APRS®& AX.25 Protocols

Presented at HamCon 2009Estes Park Colorado

30 May 2009by

Rich Painter, AB∅VOPainter Engineering, Inc.

Copyright 2009 Painter Engineering, Inc.

HamCon 2009 Copyright 2009 Painter Engineering, Inc.

APRS®& AX.25 Protocols

X.25 and AX.25 originsKISS TNC Protocol detailsAPRS Protocol detailsProblems with implementation and useReferences and additional resources

HamCon 2009 Copyright 2009 Painter Engineering, Inc.

X.25 and AX.25 origins

X.25 ITU Standard for Packet-Switched Network (CCITT 1976)Designed for use within the telephone systemLower 3 layers of OSI Net Model plus a bit more (L1 Physical, L2 Data Link, L3~Packet)Widely used 1976 to mid 1990Mostly replaced by Frame Relay in 1990’sStill used by Credit Card Terminal Processors

HamCon 2009 Copyright 2009 Painter Engineering, Inc.

X.25 and AX.25 origins

X.25 providesPart of a suite of protocols that work togetherCall setup & teardown (like a telephone call)Error detection & correction, flow controlDesigned for channels with long delays (great for communications satellites)Manages Virtual Circuits (VC) and PVCSupported billing (Phone Co)2400bps to 2Mbps, 64Kbps typical

HamCon 2009 Copyright 2009 Painter Engineering, Inc.

X.25 and AX.25 origins

AX.25 based on X.25 (minor differences)

Version 2.0 by Terry Fox WB4JFI in 1984Version 2.1 July 1993Version 2.2 1996 (11 Nov 1997)Version 2.2 July 1998

HamCon 2009 Copyright 2009 Painter Engineering, Inc.

X.25 and AX.25 origins

AX.25 based on X.25 (minor differences)

Address Field- Instead of phone numbers as in X.25 we use Call Signs, SSIDsDigipeating uses up to 8 fields for Call Sign and SSIDMic-E encodes part in DST Field

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AX.25 Frame

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AX.25 DST Address Field

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AX.25 SRC Address Field

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AX.25 Digi Path Field

HamCon 2009 Copyright 2009 Painter Engineering, Inc.

HamCon 2009 Copyright 2009 Painter Engineering, Inc.

KISS TNC Protocol details

Keep It Simple Stupid TNCBrian Lloyd WB6RQN ideaMike Chepponis K3MC & Phil Karn KA9Q Aug 1986Presented at ARRL Computer Net Conference 1987Takes the “Human Software” out of TNCRemoves AX.25 code from the TNC and expects it to be in the host computerInterfaces to computer with encapsulated HDLC framesConverts to/from sync/asyncNo error detectionPhil apologizes for leaving out flow control

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KISS TNC Protocol details

Provides KISS TNC controls:Data frameTXDELAY byte in 10ms unitsP persistence parameterSlotTime byte in 10ms unitsTxtail byte in 10ms units to hold TXFullDuplex 0 is full, nonzero is halfSetHardware for modem dependant settingsReturn leave KISS mode (optional)

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KISS TNC Protocol details

KISS Frame LayoutFEND 0xc0 Frame StartCommand & Data bytesFEND 0xc0 Frame End

EscapingFESC 0xdb EscapeTFEND 0xdc Transposed Frame EndTFESC 0xdd Transposed Frame Escape

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APRS® Protocol details

Uses AX.25 Unnumbered Information Packets (UI)Payload is in Info FieldMic-E encodes part in DST FieldTime, Position, ObjectsWeather and Telemetry DataSymbols, Queries, Responses, Status

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APRS® Protocol details

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HamCon 2009 Copyright 2009 Painter Engineering, Inc.

APRS Symbols (partial)

HamCon 2009 Copyright 2009 Painter Engineering, Inc.

Problems with implementation and use

Many TNCs allow users to place data directly into actual packets (akin to directly entering data into a TCP packet)UITRACE, RELAYBad Digi Paths (contents, too long)Path should be WIDE1-1,WIDE2-2Improper duplicate detectionRouting done by SRC and not the Net

HamCon 2009 Copyright 2009 Painter Engineering, Inc.

References and additional resources

Google APRShttp://www.tapr.org/aprs.htmlhttps://www.tapr.org/cgi-bin/mailman/listinfo/aprssighttps://lists.tapr.org/cgi-bin/mailman/listinfo/ax25-layer2http://www.tapr.org/pub_ax25.htmlhttp://en.wikipedia.org/wiki/X.25http://www.ax25.net/kiss.aspx

HamCon 2009 Copyright 2009 Painter Engineering, Inc.

References and additional resources

http://en.wikipedia.org/wiki/Automatic_Position_Reporting_Systemhttp://www.aprs.org/doc/APRS101.PDFhttp://www.aprs.org/aprs11.htmlhttp://www.aprs.org/aprs12.htmlhttp://groups.yahoo.com/search?query=aprs

APRS is a Registered Trademark of APRS Engineering LLC