=[ x25-box.com ]=

Fig. 1.18 Conceptual layout of the Data packet

• There is no length indication or termination of the data field.
This is determined from the layer two frame format, or rather
what is delivered by layer two.

Returning to Figures 1.16 and 1.17 we can now illustrate the conversation
fully (see Fig. 1.19).

Fig. 1.19 Frame layouts for the conversation showing the layer three. Data packet within the layer two Info frame

The LCI field is expressed as three hex digits and is assigned arbitrarily
by the two participating layer two nodes. In the example the first LCI to
be used was 400 and the next 401. Each LCI is a separate conversation
and has its own layer three P(R) and P(S) values. The two nodes will be
configured with a range of LCIs that may be used.

The LCI is specific to each link, and several different LCIs may be used
for the same conversation on the different links across the network. This
is shown in Fig. 1.20.

If any of the conversations were to close down then the LCIs in use
would be deallocated and would be available for new conversations.

The LCI is actually made up of two parts:

• The Logical Channel Group (LCG) which is the first hex digit.

• The Logical Channel Number (LCN) which is the last two digits.
It is normal to refer to the LCI as the LCN, even though this properly
only identifies part of the data. Network administrators usually impose
some structure on the LCN, and this is discussed in Chapter 4.

There are two ways in which LCNs may be allocated:

• Permanent connections

In this method the LCNs are allocated automatically when the
network components start up. The components have to be
configured to make appropriate connections for the desired layer
three conversation. This method would be used where a
“permanent” conversation was required such as between two
host computers that often exchange data.

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