AEA PAKRATT PK-232, Technical Reference Manual

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PK-232 TECHNICAL MANUAL APPENDIX A – AX.25 LEVEL 2 PROTOCOL
PK232TM Rev. A 5/87 A-16 Page 61
A.3.5.4.2 Timer T3 Recovery
Timer T3 is used to assure the link is still functional during periods of low information transfer.
Whenever T1 is not running (no outstanding I frames), T3 is used to periodically poll the other
DXE of a link. When T3 times out, a RR or RNR frame is transmitted as a command and with the P
bit set. The waiting acknowledgement procedure (
A.4.4.9 , below) is then executed.
A.3.5.5 Invalid Frame or FCS Error
If an invalid frame is received, or a frame is received with an FCS error, that frame will be dis-
carded with no action taken.
A.3.5.6 Frame Rejection Condition
A frame rejection condition occurs when an otherwise error-free frame has been received with one
of the conditions listed in
A.3.4.3.3 above.
Once a rejection error occurs, no more I frames are accepted (except for the examination of the
P/F bit) until the error is resolved. The error condition is reported to the other DXE by sending a
FRMR response frame. See
A.4.5 .
A.4 Description of AX.25 Procedures
The following describes the procedures used to setup, use, and disconnect a balanced link be-
tween two DXE stations.
A.4.1 Address Field Operation
A.4.1.1 Address Information
All transmitted frames shall have address fields conforming to A.2.13 , above. All frames shall have
both the destination device and the source device addresses in the address field, with the destina-
tion address coming first. This allows many links to share the same RF channel. The destination
address is always the address of the station(s) to receive the frame, while the source address con-
tains the address of the device that sent the frame.
The destination address can be a group name or club call sign if the point-to-multipoint operation
is allowed. Operation with destination addresses other than actual amateur call signs is a subject
for further study.
A.4.1.2 Command/Response Procedure
AX.25 Version 2.0 has implemented the command/response information in the address field. In or-
der to maintain compatibility with previous versions of AX.25, the command/response information
is conveyed using two bits.
An upward-compatible AX.25 DXE can determine whether it is communicating with a DXE using an
older version of this protocol by testing the command/response bit information located in bit 7 of
the SSID octets of both the destination and source address subfields. If both C bits are set to zero,
the device is using the older protocol. The newer version of the protocol always has one of these
two bits set to one and the other set to zero, depending on whether the frame is a command or a
response.