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This can be appreciated by considering what happens if a wire breaks.
Layer two will not be able to recover from the error—it cannot reestablish
a broken wire. This fact cannot be reported onwards by the other layer
two links in the circuit since each of them is still intact and functioning
correctly.

The error has to be detected and corrected by some higher level of
authority that sees the overall picture. To ensure that data is delivered
across the network correctly, layer three takes care of the end-to-end
connection and performs error detection and correction.

Going back to Fig. 1.16, layer three provides error detection and
correction between points A and C and uses several layer two links. Suppose
that there is a separate, simultaneous connection between points B and D;
this will be subject to the same process. The network would then be carrying
two layer three connections corresponding to the real conversations, and
several layer two links. The Q to R and R to S links each have a simple
layer two link which carries two layer three conections at the same time.
The conversation may look something like that in Fig. 1.17.

Fig. 1.17 Conceptual frame contents to allow multiple layer three conversations

Many of the requirements of layer three can be inferred from what we
have seen of layer two and from the need to carry several layer three
conversations over a single layer two link.

• There has to be an identification of which layer three conversation
the data in this frame refers to.

• There has to be some method of ensuring that all data arrives in
sequence. This is done by P(R) and P(S) values which are akin to
the N(R) and N(S) sequence numbers of layer two.

• There has to be a means of error recovery.

These items are carried in a data structure called the packet which is
reminiscent of the layer two frame. As in layer two, there are several
types of packet allowing the layer three conversation to proceed with all
the required features. The packet type for carrying data is the Data packet
as shown in Fig. 1.18. This is a conceptual illustration and does not
show the actual packet layout.

There are two points to note:

• The LCI (Logical Channel Identification) field identifies which
of the possible conversations this is.

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