Basic call model
In order for services to be created, deployed, and provisioned quickly, a number of assumptions must be made about the way a call is processed. The first assumption is that call processing will take place in a similar fashion regardless of the manufacturer of the telephone switch. The second is that call processing has a known sequence that can be interrupted as necessary. For these two assumptions to hold, a generic call model must be defined and accepted by software and hardware manufacturers. This generic call model is called the basic call model (BCM) and is the basis for call features and services in an intelligent network.
The BCM is composed of two parts, the originating part and the terminating part. Together, these form the end-to-end call processing used in telephony. The two parts are composed in a similar manner, each beginning with monitored events. When an event is recognized, a query is sent to a service control point (SCP). The SCP responds to the query with instructions about the disposition of the call. Once the response to the query is received, a response to the query is carried out.
In the originating part, the monitored events are based on the originating station. On/off hook status changes, dialed digits, and class of service entries are all monitored events that can generate a query to the SCP. In some cases (e.g., for normal POTS calls) these monitored events are analyzed locally and do not include the SCP. Once the SCP query is generated, the call is put in a wait state, where it will remain until a response from the SCP is received. This response indicates the disposition of the call. One of many states could be returned, for example, gather more information, play a recording, terminate the call, or process the call according to the instructions received. If the call is processed, the second part of the call model is initiated.
The terminating part of the BCM handles all destination side events of the call. Once the call is processed and routed to the destination, a new set of monitored events can take place. These events are outgoing ring/seize, class of service indicators, and the state of the called terminal. If one of the monitored events is recognized, the call is again put into a wait state as the SCP is queried. When a response is received, the call is handled as indicated. This is the terminating processing, which might include normal connections, call recorders, announcements, call forwarding, etc. The terminating process ultimately ends in the call termination.