Welltech Dialogic SS7 Protocols ISUP - Manual (Page 107)

Dialogic ® SS7 Protocols ISUP Programmer's Manual Issue 15
Call collision
A continuity test call may be initiated at both ends of a circuit simultaneously.
When a call collision condition occurs, the ISUP module checks the value of
bits 0 and 1 of the circuit group options field to determine which call should
be allowed to succeed. As far as the application is concerned, the call
collision conditions described in
Section : 6.8.2 Call Collision Procedure on
page 91 apply. For example, if the incoming call has priority, the application
will accept the Circuit seized indication and the outgoing continuity test will
be abandoned and subsequently the incoming call will be processed. If the
outgoing call has priority, the application will not be aware that a call collision
condition has occurred because the CCR message received from the network
shall be ignored by the ISUP module.
Call collision may also occur between an outgoing continuity test and an
incoming normal call (i.e. IAM received). If this situation should occur, the
normal call will always have priority and the application will receive a Setup
indication and consequently the outgoing continuity test attempt will be
discarded.
Figure 23. Example of a Call collision where the Incoming Call has Priority
SZE CCR
RLC
REL REL
CCR SZE
RLC
RLC
ISUP
MODULE
USER
APPLICATION NETWORK
The incoming
continuity test call
is processed and
the outgoing call is
discarded
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