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Thus a user anywhere on the network subscribing to five bothway LCNs
would be allocated LCNs 400 to 404.

If both the DTE and the DCE initiate a call simultaneously by
transmitting a call packet, then if they both use the same LCN there will
be a collision. In this case the DCE resolves the conflict by issuing a Clear
Request back to the remote caller. However, it is better if the conflict,
and therefore the delay, can be avoided.

The X.25 recommendation suggests that the DCE uses LCNs from
the bottom of the incoming-only (with respect to the DTE) range first,
moving up to the top of the range, and then using bothway LCNs
from the bottom to the top of the range. The DTE should allocate
from the top of its outgoing-only range first, moving down to the
bottom of the range, and then using bothway LCNs from the top to
the bottom of the range.

4.3.2 Initializing the packet layer

Once layer two has been initialized by the exchange of SABM and UA,
then layer three will go through a similar process so that the DTE and
DCE synchronize their packet activities. At this startup time, the only
real effect of the exchange is to have a confirmatory handshake and go
through internal procedures to initialize LCN tables. If the packet level
initialization follows an error then the effect is serious since all calls in
progress will be lost.

The layer three initialization is normally performed by the DCE in this
startup situation, and is achieved by sending a Restart Indication packet
(RES). The DTE will respond with a Restart Confirmation (REC). This is
shown in Fig. 4.13.

Calls can now be made. If the DTE fails to respond with a REC within

Fig. 4.13 Starting the packet layer activities

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