L-band Digital Aeronautical Communications System (LDACS)
draft-maeurer-raw-ldacs-00

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RAW                                                      N. Maeurer, Ed.
Internet-Draft                                           T. Graeupl, Ed.
Intended status: Informational             German Aerospace Center (DLR)
Expires: 8 May 2020                                      C. Schmitt, Ed.
                                         Research Institute CODE, UniBwM
                                                         5 November 2019

       L-band Digital Aeronautical Communications System (LDACS)
                       draft-maeurer-raw-ldacs-00

Abstract

   This document provides an overview of the architecture of the L-band
   Digital Aeronautical Communications System (LDACS), which provides a
   secure, scalable and spectrum efficient terrestrial data link for
   civil aviation.  LDACS is a scheduled, reliable multi-application
   cellular broadband system with support for IPv6.

Status of This Memo

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   as described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
     2.1.  Terms used in this document . . . . . . . . . . . . . . .   3
   3.  Motivation and Use Cases  . . . . . . . . . . . . . . . . . .   4
     3.1.  Voice Communications Today  . . . . . . . . . . . . . . .   5
     3.2.  Data Communications Today . . . . . . . . . . . . . . . .   5
   4.  Provenance and Documents  . . . . . . . . . . . . . . . . . .   6
   5.  Characteristics . . . . . . . . . . . . . . . . . . . . . . .   7
     5.1.  LDACS Physical Layer  . . . . . . . . . . . . . . . . . .   7
     5.2.  LDACS Data Link Layer . . . . . . . . . . . . . . . . . .   8
     5.3.  LDACS Data Rates  . . . . . . . . . . . . . . . . . . . .   8
     5.4.  Reliability and Availability  . . . . . . . . . . . . . .   8
       5.4.1.  LDACS Medium Access . . . . . . . . . . . . . . . . .   8
       5.4.2.  LDACS Resource Allocation . . . . . . . . . . . . . .   9
       5.4.3.  LDACS Handovers . . . . . . . . . . . . . . . . . . .   9
   6.  Architecture  . . . . . . . . . . . . . . . . . . . . . . . .  10
     6.1.  Protocol Stack  . . . . . . . . . . . . . . . . . . . . .  10
       6.1.1.  Medium Access Control (MAC) Entity Services . . . . .  12
       6.1.2.  Data Link Service (DLS) Entity Services . . . . . . .  13
       6.1.3.  Voice Interface (VI) Services . . . . . . . . . . . .  13
       6.1.4.  Link Management Entity (LME) Services . . . . . . . .  13
       6.1.5.  Sub-Network Protocol (SNP) Services . . . . . . . . .  13
     6.2.  LDACS Logical Communication Channels  . . . . . . . . . .  14
     6.3.  LDASC Framing Structure . . . . . . . . . . . . . . . . .  15
       6.3.1.  Forward Link  . . . . . . . . . . . . . . . . . . . .  15
       6.3.2.  Reverse Link  . . . . . . . . . . . . . . . . . . . .  15
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  16
   8.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .  17
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  17
   10. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  17
   11. Normative References  . . . . . . . . . . . . . . . . . . . .  17
   12. Informative References  . . . . . . . . . . . . . . . . . . .  17
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  19

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1.  Introduction

   One of the main pillars of the modern Air Traffic Management (ATM)
   system is the existence of a communication infrastructure that
   enables efficient aircraft guidance and safe separation in all phases
   of flight.  Current systems are technically mature but suffering from
   the VHF band's increasing saturation in high-density areas and the
   limitations posed by analogue radio.  Therefore, aviation globally
   and the European Union (EU) in particular, strives for a sustainable
   modernization of the aeronautical communication infrastructure.

   In the long-term, ATM communication shall transition from analogue
   VHF voice and VDL2 communication to more spectrum efficient digital
   data communication.  The European ATM Master Plan foresees this
   transition to be realized for terrestrial communications by the
   development and implementation of the L-band Digital Aeronautical
   Communications System (LDACS).  LDACS shall enable IPv6 based air-
   ground communication related to the safety and regularity of the
   flight.  The particular challenge is that no new frequencies can be
   made available for terrestrial aeronautical communication.  It was
   thus necessary to develop procedures to enable the operation of LDACS
   in parallel with other services in the same frequency band.

2.  Terminology

2.1.  Terms used in this document

   The following terms are used in the context of DetNet in this
   document:

   A/A  Air-To-Air
   AeroMACS  Aeronautical Mobile Airport Communication System
   A/G  Air-To-Ground
   AM(R)S  Aeronautical Mobile (Route) Service
   ANSP  Air traffic Network Service Provider
   AOC  Aeronautical Operational Control
   AS  Aircraft Station
   ATC  Air-Traffic Control
   ATM  Air-Traffic Management
   ATN  Aeronautical Telecommunication Network
   ATS  Air Traffic Service
   CCCH  Common Control Channel
   DCCH  Dedicated Control Channel
   DCH  Data Channel
   DLL  Data Link Layer
   DLS  Data Link Service
   DME  Distance Measuring Equipment
   DSB-AM  Double Side-Band Amplitude Modulation

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   FAA  Federal Aviation Administration
   FCI  Future Communication Infrastructure
   FDD  Frequency Division Duplex
   FL  Forward Link
   GANP  Global Air Navigation Plan
   GNSS  Global Navigation Satellite System
   GS  Ground Station
   GSC  Ground-Station Controller
   HF  High Frequency
   ICAO  International Civil Aviation Organization
   IWF  Interworking Function
   kbit/s  kilobit per secong
   LDACS  L-band Digital Aeronautical Communications System
   LLC  Logical Link Layer
   LME  LDACS Management Entity
   MAC  Medium Access Layer
   MF  Multi Frame
   MIMO  Multiple Input Multiple Output
   OFDM  Orthogonal Frequency-Division Multiplexing
   OFDMA  Orthogonal Frequency-Division Multiplexing Access
   PDU  Protocol Data Units
   PHY  Physical Layer
   QoS  Quality of Service
   RL  Reverse Link
   SARPs  Standards And Recommended Practices
   SESAR  Single European Sky ATM Research
   SF  Super-Frame
   SNP  Sub-Network Protocol
   SSB-AM  Single Side-Band Amplitude Modulation
   SNDCF  Sub-Network Dependent Convergence Function
   TBO  Trajectory-Based Operations
   TDM  Time Division Multiplexing
   TDMA  Time-Division Multiplexing-Access
   VDL2  VHF Data Link mode 2
   VHF  Very High Frequency
   VI  Voice Interface

3.  Motivation and Use Cases

   Aircraft are currently connected to Air-Traffic Control (ATC) and
   Airline Operational Control (AOC) via voice and data communications
   systems through all phases of a flight.  Within the airport terminal,
   connectivity is focused on high bandwidth communications, while
   during en-route high reliability, robustness, and range is the main
   focus.  Voice communications may use the same or different equipment
   as data communications systems.  In the following the main
   differences between voice and data communications capabilities are

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   summarized.  The assumed use cases for LDACS completes the list of
   use cases stated in [RAW-USE-CASES] and the list of reliable and
   available wireless technologies presented in [RAW-TECHNOS].

3.1.  Voice Communications Today

   Voice links are used for Air-To-Ground (A/G) and Air-To-Air (A/A)
   communications.  The communication equipment is either ground-based
   working in the High Frequency (HF) or Very High Frequency (VHF)
   frequency band or satellite-based.  All voice communications is
   operated via open broadcast channels without any authentication,
   encryption or other protective measures.  The use of well-proven
   communication procedures via broadcast channels helps to enhance the
   safety of communications by taking into account that other users may
   encounter communication problems and may be supported, if required.
   The main voice communications media is still the analogue VHF Double
   Side-Band Amplitude Modulation (DSB-AM) communications technique,
   supplemented by HF Single Side-Band Amplitude Modulation (SSB-AM) and
   satellite communications for remote and oceanic areas.  DSB-AM has
   been in use since 1948, works reliably and safely, and uses low-cost
   communication equipment.  These are the main reasons why VHF DSB-AM
   communications is still in use, and it is likely that this technology
   will remain in service for many more years.  This however results in
   current operational limitations and becomes impediments in deploying
   new Air-Traffic Management (ATM) applications, such as flight-centric
   operation with point-to-point communications.

3.2.  Data Communications Today

   Like for voice, data communications into the cockpit is currently
   provided by ground-based equipment operating either on HF or VHF
   radio bands or by legacy satellite systems.  All these communication
   systems are using narrowband radio channels with a data throughput
   capacity of some kilobits per second.  While the aircraft is on
   ground some additional communications systems are available, like
   Aeronautical Mobile Airport Communication System (AeroMACS),
   operating in the Airport (APT) domain and able to deliver broadband
   communication capability.

   The data communication networks used for the transmission of data
   relating to the safety and regularity of the flight must be strictly
   isolated from those providing entertainment services to passengers.
   This leads to a situation that the flight crews are supported by
   narrowband services during flight while passengers have access to
   inflight broadband services.  The current HF and VHF data links
   cannot provide broadband services now or in the future, due to the
   lack of available spectrum.  This technical shortcoming is becoming a

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   limitation to enhanced ATM operations, such as Trajectory-Based
   Operations (TBO) and 4D trajectory negotiations.

   Satellite-based communications are currently under investigation and
   enhanced capabilities are under development which will be able to
   provide inflight broadband services and communications supporting the
   safety and regularity of the flight.  In parallel, the ground-based
   broadband data link technology LDACS is being standardized by ICAO
   and has recently shown its maturity during flight tests [SCH191].
   The LDACS technology is scalable, secure and spectrum efficient and
   provides significant advantages to the users and service providers.
   It is expected that both - satellite systems and LDACS - will be
   deployed to support the future aeronautical communication needs as
   envisaged by the ICAO Global Air Navigation Plan (GANP).

4.  Provenance and Documents

   The development of LDACS has already made substantial progress in the
   Single European Sky ATM Research (SESAR) framework, and is currently
   being continued in the follow-up program, SESAR2020 [RIH18].  A key
   objective of the SESAR activities is to develop, implement and
   validate a modern aeronautical data link able to evolve with aviation
   needs over long-term.  To this end, an LDACS specification has been
   produced [GRA19] and is continuously updated; transmitter
   demonstrators were developed to test the spectrum compatibility of
   LDACS with legacy systems operating in the L-band [SAJ14]; and the
   overall system performance was analyzed by computer simulations,
   indicating that LDACS can fulfil the identified requirements [GRA11].

   LDACS standardization within the framework of the ICAO started in
   December 2016.  The ICAO standardization group has produced an
   initial Standards and Recommended Practices (SARPs) document
   [ICAO18].  The SARPs document defines the general characteristics of
   LDACS.  The ICAO standardization group plans to produce an ICAO
   technical manual - the ICAO equivalent to a technical standard -
   within the next years.  Generally, the group is open to input from
   all sources and develops LDACS in the open.

   Up to now the LDACS standardization has been focused on the
   development of the physical layer and the data link layer, only
   recently have higher layers come into the focus of the LDACS
   development activities.  There is currently no "IPv6 over LDACS"
   specification; however, SESAR2020 has started the testing of
   IPv6-based LDACS testbeds.  The IPv6 architecture for the
   aeronautical telecommunication network is called the Future
   Communications Infrastructure (FCI).  FCI shall support quality of
   service, diversity, and mobility under the umbrella of the "multi-
   link concept".  This work is conducted by ICAO working group WG-I.

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   In addition to standardization activities several industrial LDACS
   prototypes have been built.  One set of LDACS prototypes has been
   evaluated in flight trials confirming the theoretical results
   predicting the system performance [GRA18] [SCH191].

5.  Characteristics

   LDACS will become one of several wireless access networks connecting
   aircraft to the Aeronautical Telecommunications Network (ATN).
   Access to the ATN is handled by the Ground-Station Controller (GSC),
   while several Ground-Stations (GS) are connected to one GSC.  Thus
   the LDACS access network contains several GS, each of them providing
   one LDACS radio cell.  LDACS can be therefore considered a cellular
   data link with a star-topology connecting Aircraft-Stations (AS) to
   GS with a full duplex radio link.  Each GS is the centralized
   instance controlling all A/G communications within its radio cell.  A
   GS supports up to 512 aircraft.  All of this is depicted in Figure 1.

   AS11--------------+
                     |
   AS12-------------GS1------GSC------>ATN
   .                 |        |
   .                 |        |
   AS1n--------------+        |
                              |
   AS21--------------+        |
                     |        |
   AS21-------------GS2-------+
   .                 |
   .                 |
   AS2n--------------+

                     Figure 1: LDACS wireless topology

   The LDACS air interface protocol stack defines two layers, the
   physical layer and the data link layer.

5.1.  LDACS Physical Layer

   The physical layer provides the means to transfer data over the radio
   channel.  The LDACS GS supports bi-directional links to multiple
   aircraft under its control.  The forward link direction (FL; ground-
   to-air) and the reverse link direction (RL; air-to-ground) are
   separated by frequency division duplex.  Forward link and reverse
   link use a 500 kHz channel each.  The ground-station transmits a

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   continuous stream of Orthogonal Frequency-Division Multiplexing
   (OFDM) symbols on the forward link.  In the reverse link different
   aircraft are separated in time and frequency using a combination of
   Orthogonal Frequency-Division Multiple-Access (OFDMA) and Time-
   Division Multiple-Access (TDMA).  Aircraft thus transmit
   discontinuously on the reverse link with radio bursts sent in
   precisely defined transmission opportunities allocated by the ground-
   station.  LDACS does not support beam-forming or Multiple Input
   Multiple Output (MIMO) [SCH192].

5.2.  LDACS Data Link Layer

   The data-link layer provides the necessary protocols to facilitate
   concurrent and reliable data transfer for multiple users.  The LDACS
   data link layer is organized in two sub-layers: The medium access
   sub-layer and the logical link control sub-layer.  The medium access
   sub-layer manages the organization of transmission opportunities in
   slots of time and frequency.  The logical link control sub-layer
   provides acknowledged point-to-point logical channels between the
   aircraft and the ground-station using an automatic repeat request
   protocol.  LDACS supports also unacknowledged point-to-point channels
   and ground-to-air broadcast.

5.3.  LDACS Data Rates

   The user data rate of LDACS is 315 kbit/s to 1428 kbit/s on the
   forward link, and 294 kbit/s to 1390 kbit/s on the reverse link,
   depending on coding and modulation.  Due to strong interference from
   legacy systems in the L-band, the most robust coding and modulation
   should be expected for initial deployment i.e. 315/294 kbit/s on
   theforward/reverse link, respectively.

5.4.  Reliability and Availability

   LDACS has been designed with applications related to the safety and
   regularity of the flight in mind.  It has therefore been designed as
   a deterministic wireless data link (as far as possible).

5.4.1.  LDACS Medium Access

   LDACS medium access is always under the control of the ground-station
   of a radio cell.  Any medium access for the transmission of user data
   has to be requested with a resource request message stating the
   requested amount of resources and class of service.  The ground-
   station performs resource scheduling on the basis of these requests
   and grants resources with resource allocation messages.  Resource
   request and allocation messages are exchanged over dedicated
   contention-free control channels.

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5.4.2.  LDACS Resource Allocation

   LDACS has two mechanisms to request resources from the scheduler in
   the ground-station.  Resources can either be requested "on demand"
   with a given class of service.  On the forward link, this is done
   locally in the ground-station, on the reverse link a dedicated
   contention-free control channel is used called Dedicated Control
   Channel (DCCH); roughly 83 bit every 60 ms).  A resource allocation
   is always announced in the control channel of the forward link
   (Common Control Channel (CCCH); variable sized).  Due to the spacing
   of the reverse link control channels of every 60 ms, a medium access
   delay in the same order of magnitude is to be expected.

   Resources can also be requested "permanently".  The permanent
   resource request mechanism supports requesting recurring resources in
   given time intervals.  A permanent resource request has to be
   canceled by the user (or by the ground-station, which is always in
   control).  User data transmissions over LDACS are therefore always
   scheduled by the ground-station, while control data uses statically
   (i.e. at net entry) allocated recurring resources (DCCH and CCCH).
   The current specification documents specify no scheduling algorithm.
   However performance evaluations so far have used strict priority
   scheduling and round robin for equal priorities for simplicity.  In
   the current prototype implementations LDACS classes of service are
   thus realized as priorities of medium access and not as flows.  Note
   that this can starve out low priority flows.  However, this is not
   seen as a big problem since safety related message always go first in
   any case.  Scheduling of reverse link resources is done in physical
   Protocol Data Units (PDU) of 112 bit (or larger if more aggressive
   coding and modulation is used).  Scheduling on the forward link is
   done Byte-wise since the forward link is transmitted continuously
   bythe ground-station.

5.4.3.  LDACS Handovers

   In order to support diversity, LDACS supports handovers to other
   ground-stations on different channels.  Handovers may be initiated by
   the aircraft (break-before-make) or by the ground-station (make-
   before-break) if it is connected to an alternative ground-station via
   the same ground-station controller.  Beyond this, FCI diversity shall
   be implemented by the multi-link concept.

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6.  Architecture

   Aircraft-Station (AS), Ground-Station (GS) and Ground-Station
   Controller (GSC) form the basic LDACS network. 512 aircraft can be
   served by one GS where the GS sends a continuous data stream in the
   Forward Link (FL) to the AS.  The Reverse Link (RL) consists of
   individual bursts of data from each AS to GS.  This means, for every
   RL communication the AS first needs to request the respective
   resource allocation within its cell from the GS before being able to
   send.  Both FL and RL communication, including user and control data,
   is done via the air gap over the radio link between AS and GS.  On
   the ground a GSC is responsible for serving several GSs on the
   control plane, forming an LDACS sub-network with its LDACS internal
   control plane infrastructure.  The GSs are linked to an access router
   in the user plane, which in turn is linked to an Air/Ground router,
   being now the direct connection to the ground network.  The ATN is
   used for example by Air traffic Network Services Providers (ANSP) and
   airlines to exchange Air Traffic Service (ATS) or Airline Operational
   Control (AOC) data between the ground infrastructure and the
   aircraft.  Figure 2 provides a more detailed overview.

   wireless      user
   link          plane
     A--------------G-------------Access---A/G-----ATN
     S..............S             Router   Router
                   . control      . |
                   . plane        . |
                   .              . |
                  GSC.............. |
                   .                |
                   .                |
                  GS----------------+

                  Figure 2: LDACS sub-network with two GSs

6.1.  Protocol Stack

   The protocol stack of LDACS is implemented in the AS and GS as
   follows: It consists of the Physical Layer (PHY) with five major
   functional blocks above it.  Four are placed in the Data Link Layer
   (DLL) of the AS and GS: (1) Medium Access Layer (MAC), (2) Voice
   Interface (VI), (3) Data Link Service (DLS), (4) LDACS Management
   Entity (LME).  The last entity resides within the sub-network layer:
   Sub-Network Protocol (SNP).

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   The LDACS network is externally connected to voice units, radio
   control units, and the ATN network layer through a Sub-Network
   Dependent Convergence Function (SNDCF; OSI network layers),
   Convergence Sub-layer, or Interworking Function (IWF; legacy
   networks) not discussed here.

   The SNP connects the AS and GS DLL providing end-to-end user plane
   connectivity between the LDACS AS and GS.

   The DLL provides Quality of Service (QoS) assurance.  Multiplexing of
   different service classes is possible.  Except for the initial
   aircraft cell-entry and a Type 1 handover, which is not discussed
   here, medium access is deterministic, with predictable performance.
   Optional support for adaptive coding and modulation is provided as
   well.  The four functional blocks of the LDACS DLL are organised into
   two sub-layers, the MAC sub-layer and the Logical Link Control (LLC)
   sub-layer discussed in the next sections.  [GRA19].

   Figure 3 shows the protocol stack of LDACS as implemented in the AS
   and GS.

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            IPv6                   network layer
             |
             |
   +------------------+  +----+
   |        SNP       |--|    |   sub-network
   |                  |  |    |   layer
   +------------------+  |    |
             |           | LME|
   +------------------+  |    |
   |        DLS       |  |    |   logical link
   |                  |  |    |   control layer
   +------------------+  +----+
             |             |
            DCH         DCCH/CCCH
             |          RACH/BCCH
             |             |
   +--------------------------+
   |           MAC            |   medium access
   |                          |   layer
   +--------------------------+
                |
   +--------------------------+
   |           PHY            |   physical layer
   +--------------------------+
                |
                |
              ((*))
              FL/RL              radio channels
                                 separated by FDD

                       Figure 3: LDACS protocol stack

6.1.1.  Medium Access Control (MAC) Entity Services

   Time Framing Service: The MAC time framing service provides the frame
   structure necessary to realise slot-based Time Division Multiplex
   (TDM) access on the physical link.  It provides the functions for the
   synchronisation of the MAC framing structure and the PHY layer
   framing.  The MAC time framing provides a dedicated time slot for
   each logical channel.  [GRA19]

   Medium Access Service: The MAC sub-layer offers access to the
   physical channel to its service users.  Channel access is provided
   through transparent logical channels.  The MAC sub-layer maps logical
   channels onto the appropriate slots and manages the access to these

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   channels.  Logical channels are used as interface between the MAC and
   LLC sub-layers.  [GRA19]

6.1.2.  Data Link Service (DLS) Entity Services

   The DLS provides acknowledged and unacknowledged (including broadcast
   and packet mode voice) bi-directional exchange of user data.  If user
   data is transmitted using the acknowledged data link service, the
   sending DLS entity will wait for an acknowledgement from the
   receiver.  If no acknowledgement is received within a specified time
   frame, the sender may automatically try to retransmit its data.
   However, after a certain number of failed retries, the sender will
   suspend further retransmission attempts and inform its client of the
   failure.  [GRA19]

6.1.3.  Voice Interface (VI) Services

   The VI provides support for virtual voice circuits.  Voice circuits
   may either be set-up permanently by the GS (e.g. to emulate voice
   party line) or may be created on demand.  The creation and selection
   of voice circuits is performed in the LME.  The VI provides only the
   transmission services.  [GRA19]

6.1.4.  Link Management Entity (LME) Services

   Mobility Management Service: The mobility management service provides
   support for registration and de-registration (cell entry and cell
   exit), scanning RF channels of neighbouring cells and handover
   between cells.  In addition, it manages the addressing of aircraft/
   ASs within cells.  It is controlled by the network management service
   in the GSC.  [GRA19]

   Resource Management Service: The resource management service provides
   link maintenance (power, frequency and time adjustments), support for
   adaptive coding and modulation (ACM), and resource allocation.
   [GRA19]

6.1.5.  Sub-Network Protocol (SNP) Services

   Data Link Service: The data link service provides functions required
   for the transfer of user plane data and control plane data over the
   LDACS sub-network.  [GRA19]

   Security Service: The security service shall provide functions for
   secure communication over the LDACS sub-network.  Note that the SNP
   security service applies cryptographic measures as configured by the
   ground station controller.  [GRA19]

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6.2.  LDACS Logical Communication Channels

   Data Link Service: The data link service provides functions required
   for the transfer of user plane data and control plane data over the
   LDACS sub-network.  [GRA19]

   In order to communicate, LDACS uses several logical channels in the
   MAC layer [GRA19]:

   1.  The GS announces its existence and several necessary physical
      parameters in the Broadcast Channel (BCCH) to incoming AS.
   2.  The Random Access Channel (RACH) enables the AS to request access
      to an LDACS cell.
   3.  In the Forward Link (FL) the Common Control Channel (CCCH) is
      used by the GS to distribute and grant access to system resources.
   4.  The reverse direction is covered by the Reverse Link (RL), where
      aircraft need to request resources (in so called resource
      allocation) in order to be allowed to send.  This happens via the
      Dedicated Common Control Channel (DCCH).
   5.  User data itself is communicated in the Data Channel (DCH) on the
      FL and RL.

   Figure 4 shows in detail the distribution of each slot.  The LDACS
   super-frame is repeated every 240 ms and carries all control plane
   and user plane logical channels in separate slots of variable length.

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   ^
   |     +-------------+------+-------------+
   |  FL |     DCH     | CCCH |     DCH     |
   |     +-------------+------+-------------+
   |     <---- Multi-Frame (MF) - 58.32ms -->
   F
   R     +------+---------------------------+
   e  RL | DCCH |             DCH           |
   q     +------+---------------------------+
   u     <---- Multi-Frame (MF) - 58.32ms -->
   e
   n     +------+------------+------------+------------+------------+
   c  FL | BCCH |     MF     |     MF     |     MF     |     MF     |
   y     +------+------------+------------+------------+------------+
   |     <---------------- Super-Frame (SF) - 240ms ---------------->
   |
   |     +------+------------+------------+------------+------------+
   |  RL | RACH |     MF     |     MF     |     MF     |     MF     |
   |     +------+------------+------------+------------+------------+
   |     <---------------- Super-Frame (SF) - 240ms ---------------->
   |
   ----------------------------- Time ------------------------------>
   |

                      Figure 4: LDACS frame structure

6.3.  LDASC Framing Structure

   The LDACS framing structure for FL and RL is based on Super-Frames
   (SF) of 240 ms duration.  Each SF corresponds to 2000 OFDM symbols.
   The FL and RL SF boundaries are aligned in time (from the view of the
   GS).

6.3.1.  Forward Link

   In the FL, an SF contains a Broadcast Frame of duration TBC = 6.72 ms
   (56 OFDM symbols), and four Multi-Frames (MF), each of duration TMF =
   58.32 ms (486 OFDM symbols).

6.3.2.  Reverse Link

   In the RL, each SF starts with a Random Access (RA) message of length
   TRA = 6.72 ms with two opportunities for sending reverse link random
   access frames, followed by four MFs.  These MFs have the same fixed
   duration of TMF = 58.32 ms as in the FL, but a different internal
   structure.

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7.  Security Considerations

   Aviation will require secure exchanges of data and voice messages for
   managing the air-traffic flow safely through the airspaces all over
   the world.  The main communication method for ATC today is still an
   open analogue voice broadcast within the aeronautical VHF band.
   Currently, the information security is purely procedural based by
   using well-trained personnel and proven communications procedures.
   This communication method has been in service since 1948.  Future
   digital communications waveforms will need additional embedded
   security features to fulfil modern information security requirements
   like authentication and integrity.  These security features require
   sufficient bandwidth which is beyond the capabilities of a VHF
   narrowband communications system.  For voice and data communications,
   sufficient data throughput capability is needed to support the
   security functions while not degrading performance.  LDACS is a
   mature data link technology with sufficient bandwidth to support
   security.

   Security considerations for LDACS are the official ICAO SARPS
   [ICAO18]:

   1.  LDACS shall provide a capability to protect the availability and
      continuity of the system.
   2.  LDACS shall provide a capability including cryptographic
      mechanisms to protect the integrity of messages in transit.
   3.  LDACS shall provide a capability to ensure the authenticity of
      messages in transit.
   4.  LDACS should provide a capability for nonrepudiation of origin
      for messages in transit.
   5.  LDACS should provide a capability to protect the confidentiality
      of messages in transit.
   6.  LDACS shall provide an authentication capability.
   7.  LDACS shall provide a capability to authorize the permitted
      actions of users of the system and to deny actions that are not
      explicitly authorized.
   8.  If LDACS provides interfaces to multiple domains, LDACS shall
      provide capability to prevent the propagation of intrusions within
      LDACS domains and towards external domains.

   The cybersecurity architecture of LDACS [ICAO18], [MAE18] and its
   extensions [MAE191], [MAE192] regard all of the aforementioned
   requirements, since LDACS has been mainly designed for air traffic
   management communication.  Thus it supports mutual entity
   authentication, integrity and confidentiality capabilities of user
   data messages and some control channel protection capabilities
   [MAE192].

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   More details can be found here [MAE18], [MAE192] and [ICAO18].

   From the very beginning of the development process security for LDACS
   has been addressed by design and thus meets the security objectives
   as standardized by ICAO [ICAO18].

8.  Privacy Considerations

   LDACS provides a Quality of Service (QoS), and the generic
   considerations for such mechanisms apply.

9.  IANA Considerations

   This memo includes no request to IANA.

10.  Acknowledgements

   The authors want to thank all contributors to the development of
   LDACS.  Further, thanks to SBA Research Vienna for fruitful
   discussions on aeronautical communications concerning security
   incentives for industry and potential economic spillovers.

11.  Normative References

12.  Informative References

   [MAE191]   Maeurer, N., Graeupl, T., and C. Schmitt, "Evaluation of
              the LDACS Cybersecurity Implementation", IEEE 38th Digital
              Avionics Systems Conference (DACS), pp. 1-10, New York,
              NY, USA , November 2019.

   [MAE192]   Maeurer, N. and C. Schmitt, "Towards Successful
              Realization of the LDACS Cybersecurity Architecture: An
              Updated Datalink Security Threat- and Risk Analysis", IEEE
              Integrated Communications, Navigation and Surveillance
              Conference (ICNS), pp. 1-13, New York, NY, USA , November
              2019.

   [GRA19]    Graeupl, T., Rihacek, C., and B. Haindl, "LDACS A/G
              Specification", German Aerospace Center (DLR), Germany,
              SESAR2020 PJ14-02-01 D3.3.010 , 2017.

   [MAE18]    Maeurer, N. and A. Bilzhause, "A Cybersecurity
              Architecture for the L-band Digital Aeronautical
              Communications System (LDACS)", IEEE 37th Digital Avionics
              Systems Conference (DASC), pp. 1-10, New York, NY, USA ,
              2017.

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   [GRA11]    Graeupel, T. and M. Ehammer, "L-DACS1 Data Link Layer
              Evolution of ATN/IPS", 30th IEEE/AIAA Digital Avionics
              Systems Conference (DASC), pp. 1-28, New York, NY, USA ,
              2011.

   [GRA18]    Graeupel, T., Schneckenburger, N., Jost, T., Schnell, M.,
              Filip, A., Bellido-Manganell, M.A., Mielke, D.M., Maeurer,
              N., Kumar, R., Osechas, O., and G. Battista, "L-band
              Digital Aeronautical Communications System (LDACS) flight
              trials in the national German project MICONAV", Integrated
              Communications, Navigation, Surveillance Conference
              (ICNS), pp. 1-7, New York, NY, USA , 2018.

   [SCH191]   Schnell, M., "DLR Tests Digital Communications
              Technologies Combined with Additional Navigation Functions
              for the First Time", November 2019.

   [SCH192]   Schnell, M., "Update on LDACS - The FCI Terrestrial Data
              Link", 19th Integrated Communications, Navigation and
              Surveillance Conference (ICNS), pp. 1-10, New York, NY,
              USA , November 2019.

   [ICAO18]   International Civil Aviation Organization (ICAO), "L-Band
              Digital Aeronautical Communication System (LDACS)",
              International Standards and Recommended Practices Annex 10
              - Aeronautical Telecommunications, Vol. III -
              Communication Systems , 2018.

   [SAJ14]    Sajatovic, M., Guenzel, H., and S. Mueller, "WA04 D22 Test
              Report for Assessing LDACS1 Transmitter Impact upon DME/
              TACAN Receivers", 19th Integrated Communications,
              Navigation and Surveillance Conference (ICNS), pp. 1-10,
              New York, NY, USA , 2014.

   [RIH18]    Rihacek, C., Haindl, B., Fantappie, P., Pierattelli, S.,
              Graeupl, T., Schnell, M., and N. Fistas, "LDACS A/G
              Specification", Integrated Communications Navigation and
              Surveillance Conference (ICNS), pp. 1-8, New York, NY,
              USA , 2018.

   [RAW-TECHNOS]
              Thubert, P., Cavalcanti, D., Vilajosana, X., and C.
              Schmitt, "Reliable and Available Wireless Technologies",
              Work in Progress, Internet-Draft, draft-thubert-raw-
              technologies-03, 1 July 2019,
              <https://tools.ietf.org/html/draft-thubert-raw-
              technologies-03>.

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   [RAW-USE-CASES]
              Papadopoulos, G., Thubert, P., Theoleyre, F., and C.
              Bernardos, "RAW use cases", Work in Progress, Internet-
              Draft, draft-bernardos-raw-use-cases-00, 5 July 2019,
              <https://tools.ietf.org/html/draft-bernardos-raw-use-
              cases-00>.

Authors' Addresses

   Nils Maeurer (editor)
   German Aerospace Center (DLR)
   Muenchner Strasse 20
   82234 Wessling
   Germany

   Email: Nils.Maeurer@dlr.de

   Thomas Graeupl (editor)
   German Aerospace Center (DLR)
   Muenchner Strasse 20
   82234 Wessling
   Germany

   Email: Thomas.Graeupl@dlr.de

   Corinna Schmitt (editor)
   Research Institute CODE, UniBwM
   Werner-Heisenberg-Weg 28
   85577 Neubiberg
   Germany

   Email: corinna.schmitt@unibw.de

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