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.)