Networking Explained, 2nd Edition

Chapter 11: Integrated Services Digital Network (ISDN) 373

information for circuit-switched user data. The D channel transmits information related to

call initiation (call-setup) and termination (call tear-down) between an ISDN device and

the telco’s central office for each B channel. Thus, when a telephone call is made between

two sites, the D channel handles all of the call-related information for the B channels. This

is why the B channels are clear 64 kbps channels. The D channel also can be used to trans-

mit packet-switched user data (provided that no signal or control information is needed),

data from security alarm signals of remote sensing devices that detect fire or intruders, and

low-speed information acquired from telemetry services such as meter reading. (Note:

Telemetry applications involve obtaining measurements remotely and relaying them to

another site for recording or display purposes.)

The H channel is used for transmitting user data (not signal or control information) at

higher transmission rates than the B channel provides. Four H channels are defined: H0,

H10, H11, and H12. H0 comprises six B channels for a total capacity of 384 kbps. The

H10 channel is U.S.-specific and aggregates 23 B channels for a total capacity of

1.472 Mbps. The H11 channel is the equivalent of the North American DS-1 (see Chapter

7) and consists of 24 B channels for an aggregate bandwidth of 1.536 Mbps. The H12 chan-

nel, which is European-specific, comprises 30 B channels and has an aggregate bandwidth

of 1.920 Mbps. Examples of applications that might use an H channel include videocon-

ferencing high-speed fax, or high-speed packet switched data, and high-quality audio.

B and D channels are generally combined (i.e., multiplexed using TDM) by ISDN service

providers and offered to customers in different bundled configurations. The most common

package is the 2B + D arrangement, which consists of two B channels and one D channel.

This channel structure is known as basic rate interface (BRI). Two other common basic

interface structures are B + D, and D only. In the BRI structure, the D channel is 16 kbps.

A second type of channel structure is called primary rate interface (PRI), which has a

general configuration of nB + D. The two most common PRIs are 23B + D, which is equiv-

alent to the North American DS-1 rate of 1.544 Mbps, and 30B + 2D, which is equivalent

to the European E-1 rate of 2.048 Mbps. In the PRI structure, the D channel is 64 kbps.

8. Please elaborate on BRI and PRI.

OK. We will discuss these separately.

BRI The ISDN Basic Rate Interface (also known as basic access) is a 192-bit channel

that consists of two 64-kbps B channels, one 16-kbps D channel, and 48 bits of overhead

used for framing and other functions. (See Figure 11.4. See also Chapter 5 for more infor-

mation about framing.) The two B channels and the D channel are combined into a single

pair of standard copper telephone wires. Both B channels can support any combination of

voice or data transmissions (e.g., both voice, both data, one voice and the other data). BRI

provides a full-duplex data rate of 128 kbps. If call or signal information is not being car-

ried by the D channel (e.g., transmitting data via packet-switching), then the rate increases

to 144 kbps if the D channel is carrying data. Data rates also can be increased anywhere

from four to eight times more through data compression (Chapter 15). (Note: Some telcos

might still use older signaling software that requires ISDN signaling to be done in-band. In

such instances, 8 kbps per B channel must be reserved for signaling resulting in a BRI ser-

vice that provides 112 kbps instead of 128 kbps.)