In this Recommendation the terms "incoming" and "outgoing" are used to describe the call as viewed by the user side of the interface.
In the subclauses which follow states are defined for circuit switched calls in 2.1 (call states), for packet mode access connections in 2.2 (access connection states) for temporary signalling connections in 2.3 (call states), and for the interface in 2.4 (global call reference states).
This clause defines the basic call control states that individual calls may have. These definitions do not apply to the state of the interface itself, any attached equipment, the D-channel, or the logical links used for signalling on the D-channel. Because several calls may exist simultaneously at a user-network interface, and each call may be in a different state, the state of the interface itself cannot be unambiguously defined.
Note - Additional states and SDL diagrams may be defined when new procedures are developed.
A detailed description of the procedures for call control is given in clauses 5, 6, 7, and 8 in terms of:
(a) the messages defined in clause 3 which are transferred across the user-network interface; and
(b) the information processing and actions that take place at the user side and the network side.
Overview and detailed SDL diagrams for call control of circuit-switched calls are contained in Annex A.
Throughout this Recommendation, references are made to B-channels. For services using H-channels, the references to B-channels should be taken to refer to the appropriate H-channel. Further study may be needed on other enhancements to support such services.