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ISDN
The
original version of ISDN employs base band transmission.
Another version, called B-ISDN, uses broadband transmission
and is able to support transmission rates of 1.5 Mbps. B-
ISDN requires fiber optic cables and is not widely
available.
ISDN,
which stands for Integrated Services Digital Network, is a
system of digital phone connections which has been available
for over a decade. This system allows voice and data to be
transmitted simultaneously across the world using end-to-end
digital connectivity.
ISDN
(Integrated Services Digital Network) is a type of digital
phone/data and Internet service that preceded ADSL
(Asynchronous Digital Subscriber Line) and has for the most
part been superseded by it.
Normal
telephone lines carry analog signals that must be amplified
and converted to digital signals by the phone company. This
process introduces not only a slight lag time, but also
distortion in the signal.
Dial-up modems and telephones are examples of equipment that
use analog signals. ISDN makes use of digital signals
running along existing copper lines to increase the data
throughput, reduce line noise and enhance signal quality.
In the
mid 1990s, ADSL was very expensive not widely available.
Companies and individuals wanted a faster way to connect to
the Internet, but the technology behind dial-up modems had
reached its threshold.
ISDN became a viable alternative to provide speeds of up to
128 kilobits per second (kbps), versus the standard
connection of 30-53 kbps with a dial-up modem.
The
most common type of ISDN service for Internet connection is
the Basic Rate Interface, or ISDN BRI. This technology
creates two B-channels on the existing copper lines of 64
kbps each, along with a single 16 kbps D-channel for the
phone line. This separates data channels from the voice
channel, allowing telephone or fax use while online.
While
ISDN is inexpensive and about twice as fast as dial-up
service, it has been largely replaced by affordable DSL
service. An inexpensive ADSL service offers speeds up to 384
kbps, while more expensive versions are improving in speed
all the time. As of fall 2005, standard ADSL speeds range
between 1.5 and 3.0 mbps (megabits per second), or 1536-3072
kbps.
Although ISDN may not be the best choice for
packet-switching networks like the Internet, it is still
widely used for professional audio and broadcast
applications where digital clarity with integrated telephone
services is specifically required.
Small businesses that often use two voice lines, such as
phone and fax, and only require limited Internet
connectivity of, say, an hour or less per day, may prefer
ISDN. ISDN might also be a better choice for high-speed
connections to intranets for video-conferencing, or to
remote networks other than the Internet.
With
ISDN, voice and data are carried by bearer channels (B
channels) occupying a bandwidth of 64 kb/s (bits per
second). 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 (the designator "K" is
sometimes used to represent this value); therefore, a 64
kb/s channel carries data at a rate of 64000 b/s.
A new set of standard prefixes has recently been created to
handle this. Under this scheme, "k" (kilo-) means 1000
(103), "M" (mega-) means 1000000 (106), and so on, and "Ki"
(kibi-) means 1024 (210), "Mi" (mebi-) means 1048576 (220),
and so on.
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.
PRI
is intended for users with greater capacity requirements.
Typically the channel structure is 23 B channels plus one 64
kb/s D channel for a total of 1536 kb/s. In Europe, PRI
consists of 30 B channels plus one 64 kb/s D channel for a
total of 1984 kb/s.
It is also possible to support multiple PRI lines with one
64 kb/s D channel using Non-Facility Associated Signaling (NFAS).
H
channels provide a way to aggregate B channels. They are
implemented as:
H0=384 kb/s (6 B channels)
H10=1472 kb/s (23 B channels)
H11=1536 kb/s (24 B channels)
H12=1920 kb/s (30 B channels) - International (E1) only
To
access BRI service, it is necessary to subscribe to an ISDN
phone line. Customer must be within 18000 feet (about 3.4
miles or 5.5 km) 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, incorrectly, "ISDN Modems") and
ISDN
Routers.
The
early phone network consisted of a pure analog system that
connected telephone users directly by a mechanical
interconnection of wires. This system was very inefficient,
was very prone to breakdown and noise, and did not lend
itself easily to long-distance connections. Beginning in the
1960s, the telephone system gradually began converting its
internal connections to a packet-based, digital switching
system.
Today,
nearly all voice switching in the
U.S.
is digital within the telephone network. Still, the final
connection from the local central office to the customer
equipment was, and still largely is, an analog Plain-Old
Telephone Service (POTS) line.
In the
early 1990s, an industry-wide effort began to establish a
specific implementation for ISDN in the U.S. Members of the
industry agreed to create the National ISDN 1 (NI-1)
standard so that end users would not have to know the brand
of switch they are connected to in order to buy equipment
and software compatible with it.
However, there were problems agreeing on this standard. In
fact, many western states would not implement NI-1. Both
Southwestern Bell
and U.S. West (now Qwest) said that they did not plan to
deploy NI-1 software in their central office switches due to
incompatibilities with their existing ISDN networks.
Most
recently, ISDN service has largely been displaced by
broadband internet service, such as xDSL and Cable Modem
service. These services are faster, less expensive, and
easier to set up and maintain than ISDN. Still, ISDN has its
place, as backup to dedicated lines, and in locations where
broadband service is not yet available.
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