ISDN
Integrated Services Digital Network (ISDN) remains one of the
most flexible and widely offered services today. Providers all over the world
offer ISDN services to users.
ISDN basically comes in two varieties:
-
Basic Rate Interface (BRI), which consists of two 64-kbps
Bearer (B) channels and one 16-kbps Data (D) channel. This is often represented
as 2B+D, for the two B channels and one D channel. This has traditionally been
the choice of many remote users who connect to the office from their residences.
It remains popular, especially with users who do not have DSL or cable services
available at their residences.
-
Primary Rate Interface (PRI), which in the U.S. consists of 23
64-kbps B channels and one 64-kbps D channel. This is often represented as
23B+D. In Europe, PRI consists of 30 64-kbps B channels and one 64-kbps D
channel. PRI services are often used when greater bandwidth is needed, such as
when a connection is needed between a central office and a branch office.
Even though ISDN is offered all around the world, there are
differences in the switches that providers use to provide ISDN service. When
configuring ISDN, make sure that the design and configuration match the switch
type and service being offered.
ISDN makes use of the same wiring used by analog phone lines.
However, because ISDN is digital, the signal transmitted across the line is
digital instead of analog. This allows for much higher transmission speeds. In
addition, call setup for ISDN is very quick compared to that of an analog line.
This is because of the use of the separate D channel. The setup is done
out-of-band on the D channel, it does not disturb existing user traffic, and it
takes a short amount of time. The combination of these factors makes ISDN
ubiquitous, fast, and convenient.
ISDN is useful when a variety of applications need to be
supported. The higher bandwidth can support applications such as
videoconferencing, web browsing, e-mail, and voice services. Also, ISDN can
support multiple data sources, as opposed to analog, which typically can support
only one data source at a time.
ISDN lends itself to a variety of applications in the
remote-access arena. Users who want to connect from home or users in a small
office/home office (SOHO) typically can use BRI connections to do so.
However, in a scenario where a branch office needs a connection
to a central office, PRI services can be used either as a primary link or as a
backup connection that can be activated when the primary line goes down or when
additional bandwidth is needed. This is often called a remote office/branch office (ROBO) scenario.
ISDN does have some drawbacks. A variety of standards are
supported in different parts of the world. This results in a variety of
equipment needed to support these standards and interfaces. You have to be
careful when ordering, configuring, and maintaining equipment that connects to
ISDN providers in different parts of the world.
Another drawback of ISDN is its cost. Because ISDN is charged
on a per-usage basis, it can be expensive to operate. This is one of the reasons
why ISDN is used in many scenarios as a backup to a serial link that has a flat
per-month cost.
Lately ISDN has been replaced in many homes and SOHO
environments by technologies such as DSL and cable modems, which offer much
higher transfer rates. These services are also cheaper because they offer
flat-rate pricing. The combination of these factors has made these technologies
more attractive than ISDN.
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