Side_1_360

Network of Nets and

Packet Switching

Information gets transported through the net in
packets. The net is said to be packet switched.
A packet consists of a certain number of bits –
binary digits – e.g. 2048 bits plus a “packaging”
of a few bits. Packet size (number of bits) may
vary for different applications and different nets.
This packaging of header bits and trailer bits
specify how the packet is to be treated in the
transport net, and identifies the packet to the
receiver. Each digit assumes value 0 or 1. In
transmission each bit is typically represented as
“pulse or no pulse” at defined instants in time.

The initial Arpanet had permanent leased lines
connecting the nodes. The further development
comprised, among other things, possibilities for
other types of transmission carriers. Special
interest concerned packet radio nets where each
node had a radio transmitter and -receiver and all
nodes used the same radio channel – carrier fre-
quency. The purpose was to exploit such infor-
mation carrying media and their special proper-
ties. They have potential for connecting ships,
aircraft, cars, persons, as well as more or less
temporary platforms such as oil rigs and other
stations in wayless territory or in developing
areas having inadequate permanent installations.
Further developments comprised ways of using
satellite channels and special cable channels in
similar ways.

Intensive basic research and development was
carried out through the 1970s. It turned out early
that the logical rules – protocols needed to ex-
ploit each carrying medium – were rather differ-
ent from each other. An overriding goal was that
each node should function independently with-
out a common control centre.

It was discovered that advantage could be gained
by keeping each carrier medium separate as an
individual net, e.g. packet radio net, packet satel-
lite net, local broadband cable net, etc. Gateway
computers then interconnected each of these
nets. Each gateway appeared as a host computer
to each of the two nets that it connected, behav-
ing according to the protocols of the individual
nets. The gateway re-packaged each packet
accordingly. Hence transfer could be done
according to the individual network protocols
and each net was exploited well.

Stars and Masks

Many computer networks were built and oper-
ated, for practical commercial purposes, long
before the Arpanet and the Internet. Notable are
large networks for airlines, as mentioned. Private
and public organizations had been using net-
works of computers since around 1960. Perhaps
most notable were networks and associated stan-
dards developed and built by IBM.

The first computer networks were mostly if not

exclusively star shaped. The transfer channel

between any two points A and B in a star shaped

network is unique. The Arpanet/Internet devel-

opment consistently made use of mask shaped

nets. Transport between arbitrary points A and B

in a network may then travel alternative routes.

This strategy, well known in traditional telecom

networks, has many advantages. In the more

Packet

“Payload” bits

Trailer

bits

Header

bits A packet switched network

transfers streams of packets.

Each packet has a limited

amount of “payload”

information plus a

“packaging” of some header

and trailer bits for addressing

and transport specifications.

Transfer of a packet over a

line only occupies the line for

the short time of that limited

number of pulses

A

B

A

B

Star shaped network

Mask shaped network