5.3. Controlling Priority in Circuit Groups

ISUP allows the setting of different priority schemes on a per circuit group basis:

• Priority to incoming call on all circuits
• Priority to outgoing call on all circuits
• Highest point code has priority on even CICs (Circuit Identification Codes)
• Highest point code has priority on odd CICs

The third scheme is the one recommended by the ITU (Q.764). With the SS7 stack, the priority scheme can be selected in the <options> field of the ISUP_CFG_CCTGRP commands in the config.txt file. Once priority has been given to one of the calls by the SS7 stack, upper software layers (Global Call SS7 and the application) must conform.

Because of the multiple layers of the software architecture and the asynchronous nature of the communication between them, it is possible that collisions appear to exist even though there has not been a true glare condition on the signaling link. For example, if the SS7 stack has posted an IAM message for the GlobaCall SS7 call control library but that the application issues a gc_MakeCall( ) before this message is received, the application will see the equivalent of a glare condition: the outbound call will fail and the inbound call will be offered. This can happen regardless of the configured priority scheme, even with priority given to outbound calls on all circuits.

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