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Chapter 8
Troubleshooting ISDN
Some installers use RJ-11 or RJ-14 connections for ISDN terminations.
Although these connections work, it is recommended that RJ-45 be used in all
circumstances. Wires 1, 2, 7, and 8 may be used for alternate mark inversion
(AMI) encoding, and RJ-45 connections provide a visual variance from stan-
dard phone jacks.
Misconfigured Phone Switches
Administrators must consider the possibility that the service provider failed to
properly configure the ISDN switch. Although this is a very rare occurrence, the
possibility exists and should be considered, especially in new installations.
An understanding of ISDN as it relates to the OSI model can greatly assist the
network troubleshooter in locating problem causes. In addition, administrators
must be aware of the ISDN switch types and their impact on connectivity.
Troubleshooting Layer 2
There are two Layer 2 troubleshooting targets that should be identified and
analyzed when working on ISDN networks: the q.921 protocol and PPP.
q.921
ISDN maps well with the OSI reference model. Layer 2 is defined in q.921.
The q.921 signaling is carried over the D channel by using Link Access
Procedure protocol, or LAPD. This connection between the central office
switch (or the Teltone ILS-2000 in this test network) and the router must
occur and complete before Layer 3 connections are possible.
Troubleshooting q.921 problems is most frequently handled with the
debug isdn q921
command. Often, problems relate to the terminal end
point identifier, or TEI. This value uniquely identifies each terminal in the
network, and a TEI of 128 represents a broadcast. TEIs 64 through 126 are
reserved for assignment during the activation of a Layer 2 ISDN connection.
This assignment is dynamic.
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