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• Non-permanent connections

In this method the LCNs are allocated dynamically as in Fig. 1.20
when the conversation starts. This method is suitable for terminal-
to-host conversations where it is appropriate to allocate network
resources only when they are required. The disadvantage of this
approach is that there can be a delay whilst the connections are
established, during which data cannot be transferred.

In general the non-permanent approach is much preferred, and where a
permanent conversation is required then it is simply established once
and left allocated. This is simply a decision of network suppliers who
choose not to provide both methods.

To establish connections across a network that does not implement
permanent connections, a Call packet is issued. This does several things:

• The Call packet contains the destination and source addresses,
so can be routed by the nodes through the network.

• As the packet is routed throught the network the LCNs are
allocated to the conversation which we can now give its proper
name—the Call.

• When the packet arrives at the destination a Call Accept packet
(CAA) is issued which returns through the network and indicates
successful establishment of the call.

• Data can now be exchanged.

The connection is referred to as a Virtual Circuit. It is not an actual physical
end-to-end circuit as would be the case with a telephone call. The permanent
and non-permanent connections are referred to as Permanent Virtual Circuit
(PVC) or Switched Virtual Circuit (SVC) respectively.

It is important to realize that the source and destination addresses in
the Call packet are completely separate from the LCNs that may be
allocated. It is only necessary that the nodes should be able to interpret
them and know which physical link to route the call along. In the case of
an X.25 network each address may be from 0 to 15 decimal digits.

When the conversation—or Call—is finished, for instance a user logs-
off from a host, then either end can issue a Clear Request packet (CLR).
This indicates to the other end that the call is finished. The receiver will
“tidy-up” after the call and issue a Clear Confirmation packet (CLC). As
the confirmation travels through the network, the network components
release the resources allocated to the call and pass the confirmation on.
Finally the CLC arrives at the end that sent the CLR and that end can
tidy up after the call and all vestiges of it are then gone.

The layer three call, therefore, has three phases:

• Call establishment—the exchange of CALL and CAA.

• Data transfer.

• Call clear-down—the exchange of CLR and CLC.

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