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the trunk line, perhaps through intervening processors to the
destination processor. This then found a line to the required computer
that conformed to the facilities profile that the user requested, and
made a connection. Nowadays this concept is seen as natural, but it
should be appreciated that this was all happening as the mini/
mainframe battle was being fought, and represented a fairly major
change in thinking.

Although this is somewhat less sophisticated than the systems
around today—as will be shown—it has all the hallmarks of a
Network. These are:

• All services (computers) and users (terminals) connect physically
to the network, not to each other.
• The user has to tell the network which service is required to be
accessed, and which facilities such as speed of access, amount of
memory, and peripherals are required.
• The network determines whether the request can be met and, if
so, allocates the service to the user.
• When the user has finished, the network has to reset everything
that has been done back to its initial state.
• Both user and service are users of the network.

By the early eighties attention was being focussed on the network, as it
was an area with high potential growth. It would also require international
standardization to ensure that equipment from all manufacturers could
be made part of a network.

Many types of architecture were developed to implement a network
and many schemes of carrying data along wires were developed. One
such architecture and scheme is Packet Switching which is the subject of
this book. The book will explain and illustrate the subject with particular
reference to X.25 networks.

1.2 Networking solutions

Before describing packet switching and networks that conform to
international standards, it is instructive to take a brief look at other—
older—technologies that have been used to connect users to the services
that they want to access.

The use of multiplexers has already been mentioned as a tool that was
in use right from the start of remote access to computers. The multiplexer
is a straightforward data funnel that has several access points on one
side, and an aggregate or trunk port on the other. The multiplexer then
shares the transmission capability of the trunk between the terminals or
host ports connected to it. The advantage of this scheme, as shown in
Fig. 1.1, is that there is only a single communications link between the
two multiplexers, and therefore there is only a single line rental to pay
and a single pair of modems to buy. It is thuscost-effective to link say a

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