ISDN is an old but still very viable networking standard that supports voice, data, and
video. It is slowly being replaced by DSL and cable modems. Layer 2 is negotiated by using
the ITU-T Q.921 standard, and layer 3 is negotiated by using the Q.931 standard. The
ISDN reference model is set up with function groups and reference points. The function
groups classify each device in the ISDN network, and the reference points identify the connections
and electrical characteristics between each function group. Many IOS debug and
show commands are available to help you understand and troubleshoot ISDN connections.
The types of connections include dial backup, dial-on-demand routing (DDR), and
Bandwidth on Demand (BoD). There are many ways to set up a connection from one device
to another by using ISDN and analog links. The legacy method uses the physical interface to
specify IP address, dialing properties, and authentication. Dialer profiles provide more flexibility
when using dial backup and other dial-up connections. When using PPP authentication,
both Password Authentication Protocol (PAP) and Challenge Handshake Authentication Protocol
(CHAP) can be used. Some IOS debug and show commands are associated with PPP
negotiation and authentication.
Know the types of ISDN. ISDN comes in two flavors: BRI and PRI. The BRI is a standard that
runs over a 192Kbps circuit, whereas a PRI can run over a T-1 (1.544Mbps) or E-1 (2.048Mbps)
circuit. Know when to use a BRI and when to use a PRI. There are many PRI and BRI ISDN switch
types supported, and you should know which ones require SPIDs and which do not.
Understand the ISDN function groups. You need to know what function the groups NT1,
NT2, TA, LT, ET, TE1, and TE2 provide in the ISDN network.
Know the ISDN reference points. Identify the ISDN reference points of R, S, T, and U. Know
where these reference points are in the ISDN network and between which function groups they
are found.
Understand the two ITU-T Q standards used by ISDN. The Q.921 standard is used to set up
layer 2 between the router and local switch, and the Q.931 standard is used to set up layer 3.
You need to know what these protocols’ structures look like and what happens when a call is
set up and when it is torn down. You should also be familiar with the debug isdn q921 and
debug isdn q931 commands and what to look for in troubleshooting a problem.
Know how to set up dial-on-demand routing (DDR), dial backup, and Bandwidth on Demand
(BoD) by using both legacy and dialer profiles. Dialer profiles are used when you need to set
up a routing protocol over a dial-up connection; the legacy setup is used when a simple pointto-
point connection is needed between two sites. You should know how to set up authentication
and callback when security is needed on a dial-up connection. Multilink is also available when
more bandwidth is needed on a connection.
Know how to set up a channelized interface. You should know how to set up a T-1 or E-1
controller for channelized operation. You need to know the different framing and linecoding
options. The pri-group command is used when setting up a channelized interface to become
an ISDN PRI. The channel-group command is used when an interface or a portion of the interface
is used for dedicated access.
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