PDF of this tutorialThe AIN Release 0 call model has three trigger checkpoints (TCPs). At each TCP there one or more triggers. For example, the off-hook TCP includes the off-hook immediate trigger. If a subscriber's line is equipped with this trigger, communications with the SCP will occur if the switching system detects an off-hook condition. For an off-hook delayed trigger, one or more digits are dialed before triggering to the SCP. At the digit-collection and analysis TCP, collected digits are analyzed before triggering. Triggering may also occur at the routing stage of a call. This call model is shown in Figure 7.
Figure 7. AIN Release 0 Call Model
When a switching system recognizes that a call requires AIN involvement, it checks for overload conditions before communicating with the SCP. This process is called code gapping. Code gapping allows the SCP to notify the switching system to throttle back messages for certain NPAs or NPA–NXXs. When code gapping is in effect, some calls may receive final treatment. For others, a provide-instruction message is sent to the SCP. Depending on the SCP service logic, it will respond to the switching system with any of the call-processing instructions shown in the Figure 8.
Figure 8. AIN Release 0 Functions
AIN Release 0 provided 75 announcements at the switching system. Release 0 was based on American National Standards Industry (ANSI) Transaction Capability Application Part (TCAP) issue 1. TCAP is at layer 7 of the SS7 protocol stack. This means that there is only one message sent from the SSP to the SCP, no matter what trigger is hit at any of the three TCPs.
