Understanding ISDN
11-6 Configuring Integrated Services Digital Network
Call Monitoring
Call monitoring is also an vital element of the XSR’s ISDN service. Call monitoring features are
useful in terms of security, but also enable tracking of call volume and logging of all connections
so that administrators can optimize the number of ISDN lines ordered. Given that ISDN costs are
often usage-related, this checking and recording also can prevent nasty surprises that users might
receive with the monthly phone bill. At the same time, usage logs can provide managers with the
justification required to add ISDN lines as the need for additional bandwidth arises.
The show interface bri, show controllers bri, and show isdn service commands display
virtual and physical line attributes including B channel idle warnings.
The show isdn history and show isdn active commands display Cause Codes giving the
reason why a call was disconnected. These codes are detailed in Table11-2 on page 11-16.
ISDN Trace
The XSR supports protocol tracing and partial decoding of Q921 and Q931 frames for ISDN
troubleshooting with the debug isdn command. This command initiates a Layer 2 (link) or
Layer 3 (call data) ISDN debug session to trace failed calls at the D channel level. Issuing the
command has the effect of locking out debugging by any other current Telnet or Console
connection. If you select both Layer 2 (Q921) and 3 (Q931), tracing will display both layers.
You can exit the debug session either by issuing the no debug isdn command or terminating the
Telnet session. As an option, you can set a limit of up to 9999 messages which will display at the
CLI after which the debug session will end. If the limit is not specified, after 100 displayed
messages, the no debug isdn command will automatically run. The limit parameter is a global
value that is refreshed each time debug isdn is entered.

Trace Decoding

The XSR captures Q931 and Q921 packets in binary format from the D channel, decodes and
displays them. Partial decoding is performed on the most frequent messages and information
elements (IEs), with the undecoded section of the packets displayed in hexadecimal format. Be
aware that if you require full decoding, you should use an ISDN logical analyzer.
All ISDN protocol trace lines comprise the following common fields:
•Rx/Tx shows a message received or transmitted by the XSR
•ISDN-BRI/ISDN-PRI indicates the type of line monitored and XSR slot and card numbers
•slot/card/port identifies the monitored port
•hh:mm:ss:msec displays the time stamp

Q921 Decoding

At Q921 Layer 2, the XSR decodes all Link Access Protocol - D Channel (LAPD) frames up to octet
5 of the message - the header. Q931 call information - the data in UI and INFO frames - is
displayed in hex on the second line, as shown in two of the following examples. Reference codes
are described in the succeeding section:
•+ 1st line:
Note: Users with privilege level 15 only can issue this command.