J
'
d
a
X

PIA No om ME o EINEN

MARCA 27, 1989

This is the
third of a six:
part series on
Integrated Ser-
vices Digital
Network
(ISDN), a com-
munication tech.

nology that will
have great stra-
tegic impor-
tance in the
future.
In considering
the need for
| broadband ser:
| vices, the available bandwidth under a
| digital communication system such as
|3 — ISDN is much greater than with a pre:
|"... dominantly analog network. In the cur-
rent implementation of ISDN, the basic
channel available to all users has a
bandwidth of 64K bits per second, al-
though bandwidths available with fiber
- optics are considerably larger.
As copper wires in the telephone net-

-. work are progressively replaced with fi-
4... beroptic cables, bandwidths available to
ANO sadh subseriber will ine

I
|
|
|

'crease from 64K bps to 150M bps. To
the network operator, the increased ca-
pacity of a network means that more
traffic and greater diversity of services
can be put into operation—both of
which will improve revenue.

Data Communications
Another aspect of bandwidth and ser-
| vices relates to the increasing demand
for computer data communications.
Sending data over an analog telephone
| line reguires the use of a modem. With
a digital circuit, all the user needs to
| connect a computer to the telephone.
network is a standard telephone plug
and the appropriate communications
hardware and software in the computer.
While data communications still ac-
counts for only a fraction of the total
network traffic (voice is, by far, the
| largest proportion), the demand for data
communications is rising rapidly. To ful:
fill this demand both now and in the fu-
ture, networks must be available that
facilitate rapid growth in data commu-

nications at the lowest possible cost. The
best way to do this is through the use of

| digital switehing and high-bandwidth
) transmission networks.

The growing availability of high-band-
width circuits is having a tremendous
impact on users. Corporations that pre-
viously used voice-grade analog circuits

| for computer-to-computer communica-

| tions were freguently restricted to a
communications speed of 1,200 to

bps. These links were A a o
and prone to errors because of noise

|| and distortion. As digital circults be-

such as fiber
i optics, it is possible to transmit informa-

tion at.
tne Very high bit rates (currently in

PO MEEKNAPPLIGATI

bočna Sil
kA |

ON DEVELOPMENT

Broadband ISDN: High-Speed Information Highway

at speeds between 100M and 565M bps.
While 64K bps is the transmission

'rate for a standard digital communica-

tion circuit, higher-bandwidth cireuits
are needed to handle traffic as it passes
through the network. Using the tech- |
nigues of Pulse Code Modulation and
time-division multiplexing, different
multiples of 64K-bps channels are com:
bined to create higher-bandwidth
cireuits. |
In North America, the various multi
plexed 64K-bps circuits are known as T
carriers. The most common is T-1. This
carrier uses paired wires with digital
signal repeaters spaced every 6,000 feet
and has a capacity of 1.544M bps. Each

ital network. Initially, the very-high-ca-
pacity, opticalfiber circuits witha
transmission capacity of billions of bits
per second are being used on long.dis-
tance routes to carry calls between high
traffic centers such as the large cities on
the east and west, coasts of America.

As the cost of optical-fiber technolo-
gy continues to fall, more of the exist-
ing T carriers are being converted to fi-
ber, thus boosting their transmission
capacity. j ij ž

One great advantage of high-capacity
fiber'optic transmission systems is that
many different channels can be accom:
modated within a single circuit, and the
various channels supported by the cir-

ti Hi

In a Single ISDN,

AI Users Can Access
Universal Information

in the future,

both gorporate

and domestic users
will access voice,
data, video and text
on the same highway
worldwide.

Eventually, a single ISDN network will replace

Maryelen Zawatski

k

io

networks, giving users access to a universal "information
highoajj" for all domestic and corporate communications.

T-1 carrier has the capacity to transmit
24 separate 64K-bps channels.

demand for high-band-
eircuits from corporations result-
in T-1 and then T-3 circuits be-
made available to them to lease as
Kori America, T9 drena irenno

its oj
44736M bps.) k]

Both T-1 and T-3 circuits are used ex-

E]

cuit can be confi; ina wi t
Me zatre k uje

The circuit can be configured to carry /
only voice traffic with, in some cases,
many thousands of 64K-bps voice chan'
nels operating over a single fiber. Alter-
natively, the type of traffic could be
mixed; that is, some 64K-bps circuits for
voice, 1.544M-bps circuits for data, other
higner-speed for video or tele-
vision, and so on.

Pace 65

pacity, fiber-optic circuits to carry all
types of traffic at speeds from a few
bits per second to millions of bits per
second,

"The "information highway" is a single
common network to which users have
access for all voice, data, text and video
communications reguirements. Eventual-
ly, a single ISDN network will replace
all existing networks, giving users access
to a universal information highway for
all domestic and corporate communi-
cations.

'The concept of a common, single-
transmission "pipe" is fundamental to
ISDN, as this is the essence of an inte-
grated digital network. As ISDN is con-
structed at present, the information
highway is limited to a transmission
speed of 1.544M bps in North America
and Japan, and 2.048M bps in Europe,

thereby limiting the number of 64K-bps
channels available to either 23 or 30.

'The Next Generation

'The next generation of broadband
ISDN will use fiber-optic transmission
systems to link users with the network.
These fiber-optic networks will provide
a huge increase in the amount of band-
width available to users. It will be com-
mon for corporations to have access to
circuits operating at speeds from 50M to

;500M bps. When cireuits such as these

are available, the concept of a common
digital information highway will be the
building block of future networks.

Future broadband ISDN networks op-
erating at speeds in excess of 500M bps
will be constructed of multiple numbers
of 64AK-bps circuits, creating networks
that provide bandwidth "on demand."

For example, an ordinary telephone
call reguires a single 64K-bps circuit.
The transfer of a large file (containing,
perhaps, a mixture of text, high-resolu-
tion graphics and video) reguires a
much higher bandwidth—perhaps as
much as 50M bps. A common informa-
tion highway can easily accommodate

reguirements. Both high- and low-
speed data can travel over the same
network, leaving it up to the user or the
application itself to determine how
much bandwidth is reguired.

One of the most important aspects of
ISDN as presently configured is that it
En a O for future network de-
velopment. However, a major ji
its limited bandwidth. The polmera z
of the current implementation is that it
ne: promoted the pie a base set

standards upon w E
band networka will be ako

lext week, we will l Š

ic ISDN services and a Ma Ge
You can expect to be using
future. 8 ee

igh Pro-

Software [ Marble-
head, Mass. (800) 245-1240. Fur nJor-

contact (|

| region of billions of bits per second). tensi The design constrainta of analog net. the Urmited Stalen saj ee, in

(o Bit ate of a trillion bits potecini CO ing oki and Varja grla, psi link. — works (which resulted a prete svari Transfer (roki ER, St,
will be possible in the near future. across the coun-. ment of separate networl oice, 1
present, most installations o maček oi uroki peke of voice, data data and telex trattic) oj ekčee, to O a VSE TA ZE
systems in corporate networks operate digital networks. A common inf tion

| T carriers form the backbone of a dig. | highway can be developed using hieh ca. nA Tn

[ i
NEVEM zra