ISDN Protocols

Coverage of ISDN protocols and their specifications on the CCNA exam poses a particularly difficult problem for the CCNA candidate. The ITU defines the most well-known specifications for ISDN, but there are far more specifications than anyone could 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 9-10 is directly quoted from the ICND course. Be sure to learn the information in the Issue column. Knowing what each series of specifications is about will be useful.

Table 9-10 ISDN Protocols

Issue	Protocol	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 Interface (UNI)
Switching and signaling	Q.931—ISDN network layer

The OSI layers correlating to the different ISDN specifications are also mentioned in both the ITM and ICND CCNA prerequisite courses. It's also useful to memorize the specifications listed in Table 9-11, as well as which OSI layer each specification matches.

Table 9-11 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
1	ITU-T I.430 ITU-T I.431	N/A	Defines connectors, encoding, framing, and reference points.
2	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.
3	ITU-T I.450 ITU-T I.451	ITU-T Q.930 ITU-T Q.931	Defines signaling messages, such as call setup and takedown messages.

NOTE A tool to help you remember the specifications and layers is that the second digit in the Q-series numbers matches the OSI layer. For example, in ITU-T Q.920, the second digit, 2, corresponds to OSI Layer 2. In the I-series, the second digit of the specification numbers is 2 more than the corresponding OSI layer. For example, I.430, with its second digit of 3, defines OSI Layer 1 equivalent functions.

LAPD is used to deliver signaling messages to the ISDN switch, such as a call setup message. Figure 9-5 shows the use of LAPD versus PPP on B channels.

The call is established through the service provider network. PPP is used as the data-link protocol on the B channel from end to end. LAPD is used between the router and the ISDN switch at each local central office (CO) and remains up so that new signaling messages can be sent and received. Because the signals are sent outside the channel used for data, this is called out-of-band signaling.

The BRI encodes bits at 192 kbps, with most of the bandwidth (144 kbps) being used for the two B channels and the D channel. The additional bits are used for framing.

NOTE For further reading, refer to Cisco Press's Internetworking Technologies Handbook, Third Edition, ISBN 1-58705-001-3.

Figure 9-5 LAPD and PPP on D and B Channels

Figure 9-5 LAPD and PPP on D and B Channels

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 you buy ISDN for home use, the service provider personnel probably will call it the ISDN phone number instead of the 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.

Continue reading here: ISDN Function Groups and Reference Points

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