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Integrated Services
Digital Network (ISDN) is a type of
circuit switched telephone network system, designed to
allow digital transmission of voice and data over
ordinary telephone copper wires, resulting in better
quality and higher speeds than available with
analog systems. More
broadly, ISDN is a set of
protocols for establishing and breaking circuit
switched connections, and for advanced call features for
the user.
ISDN requires an ISDN Terminal Adapter and an ISDN
phone line (~$25.00 more per month than a regular phone
line). An ISDN line is a digital phone line that offers
a true 128Kbps connection and the functionality of two
individual phone lines. Compare this to regular phone
lines and V.90 modems which typically achieve 45Kbps or
less.
In a videoconference,
ISDN provides simultaneous voice, video, and text
transmission between individual desktop
videoconferencing systems and group (room)
videoconferencing systems.
In the phrase "Integrated Services Digital Network",
- Integrated Services refers to ISDN's
ability to deliver two simultaneous connections, in
any combination of data, voice, video, and fax, over a
single line. Multiple devices can be attached to the
line, and used as needed. That means an ISDN line can
take care of most people's complete communications
needs, without forcing the purchase of multiple analog
phone lines at a much higher transmission rate.
- Digital refers to its purely digital
transmission, as opposed to the analog transmission of
plain old telephone service.
If you're using a modem for Internet access at this
moment, your Internet service provider's modem has
converted this site's digital content to analog
signals before sending it to you, and your modem
converts those signals back to digital when receiving
(the same thing happens with every keystroke and mouse
click you transmit). When you connect with ISDN, there
is no analog conversion. ISDN transmits data
digitally, resulting in a very clear transmission
quality. There is none of the static and noise of
analog transmissions that can slow transmission speed.
- Network refers to the fact that ISDN is not
simply a point-to-point solution like a leased line.
ISDN networks extend from the local telephone exchange
to the remote user and include all of the
telecommunications and switching equipment in between.
Types of
communications handled
Among the kinds of data that can be moved over the 64
kbit/s channels are pulse-code
modulated voice calls, providing access to the
traditional voice PSTN.
This information can be passed between the network and
the user end-point at call set-up time. In North
America, ISDN is nowadays mostly used as an alternative
to analog connection, most commonly for Internet access.
Some of the services envisaged as being delivered over
ISDN are now delivered over the Internet instead. In
Europe, and in Germany in particular, ISDN has been
successfully marketed as a phone with features, as
opposed to a POTS phone (Plain Old Telephone Service)
with few or no features. However meanwhile features that
were first available with ISDN (such as Three-Way Call,
Call Forwarding, Caller ID, etc.) are now commonly
available for ordinary analog phones as well,
eliminating this advantage of ISDN. Another advantage of
ISDN was the possibility of multiple simultaneous calls
(one call per B channel), e.g. for big families, but
with the increased popularity and reduced prices of
mobile telephony this has become less interesting as
well, making ISDN rather unappealing to the private
customer. However, ISDN is typically more reliable than
POTS, and has a significantly faster call setup time
compared with POTS, and IP connections over ISDN
typically have some 30-35ms round trip time, as opposed
to 120-180ms (both measured with otherwise unused lines)
over 56k or V.34 modems, making ISDN more pleasant for
telecommuters.
Where an analog connection requires a
modem, an ISDN connection
requires a terminal adapter
(TA). The function of an ISDN terminal adapter is often
delivered in the form of a PC card with an S/T
interface, and single-chip solutions seem to exist,
looking at the plethora of combined ISDN- and
ADSL-routers.
With
ISDN, voice and data are carried by bearer channels (B
channels) occupying a bandwidth of 64 kbps. Some
switches limit B channels to a capacity of 56 kb/s. A
data channel (D channel) handles signaling at 16 kb/s or
64 kb/s, depending on the service type. Note that, in
ISDN terminology, "k" means 1000 (103), not
1024 (210) as in many computer applications;
therefore, a 64 kbps channel carries data at a rate of
64000 bps.
There are
two basic types of ISDN service: Basic Rate Interface (BRI)
and Primary Rate Interface (PRI). BRI consists of two 64
kb/s B channels and one 16 kb/s D channel for a total of
144 kb/s. This basic service is intended to meet the
needs of most individual users, and is what we refer to
when we speak about Dual ISDN.
To access BRI service, it is necessary to
subscribe to an ISDN phone line. Customer must be within
18000 feet (about 31/2 miles) of the
telephone company central office for BRI service; beyond
that, expensive repeater devices are required, or ISDN
service may not be available at all. Customers will also
need special equipment to communicate with the phone
company switch and with other ISDN devices. These
devices include ISDN Terminal Adapters (sometimes called
"ISDN Modems") and ISDN Routers.
Dual ISDN is a line
type that is equivalent to a BRI. It provides two B
channels and one D channel, giving an aggregate
bandwidth of 128000 bps.
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