570 Chapter 8: WAN Protocols and Design
NOTE
For further reading, refer to the ITM course, available from Cisco for a nominal fee. (The ITM
Version 2 CD is based on Cisco Press's Internetwork Technology Handbook, 2
nd
Edition, ISBN:
1-57870-102-3.)
The service profile identifier (SPID) used in signaling is important to the configuration of ISDN
and is likely to be mentioned on the exam. The SPID works like an ISDN phone number--in
fact, if buying ISDN for home use, the service provider personnel probably will call it the ISDN
phone number instead of SPID. Call setup messages refer to both the called and the calling
SPIDs. If a router wants to call another router, a SPID is used for call setup.
ISDN Function Groups and Reference Points
Many people are confused about the terms ISDN reference point and ISDN function group. One
key reason for the confusion is that only some function groups--and therefore some reference
points--are used in a single topology. Cisco expects CCNAs to be familiar with all function
groups and reference points. In an effort to clear up these two topics, consider the following
inexact but more familiar definitions of the two:
·
Function groups--A set of functions implemented by a device and software.
·
Reference points--The interface between two function groups; this includes cabling
details.
Most people understand concepts better if they can visualize a network or actually implement
the network. However, for a good understanding of function groups and reference points, keep
the following facts in mind:
·
Not all reference points are used in any one topology; in fact, one or two might never be
used in a particular part of the world.
·
After the equipment is ordered and working, there is no need to think about function
groups and reference points.
·
The router configuration does not refer to reference points and function groups, so many
people ignore these details.
A cabling diagram is helpful for examining the reference points and the function groups. Figure
8-22 shows the cabling diagram for several examples.
ch08.fm Page 570 Monday, March 20, 2000 5:17 PM