ISDN Channels

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

B channels transport data. They operate at speeds of up to 64 kbps, although the speed might be lower, depending on the service provider, or might be based on standards in some parts of the world. For instance, some national standards outside the United States call for 56-kbps B channels.

ISDN uses another channel inside the same single physical line to ask the telco to set up and tear down circuits. The signaling channel, called the D channel, signals new data calls. When a router wants to create a B-channel call to another device using a BRI or PRI, it sends the phone number that 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 when you pick up the phone and dial a number to create a voice call.

The different types of ISDN lines often are descirbed 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 T/1 framing, as in the United States, are referred to as 23B+D, and PRIs based on E/1 framing, typically found in Europe, are referred to as 30B+D. E/1s have 32 DS0 channels, with 1 reserved for framing and 1 used for the D channel when used as a PRIā€”that leaves 30 DS0 channels as B channels. Table 15-4 lists the number of channels for each type of ISDN line and the terminology used to describe them.

Table 15-4 BRI and PRI B and D Channels

Type of Interface

Number of Bearer Channels (B Channels)

Number of Signaling Channels (D Channels)

Descriptive Term

BRI

2

1 (16 kbps)

2B+D

PRI (T1)

23

1 (64 kbps)

23B+D

PRI (E1)

30

1 (64 kbps)

30B+D

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