Integrated Services Digital Network (ISDN)

Integrated Services Digital Network (ISDN)
is the result of efforts to remove analog services
from the telecommunications network. In the 1960s, the American phone company, AT&T,
realized that their network would be more efficient with digital services throughout. This
included the residence, where most ISDN BRI (explained next) is found. However, the model
scaled beyond this, and included aggregation and other interfaces that allowed efficient
MUXing
, or the consolidation of multiple small links into one large one.
Two types of ISDN services are available. The first,
ISDN
Basic Rate Interface (BRI)
,
provides for two 64Kbps channels (the bearer, or B, channels) and one 16Kbps channel (the
D channel), which can carry user data. The second type of ISDN service, called
Primary Rate
Interface (PRI)
, provides 23 64Kbps B channels for user data and one 64Kbps channel
(D channel) for signaling, based on the North American T-1 standard. The E-1 European
standard provides 2.048Mbps worth of bandwidth and a corresponding increase to 30 in the
number of B channels.
Please note that the 16Kbps channel in ISDN BRI is used for signaling; however, many
providers permit the transit of user data using this bandwidth. This is frequently marketed
as “always-on” ISDN. ISDN PRI uses a single 64Kbps channel for signaling.

Some ISDN BRI installations limit each B channel to 56Kbps.
The primary advantage of ISDN is its capability to provide faster access than would be available
from traditional asynchronous dial-up connections. Unfortunately, the service is not as
widely available as traditional analog services, and it tends to be more costly. ISDN is typically
used in scenarios including low-bandwidth video, low-bandwidth data, and voice services. It is
important to note that each of the two ISDN channels can provide the user with a traditional
analog dial-up connection.

note:
ISDN services are quickly being replaced in the United States by DSL services.
Digital subscriber line connections are currently available at over 1Mbps, and
some provide over three times this rate. However, substantial restrictions exist
regarding the distance over which these connections can be set up (the maximum
distance is 18,000 feet, or under 3 miles from the central office to the residence),
and some sources predict that up to 40 percent of homes will be too far
from the central office to receive the service. As of this writing, DSL still failed
to compete with cable modem and ISDN installations in terms of number of
deployments in the United States. In Europe, however, where more people live
closer to their exchanges, ISDN is rapidly being killed off by ADSL.
ISDN is well suited for most applications, including file transfers. However, its high perminute
pricing (depending on service package) makes it impractical when it is needed for more
than a couple hours per day. Frame Relay, which you will learn about next, is typically a better
solution for higher bandwidth, long-duration connections.