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letter is part of an individual conversation between the sender and the
recipient, and we can imagine it as part of an ongoing series.

As part of the delivery process the letter has to be carried between
various sorting offices. Each pair of offices will have individual control
of the traffic flowing between them but will only be interested in the
information written on the envelope, not the contents. The van on the
road is only interested in the fact that it is going from A to B; it is not
interested in the individual letters or their envelopes. In fact any carrier
could be used equally effectively though the delivery times may be
different.

This hierarchical arrangement—where each layer offers a service to
the next—is similar to the way in which X.25 works.

1.4.1 Layer one

Layer one simply states how data bits are transferred. This could be
implemented by emitting a squeak to indicate 0 and a squawk to
indicate 1. Such a scheme would be less than satisfactory in most
environments and the normal method is to send an electrical signal.
Chapter 7 covers details of what happens, but essentially there are
two signal paths between each two components. One signal path is
for data in one direction, and the other path is for data in the other
direction. The voltage level determines whether a 0 or 1 is indicated.
Having two signal paths allows data to be sent in both directions
simultaneously.

X.21 is the electrical system used and is a synchronous method of
transmission. This is examined in Chapter 7, but for the moment this
only needs to have one consequence to us which is that there is a
continual “beat” called the clock and it is compulsory to send a bit at
every beat.

1.4.2 Layer two

X.25 layer two defines methods for detecting and correcting errors in
the transmission of bits from one component to another. In fact this is
not precisely true as will be shown in Chapter 6, but it is nearly always
the case.

At this point we have to look at a network with more than one
conversation taking place (see Fig. 1.5). If there is a conversation between
A and P, and one between B and Q, then it is clear that on the wire
between M and N the two conversations have to be multiplexed. The
multiplexing is nothing to do with layer two. Layer two is only concerned
with the correct transmission of data along a wire; it is only concerned
with the carrying of data between M and N. The combination of layer
one and layer two provides a link between the nodes of the network, and
layer two is often referred to as the data link layer.

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