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THE AERONAUTICAL TELECOMMUNICATION NETWORK (ATN)T. L.
Signore
The MITRE CorporationCivil Center for Advanced Aviation System
Development
McLean, VirginiaMary Girard
The MITRE CorporationCenter for Air Force C2 Systems
Bedford, MassachusettsABSTRACT
The International Civil Aviation Organization (ICAO) hasendorsed
a concept for future aeronauticalcommunications, which uses digital
data links tosupplement voice communications and provide
improvedair trafiic services. A necessary part of this concept is
aninternational infrastructure that manages digital datatransfer
between aircrafi and civil air trafic controlfacilities. This
communications infrastructure is theAeronautical Telecommunication
Network (ATN). It isthe intention of certain ICAO member states to
provideair trafic services (ATS) that require ATN avionics. Asthe
civil-controlled airspace moves toward requiring newcommunication
capabilities, the militaq will need toaccommodate the A TN if
worldwide-unrestricted airspaceaccess is to be maintained. A
description of the ATN isprovided in this paper. This description
includes the ATNsolutions for mobility, low-bandwidth, and
preferenceservices. Typical applications of the ATN are
alsodescribed, as well as implementation status. The
paperconclt~des with a review of present military stratagems forthe
ATN.
THE ATN ENVIRONMENTCommercial aircraft are expected to be
equipped withmultiple air/ground data links. This is a consequence
ofthe 1imited spatial availability of the links and
thecomparatively higher cost of sending data over some linksrather
than others. Presently very high frequency (VHF)systems, which
include the Aircraft CommunicationsAddressing and Reporting System
(ACARS) and itsplanned replacement, the VHF Digital Link Mode
2(VDL2), are the cheapest to use. However, these are line-of-site
systems and consequently have no coverage overthe ocean. Satellite
links such as those operated byInmarsat and Iridium Corporations,
and high frequency(HF) links operated by ARINC Corporation, are
used tofill this gap. The Federal Aviation Administration (FAA)is
planning to replace the present analogue voice radiosystem with one
which can handle both voice and data.
This system is known as the VHF Digital Link Mode 3(VDL3).
Finally, a communication link called Gatelink,designed for
high-speed data transfer while the aircraft isat the airport gate,
is being planned.To operate within this environment, ATN is
designed withfour major elements. The first element is the ability
totransfer data to an aircraft without sender knowledge ofthe
aircraft location (network mobility). Networkmobility is equivalent
to a car that has two cellularphones, each phone operated by a
different provider andeach with different service volumes. A caller
wishing totalk to the driver would be automatically routed
throughthe correct service provider without the caller knowingwhere
the car is or which service provider is connected.Notice that
within the service volume, it is up to theservice provider to
locate the car. The second majorelement is the ability to
simultaneously use the multipleair/ground links that are installed
in an aircraft. Thisrequires applications to specify cost, link, or
speedpreferences, which are used by the ATN when forwardingdata.
Based on these preferences, one link Vra:Id b.chosen to transfer
data over othc~s v:hc; 1z;.:;;: ple links areavailable. The third
element is the ability to account forthe low bandwidth air/ground
data links available todayand in the near future. Low bandwidth
air/ground linksrequire the use of data compression. The fourth
elementis the standardization of the services required by
ATSapplications (i.e., transport, session, presentation,
andapplication functions) and the applications themselves, sothat
they are the same worldwide.
THE ATN MOBILITY SOLUTIONAn essential feature of the ATN design
can be gleanedbecause of the requirements for network mobility and
useof multiple data links. There must be a method toadvertise to
all ground systems the availability of themultiple paths that can
reach an aircraft. The obviousmethod to do this is to promulgate
the paths to all ATNground systems in the world. However, this
approach,
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termed flooding, is to be avoided. It introduces
severemanagement overhead problems and growth constraints.An
intuitive understanding of these problems is easilygrasped by
asking if the availability of a general aviationaircraft flying in
Kansas should be advertised to allground systems in China. The ATN
designers [1] haveintroduced the concept of a backbone router
system. Thisconcept is designed to restrict the distribution of
aircraftpath information to avoid the problems of flooding,
butstill allows every ground system the ability to send data toany
aircraft.Figure 1 is an illustration of a simple
backboneimplementation. Multiple routers, represented by
circles,are a part of the ATN system. The routers in the
boldcircles are designated backbone routers. Each aircraft hasan
AIN address assigned to it. Data destined for anaircraft use this
address. The address also uniquelydetermines the home domain of the
aircraft. Every homedomain has at least one backbone router in
it.
Aircraft
ooVHF Connect ion
{tSatel l ita Connect ion
mm
L--
Figure 1. Distribution of Mobile Information in the ATNWhen an
aircraft router, shown in the figure as RO, is incontact with a
ground router, R7, it is the ground routersresponsibility to inform
its associated backbone router,RI, that the aircraft is available.
It is the function of thebackbone routers to distribute this
information to otherbackbone routers. However, not all backbone
routers areinformed; only those backbone routers between the
firstbackbone router and the backbone router in the home
domain for that aircraft being advertised are updated. Theother
backbone routers receive no information about theaircraft. Thus in
the figure, only routers R1, R2, and R3are informed of the VHF
connection to the aircraft.Routers which are not on the backbone
never receiveadvertisements.When an application existing on a host
machine(represented by squares in Figure 1) wishes to transferdata
to an aircraft, it provides the destination address ofthe data and
delivers the resulting network packet to itsnearest router. In the
example, assume that HI hasdelivered the packet to R8. R8 has no
information aboutthe aircraft. Under these conditions, R8 always
sends thepacket to its nearest backbone router, which is R2. R2does
have information about the aircraft and forwards thepacket to R1,
which in turn sends the packet to R7 fordelivery.A more informative
case is that of host machine H2wishing to contact the aircraft. As
described above, RI 8receives the packet and forwards it (through
R16) to R5.This backbone element has no information about
theaircraft. The ATN rule in this situation is; Forward thepacket
in the direction of the home domain for thisaircraft. In this case,
R5 forwards the packet to R4. Thesame rule applies at R4, and the
data is forwarded to therouter in the Home Domain, which does have
routinginformation about the aircraft. Subsequently, the data
isforwarded to router R2, then through R1 and R7 foreventual
delivery to the aircraft.
In the ATN, the protocol which distributes the aircraftpath
information to the backbone muter is the ~nter-Dormcin Routing
Protocol (IDRP) [2]. This protocol isalso used by the aircraft to
inform the ground routers thatit is available for network
communications.
PREFERENCE BASED SERVICESThe ATN mobile routing solution must
allow for multiplepaths to an aircraft and for these paths to be
usedsimultaneously. Applications may require delivery viaspecific
links or with specific Quality of Service (QOS) tocontrol costs or
for safety andperformance reasons.Consider the case in which an
aircraft is simultaneously inthe service area of both VHF and
satellite communication.In Figure 1, the aircraft router, RO, is in
contact with theground via both satellite (through R9) and VHF
(throughR7). Applying ATN backbone dissemination rules to
thesatellite path, backbone router R4 has an entry for theaircraft.
The entry shows that the aircraft is available
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through R9 over a satellite connection. R3, the homedomain
backbone router, now has two routes available forthe aircraftone
through R3 over VHF, the other throughR4 over satellite.ATN allows
a ground application to indicate the specificQOS/priority desired
and a list of preferences for data linkdelivery. For example, the
packet should be shipped viaVHF and if VHF is not available,
satellite. Thisinformation is recorded in each packet emitted by an
end-system destined for an aircraft. Each ATN router can
thenexamine the information and choose the best path to usewhen
forwarding the packet. For example, if end-systemH2 sends a packet
to the aircraft, the packet willeventually reach R4, which will
send the packet to R9 orR3, depending on the QOS/preference
indicated.ACCOMMODATING LOW BANDWIDTH LINKS
The throughput available for the air/ground links expectedto
operate with the ATN is much smaller than the typicalground/ground
links used today. In the case of VDL2,32k bits per second are
shared among multiple aircraft,using carrier sensed multiple access
(CSMA) technology.Because of these low bandwidths, it is required
that theATN perform compression. The ATN standard allows
formultiple types of compression to occur within ATNrouters. The
exact compression or compressionsperformed on a data unit
transferred between air andground is negotiable and determined when
an aircraftenters the jurisdiction of an ATN router.
Headercompression of the network protocol information ispresently
required. The ATN uses the open systemsinterconnection (0S1)
connectionless network protocol(CLNP) and typicaily ~i; ! .mduce
the 100 bytes of CLNP;Ir::dc; to 6 bytes. A second available
technique uses thedeflate algorithm, originally developed for use
over theInternet. This technique is similar to that used for
thegzip and pkzip compression utilities available to Unix
andMicrcjsoft users. Deflate compresses the entire data
unittransferred, both protocol headers and user data.
ATN APPLICATIONSAs part of the international standardization of
ATN, fourair/gr{ound applications have been standardized:
ContextManagement (CM); Automatic Dependent Surveillance(ADS);
Controller Pilot Data Link Communications(CPDLC); and Digital
Flight Information Services(D-FIS). The D-FIS application set
allows pilots torequest continual updates about flight conditions,
forexample, weather, airport conditions, etc. The CPDLCapplication
set is meant to replace the presentcontroller/pilot voice
interactions with digital messages to
the extent possible. CPDLC includes over 200 uplink and100
downlink messages. The ADS message set idesigned to allow position
information determined bonboard navigation equipment to be
transferred to thground in order to supplement present radar
positionreporting. CM messages support a directo~ service thaallows
the aircraft to login whenever it enters a newATS authority (e.g.,
the U.S.) and exchange applicationand associated network address
information with theground CM application server. CM messages
control thisprocess. The resulting directory is used to determine
thenetwork addresses of aircraft and the installedapplications.
ATN IMPLEMENTATIONThe ATN is an international civil aviation
effort; as suchthe standards for the ATN are being determined by
ICAO.Presently, all-necessary documentation has beencompleted and
approved by the ICAO. The ATNStandards and Recommended Practices
(SARPS) will bepublished in August 1998 with an applicability date
oNovember 1998. Once this occurs, as ICAO isUnited Nations treaty
organization, ICAO states mustadhere to the SARPS when implementing
ArN.Within the U. S., interim data link implementations arebeing
scheduled. This is because it is felt that a completeATN
installation, which supports all applications, wouldnot be
available soon enough due to the significantchanges required in FAA
ground infrastructure, FAAcontroller workstations, and avionics.
The first steptoward full ATN capabilities will be an IICAO
SARPScompliant communications infrastructure. Succeedingsteps will
build up the supported message set andwailable aidgrounc! ;~ata
links. The first step, whichincludes limited CPDLC and CM
application capability,is to be installed at a key site for
operational evaluation in2001. This system will use a single data
link (VDL2) andwill employ four classes of CPDLC ATN messages.These
are:
. Transfer of Communication (TOC) - Providespilot with voice
frequency of next sector;
q Initial Contact (IC) - Informs controller thaaircraft has data
link capability;
. Altimeter Setting Message (ASM) - Providesaltimeter assignment
to the aircraft;
. Pre-defined Message (PDM) - Provide texmessage transfer
between controller and pilot.
Subsequent phases will support multiple data links andincrease
the number of messages used. It is intended thathe full CPDLC
message set, applicable to en route
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airspace, will be implemented by 2004 and support foradditional
air/ground data links will be added by 2007.Eleven U.S. air
carriers in cooperation with the FAA haveformed a consortium, ATN
Systems Inc. (ATNSI), toaddress institutional issues involved with
developing andfielding ATN software which requires peer elements
inboth the public and private sector. The consortium willlicense
ATN software to interested manufacturers to(1) reduce development
costs, (2) guarantee interoperableinstallations, and (3) reduce
implementation time. ATNSIhas contracted for delivery of software
by 1999 with theexpectation that certification and operational
evaluationwill be completed by the early part of 2001. The
productsof ATNS1 are expected to be the basis for the initial
ATNairborne implementations.The European Civil Aviation Conference
member stateshave committed to a program to increase the capacity
ofthe present air traffic management system and to optimizethe
interface between ATS and airports using extensiveautomation and
enhanced data communications. Theprogram is denoted the European
Air Traffic ControlHarmonization and Integration Programme, or
EATCHIP.ATN is listed as one component within this program.
TheEuropean ATN project includes standardization activities,ATN
trials, prototypes, and validation programs. TheEuropean
prototyping, testing, and validation efforts ofATN are expected to
conclude by 2005. It is thusexpected that ATN European
implementation will occurin the 2005+ time frame [3]. However, the
U.K. isplanniing on having operational ATN capability in placeover
the North Atlantic by 2001.
MILITAIRY ISSUESThere are a number of Federal, Department of
Defense,and Air Force documents that require military aircraft
tocomplly in peacetime with civil aviation requirements
incivil-controlled airspace. In addition the U. S., as amember of
ICAO, accepts ICAO standards as binding.The TJ.S. Air Forces
Electronic Systems Center (ESC)global air traffic
operations/mobility command andcontrol (GATO/MC*) program office
carried out a studywhich assessed the requirements, primarily for
largetransports, for operation in the new communicationenvironment
[4-6]. The need to conduct operationsworldwide, in peacetime and in
times of crisis, means thatmilitary aircraft must operate in
civil-controlled airspaceand comply with future ICAO requirements.
In order tocomply, an ATN compatible avionics system withintegrated
data links may be required.
To achieve worldwide air/ground link capability certainmilitary
aircraft will be required to equip their aircraftwith dual
beyond-line-of-sight (BLOS) data links, andline-of-site (LOS) data
links. The links must comply withICAO requirements for aeronautical
mobile service.Currently, only the Inmarsat satellite system has
beenapproved for use as a BLOS ATS data link, ICAO hasbeen
developing the SARPS for an HF data link (HFDL)which will allow it
to be used as an ATS data link. Themilitary, along with several
airlines, are encouraging thisuse of HFDL due to the large
installation cost of theInmarsat system and since many military
aircraft,particularly the cargo carriers, are already equipped
withHF antennas for voice operation. The management of thedata
links, including choice of media, will require theaddition of a
communications management function(CMF) that contains an ATN
router, and will requiresupporting applications.Whenever future
data link military requirements arediscussed, the possibility of
the use of existing andplanned military data links has been raised.
The idea is tosave money by using the existing equipment and
byproviding a ground entry point into the ATN using atranslation
process. The two data links typicallymentioned are UHF SATCOM for
BLOS and Linkl 6(Joint Tactical Information Distribution System
[JTIDS]or Tactical Digital Information Link [TADIL] J) for LOS.This
approach, while feasible, presents a number ofproblems to be
overcome. To be approved as an ATSdata link, the military data
links would have to meet theservice and performance requirements in
the ICAOSARPS for safe:y r~-,mmnications. This implies, in
part,that UHF SATCOM would have to meet delay rec ~.,wytime
requirements (90 seconds after satellite failure).
Thedemandassigned multiple access (DAMA) protocolswould need to
provide ATS data a higher priority. TheUHF SATCOM avionics
equipment would need toprovide an input to the CMF and would have
to bemodified to implement current CMF protocols. The effectof
added ATS traffic on the UHF SATCOM data link mayresult in poor
performance for both military and ATS datawhen using this approach.
ATS applications would alsoneed to be implemented in the avionics.
A ground entrypoint into ATN would need to be implemented in
UHFSATCOM ground terminals. In addition to thesetechnical issues,
programmatic issues regarding themodification of the system
requires Air Force-wide andjoint service concurrence and
funding.
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The use of Link16 has been examined in enough detail toindicate
that the ATN networking issues may beproblematic, while the link
performance is adequate tomeet .ATS data link throughput
requirements. The Link 16terminal employs a MIL-STD- 1553B data bus
that doesnot support ATN communications without extensiveprotocol
changes. The Link 16 network managementsoftware and hardware are
inadequate to support ATNnetwcrk administration and would need to
be modified.Allocating Linkl 6 time slots for ATN would be
difficultand even inefficient, since TADIL J messages
definingpositional and navigational data are already sent into
thenetwork, and ATN messages would duplicate thesemessages. Once
the technical issues were solved, SARPSWOUICIave to be developed in
order to use Link 16 as anATS data link. An approach that employs a
ground entryinto ATN and translation of the TADIL J positional
datais conceivable. But, since Link16 is a line-of-sighttactical
system, it would require installation of aninfrastructure of
approximately 300 Link 16 terminals forU.S. en route and terminal
areas at an estimated cost of$100M. It is also unlikely that other
countries wouldadopt a U.S. DoD-unique infrastructure which has
notbeen standardized by ICAO.The issues related to the use of
current data links and theinteroperability requirements with civil
avionics has ledmany in the military to conclude that the most
feasibleapproach is to equip with civil ATS data
links.Manufacturers are addressing the use of civil data
linksystems on military aircraft by developing solutions thatport
the civilian HFDL hardware and software onto circuitcards for
installation into military HF radios. Theaddition of VDL2 and VDL3
protocois to military radiosis being handled in a similar manner
for VHF radios.Interfaces to a commercial off-the-shelf CMU are
alsobeing developed for military radios and for the
military-specific flight management systems. Once ATN
isimplemented, the expected upgrades to the commercialavionics
would also benefit the Air Force, provided theywere carrying the
appropriate equipment.The primary military issue for use of ATN is
security.Militamy aircraft will require the ability to
inhibittransmission of data using ATS data links and ATN.
Inaddition, the military use of ATN for data transmissionmay
require encryption of the data. Preliminary effortshave begun to
test the ability of the current ACARSnetwork to handle encrypted
data. It is expected that theseefforts will continue as ATN is
developed. There are anumber of proposals that address the issue of
encryptionof the aircrafts tail number. The primary issue is that
the
tail number can be extracted from the network address andthe
aircrafts future position may become compromised.One proposal
employs a ground server to dynamicallyassign tail numbers from a
fixed pool of tail numbers. Asecond proposal implements an
encryption algorithm tothe database of aircraft addresses.The issue
of authentication between controller and pilotdigital data messages
is another important area foconsideration by the military. ICAO is
currentlydeveloping enhancements to the ATN standards to allowfor
authentication of application information, routinginformation, and
systems management informationexchanges. These enhancements to ICAO
ATN standardsare expected to be completed/approved in 2000.
Theauthentication standards may need to be completed
beforewidespread implementation of ATN occurs by themilitary.
REFERENCES[1] Aeronautical Telecommunication Network,Appendix A,
Technical Requirement, Sub-Volume
VInternet Communications Service, Version 2.2,ATN Panel, ICAO,
January 16, 1998.
[2] ISO/IEC 10747:1994, Protocol for Exchange of Inter-Domain
Routing Information among IntermediateSystems to Support Forwarding
of ISO/IEC 8473PDUS.
[3] ATN Workshop, December 16-19, 1997,Luxembourg, Proceedings,
Volume Two, Chapter 6,Current ATN Activities.
[4] AMC CNS/ATM Team, Air Mobility Command(AMC) Communications,
Navigation, andSurveillance/Air Traffic Management (C51SIkIMStudy:
Avionics Survey Report, MP 96BO0001 12The MITRE Corporation,
Bedford, MA, November1996.
[5] Salamone, et al., Air Mobility Command (AMC)Communications,
Navigation, and Surveillance/AirTraffic Management (CNS/ATM)
Study:Requirements Report, MP 96BOOO0129, The MITRECorporation,
Bedford, MA, December 1996.
[6] Salamone, et al., Air Mobility Command (AMC)Communications,
Navigation, and Surveillance/AirTraffic Management (CNS/ATM)
Study:Architecture and Cost Report, MP 97BOOOOO01,TheMITRE
Corporation, Bedford, MA, January 1997.
The contents of this material reflect the views of the
authorsNeither the Federal Aviation Administration nor thDepartment
of Defense makes any warranty or guarantee, opromise, expressed or
implied, concerning the content oaccuracy of the views expressed
herein.
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