverage can be installed at several adjacent LDACS cells. The 312 handover between the involved LDACS cells is seamless, automatic, and 313 transparent to the user. Therefore, the LDACS A2G communications 314 concept enables the aeronautical communication infrastructure to 315 support future dynamic airspace management concepts. 317 5.1. Advances Beyond the State-of-the-Art 319 LDACS offers several capabilities that are not provided in 320 contemporarily deployed aeronautical communication systems. 322 5.1.1. Priorities 324 LDACS is able to manage services priorities, an important feature not 325 available in some of the current data link deployments. Thus, LDACS 326 guarantees bandwidth, low latency, and high continuity of service for 327 safety critical ATS applications while simultaneously accommodating 328 less safety-critical AOC services. 330 5.1.2. Security 332 LDACS is a secure data link with built-in security mechanisms. It 333 enables secure data communications for ATS and AOC services, 334 including secured private communications for aircraft operators and 335 ANSPs (Air Navigation Service Providers). This includes concepts for 336 key and trust management, mutual authenticated key exchange 337 protocols, key derivation measures, user and control message-in- 338 transit confidentiality and authenticity protection, secure logging 339 and availability and robustness measures [MAE18], [MAE191], [MAE192]. 341 5.1.3. High Data Rates 343 The user data rate of LDACS is 315 kbit/s to 1428 kbit/s on the 344 forward link (Ground-to-Air), and 294 kbit/s to 1390 kbit/s on the 345 reverse link (Air-to-Ground), depending on coding and modulation. 346 This is 50 times the amount terrestrial digital aeronautical 347 communications systems such as VDLm2 provide [SCH191]. 349 5.2. Application 351 LDACS shall be used by several aeronautical applications ranging from 352 enhanced communication protocol stacks (multi-homed mobile IPv6 353 networks in the aircraft and potentially ad-hoc networks between 354 aircraft) to classical communication applications (sending GBAS 355 correction data) and integration with other service domains (using 356 the communication signal for navigation). 358 5.2.1. Air-to-Ground Multilink 360 It is expected that LDACS together with upgraded satellite-based 361 communications systems will be deployed within the Future 362 Communication Infrastructure (FCI) and constitute one of the main 363 components of the multilink concept within the FCI. 365 Both technologies, LDACS and satellite systems, have their specific 366 benefits and technical capabilities which complement each other. 367 Especially, satellite systems are well-suited for large coverage 368 areas with less dense air traffic, e.g. oceanic regions. LDACS is 369 well-suited for dense air traffic areas, e.g. continental areas or 370 hot-spots around airports and terminal airspace. In addition, both 371 technologies offer comparable data link capacity and, thus, are well- 372 suited for redundancy, mutual back-up, or load balancing. 374 Technically the FCI multilink concept shall be realized by multi- 375 homed mobile IPv6 networks in the aircraft. The related protocol 376 stack is currently under development by ICAO and SESAR. 378 5.2.2. Air-to-Air Extension for LDACS 380 A potential extension of the multi-link concept is its extension to 381 ad-hoc networks between aircraft. 383 Direct Air-to-Air (A2A) communication between aircrafts in terms of 384 ad-hoc data networks is currently considered a research topic since 385 there is no immediate operational need for it, although several 386 possible use cases are discussed (digital voice, wake vortex 387 warnings, and trajectory negotiation) [BELL19]. It should also be 388 noted that currently deployed analog VHF voice radios support direct 389 voice communication between aircraft, making a similar use case for 390 digital voice plausible. 392 LDACS direct A2A is currently not part of standardization. 394 5.2.3. Flight Guidance 396 The FCI (and therefore LDACS) shall be used to host flight guidance. 397 This is realized using three applications: 399 1. Context Management (CM): The CM application shall manage the 400 automatic logical connection to the ATC center currently 401 responsible to guide the aircraft. Currently this is done by the 402 air crew manually changing VHF voice frequencies according to the 403 progress of the flight. The CM application automatically sets up 404 equivalent sessions. 405 2. Controller Pilot Data Link Communication (CPDLC): The CPDLC 406 application provides the air crew with the ability to exchange 407 data messages similar to text messages with the currently 408 responsible ATC center. The CPDLC application shall take over 409 most of the communication currently performed over VHF voice and 410 enable new services that do not lend themselves to voice 411 communication (e.g., trajectory negotiation). 412 3. Automatic Dependent Surveillance - Contract (ADS-C): ADS-C 413 reports the position of the aircraft to the currently active ATC 414 center. Reporting is bound to "contracts", i.e. pre-defined 415 events related to the progress of the flight (i.e. the 416 trajectory). ADS-C and CPDLC are the primary applications used to 417 implement in-flight trajectory management. 419 CM, CPDLC, and ADS-C are available on legacy datalinks, but not 420 widely deployed and with limited functionality. 422 Further ATC applications may be ported to use the FCI or LDACS as 423 well. A notable application is GBAS for secure, automated landings: 424 The Global Navigation Satellite System (GNSS) based Ground Based 425 Augmentation System (GBAS) is used to improve the accuracy of GNSS to 426 allow GNSS based instrument landings. This is realized by sending 427 GNSS correction data (e.g., compensating ionospheric errors in the 428 GNSS signal) to the airborne GNSS receiver via a separate data link. 429 Currently the VDB data link is used. VDB is a narrow-band single- 430 purpose datalink without advanced security only used to transmit GBAS 431 correction data. This makes VDB a natural candidate for replacement 432 by LDACS. 434 5.2.4. Business Communication of Airlines 436 In addition to air traffic services airline operational control (AOC) 437 services shall be transmitted over LDACS. AOC is a generic term 438 referring to the business communication of airlines. Regulatory this 439 is considered related to the safety and regularity of flight and may 440 therefore be transmitted over LDACS. 442 AOC communication is considered the main business case for LDACS 443 communication service providers since modern aircraft generate 444 significant amounts of data (e.g., engine maintenance data). 446 5.2.5. LDACS Navigation 448 Beyond communication radio signals can always also be used for 449 navigation. LDACS takes this into account. 451 For future aeronautical navigation, ICAO recommends the further 452 development of Global Navigation Satellite System (GNSS) based 453 technologies as primary means for navigation. However, the drawback 454 of GNSS is its inherent single point of failure - the satellite. Due 455 to the large separation between navigational satellites and aircraft, 456 the received power of GNSS signals on the ground is very low. As a 457 result, GNSS disruptions might occasionally occur due to 458 unintentional interference, or intentional jamming. Yet the 459 navigation services must be available with sufficient performance for 460 all phases of flight. Therefore, during GNSS outages, or blockages, 461 an alternative solution is needed. This is commonly referred to as 462 Alternative Positioning, Navigation, and Timing (APNT). 464 One of such APNT solution consists of integrating the navigation 465 functionality into LDACS. The ground infrastructure for APNT is 466 deployed through the implementation of LDACS ground stations and the 467 navigation capability comes "for free". 469 LDACS navigation has already been demonstrated in practice in a 470 flight measurement campaign [SCH191]. 472 6. Characteristics of LDACS 474 LDACS will become one of several wireless access networks connecting 475 aircraft to the Aeronautical Telecommunications Network (ATN) 476 implemented by the FCI and possibly ACARS/FANS networks [FAN19]. 478 6.1. LDACS Sub-Network 480 An LDACS sub-network contains an Access Router (AR), a Ground-Station 481 Controller (GSC), and several Ground-Stations (GS), each of them 482 providing one LDACS radio cell. 484 User plane interconnection to the ATN is facilitated by the Access 485 Router (AR) peering with an Air-to-Ground Router (A2G Router) 486 connected to the ATN. It is up to implementer's choice to keep 487 Access Router and Air-Ground Router functions separated, or to merge 488 them. 490 The internal control plane of an LDACS sub-network is managed by the 491 Ground-Station Controller (GSC). An LDACS sub-network is illustrated 492 in Figure 1. 494 wireless user 495 link plane 496 A--------------G-------------Access---A2G-----ATN 497 S..............S Router Router 498 . control . | 499 . plane . | 500 . . | 501 GSC..............| 502 . | 503 . | 504 GS---------------+ 506 Figure 1: LDACS sub-network with two GSs and one AS 508 6.2. Topology 510 LDACS operating in A2G mode is a cellular point-to-multipoint system. 511 The A2G mode assumes a star-topology in each cell where Airborne 512 Stations (AS) belonging to aircraft within a certain volume of space 513 (the LDACS cell) is connected to the controlling GS. The LDACS GS is 514 a centralized instance that controls LDACS A2G communications within 515 its cell. The LDACS GS can simultaneously support multiple bi- 516 directional communications to the ASs under its control. LDACS 517 ground stations themselves are connected to a ground station 518 controller (GSC) controlling the LDACS sub-network. 520 Prior to utilizing the system an AS has to register with the 521 controlling GS to establish dedicated logical channels for user and 522 control data. Control channels have statically allocated resources, 523 while user channels have dynamically assigned resources according to 524 the current demand. Logical channels exist only between the GS and 525 the AS. 527 The LDACS wireless link protocol stack defines two layers, the 528 physical layer and the data link layer. 530 6.3. LDACS Physical Layer 532 The physical layer provides the means to transfer data over the radio 533 channel. The LDACS GS supports bi-directional links to multiple 534 aircraft under its control. The forward link direction (FL; G2A) and 535 the reverse link direction (RL; A2G) are separated by frequency 536 division duplex. Forward link and reverse link use a 500 kHz channel 537 each. The ground-station transmits a continuous stream of Orthogonal 538 Frequency-Division Multiplexing (OFDM) symbols on the forward link. 539 In the reverse link different aircraft are separated in time and 540 frequency using a combination of Orthogonal Frequency-Division 541 Multiple-Access (OFDMA) and Time-Division Multiple-Access (TDMA). 542 Aircraft thus transmit discontinuously on the reverse link with radio 543 bursts sent in precisely defined transmission opportunities allocated 544 by the ground-station. 546 6.4. LDACS Data Link Layer 548 The data-link layer provides the necessary protocols to facilitate 549 concurrent and reliable data transfer for multiple users. The LDACS 550 data link layer is organized in two sub-layers: The medium access 551 sub-layer and the logical link control sub-layer. The medium access 552 sub-layer manages the organization of transmission opportunities in 553 slots of time and frequency. The logical link control sub-layer 554 provides acknowledged point-to-point logical channels between the 555 aircraft and the ground-station using an automatic repeat request 556 protocol. LDACS supports also unacknowledged point-to-point channels 557 and G2A broadcast. 559 6.5. LDACS Mobility 561 LDACS supports layer 2 handovers to different LDACS channels. 562 Handovers may be initiated by the aircraft (break-before-make) or by 563 the GS (make-before-break). Make-before-break handovers are only 564 supported for ground-stations connected to the same GSC. 566 External handovers between non-connected LDACS sub-networks or 567 different aeronautical data links shall be handled by the FCI multi- 568 link concept. 570 7. Reliability and Availability 572 LDACS has been designed with applications related to the safety and 573 regularity of flight in mind. It has therefore been designed as a 574 deterministic wireless data link (as far as this is possible). 576 Based on channel measurements of the L-band channel [SCHN2016] and 577 respecting the specific nature of the area of application, LDACS was 578 designed from the PHY layer up with robustness in mind. 580 In order to maximize the capacity per channel and to optimally use 581 the available spectrum, LDACS was designed as an OFDM-based FDD 582 system, supporting simultaneous transmissions in Forward Link (FL; 583 G2A) and Reverse Link (RL; A2G). The legacy systems already deployed 584 in the L-band limit the bandwidth of both channels to approximately 585 500 kHz. 587 The LDACS physical layer design includes propagation guard times 588 sufficient for the operation at a maximum distance of 200 nautical 589 miles from the GS. In actual deployment, LDACS can be configured for 590 any range up to this maximum range. 592 The LDACS FL physical layer is a continuous OFDM transmission. LDACS 593 RL transmission is based on OFDMA-TDMA bursts, with silence between 594 such bursts. The RL resources (i.e. bursts) are assigned to 595 different users (ASs) on demand by the ground station (GS). 597 The LDACS physical layer supports adaptive coding and modulation for 598 user data. Control data is always encoded with the most robust 599 coding and modulation (QPSK coding rate 1/2). 601 LDACS medium access on top of the physical layer uses a static frame 602 structure to support deterministic timer management. As shown in 603 figure 3 and 4, LDACS framing structure is based on Super-Frames (SF) 604 of 240ms duration corresponding to 2000 OFDM symbols. FL and RL 605 boundaries are aligned in time (from the GS perspective) allowing for 606 deterministic sending windows for KEEP ALIVE messages and control and 607 data channels in general. 609 LDACS medium access is always under the control of the GS of a radio 610 cell. Any medium access for the transmission of user data has to be 611 requested with a resource request message stating the requested 612 amount of resources and class of service. The GS performs resource 613 scheduling on the basis of these requests and grants resources with 614 resource allocation messages. Resource request and allocation 615 messages are exchanged over dedicated contention-free control 616 channels. 618 LDACS has two mechanisms to request resources from the scheduler in 619 the GS. 621 Resources can either be requested "on demand" with a given priority. 622 On the forward link, this is done locally in the GS, on the reverse 623 link a dedicated contention-free control channel is used called 624 Dedicated Control Channel (DCCH; roughly 83 bit every 60 ms). A 625 resource allocation is always announced in the control channel of the 626 forward link (Common Control Channel (CCCH); variably sized). Due to 627 the spacing of the reverse link control channels every 60 ms, a 628 medium access delay in the same order of magnitude is to be expected. 630 Resources can also be requested "permanently". The permanent 631 resource request mechanism supports requesting recurring resources in 632 given time intervals. A permanent resource request has to be 633 canceled by the user (or by the ground-station, which is always in 634 control). 636 User data transmissions over LDACS are therefore always scheduled by 637 the GS, while control data uses statically (i.e. at cell entry) 638 allocated recurring resources (DCCH and CCCH). The current 639 specification specifies no scheduling algorithm. Scheduling of 640 reverse link resources is done in physical Protocol Data Units (PDU) 641 of 112 bit (or larger if more aggressive coding and modulation is 642 used). Scheduling on the forward link is done Byte- wise since the 643 forward link is transmitted continuously by the GS. 645 In addition to having full control over resource scheduling, the GS 646 can send forced Handover (HO) commands for off-loading or RF channel 647 management, e.g. when the signal quality declines and a more suitable 648 GS is in the AS reach. With robust resource management of the 649 capacities of the radio channel, reliability and robustness measures 650 are therefore also anchored in the LDACS management entity. 652 The LDACS data link layer protocol running on top of the medium 653 access sub-layer uses ARQ to provide reliable data transmission on 654 layer 2. 656 It employs selective repeat ARQ with transparent fragmentation and 657 reassembly to the resource allocation size to achieve low latency and 658 a low overhead without losing reliability. It ensures correct order 659 of packet delivery without duplicates. In case of transmission 660 errors it identifies lost fragments with deterministic timers synced 661 to the medium access frame structure and initiates retransmission. 662 Additionally the priority mechanism of LDACS ensures the timely 663 delivery of messages with high importance. 665 As of now no reliability and availability mechanisms for layer 3 and 666 above have been specified. 668 8. Protocol Stack 670 The protocol stack of LDACS is implemented in the AS, GS, and GSC: It 671 consists of the Physical Layer (PHY) with five major functional 672 blocks above it. Four are placed in the Data Link Layer (DLL) of the 673 AS and GS: (1) Medium Access Layer (MAC), (2) Voice Interface (VI), 674 (3) Data Link Service (DLS), (4) LDACS Management Entity (LME). The 675 last entity resides within the sub-network layer: Sub-Network 676 Protocol (SNP). The LDACS network is externally connected to voice 677 units, radio control units, and the ATN network layer. 679 Figure 2 shows the protocol stack of LDACS as implemented in the AS 680 and GS. 682 IPv6 network layer 683 | 684 | 685 +------------------+ +----+ 686 | SNP |--| | sub-network 687 | | | | layer 688 +------------------+ | | 689 | | LME| 690 +------------------+ | | 691 | DLS | | | logical link 692 | | | | control layer 693 +------------------+ +----+ 694 | | 695 DCH DCCH/CCCH 696 | RACH/BCCH 697 | | 698 +--------------------------+ 699 | MAC | medium access 700 | | layer 701 +--------------------------+ 702 | 703 +--------------------------+ 704 | PHY | physical layer 705 +--------------------------+ 706 | 707 | 708 ((*)) 709 FL/RL radio channels 710 separated by FDD 712 Figure 2: LDACS protocol stack in AS and GS 714 8.1. Medium Access Control (MAC) Entity Services 716 The MAC time framing service provides the frame structure necessary 717 to realize slot-based Time Division Multiplex (TDM) access on the 718 physical link. It provides the functions for the synchronization of 719 the MAC framing structure and the PHY layer framing. The MAC time 720 framing provides a dedicated time slot for each logical channel. 722 The MAC sub-layer offers access to the physical channel to its 723 service users. Channel access is provided through transparent 724 logical channels. The MAC sub-layer maps logical channels onto the 725 appropriate slots and manages the access to these channels. Logical 726 channels are used as interface between the MAC and LLC sub-layers. 728 The LDACS framing structure for FL and RL is based on Super-Frames 729 (SF) of 240 ms duration. Each SF corresponds to 2000 OFDM symbols. 730 The FL and RL SF boundaries are aligned in time (from the view of the 731 GS). 733 In the FL, an SF contains a Broadcast Frame of duration 6.72 ms (56 734 OFDM symbols) for the Broadcast Control Channel (BCCH), and four 735 Multi-Frames (MF), each of duration 58.32 ms (486 OFDM symbols). 737 In the RL, each SF starts with a Random Access (RA) slot of length 738 6.72 ms with two opportunities for sending reverse link random access 739 frames for the Random Access Channel (RACH), followed by four MFs. 740 These MFs have the same fixed duration of 58.32 ms as in the FL, but 741 a different internal structure 743 Figure 3 and Figure 4 illustrates the LDACS frame structure. 745 ^ 746 | +------+------------+------------+------------+------------+ 747 | FL | BCCH | MF | MF | MF | MF | 748 F +------+------------+------------+------------+------------+ 749 r <---------------- Super-Frame (SF) - 240ms ----------------> 750 e 751 q +------+------------+------------+------------+------------+ 752 u RL | RACH | MF | MF | MF | MF | 753 e +------+------------+------------+------------+------------+ 754 n <---------------- Super-Frame (SF) - 240ms ----------------> 755 c 756 y 757 | 758 ----------------------------- Time ------------------------------> 759 | 761 Figure 3: LDACS super-frame structure 763 ^ 764 | +-------------+------+-------------+ 765 | FL | DCH | CCCH | DCH | 766 F +-------------+------+-------------+ 767 r <---- Multi-Frame (MF) - 58.32ms --> 768 e 769 q +------+---------------------------+ 770 u RL | DCCH | DCH | 771 e +------+---------------------------+ 772 n <---- Multi-Frame (MF) - 58.32ms --> 773 c 774 y 775 | 776 ----------------------------- Time ------------------------------> 777 | 779 Figure 4: LDACS multi-frame (MF) structure 781 8.2. Data Link Service (DLS) Entity Services 783 The DLS provides acknowledged and unacknowledged (including broadcast 784 and packet mode voice) bi-directional exchange of user data. If user 785 data is transmitted using the acknowledged data link service, the 786 sending DLS entity will wait for an acknowledgement from the 787 receiver. If no acknowledgement is received within a specified time 788 frame, the sender may automatically try to retransmit its data. 789 However, after a certain number of failed retries, the sender will 790 suspend further retransmission attempts and inform its client of the 791 failure. 793 The data link service uses the logical channels provided by the MAC: 795 1. A ground-stations announces its existence and access parameters 796 in the Broadcast Channel (BC). 797 2. The Random Access Channel (RA) enables AS to request access to an 798 LDACS cell. 799 3. In the Forward Link (FL) the Common Control Channel (CCCH) is 800 used by the GS to grant access to data channel resources. 801 4. The reverse direction is covered by the Reverse Link (RL), where 802 aircraft-stations need to request resources before sending. This 803 happens via the Dedicated Common Control Channel (DCCH). 804 5. User data itself is communicated in the Data Channel (DCH) on the 805 FL and RL. 807 8.3. Voice Interface (VI) Services 809 The VI provides support for virtual voice circuits. Voice circuits 810 may either be set-up permanently by the GS (e.g., to emulate voice 811 party line) or may be created on demand. The creation and selection 812 of voice circuits is performed in the LME. The VI provides only the 813 transmission services. 815 8.4. LDACS Management Entity (LME) Services 817 The mobility management service in the LME provides support for 818 registration and de-registration (cell entry and cell exit), scanning 819 RF channels of neighboring cells and handover between cells. In 820 addition, it manages the addressing of aircraft/ ASs within cells. 821 It is controlled by the network management service in the GSC. 823 The resource management service provides link maintenance (power, 824 frequency and time adjustments), support for adaptive coding and 825 modulation (ACM), and resource allocation. 827 8.5. Sub-Network Protocol (SNP) Services 829 The data link service provides functions required for the transfer of 830 user plane data and control plane data over the LDACS sub-network. 832 The security service provides functions for secure communication over 833 the LDACS sub-network. Note that the SNP security service applies 834 cryptographic measures as configured by the ground station 835 controller. 837 9. Security Considerations 839 Aviation will require secure exchanges of data and voice messages for 840 managing the air-traffic flow safely through the airspaces all over 841 the world. The main communication method for ATC today is still an 842 open analogue voice broadcast within the aeronautical VHF band. 843 Currently, the information security is purely procedural based by 844 using well-trained personnel and proven communications procedures. 845 This communication method has been in service since 1948. Future 846 digital communications waveforms will need additional embedded 847 security features to fulfill modern information security requirements 848 like authentication and integrity. These security features require 849 sufficient bandwidth which is beyond the capabilities of a VHF 850 narrowband communications system. For voice and data communications, 851 sufficient data throughput capability is needed to support the 852 security functions while not degrading performance. LDACS is a 853 mature data link technology with sufficient bandwidth to support 854 security. 856 Security considerations for LDACS are defined by the official ICAO 857 SARPS [ICAO18]: 859 1. LDACS shall provide a capability to protect the availability and 860 continuity of the system. 861 2. LDACS shall provide a capability including cryptographic 862 mechanisms to protect the integrity of messages in transit. 863 3. LDACS shall provide a capability to ensure the authenticity of 864 messages in transit. 865 4. LDACS should provide a capability for nonrepudiation of origin 866 for messages in transit. 867 5. LDACS should provide a capability to protect the confidentiality 868 of messages in transit. 869 6. LDACS shall provide an authentication capability. 870 7. LDACS shall provide a capability to authorize the permitted 871 actions of users of the system and to deny actions that are not 872 explicitly authorized. 873 8. If LDACS provides interfaces to multiple domains, LDACS shall 874 provide capability to prevent the propagation of intrusions within 875 LDACS domains and towards external domains. 877 The cybersecurity architecture of LDACS [ICAO18], [MAE18] and its 878 extensions [MAE191], [MAE192] regard all of the aforementioned 879 requirements, since LDACS has been mainly designed for air traffic 880 management communication. Thus it supports mutual entity 881 authentication, integrity and confidentiality capabilities of user 882 data messages and some control channel protection capabilities 883 [MAE192]. 885 10. Privacy Considerations 887 LDACS provides a Quality of Service (QoS), and the generic 888 considerations for such mechanisms apply. 890 11. IANA Considerations 892 This memo includes no request to IANA. 894 12. Acknowledgements 896 Thanks to all contributors to the development of LDACS and ICAO PT-T. 898 Thanks to Klaus-Peter Hauf, Bart Van Den Einden, and Pierluigi 899 Fantappie for further input to this draft. 901 Thanks to SBA Research Vienna for fruitful discussions on 902 aeronautical communications concerning security incentives for 903 industry and potential economic spillovers. 905 13. Normative References 907 14. Informative References 909 [SCHN2016] Schneckenburger, N., Jost, T., Shutin, D., Walter, M., 910 Thiasiriphet, T., Schnell, M., and U.C. Fiebig, 911 "Measurement of the L-band Air-to-Ground Channel for 912 Positioning Applications", IEEE Transactions on Aerospace 913 and Electronic Systems, 52(5), pp.2281-229 , 2016. 915 [MAE191] Maeurer, N., Graeupl, T., and C. Schmitt, "Evaluation of 916 the LDACS Cybersecurity Implementation", IEEE 38th Digital 917 Avionics Systems Conference (DACS), pp. 1-10, San Diego, 918 CA, USA , 2019. 920 [MAE192] Maeurer, N. and C. Schmitt, "Towards Successful 921 Realization of the LDACS Cybersecurity Architecture: An 922 Updated Datalink Security Threat- and Risk Analysis", IEEE 923 Integrated Communications, Navigation and Surveillance 924 Conference (ICNS), pp. 1-13, Herndon, VA, USA , 2019. 926 [GRA19] Graeupl, T., Rihacek, C., and B. Haindl, "LDACS A/G 927 Specification", SESAR2020 PJ14-02-01 D3.3.030 , 2019. 929 [FAN19] Pierattelli, S., Fantappie, P., Tamalet, S., van den 930 Einden, B., Rihacek, C., and T. Graeupl, "LDACS Deployment 931 Options and Recommendations", SESAR2020 PJ14-02-01 932 D3.4.020 , 2019. 934 [MAE18] Maeurer, N. and A. Bilzhause, "A Cybersecurity 935 Architecture for the L-band Digital Aeronautical 936 Communications System (LDACS)", IEEE 37th Digital Avionics 937 Systems Conference (DASC), pp. 1-10, London, UK , 2017. 939 [GRA11] Graeupl, T. and M. Ehammer, "L-DACS1 Data Link Layer 940 Evolution of ATN/IPS", 30th IEEE/AIAA Digital Avionics 941 Systems Conference (DASC), pp. 1-28, Seattle, WA, USA , 942 2011. 944 [GRA18] Graeupl, T., Schneckenburger, N., Jost, T., Schnell, M., 945 Filip, A., Bellido-Manganell, M.A., Mielke, D.M., Maeurer, 946 N., Kumar, R., Osechas, O., and G. Battista, "L-band 947 Digital Aeronautical Communications System (LDACS) flight 948 trials in the national German project MICONAV", Integrated 949 Communications, Navigation, Surveillance Conference 950 (ICNS), pp. 1-7, Herndon, VA, USA , 2018. 952 [SCH191] Schnell, M., "DLR Tests Digital Communications 953 Technologies Combined with Additional Navigation Functions 954 for the First Time", 2019. 956 [ICAO18] International Civil Aviation Organization (ICAO), "L-Band 957 Digital Aeronautical Communication System (LDACS)", 958 International Standards and Recommended Practices Annex 10 959 - Aeronautical Telecommunications, Vol. III - 960 Communication Systems , 2018. 962 [SAJ14] Haindl, B., Meser, J., Sajatovic, M., Mueller, S., 963 Arthaber, H., Faseth, T., and M. Zaisberger, "LDACS1 964 Conformance and Compatibility Assessment", IEEE/AIAA 33rd 965 Digital Avionics Systems Conference (DASC), pp. 1-11, 966 Colorado Springs, CO, USA , 2014. 968 [RIH18] Rihacek, C., Haindl, B., Fantappie, P., Pierattelli, S., 969 Graeupl, T., Schnell, M., and N. Fistas, "L-band Digital 970 Aeronautical Communications System (LDACS) Activities in 971 SESAR2020", Integrated Communications Navigation and 972 Surveillance Conference (ICNS), pp. 1-8, Herndon, VA, 973 USA , 2018. 975 [BELL19] Bellido-Manganell, M. A. and M. Schnell, "Towards Modern 976 Air-to-Air Communications: the LDACS A2A Mode", IEEE/AIAA 977 38th Digital Avionics Systems Conference (DASC), pp. 1-10, 978 San Diego, CA, USA , 2019. 980 [RAW-TECHNOS] 981 Thubert, P., Cavalcanti, D., Vilajosana, X., Schmitt, C., 982 and J. Farkas, "Reliable and Available Wireless 983 Technologies", Work in Progress, Internet-Draft, draft- 984 thubert-raw-technologies-05, 18 May 2020, 985 . 988 [RAW-USE-CASES] 989 Papadopoulos, G., Thubert, P., Theoleyre, F., and C. 990 Bernardos, "RAW use cases", Work in Progress, Internet- 991 Draft, draft-bernardos-raw-use-cases-03, 8 March 2020, 992 . 995 Authors' Addresses 997 Nils Maeurer (editor) 998 German Aerospace Center (DLR) 999 Muenchner Strasse 20 1000 82234 Wessling 1001 Germany 1003 Email: Nils.Maeurer@dlr.de 1005 Thomas Graeupl (editor) 1006 German Aerospace Center (DLR) 1007 Muenchner Strasse 20 1008 82234 Wessling 1009 Germany 1011 Email: Thomas.Graeupl@dlr.de 1013 Corinna Schmitt (editor) 1014 Research Institute CODE, UniBwM 1015 Werner-Heisenberg-Weg 28 1016 85577 Neubiberg 1017 Germany 1019 Email: corinna.schmitt@unibw.de