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568 Chapter 8: WAN Protocols and Design

ISDN Protocols

Coverage of ISDN protocols and their specifications on the CCNA exam poses a particularly
difficult problem for the CCNA candidate. The International Telecommunications Union (ITU)
defines the most well-known specifications for ISDN, but there are far more specifications than
anyone would want to try to memorize. The problem is choosing what to memorize and what
to ignore. My personal philosophy is that standards information is best kept in a book rather
than in my own memory. With Cisco's emphasis on proving your hands-on skills using the
CCNA and CCNP exams, hopefully a de-emphasis on memorizing standards will be a
convenient side effect. However, these standards are fair game for the exam.

The characterizations of several key protocols made by the Cisco ICND course are important
for the exam. Table 8-25 is directly quoted from the ICND course. Take care to learn the
information in the Issue column--knowing what each series of specifications is about will be
useful.

The OSI layers correlating to the different ISDN specifications are also mentioned in both the
ITM and the ICND CCNA prerequisite courses. Memorizing the specifications in Table 8-26
and the OSI layer each specification matches is also useful.

Table 8-25

ISDN Protocol Table

Issue

Protocols

Key Examples

Telephone network and ISDN

E-series

E.163: international telephone

numbering plan

E.164: international ISDN addressing

ISDN concepts, aspects, and
interfaces

I-series

I.100 series: concepts, structures,
terminology

I.400 series: User-Network Interfaces
(UNI)

Switching and signaling

Q-series

Q.921: LAPD

Q.931: ISDN network layer

Table 8-26

ISDN I-Series and Q-Series Mentioned in ICND and ITM: OSI Layer Comparison

Layer, as
Compared to OSI

I-Series

Equivalent
Q-Series
Specification

General Purpose

ITU-T I.430

ITU-T I.431

Defines connectors, encoding, framing,
and reference points

ITU-T I.440

ITU-T I.4411

ITU-T Q.920

ITU-T Q.921

Defines the LAPD protocol used on the
D channel to encapsulate signaling
requests

ch08.fm Page 568 Monday, March 20, 2000 5:17 PM