X25AM Configuration and Management Manual

PTrace for the X25AM Subsystem
X25AM Configuration and Management Manual—523424-004
6-10
What to Look for in Level 2
probably have a level-2 problem. You should look for the following states in the
level-2 frames from PTrace:
1. On startup, you should see one side sending SABM and the other responding with
UA, which synchronizes level 2. If you do not see the UA response, there is a
level-2 problem or a modem problem. Check to see whether the SABM is being
retried every timeout interval. X25AM may be trying to send the UA in response to
the SABM, but the modem may not be supplying the transmit clock to the CLIP. A
quick look at a datascope or breakout box should show DTR, DSR, RTS, CTS, and
Carrier Detect pins (20, 6, 4, 5, and 8) high, with Transmit and Receive Clocks
fluctuating at the proper speeds (pins 15 and 17). Also, TD and RD (pins 2 and 3)
should show some action every T1 timer interval. If they do not show any action,
there is a problem with the CLIP, cable, or modem. Run the diagnostics to isolate
the problem.
2. If both sides are sending SABMs, check the addresses. If both sides are sending
commands with the same address, then most likely they are both set as DTEs or
DCEs. In a typical X.25 setup, one side must be set for DCE (usually the network)
and the other side set for DTE (usually X25AM).
3. L2DISC is an option for DCEs that require a DISC command before the DCEs can
handle SABM commands. The default option is L2NODISC (do not send DISC
commands on SCF START). The default works for most networks.
4. An error-free level 2 should show I-frames (data) getting RR responses. You
generally do not want to encounter RNRs (buffer problems), FRMRs (illegal or
unaccepted command frames), REJs (out of sequence I-frames received), or
DISCs and DMs (one side disconnecting). If you encounter only I-frames and RRs
being sent and received, you can usually assume there is nothing wrong with level
2. Also, you can determine that level 2 is error-free by examining the level-2
statistics using the SCF command STATUS LINE $ldev, DETAIL.
5. The level-2 trace is the last place where you can see the frame written to the trace
file before it is sent to the communications line. However, seeing the frame traced
does not mean that it has been successfully sent.
Because the software does all the tracing and accumulates the statistics, the only
way to actually see what is sent on the line is to use a line monitor. You should
always look for a response to level-2 frames that the CLIP sends. If level-2 frames
are retried, a bad modem or cable may be the problem: either the data is not being
sent correctly or not being acknowledged from the remote side.
Note. The examples shown in this section are for illustrative purposes only.