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-17 Page 62
The command/response information is encoded into the address field as shown in Fig. 10.
Fig. 10 – Command/Response encoding
Frame Type Dest. SSID C-Bit Source SSID C-Bit
Previous versions 0 0
Command (V.2.0) 1 0
Response (V.2.0) 0 1
Previous versions 1 1
Since all frames are considered either commands or responses, a device shall always have one of
the bits set to one, and the other bit set to zero.
The use of the command/response information in AX.25 allows S frames to be either commands or
responses. This aids maintenance of proper control over the link during the information transfer
state.
A.4.2 P/F Bit Procedures
The next response frame returned by the DXE to a SABM or DISC command with the P bit set to 1
will be a UA or DM response with the F bit set to 1.
The next response frame returned to an I frame with the P bit set to 1, received during the infor-
mation transfer state, will be a RR, RNR, or REJ response with the F bit set to 1.
The next response frame returned to a supervisory command frame with the P bit set to 1, re-
ceived during the information transfer state, will be a RR, RNR, or REJ response frame with the F
bit set to 1.
The next response frame returned to a S or I command frame with the P bit set to 1, received in
the disconnected state, will be a DM response frame with the F bit set to 1.
The P bit is used in conjunction with the time-out recovery condition discussed in A.3.5.4 , above.
When not used, the P/F bit is set to zero.
A.4.3 Procedures For Link Set-Up and Disconnection
A.4.3.1 LAPB Link Connection Establishment
When one DXE wishes to connect to another DXE, it will send a SABM command frame to that de-
vice and start timer (T1). If the other DXE is there and able to connect, it will respond with a UA
response frame, and reset both of its internal state variables (V(S) and V(R)). The reception of the
UA response frame at the other end will cause the DXE requesting the connection to cancel the T1
timer and set its internal state variables to 0.
If the other DXE doesn't respond before T1 times out, the device requesting the connection will re-
send the SABM frame, and start T1 running again. The DXE should continue to try to establish a
connection until it has tried unsuccessfully N2 times. N2 is defined in
A.4.7.2 , below.
If, upon reception of a SABM command, the DXE decides that it cannot enter the indicated state, it
should send a DM frame.
When receiving a DM response, the DXE sending the SABM should cancel its T1 timer, and not en-
ter the information- transfer state.