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326 Chapter 10: ISDN and Dial-on-Demand Routing

Foundation Topics

ISDN Protocols and Design

Integrated Services Digital Network (ISDN) provides switched (dialed) digital WAN services in increments of 64 kbps. Before ISDN, most dial services used the same analog lines that were connected to phones. Before ISDN was created, data rates using modems and analog phone lines typically did not exceed 9600 bits per second. The phone companies of the world created ISDN as a key building block for digital services of the future. They wanted to have a dialed digital service that not only allowed faster transmission rates but that also was pervasive as a simple analog line used for voice. Today, you could argue that the collective phone companies of the world were ultimately successful with these goals, but not totally successful: ISDN availability is widespread, but you can still find places where it is simply not available.

ISDN was created more than 20 years ago, and it began being widely deployed in the U.S. by the early 1990s. Competing technologies might eventually overtake the need for ISDN. As covered in Chapter 15, “Remote Access Technologies,” of CCNA INTRO Exam Certification Guide, for Internet access, ISDN has been usurped by competing technologies such as DSL, cable modems, and simply faster analog modems. ISDN remains a popular option for temporary connectivity between routers.

This chapter begins with some perspectives on when to use ISDN, followed by the technical details of the ISDN specifications. After that, you will read about how to configure ISDN.

Typical Uses of ISDN

One key reason to use dialed connections of any kind, including ISDN, might be to send and receive data for only short periods of time. “Occasional” connections might be used by a site for which instant access to data is not needed, but for which access is needed a few times per day. For example, a store might send in sales and resupply information overnight.

Routers frequently use ISDN to create a backup link when their primary leased line or Frame Relay connection is lost. Although the leased line or Frame Relay access link might seldom fail, when it does, a remote site might be completely cut off from the rest of the network.

Depending on the network’s business goals, long outages might not be acceptable, so ISDN could be used to dial back to the main site.

ISDN Protocols and Design 327

Figure 10-1 shows some typical network topologies when you’re using ISDN. These scenarios can be described as follows:

Case 1 shows dial-on-demand routing. Logic is configured in the routers to trigger the dial when the user sends traffic that needs to get to another site.

Case 2 shows a typical telecommuting environment.

Case 3 shows a typical dial-backup topology. The leased line fails, so an ISDN call is established between the same two routers.

Case 4 shows where an ISDN BRI can be used to dial directly to another router to replace a Frame Relay access link or a failed virtual circuit (VC).

Figure 10-1 Typical Occasional Connections Between Routers











Frame Relay



ISDN Channels

ISDN includes two types of interfaces: Basic Rate Interface (BRI) and Primary Rate Interface (PRI). Both BRI and PRI provide multiple digital bearer channels (B channels), over which temporary connections can be made and data can be sent. Because both BRI and PRI have multiple B channels, a single BRI or PRI line can have concurrent digital dial circuits to multiple sites, or multiple circuits to the same remote router to increase available bandwidth to that site.

B channels are used to transport data. B channels are called bearer channels because they bear or carry the data. B channels operate at speeds of up to 64 kbps, although the speed might be lower depending on the service provider.

328 Chapter 10: ISDN and Dial-on-Demand Routing

ISDN signals new data calls using the D channel. When a router creates a B channel call to another device using a BRI or PRI, it sends the phone number it wants to connect to inside a message sent across the D channel. The phone company’s switch receives the message and sets up the circuit. Signaling a new call over the D channel is effectively the same thing as picking up the phone and dialing a number to create a voice call.

The different types of ISDN lines are often described with a phrase that implies the number of each type of channel. For instance, BRIs are referred to as 2B+D, meaning two B channels and one D channel. PRIs based on T1 framing, as in the U.S., are referred to as 23B+D, and PRIs based on E1 framing are referred to as 30B+D. Table 10-2 lists the number of channels for each type of ISDN line and the terminology used to describe them.

Table 10-2 BRI and PRI B and D Channels


Number of Bearer (B)

Number of Signaling


Type of Interface



Descriptive Term







1 (16 kbps)






PRI (T1)


1 (64 kbps)






PRI (E1)


1 (64 kbps)






ISDN Protocols

The characterizations of several key protocols made by the Cisco ICND course are important for the exam. Table 10-3 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 is useful.

Table 10-3 ISDN Protocols



Key Examples




Telephone network and


E.163—International telephone numbering plan






E.164—International ISDN addressing




ISDN concepts, aspects,


I.100 series—Concepts, structures, and terminology

and interfaces





I.400 series—User-Network Interface (UNI)




Switching and signaling


Q.921—Link Access Procedure on the D channel






Q.931—ISDN network layer




ISDN Protocols and Design 329

The OSI layers correlating to the different ISDN specifications are also mentioned in the ICND course. It’s also useful to memorize the specifications listed in Table 10-4, as well as which OSI layer each specification matches.

Table 10-4 ISDN I-Series and Q-Series Mentioned in ICND: OSI Layer Comparison

Layer as






Equivalent Q-Series


to OSI









ITU-T I.430

Defines connectors, encoding, framing,




and reference points.


ITU-T I.431








ITU-T I.440

ITU-T Q.920

Defines the LAPD protocol used on




the D channel to encapsulate signaling


ITU-T I.441

ITU-T Q.921







ITU-T I.450

ITU-T Q.930

Defines signaling messages, such as call




setup and teardown messages.


ITU-T I.451

ITU-T Q.931






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.

Now that you have at least seen the names and numbers behind some of the ISDN protocols, you can concentrate on the more important protocols. The first of these is LAPD, defined in

Q.921, which is used as a data-link protocol across an ISDN D channel. Essentially, a router with an ISDN interface needs to send and receive signaling messages to and from the local ISDN switch to which it is connected. LAPD provides the data-link protocol that allows delivery of messages across that D channel to the local switch. Note that LAPD does not define the signaling messages. It just provides a data-link protocol that can be used to send the signaling messages.

The call setup and teardown messages themselves are defined by the Q.931 protocol. So, the local switch can receive a Q.931 call setup request from a router over the LAPD-controlled D channel, and it should react to that Q.931 message by setting up a circuit over the public network, as shown in Figure 10-2.