Digital Networks

Elegant though these solutions are, though, they are rapidly being
replaced by totally digital schemes. Analogue systems would be very
wasteful when all that is being transmitted are the discrete audio
tones of the output of a modem. In a speech circuit, the technology
has to be able to 'hear', receive, digitize and reassemble the entire
audio spectrum between 100 Hz and 3000 Hz, which is the usual
passband of what we have come to expect from the audio quality of the
telephone. Moreover, the technology must be sensitive to a wide range
of amplitude; speech is made up of pitch and associated loudness. In
a digital network, however, all one really wants to transmit are the
digits, and it doesn't matter whether they are signified by audio
tones, radio frequency values, voltage conditions or light pulses,
just so long as there is circuitry at either end which can encode and
decode.
There are other problems with voice transmission: once two parties
have made a connection with each other (by the one dialling a number
and the other lifting a handset), good sense has suggested that it
was desirable to keep a total physical path open between them, it not
being practical to close down the path during silences and re-open it
when someone speaks. In any case the electromechanical nature of most
of today's phone exchanges would make such turning off and on very
cumbersome and noisy.
But with a purely digital transmission, routing of a 'call'
doesn't have to be physical--individual blocks merely have to bear an
electronic label of their originating and destination addresses, such
addresses being 'read' in digital switching exchanges using chips,
rather than electromechanical ones. Two benefits are thus
simultaneously obtained: the valuable physical path (the cable or
satellite link) is only in use when some intelligence is actually
being transmitted and is not in use during 'silence'; secondly,
switching can be much faster and more reliable.

Packet Switching
These ideas were synthesised into creating what has now become
packet switching. The methods were first described in the mid-1960's
but it was not until a decade later that suitable cheap technology
existed to create a viable commercial service.

The British Telecom product is called Packet SwitchStream (PSS) and
notable comparable US services are Compuserve, Telenet and Tymnet.
Many other countries have their own services and international packet
switching is entirely possible--the UK service is called,
unsurprisingly, IPSS.
International Packet Switched Services and DNICs
INTERNATIONAL NETWORKS
Datacalls can be made to hosts on any listed International Networks.
The NIC (Data Network Identification Code) must precede the
international host's NUA. Charges quoted are for duration (per hour)
and volume (per Ksegment) and are raised in steps of 1 minute and 10
segments respectively.

Country                         Network                   DNIC
Australia                          Midas                       5053
8elgium                           Euronet                     2062
Belgium  Euronet 2063
Canada Datapac 3020
Canada Globedat 3025
Canada Infoswitch 3029
Denmark Euronet 2383
France Transpac 2080
French Antilles Euronet 3400
Germany (FDR) Datex P 2624
Germany (FDR) Euronet 2623
Hong Kong IDAS 4542
Irish Republic Euronet 2723
Italy Euronet 2223
Japan DDX-P 4401
Japan Venus-P 4408
Luxembourg Euronet 2703
** Page 72
Netherlands Euronet 2043
Country Network DNIC
Norway Norpak 2422
Portugal N/A 2682
Singapore Telepac 5252
South Africa Saponet 6550
Spain TIDA 2141
Sweden Telepak 2405
Switzerland Datalink 2289
Switzerland Euronet 2283
U.S.A. Autonet 3126
U.S.A. Compuserve 3132
U.S.A. ITT (UDTS) 3103
U.S.A. RCA (LSDS) 3113
U.S.A. Telenet 3110
U.S.A. Tymnet 3106
U.S.A. Uninet 3125
U.S.A. WUI (DBS) 3104
Additionally, Datacalls to the U.K. may be initiated from:
Bahrain, Barbados, Bermuda, Israel, New Zealand and the United Arabs
Emirates.
Up to date Information can be obtained from IPSS Marketing on
01-9362743
In essence, the service operates at 48kbits/sec full duplex (both
directions simultaneously) and uses an extension of time division
multiplexing Transmission streams are separated in convenient- sized
blocks or packets, each one of which contains a head and tail
signifying origination and destination. The packets are assembled
either by the originating computer or by a special facility supplied
by the packet switch system. The packets in a single transmission
stream may all follow the same physical path or may use alternate
routes depending on congestion. The packets from one 'conversation'
are very likely to be interleaved with packets from many Other
'conversations'. The originating and receiving computers see none of
this. At the receiving end, the various packets are stripped of their
routing information, and re-assembled in the correct order before
presentation to the computer's VDU or applications program.