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172 Chapter 6: Using ISDN and DDR Technologies
E1 Line Code
E1 deployments can implement AMI; however, the issues with AMI and data transmission still
hold true: data transmissions are limited to 56 kbps. To remedy this, high-density bit, level 3
(HDB3) was created. It operates similarly to B8ZS, but in a slightly more efficient manner. If a
long string of 0s is detected, a number of them are changed to 1s for the duration of their trip
across the provider network. They are then changed back to their original values at the remote
CSU/DSU.
Obviously, T1/E1 framing and line coding have not been discussed at length. It is an important
topic to understand, however, even though it is not touched on in depth on the CCNP Remote
Access Exam. For more information on this topic, check out www.cisco.com.
PRI Layers
PRI is based in the same technologies as BRI. In fact, PRI implements ISDN Q.921 (Layer 2)
and Q.931 (Layer 3) in the same manner as BRI. In addition, the call setup messages are
identical, as are the call release messages.
There are some basic differences between BRI and PRI, however. PRI relies on the assignment
of a TEI. This TEI, however, is always 0 in Cisco's implementation.
Example 6-30 shows sample output of the show isdn status command. Notice the TEI and the
fact that the state is MULTIPLE_FRAME_ESTABLISHED. This verifies the existence of
Layer 2 connectivity.
PRI Configuration
The configuration of the PRI service is quite simple. Although the command variations and
options are very similar to that of its BRI counterpart, a PRI configuration has additional
requirements.
Example 6-30
show isdn status Command Output
RouterA#show isdn status
The current ISDN Switchtype = primary-ni
ISDN Serial0:23 interface
Layer 1 Status:
ACTIVE
Layer 2 Status:
TEI = 0, State = MULTIPLE_FRAME_ESTABLISHED
Layer 3 Status:
No Active Layer 3 Call(s)
Activated dsl 0 CCBs = 0