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# Aeronautical Telecommunication Networks
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The ATN protocol family (along with VDL-Mode2) was designed as a successor for the Aircraft Communications Addressing and Reporting System (ACARS). It implements a reliable air ground datalink based on OSI protocols (which were considered "state of the art" when the standardisation began) and VDL-Mode2.
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The system is currently deployed along with VDL-Mode2 in Europe in the scope of the LINK2000 plus program.
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At present only CMv1 and CPDLCv1 (PO and PM) applications are suported by Wireshark; the dissector is based on ICAO doc 9705 Ed2 including ammendements for Protected Mode CPDLC.
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A simplified overview of the core ATN protocols looks like this:
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<div>
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<table>
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<tbody>
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<tr>
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<td>
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7
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</div>
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</td>
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<td colspan="2" style="&quot; text-align:center&quot;">
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Application layer (**ULCS/ACSE**,**CM**,**CPDLC**, ...)
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</td>
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</tr>
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<tr>
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<td>
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6
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</td>
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<td colspan="2" style="&quot; text-align:center&quot;">
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**ULCS/Presentation**,**ULCS/PDV**
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</td>
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</tr>
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<tr>
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<td>
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5
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</td>
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<td colspan="2" style="&quot; text-align:center&quot;">
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**ULCS/Session**
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</td>
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</tr>
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<tr>
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<td>
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4
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</td>
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<td>
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**COTP** (TP4 connections)
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</td>
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</tr>
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<tr>
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<td>
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3
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</td>
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<td colspan="2" style="&quot; text-align:center&quot;">
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Internet (**CLNP**, **ES-IS**,**IS-IS**,**IDRP**)
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</td>
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</tr>
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<tr>
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<td>
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2
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</td>
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<td colspan="2" style="&quot; text-align:center&quot;">
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Link layer (**LLC**,**IP**,**X.25**, ...)
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</td>
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</tr>
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</tbody>
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</table>
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</div>
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## Wireshark
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ATN reuses CLNP and COTP dissectors and stacks the "atn-ulcs" dissector on top of COTP. Technically "atn-ulcs" implements the "ATN upper layers" containing the SES, PRES and ACSE protocols (ASN.1 Packed Encoding Rules).
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The "atn-ulcs" dissector then calls application-level dissecctors like CM("atn-cm") and CPDLC("atn-cpdlc").
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## Enable ATN support in wireshark
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The dissector should be compiled in by default (developper version) and can be enabled by tuning the Wireshark protocol Preferences/Settings.
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The following settings are required to decode ATN traffic: CLNP dissector:
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- "Always try to decode NSDU as Transport PDU's" -\> Enabled
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- "Decode ATN security label" -\> Enabled
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COTP dissector:
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- "Decode ATN TPDU's" -\> Enabled
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T.125 dissector
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- the T.125 dissector should be disabled, for some CM/CPDLC PDU may be incorrectly decoded as "T.125" (Main Menu: "Analyze"-\>"Enabled Protocols" disable checkbox on "T.125 Multipoint-communication" entry).
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No settings tabs are available for "atn-ulcs", "atn-cm" and "atn-cpdlc" dissectors.
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## Bugs
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plenty ;-). IMHO the dissector is far from beeing usable in production systems (it is better than nothing though, as network monitors for OSI are expensive/inappropriate when compared to tools in the TCP/IP world).
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Layers up to OSI Layer 6 (ATN-ULCS/ACSE) are working in most cases; decoding of CM and CPDLC is flawed.
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The main problem is that there is no working "conversation" function for the TP4 layer which is needed do differentiate between PER encoded CM and CPDLC PDU's; this function would allow to decode all PDU's of a transport connection an CM or CPDLC, for some PDU's share the same bit encoding.
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Known bugs:
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- atn-ulcs (ACSE): AARE incorrectly decoded; probably related to the incorrect handling of "[CpdlcStart](/CpdlcStart) confirmation"
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- atn-cpdlc (CPDLC): the dissector incorrectly decodes "[CpdlcStart](/CpdlcStart) confirmation" as "Cpdlc message"
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- some PDU's of CM and CPDLC have the same PER encoding, which may lead to a wrong encoding.
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## External Links
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### Background:
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- ACARS \[<http://en.wikipedia.org/wiki/ACARS>\]
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- VDL-Mode2 \[<http://en.wikipedia.org/wiki/VHF_Data_Link>\]
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- Controller Pilot Data Link Communications \[<http://en.wikipedia.org/wiki/CPDLC>\]
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- A short introduction to air/ground datalink \[<http://members.optusnet.com.au/~cjr/introduction.htm>\]
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### Standards
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- ICAO standards \[<http://legacy.icao.int/anb/panels/acp/repository.cfm>\]
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- OSI standards \[<http://standards.iso.org/ittf/PubliclyAvailableStandards/index.html>\]
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### Protocol Traces
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- ATN over ethernet SNDCF \[<https://bugs.wireshark.org/bugzilla/attachment.cgi?id=5808>\]
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- ATN over IP SNDCF \[<https://bugs.wireshark.org/bugzilla/attachment.cgi?id=5809>\]
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- ATN over IP SNDCF, Airbus A350 avionics evaluation \[<https://bugs.wireshark.org/bugzilla/attachment.cgi?id=9925>\]
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---
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Imported from https://wiki.wireshark.org/ATN on 2020-08-11 23:11:26 UTC |