=[ x25-box.com ]=

Note that further encodings are defined in the X.25 recommendationbut
have been excluded here for clarity.

Many of these clearing causes are the same as may be given for Resets
or Restarts, for instance any of the three types of packet may result from
a network congestion. The type of packet sent in any given situation will
depend to some extent on the severity of the conditions, and to some
extent on the decisions made by individual implementors.

The second field is called the Diagnostic Code. It may be set by the
initiator of the Clear procedure to give further details of the reason—
perhaps the precise nature of a protocol violation—and if not set explicitly
will be delivered as zero by the network.

If a DTE and its local DCE transmit a Clear Request and a Clear
Indication to each other on the same LCN, because both want to clear
the call, then the procedure at that end is complete and no confirmations
at that end are issued.

The Clear Request is not subject to the flow control mechanism of
Data packets, and in general will overtake Data packets queued in the
network. This is illustrated in Fig. 4.21.

Fig. 4.21 The Clear procedure; the Clear Request may “overtake” data already transmitted and thus clear the call and cause the data to be discarded

The network will usually discard Data packets and Interrupts for the
virtual circuit once the Clear procedure has started, thus, in general,
sending a Clear Request will cause loss of data. This situation can be
remedied by the use of higher-level protocols. An example of such a
remedy is the Invitation to Clear message of X.29 which is described in
Chapter 2.

4.3.5 Permanent virtual circuits

A Permanent Virtual Circuit (PVC) is a circuit permanently assigned
between two DTEs. As long as all equipment involved in the circuit is
operational then data can be exchanged between the two DTEs. No Call

НАЗАД