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Checking references for intended status: Informational ---------------------------------------------------------------------------- == Outdated reference: A later version (-05) exists of draft-thubert-raw-technologies-03 == Outdated reference: A later version (-04) exists of draft-bernardos-raw-use-cases-00 Summary: 0 errors (**), 0 flaws (~~), 3 warnings (==), 1 comment (--).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: 8 May 2020 C. Schmitt, Ed. 6 Research Institute CODE, UniBwM 7 5 November 2019 9 L-band Digital Aeronautical Communications System (LDACS) 10 draft-maeurer-raw-ldacs-00 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 8 May 2020. 37 Copyright Notice 39 Copyright (c) 2019 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 2.1. Terms used in this document . . . . . . . . . . . . . . . 3 56 3. Motivation and Use Cases . . . . . . . . . . . . . . . . . . 4 57 3.1. Voice Communications Today . . . . . . . . . . . . . . . 5 58 3.2. Data Communications Today . . . . . . . . . . . . . . . . 5 59 4. Provenance and Documents . . . . . . . . . . . . . . . . . . 6 60 5. Characteristics . . . . . . . . . . . . . . . . . . . . . . . 7 61 5.1. LDACS Physical Layer . . . . . . . . . . . . . . . . . . 7 62 5.2. LDACS Data Link Layer . . . . . . . . . . . . . . . . . . 8 63 5.3. LDACS Data Rates . . . . . . . . . . . . . . . . . . . . 8 64 5.4. Reliability and Availability . . . . . . . . . . . . . . 8 65 5.4.1. LDACS Medium Access . . . . . . . . . . . . . . . . . 8 66 5.4.2. LDACS Resource Allocation . . . . . . . . . . . . . . 9 67 5.4.3. LDACS Handovers . . . . . . . . . . . . . . . . . . . 9 68 6. Architecture . . . . . . . . . . . . . . . . . . . . . . . . 10 69 6.1. Protocol Stack . . . . . . . . . . . . . . . . . . . . . 10 70 6.1.1. Medium Access Control (MAC) Entity Services . . . . . 12 71 6.1.2. Data Link Service (DLS) Entity Services . . . . . . . 13 72 6.1.3. Voice Interface (VI) Services . . . . . . . . . . . . 13 73 6.1.4. Link Management Entity (LME) Services . . . . . . . . 13 74 6.1.5. Sub-Network Protocol (SNP) Services . . . . . . . . . 13 75 6.2. LDACS Logical Communication Channels . . . . . . . . . . 14 76 6.3. LDASC Framing Structure . . . . . . . . . . . . . . . . . 15 77 6.3.1. Forward Link . . . . . . . . . . . . . . . . . . . . 15 78 6.3.2. Reverse Link . . . . . . . . . . . . . . . . . . . . 15 79 7. Security Considerations . . . . . . . . . . . . . . . . . . . 16 80 8. Privacy Considerations . . . . . . . . . . . . . . . . . . . 17 81 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17 82 10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 17 83 11. Normative References . . . . . . . . . . . . . . . . . . . . 17 84 12. Informative References . . . . . . . . . . . . . . . . . . . 17 85 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 19 87 1. Introduction 89 One of the main pillars of the modern Air Traffic Management (ATM) 90 system is the existence of a communication infrastructure that 91 enables efficient aircraft guidance and safe separation in all phases 92 of flight. Current systems are technically mature but suffering from 93 the VHF band's increasing saturation in high-density areas and the 94 limitations posed by analogue radio. Therefore, aviation globally 95 and the European Union (EU) in particular, strives for a sustainable 96 modernization of the aeronautical communication infrastructure. 98 In the long-term, ATM communication shall transition from analogue 99 VHF voice and VDL2 communication to more spectrum efficient digital 100 data communication. The European ATM Master Plan foresees this 101 transition to be realized for terrestrial communications by the 102 development and implementation of the L-band Digital Aeronautical 103 Communications System (LDACS). LDACS shall enable IPv6 based air- 104 ground communication related to the safety and regularity of the 105 flight. The particular challenge is that no new frequencies can be 106 made available for terrestrial aeronautical communication. It was 107 thus necessary to develop procedures to enable the operation of LDACS 108 in parallel with other services in the same frequency band. 110 2. Terminology 112 2.1. Terms used in this document 114 The following terms are used in the context of DetNet in this 115 document: 117 A/A Air-To-Air 118 AeroMACS Aeronautical Mobile Airport Communication System 119 A/G Air-To-Ground 120 AM(R)S Aeronautical Mobile (Route) Service 121 ANSP Air traffic Network Service Provider 122 AOC Aeronautical Operational Control 123 AS Aircraft Station 124 ATC Air-Traffic Control 125 ATM Air-Traffic Management 126 ATN Aeronautical Telecommunication Network 127 ATS Air Traffic Service 128 CCCH Common Control Channel 129 DCCH Dedicated Control Channel 130 DCH Data Channel 131 DLL Data Link Layer 132 DLS Data Link Service 133 DME Distance Measuring Equipment 134 DSB-AM Double Side-Band Amplitude Modulation 135 FAA Federal Aviation Administration 136 FCI Future Communication Infrastructure 137 FDD Frequency Division Duplex 138 FL Forward Link 139 GANP Global Air Navigation Plan 140 GNSS Global Navigation Satellite System 141 GS Ground Station 142 GSC Ground-Station Controller 143 HF High Frequency 144 ICAO International Civil Aviation Organization 145 IWF Interworking Function 146 kbit/s kilobit per secong 147 LDACS L-band Digital Aeronautical Communications System 148 LLC Logical Link Layer 149 LME LDACS Management Entity 150 MAC Medium Access Layer 151 MF Multi Frame 152 MIMO Multiple Input Multiple Output 153 OFDM Orthogonal Frequency-Division Multiplexing 154 OFDMA Orthogonal Frequency-Division Multiplexing Access 155 PDU Protocol Data Units 156 PHY Physical Layer 157 QoS Quality of Service 158 RL Reverse Link 159 SARPs Standards And Recommended Practices 160 SESAR Single European Sky ATM Research 161 SF Super-Frame 162 SNP Sub-Network Protocol 163 SSB-AM Single Side-Band Amplitude Modulation 164 SNDCF Sub-Network Dependent Convergence Function 165 TBO Trajectory-Based Operations 166 TDM Time Division Multiplexing 167 TDMA Time-Division Multiplexing-Access 168 VDL2 VHF Data Link mode 2 169 VHF Very High Frequency 170 VI Voice Interface 172 3. Motivation and Use Cases 174 Aircraft are currently connected to Air-Traffic Control (ATC) and 175 Airline Operational Control (AOC) via voice and data communications 176 systems through all phases of a flight. Within the airport terminal, 177 connectivity is focused on high bandwidth communications, while 178 during en-route high reliability, robustness, and range is the main 179 focus. Voice communications may use the same or different equipment 180 as data communications systems. In the following the main 181 differences between voice and data communications capabilities are 182 summarized. The assumed use cases for LDACS completes the list of 183 use cases stated in [RAW-USE-CASES] and the list of reliable and 184 available wireless technologies presented in [RAW-TECHNOS]. 186 3.1. Voice Communications Today 188 Voice links are used for Air-To-Ground (A/G) and Air-To-Air (A/A) 189 communications. The communication equipment is either ground-based 190 working in the High Frequency (HF) or Very High Frequency (VHF) 191 frequency band or satellite-based. All voice communications is 192 operated via open broadcast channels without any authentication, 193 encryption or other protective measures. The use of well-proven 194 communication procedures via broadcast channels helps to enhance the 195 safety of communications by taking into account that other users may 196 encounter communication problems and may be supported, if required. 197 The main voice communications media is still the analogue VHF Double 198 Side-Band Amplitude Modulation (DSB-AM) communications technique, 199 supplemented by HF Single Side-Band Amplitude Modulation (SSB-AM) and 200 satellite communications for remote and oceanic areas. DSB-AM has 201 been in use since 1948, works reliably and safely, and uses low-cost 202 communication equipment. These are the main reasons why VHF DSB-AM 203 communications is still in use, and it is likely that this technology 204 will remain in service for many more years. This however results in 205 current operational limitations and becomes impediments in deploying 206 new Air-Traffic Management (ATM) applications, such as flight-centric 207 operation with point-to-point communications. 209 3.2. Data Communications Today 211 Like for voice, data communications into the cockpit is currently 212 provided by ground-based equipment operating either on HF or VHF 213 radio bands or by legacy satellite systems. All these communication 214 systems are using narrowband radio channels with a data throughput 215 capacity of some kilobits per second. While the aircraft is on 216 ground some additional communications systems are available, like 217 Aeronautical Mobile Airport Communication System (AeroMACS), 218 operating in the Airport (APT) domain and able to deliver broadband 219 communication capability. 221 The data communication networks used for the transmission of data 222 relating to the safety and regularity of the flight must be strictly 223 isolated from those providing entertainment services to passengers. 224 This leads to a situation that the flight crews are supported by 225 narrowband services during flight while passengers have access to 226 inflight broadband services. The current HF and VHF data links 227 cannot provide broadband services now or in the future, due to the 228 lack of available spectrum. This technical shortcoming is becoming a 229 limitation to enhanced ATM operations, such as Trajectory-Based 230 Operations (TBO) and 4D trajectory negotiations. 232 Satellite-based communications are currently under investigation and 233 enhanced capabilities are under development which will be able to 234 provide inflight broadband services and communications supporting the 235 safety and regularity of the flight. In parallel, the ground-based 236 broadband data link technology LDACS is being standardized by ICAO 237 and has recently shown its maturity during flight tests [SCH191]. 238 The LDACS technology is scalable, secure and spectrum efficient and 239 provides significant advantages to the users and service providers. 240 It is expected that both - satellite systems and LDACS - will be 241 deployed to support the future aeronautical communication needs as 242 envisaged by the ICAO Global Air Navigation Plan (GANP). 244 4. Provenance and Documents 246 The development of LDACS has already made substantial progress in the 247 Single European Sky ATM Research (SESAR) framework, and is currently 248 being continued in the follow-up program, SESAR2020 [RIH18]. A key 249 objective of the SESAR activities is to develop, implement and 250 validate a modern aeronautical data link able to evolve with aviation 251 needs over long-term. To this end, an LDACS specification has been 252 produced [GRA19] and is continuously updated; transmitter 253 demonstrators were developed to test the spectrum compatibility of 254 LDACS with legacy systems operating in the L-band [SAJ14]; and the 255 overall system performance was analyzed by computer simulations, 256 indicating that LDACS can fulfil the identified requirements [GRA11]. 258 LDACS standardization within the framework of the ICAO started in 259 December 2016. The ICAO standardization group has produced an 260 initial Standards and Recommended Practices (SARPs) document 261 [ICAO18]. The SARPs document defines the general characteristics of 262 LDACS. The ICAO standardization group plans to produce an ICAO 263 technical manual - the ICAO equivalent to a technical standard - 264 within the next years. Generally, the group is open to input from 265 all sources and develops LDACS in the open. 267 Up to now the LDACS standardization has been focused on the 268 development of the physical layer and the data link layer, only 269 recently have higher layers come into the focus of the LDACS 270 development activities. There is currently no "IPv6 over LDACS" 271 specification; however, SESAR2020 has started the testing of 272 IPv6-based LDACS testbeds. The IPv6 architecture for the 273 aeronautical telecommunication network is called the Future 274 Communications Infrastructure (FCI). FCI shall support quality of 275 service, diversity, and mobility under the umbrella of the "multi- 276 link concept". This work is conducted by ICAO working group WG-I. 278 In addition to standardization activities several industrial LDACS 279 prototypes have been built. One set of LDACS prototypes has been 280 evaluated in flight trials confirming the theoretical results 281 predicting the system performance [GRA18] [SCH191]. 283 5. Characteristics 285 LDACS will become one of several wireless access networks connecting 286 aircraft to the Aeronautical Telecommunications Network (ATN). 287 Access to the ATN is handled by the Ground-Station Controller (GSC), 288 while several Ground-Stations (GS) are connected to one GSC. Thus 289 the LDACS access network contains several GS, each of them providing 290 one LDACS radio cell. LDACS can be therefore considered a cellular 291 data link with a star-topology connecting Aircraft-Stations (AS) to 292 GS with a full duplex radio link. Each GS is the centralized 293 instance controlling all A/G communications within its radio cell. A 294 GS supports up to 512 aircraft. All of this is depicted in Figure 1. 296 AS11--------------+ 297 | 298 AS12-------------GS1------GSC------>ATN 299 . | | 300 . | | 301 AS1n--------------+ | 302 | 303 AS21--------------+ | 304 | | 305 AS21-------------GS2-------+ 306 . | 307 . | 308 AS2n--------------+ 310 Figure 1: LDACS wireless topology 312 The LDACS air interface protocol stack defines two layers, the 313 physical layer and the data link layer. 315 5.1. LDACS Physical Layer 317 The physical layer provides the means to transfer data over the radio 318 channel. The LDACS GS supports bi-directional links to multiple 319 aircraft under its control. The forward link direction (FL; ground- 320 to-air) and the reverse link direction (RL; air-to-ground) are 321 separated by frequency division duplex. Forward link and reverse 322 link use a 500 kHz channel each. The ground-station transmits a 323 continuous stream of Orthogonal Frequency-Division Multiplexing 324 (OFDM) symbols on the forward link. In the reverse link different 325 aircraft are separated in time and frequency using a combination of 326 Orthogonal Frequency-Division Multiple-Access (OFDMA) and Time- 327 Division Multiple-Access (TDMA). Aircraft thus transmit 328 discontinuously on the reverse link with radio bursts sent in 329 precisely defined transmission opportunities allocated by the ground- 330 station. LDACS does not support beam-forming or Multiple Input 331 Multiple Output (MIMO) [SCH192]. 333 5.2. LDACS Data Link Layer 335 The data-link layer provides the necessary protocols to facilitate 336 concurrent and reliable data transfer for multiple users. The LDACS 337 data link layer is organized in two sub-layers: The medium access 338 sub-layer and the logical link control sub-layer. The medium access 339 sub-layer manages the organization of transmission opportunities in 340 slots of time and frequency. The logical link control sub-layer 341 provides acknowledged point-to-point logical channels between the 342 aircraft and the ground-station using an automatic repeat request 343 protocol. LDACS supports also unacknowledged point-to-point channels 344 and ground-to-air broadcast. 346 5.3. LDACS Data Rates 348 The user data rate of LDACS is 315 kbit/s to 1428 kbit/s on the 349 forward link, and 294 kbit/s to 1390 kbit/s on the reverse link, 350 depending on coding and modulation. Due to strong interference from 351 legacy systems in the L-band, the most robust coding and modulation 352 should be expected for initial deployment i.e. 315/294 kbit/s on 353 theforward/reverse link, respectively. 355 5.4. Reliability and Availability 357 LDACS has been designed with applications related to the safety and 358 regularity of the flight in mind. It has therefore been designed as 359 a deterministic wireless data link (as far as possible). 361 5.4.1. LDACS Medium Access 363 LDACS medium access is always under the control of the ground-station 364 of a radio cell. Any medium access for the transmission of user data 365 has to be requested with a resource request message stating the 366 requested amount of resources and class of service. The ground- 367 station performs resource scheduling on the basis of these requests 368 and grants resources with resource allocation messages. Resource 369 request and allocation messages are exchanged over dedicated 370 contention-free control channels. 372 5.4.2. LDACS Resource Allocation 374 LDACS has two mechanisms to request resources from the scheduler in 375 the ground-station. Resources can either be requested "on demand" 376 with a given class of service. On the forward link, this is done 377 locally in the ground-station, on the reverse link a dedicated 378 contention-free control channel is used called Dedicated Control 379 Channel (DCCH); roughly 83 bit every 60 ms). A resource allocation 380 is always announced in the control channel of the forward link 381 (Common Control Channel (CCCH); variable sized). Due to the spacing 382 of the reverse link control channels of every 60 ms, a medium access 383 delay in the same order of magnitude is to be expected. 385 Resources can also be requested "permanently". The permanent 386 resource request mechanism supports requesting recurring resources in 387 given time intervals. A permanent resource request has to be 388 canceled by the user (or by the ground-station, which is always in 389 control). User data transmissions over LDACS are therefore always 390 scheduled by the ground-station, while control data uses statically 391 (i.e. at net entry) allocated recurring resources (DCCH and CCCH). 392 The current specification documents specify no scheduling algorithm. 393 However performance evaluations so far have used strict priority 394 scheduling and round robin for equal priorities for simplicity. In 395 the current prototype implementations LDACS classes of service are 396 thus realized as priorities of medium access and not as flows. Note 397 that this can starve out low priority flows. However, this is not 398 seen as a big problem since safety related message always go first in 399 any case. Scheduling of reverse link resources is done in physical 400 Protocol Data Units (PDU) of 112 bit (or larger if more aggressive 401 coding and modulation is used). Scheduling on the forward link is 402 done Byte-wise since the forward link is transmitted continuously 403 bythe ground-station. 405 5.4.3. LDACS Handovers 407 In order to support diversity, LDACS supports handovers to other 408 ground-stations on different channels. Handovers may be initiated by 409 the aircraft (break-before-make) or by the ground-station (make- 410 before-break) if it is connected to an alternative ground-station via 411 the same ground-station controller. Beyond this, FCI diversity shall 412 be implemented by the multi-link concept. 414 6. Architecture 416 Aircraft-Station (AS), Ground-Station (GS) and Ground-Station 417 Controller (GSC) form the basic LDACS network. 512 aircraft can be 418 served by one GS where the GS sends a continuous data stream in the 419 Forward Link (FL) to the AS. The Reverse Link (RL) consists of 420 individual bursts of data from each AS to GS. This means, for every 421 RL communication the AS first needs to request the respective 422 resource allocation within its cell from the GS before being able to 423 send. Both FL and RL communication, including user and control data, 424 is done via the air gap over the radio link between AS and GS. On 425 the ground a GSC is responsible for serving several GSs on the 426 control plane, forming an LDACS sub-network with its LDACS internal 427 control plane infrastructure. The GSs are linked to an access router 428 in the user plane, which in turn is linked to an Air/Ground router, 429 being now the direct connection to the ground network. The ATN is 430 used for example by Air traffic Network Services Providers (ANSP) and 431 airlines to exchange Air Traffic Service (ATS) or Airline Operational 432 Control (AOC) data between the ground infrastructure and the 433 aircraft. Figure 2 provides a more detailed overview. 435 wireless user 436 link plane 437 A--------------G-------------Access---A/G-----ATN 438 S..............S Router Router 439 . control . | 440 . plane . | 441 . . | 442 GSC.............. | 443 . | 444 . | 445 GS----------------+ 447 Figure 2: LDACS sub-network with two GSs 449 6.1. Protocol Stack 451 The protocol stack of LDACS is implemented in the AS and GS as 452 follows: It consists of the Physical Layer (PHY) with five major 453 functional blocks above it. Four are placed in the Data Link Layer 454 (DLL) of the AS and GS: (1) Medium Access Layer (MAC), (2) Voice 455 Interface (VI), (3) Data Link Service (DLS), (4) LDACS Management 456 Entity (LME). The last entity resides within the sub-network layer: 457 Sub-Network Protocol (SNP). 459 The LDACS network is externally connected to voice units, radio 460 control units, and the ATN network layer through a Sub-Network 461 Dependent Convergence Function (SNDCF; OSI network layers), 462 Convergence Sub-layer, or Interworking Function (IWF; legacy 463 networks) not discussed here. 465 The SNP connects the AS and GS DLL providing end-to-end user plane 466 connectivity between the LDACS AS and GS. 468 The DLL provides Quality of Service (QoS) assurance. Multiplexing of 469 different service classes is possible. Except for the initial 470 aircraft cell-entry and a Type 1 handover, which is not discussed 471 here, medium access is deterministic, with predictable performance. 472 Optional support for adaptive coding and modulation is provided as 473 well. The four functional blocks of the LDACS DLL are organised into 474 two sub-layers, the MAC sub-layer and the Logical Link Control (LLC) 475 sub-layer discussed in the next sections. [GRA19]. 477 Figure 3 shows the protocol stack of LDACS as implemented in the AS 478 and GS. 480 IPv6 network layer 481 | 482 | 483 +------------------+ +----+ 484 | SNP |--| | sub-network 485 | | | | layer 486 +------------------+ | | 487 | | LME| 488 +------------------+ | | 489 | DLS | | | logical link 490 | | | | control layer 491 +------------------+ +----+ 492 | | 493 DCH DCCH/CCCH 494 | RACH/BCCH 495 | | 496 +--------------------------+ 497 | MAC | medium access 498 | | layer 499 +--------------------------+ 500 | 501 +--------------------------+ 502 | PHY | physical layer 503 +--------------------------+ 504 | 505 | 506 ((*)) 507 FL/RL radio channels 508 separated by FDD 510 Figure 3: LDACS protocol stack 512 6.1.1. Medium Access Control (MAC) Entity Services 514 Time Framing Service: The MAC time framing service provides the frame 515 structure necessary to realise slot-based Time Division Multiplex 516 (TDM) access on the physical link. It provides the functions for the 517 synchronisation of the MAC framing structure and the PHY layer 518 framing. The MAC time framing provides a dedicated time slot for 519 each logical channel. [GRA19] 521 Medium Access Service: The MAC sub-layer offers access to the 522 physical channel to its service users. Channel access is provided 523 through transparent logical channels. The MAC sub-layer maps logical 524 channels onto the appropriate slots and manages the access to these 525 channels. Logical channels are used as interface between the MAC and 526 LLC sub-layers. [GRA19] 528 6.1.2. Data Link Service (DLS) Entity Services 530 The DLS provides acknowledged and unacknowledged (including broadcast 531 and packet mode voice) bi-directional exchange of user data. If user 532 data is transmitted using the acknowledged data link service, the 533 sending DLS entity will wait for an acknowledgement from the 534 receiver. If no acknowledgement is received within a specified time 535 frame, the sender may automatically try to retransmit its data. 536 However, after a certain number of failed retries, the sender will 537 suspend further retransmission attempts and inform its client of the 538 failure. [GRA19] 540 6.1.3. Voice Interface (VI) Services 542 The VI provides support for virtual voice circuits. Voice circuits 543 may either be set-up permanently by the GS (e.g. to emulate voice 544 party line) or may be created on demand. The creation and selection 545 of voice circuits is performed in the LME. The VI provides only the 546 transmission services. [GRA19] 548 6.1.4. Link Management Entity (LME) Services 550 Mobility Management Service: The mobility management service provides 551 support for registration and de-registration (cell entry and cell 552 exit), scanning RF channels of neighbouring cells and handover 553 between cells. In addition, it manages the addressing of aircraft/ 554 ASs within cells. It is controlled by the network management service 555 in the GSC. [GRA19] 557 Resource Management Service: The resource management service provides 558 link maintenance (power, frequency and time adjustments), support for 559 adaptive coding and modulation (ACM), and resource allocation. 560 [GRA19] 562 6.1.5. Sub-Network Protocol (SNP) Services 564 Data Link Service: The data link service provides functions required 565 for the transfer of user plane data and control plane data over the 566 LDACS sub-network. [GRA19] 568 Security Service: The security service shall provide functions for 569 secure communication over the LDACS sub-network. Note that the SNP 570 security service applies cryptographic measures as configured by the 571 ground station controller. [GRA19] 573 6.2. LDACS Logical Communication Channels 575 Data Link Service: The data link service provides functions required 576 for the transfer of user plane data and control plane data over the 577 LDACS sub-network. [GRA19] 579 In order to communicate, LDACS uses several logical channels in the 580 MAC layer [GRA19]: 582 1. The GS announces its existence and several necessary physical 583 parameters in the Broadcast Channel (BCCH) to incoming AS. 584 2. The Random Access Channel (RACH) enables the AS to request access 585 to an LDACS cell. 586 3. In the Forward Link (FL) the Common Control Channel (CCCH) is 587 used by the GS to distribute and grant access to system resources. 588 4. The reverse direction is covered by the Reverse Link (RL), where 589 aircraft need to request resources (in so called resource 590 allocation) in order to be allowed to send. This happens via the 591 Dedicated Common Control Channel (DCCH). 592 5. User data itself is communicated in the Data Channel (DCH) on the 593 FL and RL. 595 Figure 4 shows in detail the distribution of each slot. The LDACS 596 super-frame is repeated every 240 ms and carries all control plane 597 and user plane logical channels in separate slots of variable length. 599 ^ 600 | +-------------+------+-------------+ 601 | FL | DCH | CCCH | DCH | 602 | +-------------+------+-------------+ 603 | <---- Multi-Frame (MF) - 58.32ms --> 604 F 605 R +------+---------------------------+ 606 e RL | DCCH | DCH | 607 q +------+---------------------------+ 608 u <---- Multi-Frame (MF) - 58.32ms --> 609 e 610 n +------+------------+------------+------------+------------+ 611 c FL | BCCH | MF | MF | MF | MF | 612 y +------+------------+------------+------------+------------+ 613 | <---------------- Super-Frame (SF) - 240ms ----------------> 614 | 615 | +------+------------+------------+------------+------------+ 616 | RL | RACH | MF | MF | MF | MF | 617 | +------+------------+------------+------------+------------+ 618 | <---------------- Super-Frame (SF) - 240ms ----------------> 619 | 620 ----------------------------- Time ------------------------------> 621 | 623 Figure 4: LDACS frame structure 625 6.3. LDASC Framing Structure 627 The LDACS framing structure for FL and RL is based on Super-Frames 628 (SF) of 240 ms duration. Each SF corresponds to 2000 OFDM symbols. 629 The FL and RL SF boundaries are aligned in time (from the view of the 630 GS). 632 6.3.1. Forward Link 634 In the FL, an SF contains a Broadcast Frame of duration TBC = 6.72 ms 635 (56 OFDM symbols), and four Multi-Frames (MF), each of duration TMF = 636 58.32 ms (486 OFDM symbols). 638 6.3.2. Reverse Link 640 In the RL, each SF starts with a Random Access (RA) message of length 641 TRA = 6.72 ms with two opportunities for sending reverse link random 642 access frames, followed by four MFs. These MFs have the same fixed 643 duration of TMF = 58.32 ms as in the FL, but a different internal 644 structure. 646 7. Security Considerations 648 Aviation will require secure exchanges of data and voice messages for 649 managing the air-traffic flow safely through the airspaces all over 650 the world. The main communication method for ATC today is still an 651 open analogue voice broadcast within the aeronautical VHF band. 652 Currently, the information security is purely procedural based by 653 using well-trained personnel and proven communications procedures. 654 This communication method has been in service since 1948. Future 655 digital communications waveforms will need additional embedded 656 security features to fulfil modern information security requirements 657 like authentication and integrity. These security features require 658 sufficient bandwidth which is beyond the capabilities of a VHF 659 narrowband communications system. For voice and data communications, 660 sufficient data throughput capability is needed to support the 661 security functions while not degrading performance. LDACS is a 662 mature data link technology with sufficient bandwidth to support 663 security. 665 Security considerations for LDACS are the official ICAO SARPS 666 [ICAO18]: 668 1. LDACS shall provide a capability to protect the availability and 669 continuity of the system. 670 2. LDACS shall provide a capability including cryptographic 671 mechanisms to protect the integrity of messages in transit. 672 3. LDACS shall provide a capability to ensure the authenticity of 673 messages in transit. 674 4. LDACS should provide a capability for nonrepudiation of origin 675 for messages in transit. 676 5. LDACS should provide a capability to protect the confidentiality 677 of messages in transit. 678 6. LDACS shall provide an authentication capability. 679 7. LDACS shall provide a capability to authorize the permitted 680 actions of users of the system and to deny actions that are not 681 explicitly authorized. 682 8. If LDACS provides interfaces to multiple domains, LDACS shall 683 provide capability to prevent the propagation of intrusions within 684 LDACS domains and towards external domains. 686 The cybersecurity architecture of LDACS [ICAO18], [MAE18] and its 687 extensions [MAE191], [MAE192] regard all of the aforementioned 688 requirements, since LDACS has been mainly designed for air traffic 689 management communication. Thus it supports mutual entity 690 authentication, integrity and confidentiality capabilities of user 691 data messages and some control channel protection capabilities 692 [MAE192]. 694 More details can be found here [MAE18], [MAE192] and [ICAO18]. 696 From the very beginning of the development process security for LDACS 697 has been addressed by design and thus meets the security objectives 698 as standardized by ICAO [ICAO18]. 700 8. Privacy Considerations 702 LDACS provides a Quality of Service (QoS), and the generic 703 considerations for such mechanisms apply. 705 9. IANA Considerations 707 This memo includes no request to IANA. 709 10. Acknowledgements 711 The authors want to thank all contributors to the development of 712 LDACS. Further, thanks to SBA Research Vienna for fruitful 713 discussions on aeronautical communications concerning security 714 incentives for industry and potential economic spillovers. 716 11. Normative References 718 12. Informative References 720 [MAE191] Maeurer, N., Graeupl, T., and C. Schmitt, "Evaluation of 721 the LDACS Cybersecurity Implementation", IEEE 38th Digital 722 Avionics Systems Conference (DACS), pp. 1-10, New York, 723 NY, USA , November 2019. 725 [MAE192] Maeurer, N. and C. Schmitt, "Towards Successful 726 Realization of the LDACS Cybersecurity Architecture: An 727 Updated Datalink Security Threat- and Risk Analysis", IEEE 728 Integrated Communications, Navigation and Surveillance 729 Conference (ICNS), pp. 1-13, New York, NY, USA , November 730 2019. 732 [GRA19] Graeupl, T., Rihacek, C., and B. Haindl, "LDACS A/G 733 Specification", German Aerospace Center (DLR), Germany, 734 SESAR2020 PJ14-02-01 D3.3.010 , 2017. 736 [MAE18] Maeurer, N. and A. Bilzhause, "A Cybersecurity 737 Architecture for the L-band Digital Aeronautical 738 Communications System (LDACS)", IEEE 37th Digital Avionics 739 Systems Conference (DASC), pp. 1-10, New York, NY, USA , 740 2017. 742 [GRA11] Graeupel, T. and M. Ehammer, "L-DACS1 Data Link Layer 743 Evolution of ATN/IPS", 30th IEEE/AIAA Digital Avionics 744 Systems Conference (DASC), pp. 1-28, New York, NY, USA , 745 2011. 747 [GRA18] Graeupel, T., Schneckenburger, N., Jost, T., Schnell, M., 748 Filip, A., Bellido-Manganell, M.A., Mielke, D.M., Maeurer, 749 N., Kumar, R., Osechas, O., and G. Battista, "L-band 750 Digital Aeronautical Communications System (LDACS) flight 751 trials in the national German project MICONAV", Integrated 752 Communications, Navigation, Surveillance Conference 753 (ICNS), pp. 1-7, New York, NY, USA , 2018. 755 [SCH191] Schnell, M., "DLR Tests Digital Communications 756 Technologies Combined with Additional Navigation Functions 757 for the First Time", November 2019. 759 [SCH192] Schnell, M., "Update on LDACS - The FCI Terrestrial Data 760 Link", 19th Integrated Communications, Navigation and 761 Surveillance Conference (ICNS), pp. 1-10, New York, NY, 762 USA , November 2019. 764 [ICAO18] International Civil Aviation Organization (ICAO), "L-Band 765 Digital Aeronautical Communication System (LDACS)", 766 International Standards and Recommended Practices Annex 10 767 - Aeronautical Telecommunications, Vol. III - 768 Communication Systems , 2018. 770 [SAJ14] Sajatovic, M., Guenzel, H., and S. Mueller, "WA04 D22 Test 771 Report for Assessing LDACS1 Transmitter Impact upon DME/ 772 TACAN Receivers", 19th Integrated Communications, 773 Navigation and Surveillance Conference (ICNS), pp. 1-10, 774 New York, NY, USA , 2014. 776 [RIH18] Rihacek, C., Haindl, B., Fantappie, P., Pierattelli, S., 777 Graeupl, T., Schnell, M., and N. Fistas, "LDACS A/G 778 Specification", Integrated Communications Navigation and 779 Surveillance Conference (ICNS), pp. 1-8, New York, NY, 780 USA , 2018. 782 [RAW-TECHNOS] 783 Thubert, P., Cavalcanti, D., Vilajosana, X., and C. 784 Schmitt, "Reliable and Available Wireless Technologies", 785 Work in Progress, Internet-Draft, draft-thubert-raw- 786 technologies-03, 1 July 2019, 787 . 790 [RAW-USE-CASES] 791 Papadopoulos, G., Thubert, P., Theoleyre, F., and C. 792 Bernardos, "RAW use cases", Work in Progress, Internet- 793 Draft, draft-bernardos-raw-use-cases-00, 5 July 2019, 794 . 797 Authors' Addresses 799 Nils Maeurer (editor) 800 German Aerospace Center (DLR) 801 Muenchner Strasse 20 802 82234 Wessling 803 Germany 805 Email: Nils.Maeurer@dlr.de 807 Thomas Graeupl (editor) 808 German Aerospace Center (DLR) 809 Muenchner Strasse 20 810 82234 Wessling 811 Germany 813 Email: Thomas.Graeupl@dlr.de 815 Corinna Schmitt (editor) 816 Research Institute CODE, UniBwM 817 Werner-Heisenberg-Weg 28 818 85577 Neubiberg 819 Germany 821 Email: corinna.schmitt@unibw.de