© 2006 DIGIFON
From "The ISDN Studio" by Dave Immer
Audio Engineering Society 99th Convention
Oct. 8, 1995 , New York City
ISDN stands for "Integrated Services Digital Network"
and is so named because with it one can set up calls that handle various combinations
of voice, video, audio, data and control signals on a single digital subscriber
loop connected to the local central office of your telephone company. ISDN is
a worldwide standard accepted in over 100 nations at this time. ISDN service
is delivered to you over a single "twisted pair" of copper wires,
identical to Plain Old Telephone Service (POTS) lines. A POTS line can be converted
to ISDN.
New Affordability
For those of us involved in creating, producing and delivering audio for broadcast,
theatrical, compact disc/tape and other uses, the worldwide public telephone
network is making the process faster and cheaper. Any information that can be
digitized such as still images, video, audio, text, as well as control and sync
signals can be sent over cost effective dial-up temporary "virtual private
circuits". Digital end to end wide-area networking has been around for
some time and has been employed by the military, large corporations, universities
and research groups. But until recently the cost of access and terminal equipment
has made it too expensive for smaller businesses and individuals to participate
in the technology. With the growth of "Switched56" and ISDN telephone
service in combination with cheaper terminal equipment, high speed digital dial-up
access is now cost-effective for many small businesses and individuals.
Increasing Deployment
ISDN telephone service is finally seeing wide deployment throughout the US,
and is getting a lot of publicity in trade publications and the general media.
More and more people involved with professional audio and video production are
becoming interested in the possibilities presented by digital delivery of information
over simple dial-up telephone calls.
ISDN, as a worldwide transmission standard, has been evolving since the early
80's. Billions of dollars have been invested by telephone companies and equipment
manufacturers in developing those standards, designing, building and installing
hardware, and marketing it all. Many of those involved in telecommunications
believe that ISDN is destined to become the "Plain Old Telephone Service"
of the near future. 1995 has seen a faster rate of deployment by local and long
distance service providers than ever. ISDN is being embraced by increasing numbers
of telephone companies worldwide and there is an inertia being built up that
will continue well into the 21st century. Even as faster transmission methods,
such as ATM, are developed, ISDN will continue to be used as a popular and inexpensive
"sub-layer" of these new approaches. In short, ISDN is here to stay.
Network Bandwidth
The marketing of network access services is largely about bandwidth - that is,
how much information per second can be delivered per dollar spent. It doesn't
really matter whether the transmission occurs in free space - as in microwave,
satellite or cellular - or over physical cables such as copper, coaxial or fiber-optics.
The important issue here is: How much will it cost to reliably deliver a given
amount of material in a given amount of time. If existing copper wire can deliver
the goods just as cheap as fiber, why wait for a new transmission medium to
be offered when all you need is here now? Admittedly, some users may have such
high bandwidth requirements that copper wire may not be as cost effective as
fiber, but the crossover point at which that occurs is still pretty high. For
the great majority of pro audio users, at this time, the installed infrastructure
of local copper lines is adequate. The phone companies are able to offer ISDN
service at very reasonable prices for installation, monthly access and usage.
Major Improvements while Maintaining Backwards Compatibility
ISDN is fully compatible with Plain Old Telephone Service (POTS), all the conversions
being done transparently within the network. Voice calls can be made to and
from all other phones in the world. But the real cost effectiveness of ISDN
becomes apparent when you are using it to send data directly from one ISDN user
to another. The ISDN standard specifies an extremely low bit-error rate. When
you create a connection at 64,000 bits per second (64 Kbps), that is the speed
at which your transmission or file-transfer will occur. This is not the case
with analog modems. Since there may be intermittent noise or weak connections
with analog circuits, a modem rated at 28.8 Kbps will "fall back"
to whatever speed the connection can reliably transmit and receive the data.
Many calls will only be able to operate at under 13 Kbps, so it would take you
4-5 times longer than a 64 Kbps connection costing roughly the same per minute
(domestic). If you are sending lots of data (large files, graphics, screen sharing),
this will save a lot of time and money on the phone. When you dial a telephone
call (POTS or ISDN), you are creating a "temporary virtual private circuit"
between yourself and the called party.
In the case of an ISDN call for sending real-time audio or video, the data is
synchronized with a clocking signal. This is an option selected by the users
terminal adapter. Alternately, one can choose to use an asynchronous - or packet
- mode of data delivery if the nature of the material is not time sensitive,
such as text, graphics, code, control signals, etc. The terminal adapter handles
the process of selecting the service options you require for a particular call,
dialing or receiving the call, setting up and maintaining the call, and tearing
down the call (hanging up).