Integrated Services Digital Network, called ISDN for short, is an
all-digital telecommunications technology that can simultaneously transmit voice
conversations and data calls over the same pair of copper telephone wires. What's
important about ISDN and what makes it different from the analog phone lines that users'
probably using today is the speed with which it transfers data and the flexibility it
providers to users.
ISDN is part of an evolutionary process that started back in the 1950s
to upgrade the telephone network from analog to digital technology. The ultimate goal of
ISDN, designed over a decade ago by international standards organizations, allows users to
access voice, data, fax, video and graphics over the same phone line and enjoy the
error-free performance that digital technology provides. Ultimately, this means that any
communications equipment could be plugged into any phone jack, anywhere in the world.
Although this goal is still a long way off, ISDN has already been widely implemented by
telephone companies in both Europe and Japan. Until recently the lack of ISDN standards
and ISDN-compatible switching equipment had hampered growth in the U.S.
But all that is changing. Four years ago, switch and terminal equipment
vendors led by Bellcore agreed on a firm set of technical specifications called National
ISDN-1, a suite of interfaces, capabilities and functions which ensure that ISDN equipment
manufactured by different vendors can inter-operate. The agreement on standards was the
catalyst needed for the telecommunications industry to begin aggressively developing and
deploying ISDN products and services. Over the last year, demand for ISDN has grown
dramatically and now ISDN sales are booming. Its now easy to have ISDN installed in either
home or office almost anywhere in the world. -- it is simply ordered from the local phone
company in the same manner as regular telephone service.
ISDN Basic Rate Interface (ISDN BRI) operates over most of the
telephone wiring in place today - the copper twisted pair cables that have carried
telephone conversations for most of this century. However, ISDN brings the speed,
flexibility and reliability of digital communications right into users home or small
business, delivering the total bandwidth of a 144 kbit/s digital line into three separate
channels. Two of the channels, called B (Bearer) channels, operate at 64 kbit/s and are
always used to carry voice or data traffic. A third channel, the D (Data) channel, is a 16
kbit/s signaling channel used to carry instructions which tell the telephone network how
to handle each of the B channels. The combined BRI is often referred to simply as "2B
+ D".
The flexibility of ISDN comes from its ability to use each of the B
channels for separate voice or data applications. With ISDN in place, for example, a long
document can downloaded from the corporate network over one ISDN 64 kbit/s B channel
connection. At the same time that the document is downloading, another B channel may be
connected to the Internet to browse a competitor's World Wide Web site.
Even higher ISDN performance can be achieved by using inverse
multiplexing, a feature available on many types of ISDN access equipment. Inverse
multiplexing creates extra bandwidth by aggregating the two B channels of a BRI line into
a single "virtual" channel of 128 kbit/s or by combining multiple BRI lines for
even higher bandwidth. Under most circumstances, speeds of 256 or 512 kbit/s over a single
BRI line can be achieved using two-to-one or four-to-one data compression.
Another available ISDN service is ISDN Primary Rate Interface (ISDN
PRI), also called "30B + D". PRI is designed to handle large numbers of incoming
ISDN calls at corporate offices or other central site locations. All of the reliability
and performance features of ISDN BRI apply to ISDN PRI, but with 30 B channels of 64
kbit/s each that carry user traffic and a shared 64 kbit/s D channel that carries
signaling traffic.
| What are the advantages of ISDN
over Analog |
With a basic understanding of how ISDN works, it's advantages over
traditional analog technology become readily evident.
Based on speed alone, ISDN overwhelmingly outperforms analog
communications. Top data rates for ISDN are four times faster than what the latest V.34
modem technology can manage over regular phone lines (28.8 kbit/s). And ISDN is nearly
nine times faster than V.32bis modems, the most commonly used modems in the marketplace
which transmit data at just 14.4 kbit/s.
ISDN offers incomparable benefit when it comes to call setup time. By
using digital signaling capabilities, ISDN sets up calls at speeds of just 300
milliseconds. By comparison, it takes an analog line and a modem between eight and fifteen
seconds to set up a call. These few seconds can amount to considerable expense in terms of
connect time and aggravation, particularly for users whose applications require dial-up
network connections repeatedly throughout the day. By comparison, fast setup lends the
impression of maintaining a 'virtual dedicated' connection to the Internet, whereby
connections are established only as they are needed.
Another advantage of ISDN is its reliability. Digital lines such as
ISDN are virtually error-free, since digital signal does not degrade along the length of
its transmission. Analog calls, on the other hand, are subject to interference such as
static and other line problems that may corrupt data and cause analog modems to
automatically default to slower speeds. Worse yet, problems on an analog line can
frequently cause modems to suddenly terminate connections to the network.
One might conclude from the benefits outlined that ISDN is
cost-effective. For about the same price as an analog line, ISDN provides increased speed
and reliability over analog modems resulting in higher productivity and throughput.
ISDN Service offers several important advantages:
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