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772 Appendix A: Answers to the "Do I Know This Already?" Quizzes and Q&A Sections
26
How many bearer channels are in a BRI? What about a PRI in North America? What
about a PRI in Europe?
BRI uses two bearer channels and one signaling channel (2B+D). PRI uses 23B+D in
North America and 30B+D in Europe. The signaling channel on BRI is a 16-kbps channel;
on PRI, it is a 64-kbps channel.
27
True or false: ISDN defines protocols that can be functionally equivalent to OSI Layers 1,
2, and 3. Defend your answer.
True. Reference points in part define the physical interfaces. Used on the signaling
channel, LAPD is a data link protocol. SPIDs define a logical addressing structure and are
roughly equivalent to OSI Layer 3. Table 8-26 on page 568 summarizes ISDN protocols
as compared with the OSI model.
28
What reference points are used by ISDN BRI interfaces on Cisco routers?
A BRI interface with an S/T reference point, or a BRI with a U reference point, can be
bought from Cisco. With an S/T interface, an external NT1, NT2, or NT1/NT2 device is
required. With the U interface, no external device is required.
29
What do the letters LAPD represent? Is LAPD used on ISDN channels? If so, which ones?
Link Access Procedure, D-channel. LAPD is used only on ISDN D channels to deliver
signaling messages to the local ISDN switch. Many people don't understand the function
of LAPD, thinking it is used on the B channels after the dial is complete. The encap-
sulation chosen in the router configuration determines the data link protocol on the bearer
channels. There is no option on Cisco routers to turn off LAPD on the signaling channel.
30
Name the standards body that defines ISDN protocols.
International Telecommunications Union (ITU). This group was formerly the CCITT. The
ITU is governed by the United Nations.
31
What ISDN functions do standards ITU-T Q.920 and Q.930 define? Does either standard
correlate to an OSI layer?
Q.920 defines the ISDN data link specifications, such as LAPD; Q.930 defines Layer 3
functions, such as call setup messages. I.440 and I.450 are equivalent to Q.920 and Q.930,
respectively.
32
What ISDN functions does standard ITU-T I.430 define? Does it correlate to an OSI
layer?
I.430 defines ISDN BRI physical layer specifications. It is similar to OSI Layer 1. There
is no Q-series equivalent specification to I.430.
33
What do the letters SPID represent, and what does the term mean?
Service profile identifier. This is the ISDN phone number used in signaling.
apA.fm Page 772 Monday, March 20, 2000 5:24 PM