Appendix A - Ax.25 Level 2 Protocol - AEA PK-232 Technical Reference Manual

PK-232 TECHNICAL MANUAL APPENDIX A – AX.25 LEVEL 2 PROTOCOL
APPENDIX A – AX.25 LEVEL 2 PROTOCOL
3/13/83
Terry Fox, WB4JFI
Vice-President, AMRAD
1819 Anderson Road
Falls Church, VA 22043
Abstract
This paper contains the latest draft of the AX.25 protocol specification. This is the first public re-
lease of this draft. Earlier drafts have been given to specific individuls for comment and as a refer-
ence for software development. Changes should be expected. Please check the AMRD Newsletter
for announcements of later versions.
History
Over the years there have been several protcols suggested for use at layer 2 of the ISO Open Sys-
tem Interface Reference Model (OSI-RM) over Amateur Radio. The open system that has been in
use is based on the IBM SDLC protocol, and it has been working as far as it went. One of the im-
mediate problems that came up with SDLC was that the address field of SDLC is very limited
(being one byte long), causing problems if there are many amateurs on at a time.
Trying to come up with a protocol that everyone would agree to seemed like an almost impossible
task a year ago. What we at AMRAD decided to do was to go over the various protocols in use or
available to the amateur, figure out the best and worst parts of each prtocol and see id the proto-
col could be "enhanced" to work properly over the amateur radio enviroment. After reviewing the
various protocols araund and talking with people in the computer networking industry, we decided
to push the AX.25 standard, modified to allow a larger address field. A about this time, a group of
amateurs in New Jersy were coming to the same conclusion, so about mid-June of 1982 the two
groups got together and after two weekends came to an "understanding" on a level 2 protocol.
The most delicate part of the negotiations between the two groups concerned the name to be giv-
en to the protocol AX.25, which stands for Amateur X.25.
The next step in the evolvement of AX.25 was that in October of 1982, AMSAT hosted a gathering
of some leaders in amateur packet radio. AMRAD was at the meeting, along with representatives
from TAPR, SLAPR, AMSAT and PPRS.
Three days of intense discussion followed, and an agreement was finally reached on a nationwide
compaible protocol. AX.25 was then modiefied to be compatible with this new protocol (basically
the only major changes were an additional extension of the address field and the addition of a Pro-
tocol IDentifier, or PID field).
The rest of this paper will describe the basics of the AX.25 level 2 protocol.
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