AEA PAKRATT PK-232, Technical Reference Manual

Page: 64 / 101

PK-232 TECHNICAL MANUAL APPENDIX A – AX.25 LEVEL 2 PROTOCOL
PK232TM Rev. A 5/87 A-19 Page 64
A.4.3.4.4
When the DXE enters the disconnected state after an error condition or if an internal error has re-
sulted in the DXE being in the disconnected state, the DXE should indicate this by sending a DM
response rather than a DISC frame and follow the link disconnection procedure outlined in
A.4.3.3.3 , above. The DXE may then try to re-establish the link using the link set-up procedure
outlined in A.4.3.1 , above.
A.4.3.5 Collision Recovery
A.4.3.5.1 Collisions in a Half-Duplex Environment
Collisions of frames in a half-duplex environment are taken care of by the retry nature of the T1
timer and retransmission count variable. No other special action needs to be taken.
A.4.3.5.2 Collisions of Unnumbered Commands
If sent and received SABM or DISC command frames are the same, both DXEs should send a UA
response at the earliest opportunity, and both devices should enter the indicated state.
If sent and received SABM or DISC commands are different, both DXEs should enter the discon-
nected state and transmit a DM frame at the earliest opportunity.
A.4.3.5.3 Collision of a DM with a SABM or DISC
When an unsolicited DM response frame is sent, a collision between it and a SABM or DISC may
occur. In order to prevent this DM from being misinterpreted, all unsolicited DM frames should be
transmitted with the F bit set to zero. All SABM and DISC frames should be sent with the P bit set
to one. This will prevent any confusion when a DM frame is received.
A.4.3.6 Connectionless Operation
In Amateur Radio, there is an additional type of operation that is not feasible using level 2 connec-
tions. This operation is the round table, where several amateurs may be engaged in one conversa-
tion. This type of operation cannot be accommodated by AX.25 link-layer connections.
The way round-table activity is implemented is technically outside the AX.25 connection, but still
using the AX.25 frame structure.
AX.25 uses a special frame for this operation, called the Unnumbered Information (UI) frame.
When this type of operation is used, the destination address should have a code word installed in
it to prevent the users of that particular round table from seeing all frames going through the
shared RF medium. An example of this is if a group of amateurs are in a round-table discussion
about packet radio, they could put PACKET in the destination address, so they would receive
frames only from others in the same discussion. An added advantage of the use of AX.25 in this
manner is that the source of each frame is in the source address subfield, so software could be
written to automatically display who is making what comments.
Since this mode is connectionless, there will be no requests for retransmissions of bad frames. Col-
lisions will also occur, with the potential of losing the frames that collided.
A.4.4 Procedures for Information Transfer
Once a connection has been established, as outlined above, both devices are able to accept I, S,
and U frames.