A Beginner's Guide to the Internet/INTERNET.ISO/text/faqs/isdn.inf

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An Introduction to ISDN

Editor's note: This article is reprinted with permission.  It was
originally adapted and reprinted, with modifications, from Merit
Network News, March 1989, for Information Technology Digest,
University of Michigan, September 28, 1992.

What Is ISDN?

ISDN (Integrated Services Digital Network) is a public integrated
voice and data digital network that can deliver data 50 times faster
than a 2,400 bit-per-second modem.  It does this digitally, by
creating two 64 kilobit-per-second (Kbps) transmission channels that
can carry voice or data, and one 16 Kbps transmission channel that
carries signaling information or packet data.

How ISDN Evolved

To understand the evolution of ISDN, consider the current telephone
network.  The basic telephone is an analog instrument connected to a
pair of wires.  "Analog" means that signals are transmitted by varying
the frequency and intensity of the electric current analogously, in
this case, to changes in the speaker's voice.  The pair of wires from
a subscriber's premises--a private home, for example--is connected to
a local telephone company's central office.  This pair of wires is
commonly called the "local loop."

    Inside the central office, the pair of wires is attached to a
device called a switch.  The switch converts the analog signal to a
digital signal by sampling it thousands of times a second.  (Digital
signals consist of only two discrete voltage levels corresponding to
binary 0 and 1.)  The switch also routes the call by examining the
telephone number dialed.  If the call is long-distance, the central
office routes it to an Interexchange Carrier (IEC) such as AT&T, MCI,
or Sprint.  The IEC in turn routes the call to the local telephone
company at the destination, still preserving the digital nature of the
signal.

    Digital signals are easily carried over long-distance lines
because they can be sent simultaneously, or "multiplexed" for
transmission on high-capacity links.  Digital signals are also less
susceptible to line interference during amplification than analog
signals.

    When the destination switch receives the digital signal, it
converts the digital signal back to analog and sends it out over the
local loop at the other end.

    This conversion between digital and analog seems reasonable since
humans cannot hear or speak digitally.  But what if we intend to
exchange digital information between two computers?  In that case, we
must convert digital information from our computers into analog
signals using a modem.  When these signals reach the telephone
company's central office, they are converted back to digital.  The
digital signal can only be a sampling of the "noise" coming out of the
modem, not a regeneration of the original digital bit stream from the
computer.  The reverse process is used at the destination switch to
convert the digital signal back to analog and pass it over the local
loop to the destination modem, which finally turns it back for the
last time to a digital signal.

    This process is not only redundant, it is inefficient.  When voice
is converted from analog to digital, a bit rate of 56,000 bits per
second (bps) is typically dedicated to carrying it.  This rate is
required to make sure that the voice will sound natural when it is
converted back to analog.  Since the telephone network treats modems
the same way, a rate of 56,000 bps is also required to convey modem
signals.  However, since many modems still send and receive data at or
under 9,600 bps, the rest of the capacity is wasted.

    Modems serve another purpose apart from digital transmission.
Most modern modems incorporate automatic dialing and answering
functions.  We say that an autodial modem exchanges "signaling"
information with the telephone network.  The modem can be instructed
to place a call and report its progress: examples of what it can
report back are "ringing," "busy," or "no circuits available."

    Again in this case, because the telephone network is designed for
voice, computer equipment is at a disadvantage.  The modem requires
special hardware to detect (actually to listen to and guess) the sound
of a busy signal, ring, or "call incomplete" message.  This type of
signaling is not only analog but also "in-band"; that is, signals and
real transmitted information use the same channel.  Sharing a single
circuit to convey both transmission and signaling information imposes
serious limitations.

    ISDN relieves the limitations of both in-band signaling and analog
transmission.  A standard ISDN interface provides end-to-end digital
transmission and separates the signaling functions from the
transmission functions.

ISDN Basic Rate Interface

The ISDN basic rate interface is the standard interface to connect
subscribers to the ISDN.  This interface uses the existing telephone
wire pair to convey only digital information.  Three channels or
digital paths exist on this wire.  The channels are multiplexed by
giving each a time slice on the wire.

    Each ISDN circuit includes three channels:

+   Two B or bearer channels for carrying data or voice (64,000 bps
    each).

+   One D or data channel for carrying signaling information or packet
    data (16,000 bps).

There is no distinction between voice and data on the B channels; the
ISDN treats both as a stream of bits.  The bits have significance only
to the terminating equipment, such as a telephone for voice or a
computer for data.  When a subscriber wishes to place a call, the
terminating equipment sends a packet on the D channel containing the
information needed by the network to establish the call.  Assuming
that the call succeeds, the subscriber may then send either voice or
data on a B channel.  To end the call, the terminating equipment sends
a take-down packet; this is analogous to hanging up.

What Is an Integrated Service?

An integrated service can provide a wide assortment of information in
a single well-organized package.  This information may be in the form
of voice, data, or video.  Initially, services available on ISDN will
not be integrated.  Voice and data, although they may be accessed
together on an integrated terminal, have little to do with one
another.  Voice calls will involve only voice and data calls only
data.

    The second generation of ISDN services will be integrated.  For
example, consider a future bank credit card service.  A credit card
holder who disputes an entry in the credit card bill places an ISDN
call to the bank.  At the bank, a customer service representative
equipped with an ISDN terminal answers the call.  The bank teller
immediately has access to the caller's name and records, since the
ISDN passes the customer's originating address.  The bank uses this
address as a key into its customer database.  The teller can address
the customer by name when answering the phone.  When the customer
explains the nature of the problem, the teller retrieves the previous
month's bill, which appears simultaneously on both the customer's and
the teller's screens.  If the statement is in error, the balance can
be recomputed before the customer's eyes.

Implications

ISDN will do for the data network what the Communications Act of 1934
did for voice--provide a standard method for public transmission.  It
will extend the capabilities of today's telephone networks, thus
providing a market for new services.

- Dory Leifer, ITD Network Systems, University of Michigan

        Berkeley Computing, Volume 3, Number 4 (May-June 1993)

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