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The Basic Call State Machine defined in Q.1214 provides a standard abstract view of how the
processing of a basic call proceeds and the way in which basic call processing interacts with the
processing of IN queries. Q.1214 defines two BCSM entities:
• The Originating BCSM (O_BCSM), which supports IN triggering for call originations
• The Terminating BCSM (T_BCSM), which supports IN triggering for call terminations
The ITU IN SSP supports both the O_BCSM and the T_BCSM. This section describes how the
O_BCSM and T_BCSM are used to support triggering and other IN interaction for call originations
(incoming calls).
The BCSM defines the progress of a call in terms of the following:
• Points In Call (PICs), which are equivalent to states. Note, however, that PICs are not inactive,
because call processing is performed by the switch software while the BCSM is “inside” a PIC.
• Transitions between PICs, which correspond to state changes and indicate the possible flow of
call processing from one PIC to another.
• Events, which are associated with transitions, and cause them to take place. A given event, e.g.
subscriber busy, may be represented in both the O_BCSM and the T_BCSM.
• Detection Points (DPs), which provide IN entry points, i.e. points in the BCSM at which the
SCF can become involved in order to provide an IN service. DPs are located between PICs, i.e.
interaction with the IN takes place not within PICs but during transitions between them
