XX. ISDN (DIGITAL) TELEPHONE LINES ISDN or digital telephone service is a mid-point between plain
old telephone service and laser/fiberoptics. It uses the existing network of copper wires, but sends digital
JJ. What is ISDN? The following description has been given for ISDN:
ISDN (integrated services digital network) is a digital telephone line, that can be used for carrying large amounts
of computer data. An ISDN line is actually 2 lines, or channels, each of 64KB capacity. These lines can be
connected together, or aggregated, to provide 128KB of data capacity. In contrast today's fastest analog modem
technology allows for a maximum of 33.6KB of data capacity, and that assumes
a good quality telephone line. See http://www.rmplc.co.uk/rmplc/rmifl/isdnwhat.html.
The following description of ISDN is given by Digitcom Multimedia Corporation:
"ISDN" stands for Integrated Services Digital Network, and, fundamentally, it is a switch in your telephone
company's central office. In contrast to the normal analog switch that (probably) controls your telephone line now,
this switch makes the connection--from the source of your signal through to the device at the other end--digital all
the way. In this way, much, much more information can be crowded onto the copper wires running from your
office, through the phone company switches, on to its destination.
All modems transmitting at 2,400 baud through the "fast" 28.8 kilo-bits per second (Kbs) are analog modems.
These modems take the digital information coming from your computer and translate it into an analog signal
before sending it out over the "plain-old-telephone" (POTS) line. For as long as it is taking place, the circuit can
only handle only that one transmission. At the other end, another analog modem has to translate the analog signal
back into a digital stream of information.
Moving to ISDN phone service is kind of like switching from vinyl records to CDs. Even at its most basic, ISDN
allows your telecom connection to carry as many as three different "conversations" at the same time with the same
twisted-pair copper wires you are now using to have just one voice conversation, or one fax transmission, or one
online modem connection. This is accomplished by using multiple channels of information, increasing the
available bandwidth from the 4kHz provided by POTS "voice" lines to two channels carrying 64,000 bits per
second and a third data and control channel handling 16 Kbs. See
An important aspect of ISDN is the fact that various types of information are integrated into one communication
The integration of different services has become an ISDN hallmark. In the past, video, audio, voice and data
services required at least four separate networks. Video was distributed on coaxial lines, audio over balanced lines,
voice used copper cable pairs and data services required coaxial or twisted pair cables. This multiple plant
environment was expensive to install and difficult to maintain. ISDN is different. It integrates voice, video, audio
and data over the same network and cable plant with quality not available in previous switched services. It offers
features such as on demand networking, automatic bandwidth and on the fly connectivity. These advanced services
are available, in large part, because ISDN is digital. See "Broadband Integrated Service Digital Network," at
LL. Cost and Arranging for ISDN Service Southwestern Bell Corporation describes its ISDN service ("DigiLine") as follows. (The following text is a World Wide Web advertisement):
Southwestern Bell Telephone is introducing a new generation of digital phone service that turns ordinary phone
lines into high-speed digital links. It's called DigiLine(SM) Service, and it's going to change the way businesses of
all sizes do just about everything. It's perfect for home use, too.
DigiLine uses a switched technology called ISDN (Integrated Services Digital Network) to give your existing
phone lines much greater capacity.
In essence, DigiLine is Southwestern Bell's BRI (Basic Rate Interface) offering of ISDN. It reconfigures your
existing line into three channels. Two B (or bearer) channels with a speed of 64 kilobits per second can carry both
voice and data. A third D (or delta) channel carries primarily message signaling but also can be used to carry
packet-mode user data.
The result is a nearly limitless combination of voice, data and image capabilities.
With DigiLine you can send faster, higher resolution faxes to clients while you're talking to them --on the same
line. Or bring people together to edit a document jointly, each at their own terminal in their own office, yet each
with full control over the document on the screen.
Home or Office at High Speeds
From your home or office, you can retrieve valuable information from the Internet and from government,
university and business databases at speeds that would melt down an ordinary modem (up to 128 kbps).
Telecommuters and home-based businesses can have a virtual office anywhere, with complete access to everything
from the Internet to their company's e-mail. Even videoconferencing is possible.
When you integrate the equipment and software that deliver the unique abilities of DigiLine, you can start working
smarter. (Taking full advantage of ISDN technology may necessitate software and computer purchases and/or
Physicians in different hospitals can use DigiLine Service to consult on a medical case, complete with digitized
Financial planners, bankers and investment advisors thrive on information. DigiLine Service can help them search
remote databases much faster than using modems over standard phone lines.
To subscribe to ISDN service, call Southwestern Bell at 1-800-SWB-ISDN (1-800-792-4736). It costs $328.60 to
install one ISDN line, which equates to two lines ("channels") with two telephone numbers. However, if you sign
a 12-month contract the installation fee is $203.60, and if you sign a 24-month contract the installation fee drops to
$78.60 per ISDN line. The cost per month is $58.00 + tax per line. Since one ISDN line can replace two of your
regular telephone lines (which cost approx. $30 to $40 each a month apiece), one ISDN line is probably cheaper
than maintaining two of your current telephone lines.
If you use one channel of your ISDN line for your fax machine and analogue modem, they should both run faster because there is less "noise" on an ISDN line so data transmission on an ISDN line, even using analogue devices, is cleaner and therefore faster. You can use the other channel for a digital modem, digital telephone, etc.