PDF of this tutorialThe call model is a generic representation of SSP call-processing activities required to establish, maintain, and clear a basic call. The call model consists of point in calls (PICs), detection points (DPs), and triggers. These are depicted in Figure 6.
Figure 6. The Call Model: Basic Concept
PICs represent the normal switching system activities or states that a call goes through from origination to termination. For example, the null state or the idle state is when the SSP is actually monitoring the customer's line. Other examples of states, or PICs, are off-hook (or origination attempt), collecting information, analyzing information, routing, alerting, etc.
Switching systems went through similar stages before AIN was developed. However, the advent of AIN introduced a formal call model to which all switching systems must adhere. In this new call model, trigger detection points (TDPs) were added between the PICs. SSPs check TDPs to see if there are any active triggers.
There are three types of triggers: subscribed or line-based triggers, group-based triggers, and office-based triggers. Subscribed triggers are provisioned to the customer's line so that any calls originating from or terminating to that line would encounter the trigger. Group-based triggers are assigned to groups of subscribers—e.g., business or Centrex groups. Any member of a software-defined group will encounter the trigger. Office-based triggers are available to everyone who is connected to the telephone switching office or has access to the North American numbering plan. Office-based triggers are not assigned to individuals or groups.
If an active trigger is detected, normal switching system call processing is suspended until the SSP and SCP complete communications. For example, in the diagram above, suppose an AIN call has progressed through the null state the off-hook PIC and is currently at the collecting-information PIC. Normal call processing is suspended at the information-collected TDP because of an active off-hook delayed trigger. Before progressing to the next (analyze information) PIC, the SSP assembles an information-collected message and sends it to the SCP over the SS7 network. After SCP service logic acts on the message, the SCP sends an analyze-route message that tells the SSP how to handle the call before going to the next PIC (analyze information).
Essentially, when the SSP recognizes that a call has an associated AIN trigger, the SSP suspends the call processing while querying the SCP for call routing instructions. Once the SCP provides the instruction, the SSP continues the call model flow until completion of the call. This is basically how a call model works, and it is an important part of AIN. This concept differs from the pre–AIN switching concept in which calls were processed from origination state to the call-termination state without call suspension.
