X25AM Configuration and Management Manual

PTrace for the X25AM Subsystem
X25AM Configuration and Management Manual—523424-004
6-12
Level-2 Trace Analysis
Level-2 Trace Analysis
The L2-trace example comes from a single CLIP trace block that contains several
level-2 frames that were successfully sent and received. The I-frame (information) sent
at Sequence 63 shows the state of the DCE. The N(S) is sending an I-frame. The N(R)
value states that it has accepted and acknowledged the I-frame from the DTE, and it is
waiting for the next incoming frame.
At Sequence 64 in the trace block, an RR response frame is received from the DTE.
The N(R) value states that the DTE received the I-frame sent at Sequence 63 (which
contained the N(S) value of 1), and that the DTE is waiting for the next I-frame to be
sent.
The N(R) value always points to the next I-frame N(S) the sender expects to receive. It
acknowledges all frames received up to N(R)-1.
It is normal to find an RR frame returned with an N(R) that does not acknowledge the
last I-frame sent. However, if an RR frame is sent with the P bit set, the remote side
must respond with either an I-frame or RR frame that has the F bit set. This responding
relationship is called checkpointing, and it is usually done when one side’s L2 window
has been reached. Checkpointing allows the DTE to decide which frames have not
been acknowledged and, if necessary, to begin error recovery.
06/11/99 15:09:34.619807 >028.729647 #5 CLIP Trace Blk
CSS LAPB In Seq 47 Len 13 Time 15:08:54.95
A/C($0121) DCE RR P/F=0 N(R)=1
000: 0121
CSS LAPB Out Seq 54 Len 13 Time 15:09:09.95
A/C($0321) DTE RR P/F=0 N(R)=1
000: 0321
CSS LAPB In Seq 55 Len 13 Time 15:09:09.95
A/C($0121) DCE RR P/F=0 N(R)=1
000: 0121
06/11/99 15:09:34.668334 >000.048527 #6 CLIP Trace Blk
CSS LAPB In Seq 59 Len 23 Time 15:09:23.68
A/C($0322) DTE IFRAME P=0 N(S)=1 N(R)=1
000: 0322 1001 0B20 0000 0000 0000
CSS LAPB Out Seq 62 Len 13 Time 15:09:23.68
A/C($0341) DTE RR P/F=0 N(R)=2
000: 0341
CSS LAPB Out Seq 63 Len 18 Time 15:09:23.70
A/C($0142) DCE IFRAME P=0 N(S)=1 N(R)=2
000: 0142 1001 1300 81
CSS LAPB In Seq 64 Len 13 Time 15:09:23.71
A/C($0141) DCE RR P/F=0 N(R)=2
000: 0141
End of Trace file.
Note. An RR frame does not contain an N(S) field.