Bulletins from the Pacific Packet Radio Society - page 115
6. The AMRAD and TAPR groups are going to pursue development of a modem which employs Minimum Shift Keying (MSK). MSK is the considered the most promising method at this time.
7. The SSC will have to be open for different experimental approaches until there is general agreement and experimental validation of "the" method.
Considering all the alternatives which were discarded, the above resolutions represent a significant narrowing of focus. If that wasn't enough, more was yet to come!
Eric Scace reviewed the work which had be en done by the NJ & AMRAD packeteers to adapt the international X.25 protocol to amateur needs. The amateur subset has been documented by Terry Fox, and is now called AX.25. Hank Magnuski distributed a tutorial document on connectionless protocols and described work which had been done to develop Revision 4 of the TIPM and LIPM software. This is the first implementation of software utilizing amateur calls only for addressing.
By the evening of the 8th the group began to recognize that only minor differences separated the AX.25 and LIPM.04/TIPM.04 approaches. The TAPR protocol users were willing to implement an interface based on the recommendations which would come out of the meeting. The differences were resolved and led to the unanimous adoption of a subset of an internationally recognized link level protocol (specifically, ANSI X3.66 ADCCP-HDLC BA Class, with options 2, 4, 7, 8, & 11).
Why is this important? First, this link level protocol allows two AMICON groundstations to construct a packet pipeline. This same protocol can be used between two stations in a terrestrial backbone net. The same protocol can be used terminal-to-terminal and terminal-to-terminal via a simplex packet repeater. It is a slight extension of the X.25 LAPB link level protocol. Finally, it does not restrict future development of ISO Level 3 virtual circuit or datagram protocols.
The key feature of the new design is the adoption of a scheme for using call-sign addressing in a packet. The packet format looks like this:
---------------------------------------------------------------------------------------
] FLAG ] TOCALL ] FMCALL ] CTL ] PID ] INFOFIELD ] FCS ] FLAG ]
---------------------------------------------------------------------------------------where:
FLAG - Standard opening HDLC flag byte (01111110)
TOCALL - The destination call-sign (6 char's plus sub-station ID code)
FMCALL - The source call-sign of (6 characters plus sub-station ID code)
CTL - Standard HDLC control byte
PID - Protocol Identification byte for information frames.
INFO - Information field in information frames
FCS - Frame check sequence
FLAG - Closing flag byte
Click for Original Graphic Image of this page.
