Welltech Dialogic SS7 Protocols ISUP - Manual (Page 92)

Section 6 Interface to Application
External Call collision
This is when the call collision occurs between the ISUP module and the
network i.e. the network sends an IAM to ISUP at the same time as ISUP
sends an IAM to the network.
Figure 10. Example of an External Call Collision where this End of the Circuit is
Master
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IAM
ISUP
MODULE
USER
APPLICATION NETWORK
IAM
IAM
If this end of the circuit is master, the network should always accept the
outgoing call. Therefore, when an (outgoing) IAM is received from the user
application and a second (incoming) IAM is received from the network
causing an external call collision condition, the outgoing IAM received from
the application will be sent to the network. However, this means that the
incoming IAM will not be passed to the user application and consequently
shall be discarded. If this condition occurs, the user application will not be
aware that a call collision condition has occurred.