Side_1_360

sentation is easily converted by the computer
to and from decimal before being displayed,
printed, etc.

Any kind of information, e.g. speech, music,
images, etc. may be digitized. Some types of
information such as text and numbers are inher-
ently digital. In particular co-operating comput-
ers need to exchange special control information
between them. Standards exist that specify pre-
cisely how various types of information are rep-
resented. Binary information is represented by
pulse patterns. Typically, pulses are grouped in
“bytes” of eight bits each. Various modulation
methods exist for representing bits in transmis-
sion channels. Examples are pulse or no pulse,
positive or negative pulse, high or low tone, and
others.

In its earliest days telecommunications had to
live with “the facts of life” that noise and loss
along lines were inescapable limitations. Trans-
mission theory and technology have matured
over the years. Information carrying capabilities
and limitations of various media are now well
understood. Techniques have been developed to
exploit each medium according to technical and
economic criteria.

One way to look at Internet technology from a
telecommunications standpoint is that it is a uni-
fied method of supporting a large number of dif-
ferent telecom applications together. Varying
requirements of individual applications may be
specified and met accordingly. Various capabili-
ties of available carrying media may be ex-
ploited. These requirements may vary over time
and be met automatically in near optimal combi-
nation even in dynamically changing situations.
Coding methods are available for error detection
and -correction to any level of dependability.

Consider the following examples. If a packet
comprises representation of monetary amounts,
the receiver must not accept any errors inflicted
on the packet during transit. Built-in error detec-
tion and -correction are paramount. If necessary
the damaged packet must be retransmitted until
acceptance. And that is more important than time
delay. In another case a packet represents a piece
of sound, e.g. part of a spoken word. In that case
it is more important for the packet to get there in
time, perhaps distorted, than to be guaranteed
correct and too old to be of any interest.

In a packet switched net the nodes consist of
packet switches interconnected by digital chan-
nels transmitting streams of packets. The switch
has a number of at least three incoming and out-
going channels, two in the case of a gateway
between two different types of net. Each incom-
ing packet is transmitted onwards in the right

direction according to the address in the packet

header and the switch’s knowledge of the net.

That includes alternative routes and current traf-

fic load. Such knowledge is available to the

switch in the form of dynamic routing tables.

Queuing, re-packaging and retransmission may

happen according to requirements specified in

the packet and capabilities of the net, including

the current traffic situation.

Quality and Efficiency

A telecommunications network may be line

switched or packet switched and the transmis-

sion may be analog or digital. Digital informa-

tion may be transmitted in analog channels by

modem-units that modulate and demodulate to

and from analog representation. All these forms

of transmission may transfer all types of infor-

mation. But they have different qualities and

efficiencies of different significance for various

information types. Analog transmission is tradi-

tional for sound and pictures. The information in

them is inherently analog. Text and numbers are

naturally digital information. Analog transmis-

sion has quality limitations, noise and inaccu-

racy that can only be partly corrected. For nor-

mal telephone connections and some others

these limitations have little or no practical sig-

nificance.

Original

Distorted in

transmission

Symbols

recovered by

sampling at clock periods

10 1101 001

10 1101 001

Bit symbols distorted and

recovered. Here the symbol

“1” is represented by “pulse”

and “0” by “no pulse”

Decimal Binary

0 0000

1 0001

2 0010

3 0011

4 0100

5 0101

6 0110

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16 1000

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Decimal and binary number

symbols