***** BULLETIN 11 - ISDN Introduction
***** Reprinted by permission from the Merit Network News, Vol 4, No. 1
***** March, 1989
***** Special thanks to Sheila Ryle for typing this onto disk.
***** For more information, call Merit Network News at (313) 764-9430

                       An Introduction to ISDN

INTRODUCTION

    Motivated by the ever increasing public need to send digital
information in the form of voice, data or image, national governments
along with private corporations have developed a scheme called
Integrated Services Digital Network (ISDN).  Although this concept
dates back to the early 1970s, only recently have standards been
developed.  The standardization of ISDN has resulted in an emerging
market of ISDN equipment and service plans.  This technology will have
widespread impact on both suppliers and users of network equipment and
services.

    In the United States, all seven regional Bell operating companies
have initiated limited testing and deployment of ISDN.  General
deployment is expected during the mid to late 1990s.  Our European and
Japanese counterparts are committed to the nationwide implementation
of ISDN.


    ISDN will spur technological development of new and innovative
products and services for both research and business.  This article
introduces the basic concepts of telephone networks and ISDN and
explores possible applications of ISDN technology.


THE TELEPHONE NETWORK

    In order to understand why ISDN evolved, let's look at 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 in response, in this case, to changes in the speaker's voice.
Digital signals, in contrast, consist of only two discrete voltage
levels corresponding to binary 0 and 1.  The pair of wires from a
subscriber's premises, a private home for example, is connected over
approximately a mile of cable to a local telephone company's central
office.  This pair of wires is commonly called the "last mile" or
local loop.


    Inside the central office, the pair is attached to a device called
a switch.  The switch converts the analog signal to digital by
sampling it thousands of times a second.  The switch also routes the
call by examining the telephone number called.  If the call is long-
distance, it is routed by the local telephone company, Michigan Bell,
for example, to an Interexchange Carrier (IEC) such as AT&T, MCI, or
US Sprint.  The IEC routes the call to the local telephone company at
the destination, still preserving the digital nature of the signal.


    Digital signals can be carried easily over long distance lines
because they can be combined or multiplexed for transmission on high
capacity links.  Digital signals also are not very susceptible to
noise during amplication.  When the destination switch receives the
digital signal, it converts the digital signal back into analog and
sends it out over the local loop at that end.


    This conversion between digital and analog seems reasonable for
voice since humans (even programmers) cannot hear or speak digitally.
But what if we intend to exchange digital information by connecting
two computers together?  In that case, we must convert digital
information from our computers into analog signals using a modem.


    When these signals reach the 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
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 to the destination modem which finally turns it back for the
last time to a computer bit stream.


    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, most modems send and receive at or under 2400 bps.
The rest of the capacity is wasted.


    Modems serve another purpose apart from digital transmission.
Most modern modems incorporate automatic dialing and answer functions.
We say that a autodial modem exchanges signalling 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", and "no circuits available".


    Again in this case, because the telephone network is designed for
voice, computer equipment is disadvantaged.  The modem requires
special hardware to detect (actually to listen and guess) the sound of
a busy signal, ring, or call incomplete message (usually preceded by
three tones.)  This type of signalling is not only analog but it is in
band:  that is, signals and real transmitted information use the same
channel.


    On a phone line, you cannot start dialing unless you hear a dial
tone.  A dial tone means that your phone is connected to a device at
the telephone company ready to accept call initiation.  If a call is
in progress and you try to dial, the person at the on the other end
hears an upleasant tone.  Sharing a single circuit to convey both
transmissiong and signalling information imposes serious limitations.


    ISDN relieves the limitations of both in-band signalling and
analog transmission.  The next section describes a standard ISDN
interface which provides end-to-end digital transmission and separates
the signalling 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.  Instead of using this pair for analog signalling and
transmission, only digital information is converyed.  On this wire,
three channels or digital paths exist.  The channels are multiplexed
by giving each a time slice on the wire.  Since ISDN channels are half
duplex or uni-directional, a "ping-pong" method is used so that when
one end transmits, the other listens.  The ping pong happens with
every tick of some central clock so the link appears to be
bidirectional.


    Each ISDN circuit includes three channels:
          2 B or Bearer channels for data or voice (each 64,000 bps)
          1 D or Data channel for signalling or packet data (16,000
            bps)
          These channels provide both signalling and transmission.


    Notice that there is no distinction between voice and data on the
B-channel.  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 in order 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, a take-
down packet is send.  This is analogous to hanging up.


Bearer Channel Transmission

    The B-channel is referred to as a clear channel because of its
ability to pass an arbitrary bit stream transparently.  In reality, an
arbitrary bit patterns have limited uses since the B-channel must
adhere to the disciplines of existing voice and data networks.
Sending voice using some non-standard encoding would preclude placing
calls between the ISDN and the existing telephone network.  A standard
Pulse Code Modulation (PCM) scheme has been standardized for digitized
voice because it is compatible with the existing voice network.


    Correspondingly, a data protocol must be employed on the B-channel
if the subscriber is to reach hosts on the existing packet services
which are not yet on the ISDN.  Even if the host is on the ISDN, the
network provides no guarantee that the data wil be transmitted without
errors.  This is not a serious problem with terminal sessions (we live
with error-prone modems), but for computer to computer connections
(for example, performing a file transfer) an error-correction protocol
may be required.


    The B-channel itself provides services that comply with layer one
of the Open Systems Interconnection (OSI) Reference model (the
physical layer).  That is, it offers a medium through which bits may
pass.  (For information on OSI protocols, refer to the Dec. 1988-Jan.
1989 Merit Network News.)


    If a subscriber uses the ISDN to call another computer directly, a
minimum of a layer-two protocol is involved for error correction and
flow control.  In many cases, the subscriber will wish to access a
host on a packet network like Telenet.  In this case, both a link
layer (OSI layer two) and network layer (layer three) are required.
The subscriber then uses the X.25 protocol between the ISDN and his or
her machine.  An interworking unit acts as a gateway between the ISDN
and the packet network, using the X.75 protocol.


    A somewhat similar service could be deployed by Merit in the
future to provide Internet access for ISDN subscribers.  Off-campus
users could place an ISDN call to an Internet gateway.  They could
then access TCP/IP applications like file transfer, remote terminal,
and mail.  ISDN provides added support in this case:  since the ISDN
would report the caller's address, a unique Interenet address could be
associated with a particular calling address.  Other services which
require authentication of the caller would also be facilitated by this
feature.


The Data Channel

    The Data or D-Channel was originally specified by the CCITT for
signalling but later was re-specified to include both signalling and
transmission of packet data.  Unlike its sister B-channel, the D-
channel is not designed to carry an arbitrary bit stream.  The D-
channel uses both a link layer, Link Access Protocol-D (LAPD), similar
to HDLC, and a network layer, Q.931, similar to X.25.


    The D-channel may be used for packet data when data throughput is
not of high priority.  No call set-up or take-down is required when
using the D-channel to interface in packet mode.


    The signalling protocol on the D-channel is based on the set of
signalling messages needed to establish and release a simple 64,000
bps B-channel voice or data connection.  Included in call set-up are:
    Flexible addressing compatible with many standard networks
    Required data rate
    IEC (long distance carrier) selection if applicable
    Notification if line forwarded to another address
    User information text


    Signalling information is exchanged between a subscriber and the
ISDN.  But this information must also be passed within the ISDN to
assure timely circuit establishment, efficient allocation of
resources, and accurate billing and accounting between various service
providers.  A protocol called Common Channel Signalling Number Seven
(CCS7) performs these functions.  CCS7 was designed by AT&T and is
based on the international standard CCITT Signalling System Seven
(SS7).  CCS7 is already used on a wide scale for signalling in the
non-ISDN world but will be essential to support ISDN.


Equipment

    Compatibility with existing equipment is extremely important to
most of the users who will migrate from switched and private networks
to ISDN.  Therefore, most of the early ISDN equipment whcih users will
purchase will be adapters for non-ISDN devices such as asynchronous
terminals with RS-232 interfaces, 3270 style terminals with IBM SDLC
and coax interfaces, and various LANs.  An interface to connect common
analog telephones will surely be a hot seller.


    Many of these devices are quite complex because they have to
support both signalling and transmission.  For example, an adapter
which allows RS-232 attachment for terminals needs to interface with
both the B- and D-channels.


    Under development by several manufacturers are integrated
terminals that combine voice, data, and signalling into a compact
desktop pakcage.  Initially, these terminals will function as
expensive desktop space savers, replacing a separate phone and
terminal, but later they will provide access to truly integrated
services.


What is an Integrated Service?

    The concept of an intergrated service is an abstraction rather
than a set of particular CCITT recommendations.  An integrated service
is one that is capable of providing a wide assortment of information
well organized into a single package.  This information may be, for
example, in the form of voice, computer data, video, or facsimile.


    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.  We speak of this
relationship as Service Co-existence.


    The second generation of ISDN services will e integrated.  For
example, consider a future bank credit card service.  A card holder
who disputes an entry in the credit card bill places an ISDN call to
the bank.  Ah the bank, a customer representative equipped with an
ISDN terminal answers the call.  The bank representative immediately
has access to the caller's name and records since the ISDN passes the
customers's origianting address.  THe bank uses this address as a key
into its customer database.  The representative can address the
customer by name when answering the phone.  When the customer explains
the nature of the problem,  the bank representative retrieves the
previous month's bill, which appears simultaneously on both screens.
If the statement is in error, the balance can be recomputed before the
customer's eyes.  Integrated services can also facilitate research
collaboration via multi-media voice, image, and control functions
between scientists.


    Applications which require exchange of only short, infrequent
messages can use services offered by the D-channel.  Applications such
as burglary alerting, energy control, credit card verfication, cable
TV requests for service, and home shopping can be accomplished using
the D-channel packet facilities.


Advantages of Circuit Switching

    Although the data rate of 64,000 bps may be too slow for
bandwidth-intensive applications like real-time high definition
imaging, ISDN's circuit-switched capabilities do offer several
advantages to the research community over packet-switched networks
like Merit, NSFNET, or ARPANET.  Certain real-time applications which
require cross-country connectivity can be run over ISDN.  Although the
individual circuits which comprise moderm packet networks may be much
faster than 64,000 bps, the overhead involved in packet switching and
queueing is far in excess of similar circuit switching functions on an
established call.


    Packet networks try to optimize aggregate performance across the
entire network.  Real-time applications are usually interested not in
averages but rather in worst cases.  If you get a 64,000 bps ISDN
circuit,  you will be guaranteed 64,000 bps service for the duration
of the connection.  Throughput on a packet network might average
150,000 bps, for example, but might fall below 64,000 bps 10% of the
time, causing serious problems for a real-time system.


    Another advantage ISDN has over packet networks is its potential
ability to interface to a wide variety of digital laboratory
equipment.  The ISDN B-channel offers clear channel transmission.
There is no protocol oeverhead involved in order to exchange
information.  This bit pipe can be used, for example, between
detector/collector paired devices without the complication and expense
of packet protocol gateway machines at each end of the conngence, and through
frequency relationship, harmonic structures, and interdimensional
relationships, captures and recaptures, generates and regenerates
all of the infinities of life.
     The materialist who has an ant's eye-view of the infinite
cosmogony is always prone to say that this is so or not so; and
like the potter who sits at the wheel, he molds his vessel of
life from the ideas, forms, and patterns which he has gained
through countless centuries.  But seldom does he remember that
this vessel carries water only in proportion and amount to the
way in which it is fashioned; and that these few drops of water
which he contains in his vessel of life are but small drops
compared to the great oceans and seas of timeless wisdom from
which he will constantly fill each new vessel of life.  In the
pageantry and in the many colored pages of history, it is easy
for any one man to select what he considers suitable portions and
to use these things to make himself a Jacob's cloak, a patchwork
of many pieces and colors which is neither serviceable nor warm.
     To further enlarge upon the principles which cause the
resurgence of life, individually or collectively, finite or
infinite, we must at this point include another one of the
thoroughly misunderstood concepts of life called mental
telepathy, thus killing two birds with one stone, since the same
issue of "Fate" contains an article on this subject.  Here again
                                                  6


is a very fine display of the more literate forms of the English
language but which fails to prove anything, except that the
author was over-biased, his knowledge on the subject antique and
superficial and lacked the common knowledge on the basic
principles of life, just as did the article on reincarnation.  As
both of these subjects are part of these all-inclusive principles
and are synonymous in many respects, let us go into the more
scientific facts which, functionally speaking ,whether telepathy
or any other form or w
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