L-band Digital Aeronautical Communications System (LDACS)
draft-maeurer-raw-ldacs-04
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| Authors | Nils Mäurer , Thomas Gräupl , Corinna Schmitt | ||
| Last updated | 2020-07-02 (Latest revision 2020-06-03) | ||
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draft-maeurer-raw-ldacs-04
RAW N. Maeurer, Ed.
Internet-Draft T. Graeupl, Ed.
Intended status: Informational German Aerospace Center (DLR)
Expires: 3 January 2021 C. Schmitt, Ed.
Research Institute CODE, UniBwM
2 July 2020
L-band Digital Aeronautical Communications System (LDACS)
draft-maeurer-raw-ldacs-04
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. LDACS shall provide
a data link for IP network-based aircraft guidance. High reliability
and availability for IP connectivity over LDACS are therefore
essential.
Status of This Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on 3 January 2021.
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Please review these documents carefully, as they describe your rights
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 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. Applicability . . . . . . . . . . . . . . . . . . . . . . . . 7
5.1. Advances Beyond the State-of-the-Art . . . . . . . . . . 7
5.1.1. Priorities . . . . . . . . . . . . . . . . . . . . . 7
5.1.2. Security . . . . . . . . . . . . . . . . . . . . . . 8
5.1.3. High Data Rates . . . . . . . . . . . . . . . . . . . 8
5.2. Application . . . . . . . . . . . . . . . . . . . . . . . 8
5.2.1. Air-to-Ground Multilink . . . . . . . . . . . . . . . 8
5.2.2. Air-to-Air Extension for LDACS . . . . . . . . . . . 9
5.2.3. Flight Guidance . . . . . . . . . . . . . . . . . . . 9
5.2.4. Business Communication of Airlines . . . . . . . . . 10
5.2.5. LDACS Navigation . . . . . . . . . . . . . . . . . . 10
6. Characteristics of LDACS . . . . . . . . . . . . . . . . . . 11
6.1. LDACS Sub-Network . . . . . . . . . . . . . . . . . . . . 11
6.2. Topology . . . . . . . . . . . . . . . . . . . . . . . . 12
6.3. LDACS Physical Layer . . . . . . . . . . . . . . . . . . 12
6.4. LDACS Data Link Layer . . . . . . . . . . . . . . . . . . 12
6.5. LDACS Mobility . . . . . . . . . . . . . . . . . . . . . 13
7. Reliability and Availability . . . . . . . . . . . . . . . . 13
8. Protocol Stack . . . . . . . . . . . . . . . . . . . . . . . 15
8.1. Medium Access Control (MAC) Entity Services . . . . . . . 16
8.2. Data Link Service (DLS) Entity Services . . . . . . . . . 18
8.3. Voice Interface (VI) Services . . . . . . . . . . . . . . 19
8.4. LDACS Management Entity (LME) Services . . . . . . . . . 19
8.5. Sub-Network Protocol (SNP) Services . . . . . . . . . . . 19
9. Security Considerations . . . . . . . . . . . . . . . . . . . 19
10. Privacy Considerations . . . . . . . . . . . . . . . . . . . 20
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20
12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 20
13. Normative References . . . . . . . . . . . . . . . . . . . . 21
14. Informative References . . . . . . . . . . . . . . . . . . . 21
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 23
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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 control 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 communications. 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 potential implementation) of the L-band Digital
Aeronautical Communications System (LDACS). LDACS shall enable IPv6
based air- ground communication related to the aviation safety and
regularity of flight. The particular challenge is that no additional
spectrum can be made available for terrestrial aeronautical
communication. It was thus necessary to develop co-existence
mechanism/procedures to enable the interference free operation of
LDACS in parallel with other aeronautical services/systems in the
same frequency band.
Since LDACS shall be used for aircraft guidance, high reliability and
availability for IP connectivity over LDACS are essential.
2. Terminology
The following terms are used in the context of RAW in this document:
A2A Air-to-Air
LDACS A2A LDACS Air-to-Air
AeroMACS Aeronautical Mobile Airport Communication System
A2G 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
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DLL Data Link Layer
DLS Data Link Service
DME Distance Measuring Equipment
DSB-AM Double Side-Band Amplitude Modulation
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
G2A Ground-to-Air
HF High Frequency
ICAO International Civil Aviation Organization
kbit/s kilobit per second
LDACS L-band Digital Aeronautical Communications System
LLC Logical Link Layer
LME LDACS Management Entity
MAC Medium Access Layer
MF Multi Frame
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
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
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as data communications systems. In the following the main
differences between voice and data communications capabilities are
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 (A2G) and Air-to-Air (A2A)
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 VHF and HF voice
communications is operated via open broadcast channels without
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
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 in order of kilobits per second. While the aircraft is on
ground some additional communications systems are available, like
Aeronautical Mobile Airport Communication System (AeroMACS; as of now
not widely used) or public cellular networks, 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
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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
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 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 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
publicly available; however, SESAR2020 has started the testing of
IPv6-based LDACS testbeds.
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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
Communication Panel working group WG-I.
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. Applicability
LDACS is a multi-application cellular broadband system capable of
simultaneously providing various kinds of Air Traffic Services
(including ATS-B3) and Aeronautical Operational Control (AOC)
communications services from deployed Ground Stations (GS). The
LDACS A2G sub-system physical layer and data link layer are optimized
for data link communications, but the system also supports digital
air-ground voice communications.
LDACS supports communication in all airspaces (airport, terminal
maneuvering area, and en-route), and on the airport surface. The
physical LDACS cell coverage is effectively de-coupled from the
operational coverage required for a particular service. This is new
in aeronautical communications. Services requiring wide-area
coverage can be installed at several adjacent LDACS cells. The
handover between the involved LDACS cells is seamless, automatic, and
transparent to the user. Therefore, the LDACS A2G communications
concept enables the aeronautical communication infrastructure to
support future dynamic airspace management concepts.
5.1. Advances Beyond the State-of-the-Art
LDACS offers several capabilities that are not provided in
contemporarily deployed aeronautical communication systems.
5.1.1. Priorities
LDACS is able to manage services priorities, an important feature not
available in some of the current data link deployments. Thus, LDACS
guarantees bandwidth, low latency, and high continuity of service for
safety critical ATS applications while simultaneously accommodating
less safety-critical AOC services.
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5.1.2. Security
LDACS is a secure data link with built-in security mechanisms. It
enables secure data communications for ATS and AOC services,
including secured private communications for aircraft operators and
ANSPs (Air Navigation Service Providers). This includes concepts for
key and trust management, mutual authenticated key exchange
protocols, key derivation measures, user and control message-in-
transit confidentiality and authenticity protection, secure logging
and availability and robustness measures [MAE18], [MAE191], [MAE192].
5.1.3. High Data Rates
The user data rate of LDACS is 315 kbit/s to 1428 kbit/s on the
forward link (Ground-to-Air), and 294 kbit/s to 1390 kbit/s on the
reverse link (Air-to-Ground), depending on coding and modulation.
This is 50 times the amount terrestrial digital aeronautical
communications systems such as VDLm2 provide [SCH191].
5.2. Application
LDACS shall be used by several aeronautical applications ranging from
enhanced communication protocol stacks (multi-homed mobile IPv6
networks in the aircraft and potentially ad-hoc networks between
aircraft) to classical communication applications (sending GBAS
correction data) and integration with other service domains (using
the communication signal for navigation).
5.2.1. Air-to-Ground Multilink
It is expected that LDACS together with upgraded satellite-based
communications systems will be deployed within the Future
Communication Infrastructure (FCI) and constitute one of the main
components of the multilink concept within the FCI.
Both technologies, LDACS and satellite systems, have their specific
benefits and technical capabilities which complement each other.
Especially, satellite systems are well-suited for large coverage
areas with less dense air traffic, e.g. oceanic regions. LDACS is
well-suited for dense air traffic areas, e.g. continental areas or
hot-spots around airports and terminal airspace. In addition, both
technologies offer comparable data link capacity and, thus, are well-
suited for redundancy, mutual back-up, or load balancing.
Technically the FCI multilink concept shall be realized by multi-
homed mobile IPv6 networks in the aircraft. The related protocol
stack is currently under development by ICAO and SESAR.
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5.2.2. Air-to-Air Extension for LDACS
A potential extension of the multi-link concept is its extension to
ad-hoc networks between aircraft.
Direct Air-to-Air (A2A) communication between aircrafts in terms of
ad-hoc data networks is currently considered a research topic since
there is no immediate operational need for it, although several
possible use cases are discussed (digital voice, wake vortex
warnings, and trajectory negotiation) [BELL19]. It should also be
noted that currently deployed analog VHF voice radios support direct
voice communication between aircraft, making a similar use case for
digital voice plausible.
LDACS direct A2A is currently not part of standardization.
5.2.3. Flight Guidance
The FCI (and therefore LDACS) shall be used to host flight guidance.
This is realized using three applications:
1. Context Management (CM): The CM application shall manage the
automatic logical connection to the ATC center currently
responsible to guide the aircraft. Currently this is done by the
air crew manually changing VHF voice frequencies according to the
progress of the flight. The CM application automatically sets up
equivalent sessions.
2. Controller Pilot Data Link Communication (CPDLC): The CPDLC
application provides the air crew with the ability to exchange
data messages similar to text messages with the currently
responsible ATC center. The CPDLC application shall take over
most of the communication currently performed over VHF voice and
enable new services that do not lend themselves to voice
communication (e.g., trajectory negotiation).
3. Automatic Dependent Surveillance - Contract (ADS-C): ADS-C
reports the position of the aircraft to the currently active ATC
center. Reporting is bound to "contracts", i.e. pre-defined
events related to the progress of the flight (i.e. the
trajectory). ADS-C and CPDLC are the primary applications used to
implement in-flight trajectory management.
CM, CPDLC, and ADS-C are available on legacy datalinks, but not
widely deployed and with limited functionality.
Further ATC applications may be ported to use the FCI or LDACS as
well. A notable application is GBAS for secure, automated landings:
The Global Navigation Satellite System (GNSS) based Ground Based
Augmentation System (GBAS) is used to improve the accuracy of GNSS to
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allow GNSS based instrument landings. This is realized by sending
GNSS correction data (e.g., compensating ionospheric errors in the
GNSS signal) to the airborne GNSS receiver via a separate data link.
Currently the VDB data link is used. VDB is a narrow-band single-
purpose datalink without advanced security only used to transmit GBAS
correction data. This makes VDB a natural candidate for replacement
by LDACS.
5.2.4. Business Communication of Airlines
In addition to air traffic services airline operational control (AOC)
services shall be transmitted over LDACS. AOC is a generic term
referring to the business communication of airlines. Regulatory this
is considered related to the safety and regularity of flight and may
therefore be transmitted over LDACS.
AOC communication is considered the main business case for LDACS
communication service providers since modern aircraft generate
significant amounts of data (e.g., engine maintenance data).
5.2.5. LDACS Navigation
Beyond communication radio signals can always also be used for
navigation. LDACS takes this into account.
For future aeronautical navigation, ICAO recommends the further
development of Global Navigation Satellite System (GNSS) based
technologies as primary means for navigation. However, the drawback
of GNSS is its inherent single point of failure - the satellite. Due
to the large separation between navigational satellites and aircraft,
the received power of GNSS signals on the ground is very low. As a
result, GNSS disruptions might occasionally occur due to
unintentional interference, or intentional jamming. Yet the
navigation services must be available with sufficient performance for
all phases of flight. Therefore, during GNSS outages, or blockages,
an alternative solution is needed. This is commonly referred to as
Alternative Positioning, Navigation, and Timing (APNT).
One of such APNT solution consists of integrating the navigation
functionality into LDACS. The ground infrastructure for APNT is
deployed through the implementation of LDACS ground stations and the
navigation capability comes "for free".
LDACS navigation has already been demonstrated in practice in a
flight measurement campaign [SCH191].
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6. Characteristics of LDACS
LDACS will become one of several wireless access networks connecting
aircraft to the Aeronautical Telecommunications Network (ATN)
implemented by the FCI and possibly ACARS/FANS networks [FAN19].
6.1. LDACS Sub-Network
An LDACS sub-network contains an Access Router (AR), a Ground-Station
Controller (GSC), and several Ground-Stations (GS), each of them
providing one LDACS radio cell.
User plane interconnection to the ATN is facilitated by the Access
Router (AR) peering with an Air-to-Ground Router (A2G Router)
connected to the ATN. It is up to implementer's choice to keep
Access Router and Air-Ground Router functions separated, or to merge
them.
The internal control plane of an LDACS sub-network is managed by the
Ground-Station Controller (GSC). An LDACS sub-network is illustrated
in Figure 1.
wireless user
link plane
A--------------G-------------Access---A2G-----ATN
S..............S Router Router
. control . |
. plane . |
. . |
GSC..............|
. |
. |
GS---------------+
Figure 1: LDACS sub-network with two GSs and one AS
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6.2. Topology
LDACS operating in A2G mode is a cellular point-to-multipoint system.
The A2G mode assumes a star-topology in each cell where Airborne
Stations (AS) belonging to aircraft within a certain volume of space
(the LDACS cell) is connected to the controlling GS. The LDACS GS is
a centralized instance that controls LDACS A2G communications within
its cell. The LDACS GS can simultaneously support multiple bi-
directional communications to the ASs under its control. LDACS
ground stations themselves are connected to a ground station
controller (GSC) controlling the LDACS sub-network.
Prior to utilizing the system an AS has to register with the
controlling GS to establish dedicated logical channels for user and
control data. Control channels have statically allocated resources,
while user channels have dynamically assigned resources according to
the current demand. Logical channels exist only between the GS and
the AS.
The LDACS wireless link protocol stack defines two layers, the
physical layer and the data link layer.
6.3. 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; G2A) and
the reverse link direction (RL; A2G) are separated by frequency
division duplex. Forward link and reverse link use a 500 kHz channel
each. The ground-station transmits a 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.
6.4. 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
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protocol. LDACS supports also unacknowledged point-to-point channels
and G2A broadcast.
6.5. LDACS Mobility
LDACS supports layer 2 handovers to different LDACS channels.
Handovers may be initiated by the aircraft (break-before-make) or by
the GS (make-before-break). Make-before-break handovers are only
supported for ground-stations connected to the same GSC.
External handovers between non-connected LDACS sub-networks or
different aeronautical data links shall be handled by the FCI multi-
link concept.
7. Reliability and Availability
LDACS has been designed with applications related to the safety and
regularity of flight in mind. It has therefore been designed as a
deterministic wireless data link (as far as this is possible).
Based on channel measurements of the L-band channel [SCHN2016] and
respecting the specific nature of the area of application, LDACS was
designed from the PHY layer up with robustness in mind.
In order to maximize the capacity per channel and to optimally use
the available spectrum, LDACS was designed as an OFDM-based FDD
system, supporting simultaneous transmissions in Forward Link (FL;
G2A) and Reverse Link (RL; A2G). The legacy systems already deployed
in the L-band limit the bandwidth of both channels to approximately
500 kHz.
The LDACS physical layer design includes propagation guard times
sufficient for the operation at a maximum distance of 200 nautical
miles from the GS. In actual deployment, LDACS can be configured for
any range up to this maximum range.
The LDACS FL physical layer is a continuous OFDM transmission. LDACS
RL transmission is based on OFDMA-TDMA bursts, with silence between
such bursts. The RL resources (i.e. bursts) are assigned to
different users (ASs) on demand by the ground station (GS).
The LDACS physical layer supports adaptive coding and modulation for
user data. Control data is always encoded with the most robust
coding and modulation (QPSK coding rate 1/2).
LDACS medium access on top of the physical layer uses a static frame
structure to support deterministic timer management. As shown in
figure 3 and 4, LDACS framing structure is based on Super-Frames (SF)
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of 240ms duration corresponding to 2000 OFDM symbols. FL and RL
boundaries are aligned in time (from the GS perspective) allowing for
deterministic sending windows for KEEP ALIVE messages and control and
data channels in general.
LDACS medium access is always under the control of the GS 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 GS 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.
LDACS has two mechanisms to request resources from the scheduler in
the GS.
Resources can either be requested "on demand" with a given priority.
On the forward link, this is done locally in the GS, 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); variably sized). Due to
the spacing of the reverse link control channels 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 GS, while control data uses statically (i.e. at cell entry)
allocated recurring resources (DCCH and CCCH). The current
specification specifies no scheduling algorithm. 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 by the GS.
In addition to having full control over resource scheduling, the GS
can send forced Handover (HO) commands for off-loading or RF channel
management, e.g. when the signal quality declines and a more suitable
GS is in the AS reach. With robust resource management of the
capacities of the radio channel, reliability and robustness measures
are therefore also anchored in the LDACS management entity.
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The LDACS data link layer protocol running on top of the medium
access sub-layer uses ARQ to provide reliable data transmission on
layer 2.
It employs selective repeat ARQ with transparent fragmentation and
reassembly to the resource allocation size to achieve low latency and
a low overhead without losing reliability. It ensures correct order
of packet delivery without duplicates. In case of transmission
errors it identifies lost fragments with deterministic timers synced
to the medium access frame structure and initiates retransmission.
Additionally the priority mechanism of LDACS ensures the timely
delivery of messages with high importance.
As of now no reliability and availability mechanisms for layer 3 and
above have been specified.
8. Protocol Stack
The protocol stack of LDACS is implemented in the AS, GS, and GSC: 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). The LDACS network is externally connected to voice
units, radio control units, and the ATN network layer.
Figure 2 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 2: LDACS protocol stack in AS and GS
8.1. Medium Access Control (MAC) Entity Services
The MAC time framing service provides the frame structure necessary
to realize slot-based Time Division Multiplex (TDM) access on the
physical link. It provides the functions for the synchronization of
the MAC framing structure and the PHY layer framing. The MAC time
framing provides a dedicated time slot for each logical channel.
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 channels. Logical
channels are used as interface between the MAC and LLC sub-layers.
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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).
In the FL, an SF contains a Broadcast Frame of duration 6.72 ms (56
OFDM symbols) for the Broadcast Control Channel (BCCH), and four
Multi-Frames (MF), each of duration 58.32 ms (486 OFDM symbols).
In the RL, each SF starts with a Random Access (RA) slot of length
6.72 ms with two opportunities for sending reverse link random access
frames for the Random Access Channel (RACH), followed by four MFs.
These MFs have the same fixed duration of 58.32 ms as in the FL, but
a different internal structure
Figure 3 and Figure 4 illustrates the LDACS frame structure.
^
| +------+------------+------------+------------+------------+
| FL | BCCH | MF | MF | MF | MF |
F +------+------------+------------+------------+------------+
r <---------------- Super-Frame (SF) - 240ms ---------------->
e
q +------+------------+------------+------------+------------+
u RL | RACH | MF | MF | MF | MF |
e +------+------------+------------+------------+------------+
n <---------------- Super-Frame (SF) - 240ms ---------------->
c
y
|
----------------------------- Time ------------------------------>
|
Figure 3: LDACS super-frame structure
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^
| +-------------+------+-------------+
| FL | DCH | CCCH | DCH |
F +-------------+------+-------------+
r <---- Multi-Frame (MF) - 58.32ms -->
e
q +------+---------------------------+
u RL | DCCH | DCH |
e +------+---------------------------+
n <---- Multi-Frame (MF) - 58.32ms -->
c
y
|
----------------------------- Time ------------------------------>
|
Figure 4: LDACS multi-frame (MF) structure
8.2. 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.
The data link service uses the logical channels provided by the MAC:
1. A ground-stations announces its existence and access parameters
in the Broadcast Channel (BC).
2. The Random Access Channel (RA) enables 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 grant access to data channel resources.
4. The reverse direction is covered by the Reverse Link (RL), where
aircraft-stations need to request resources before sending. 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.
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8.3. 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.
8.4. LDACS Management Entity (LME) Services
The mobility management service in the LME provides support for
registration and de-registration (cell entry and cell exit), scanning
RF channels of neighboring 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.
The resource management service provides link maintenance (power,
frequency and time adjustments), support for adaptive coding and
modulation (ACM), and resource allocation.
8.5. Sub-Network Protocol (SNP) Services
The data link service provides functions required for the transfer of
user plane data and control plane data over the LDACS sub-network.
The security service provides 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.
9. 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 fulfill 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.
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Security considerations for LDACS are defined by 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].
10. Privacy Considerations
LDACS provides a Quality of Service (QoS), and the generic
considerations for such mechanisms apply.
11. IANA Considerations
This memo includes no request to IANA.
12. Acknowledgements
Thanks to all contributors to the development of LDACS and ICAO PT-T.
Thanks to Klaus-Peter Hauf, Bart Van Den Einden, and Pierluigi
Fantappie for further input to this draft.
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Thanks to SBA Research Vienna for fruitful discussions on
aeronautical communications concerning security incentives for
industry and potential economic spillovers.
13. Normative References
14. Informative References
[SCHN2016] Schneckenburger, N., Jost, T., Shutin, D., Walter, M.,
Thiasiriphet, T., Schnell, M., and U.C. Fiebig,
"Measurement of the L-band Air-to-Ground Channel for
Positioning Applications", IEEE Transactions on Aerospace
and Electronic Systems, 52(5), pp.2281-229 , 2016.
[MAE191] Maeurer, N., Graeupl, T., and C. Schmitt, "Evaluation of
the LDACS Cybersecurity Implementation", IEEE 38th Digital
Avionics Systems Conference (DACS), pp. 1-10, San Diego,
CA, USA , 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, Herndon, VA, USA , 2019.
[GRA19] Graeupl, T., Rihacek, C., and B. Haindl, "LDACS A/G
Specification", SESAR2020 PJ14-02-01 D3.3.030 , 2019.
[FAN19] Pierattelli, S., Fantappie, P., Tamalet, S., van den
Einden, B., Rihacek, C., and T. Graeupl, "LDACS Deployment
Options and Recommendations", SESAR2020 PJ14-02-01
D3.4.020 , 2019.
[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, London, UK , 2017.
[GRA11] Graeupl, T. and M. Ehammer, "L-DACS1 Data Link Layer
Evolution of ATN/IPS", 30th IEEE/AIAA Digital Avionics
Systems Conference (DASC), pp. 1-28, Seattle, WA, USA ,
2011.
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[GRA18] Graeupl, 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, Herndon, VA, USA , 2018.
[SCH191] Schnell, M., "DLR Tests Digital Communications
Technologies Combined with Additional Navigation Functions
for the First Time", 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] Haindl, B., Meser, J., Sajatovic, M., Mueller, S.,
Arthaber, H., Faseth, T., and M. Zaisberger, "LDACS1
Conformance and Compatibility Assessment", IEEE/AIAA 33rd
Digital Avionics Systems Conference (DASC), pp. 1-11,
Colorado Springs, CO, USA , 2014.
[RIH18] Rihacek, C., Haindl, B., Fantappie, P., Pierattelli, S.,
Graeupl, T., Schnell, M., and N. Fistas, "L-band Digital
Aeronautical Communications System (LDACS) Activities in
SESAR2020", Integrated Communications Navigation and
Surveillance Conference (ICNS), pp. 1-8, Herndon, VA,
USA , 2018.
[BELL19] Bellido-Manganell, M. A. and M. Schnell, "Towards Modern
Air-to-Air Communications: the LDACS A2A Mode", IEEE/AIAA
38th Digital Avionics Systems Conference (DASC), pp. 1-10,
San Diego, CA, USA , 2019.
[RAW-TECHNOS]
Thubert, P., Cavalcanti, D., Vilajosana, X., Schmitt, C.,
and J. Farkas, "Reliable and Available Wireless
Technologies", Work in Progress, Internet-Draft, draft-
thubert-raw-technologies-05, 18 May 2020,
<https://tools.ietf.org/html/draft-thubert-raw-
technologies-05>.
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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-03, 8 March 2020,
<https://tools.ietf.org/html/draft-bernardos-raw-use-
cases-03>.
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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