CISCO IOS in a Nutshell

ISDN Interfaces

ISDN is an important standard for sending digital data over telephone lines. An ISDN link can carry two different kinds of information: the data itself, and control information for the ISDN circuit. Data channels are called B (“bearer”) channels and carry either 56 or 64 kilobits/second; the control channels are called D (“data”) channels.

ISDN is packaged in two different ways. BRI (Basic Rate Interface) is commonly used over residential phone lines. It provides two B channels plus a D channel, for a maximum B-channel capacity of 128 kbps. PRI (Primary Rate Interface) is more like a T1 connection—in fact, it’s really just a repackaged T1 connection. PRI provides 23 B channels and 1 D channel, with a maximum rate of 1.544 Mbps in the U.S. In Europe, on E1 lines you get 30 B channels, for 2.048 Mbps.

BRI is a reasonably common technology for providing home or small-office connectivity. Although it’s being displaced by technologies like ADSL and cable modems, there are many places where those technologies are unavailable. ISDN reaches almost anywhere. Let’s look briefly at what you need to understand to configure a BRI connection:

• Because ISDN connections are dial-up connections, they rely on DDR (dial-on-demand routing). DDR is discussed in Chapter 11, but we’ll look at a simple example in this section.

• It shouldn’t be a surprise that PPP is almost always used as an encapsulation protocol for ISDN links. HDLC is also possible, but it’s supported only by Cisco ...