This clause provides the D-channel signalling procedures in support of circuit-mode bearer capabilities other than multirate (64 kbit/s base rate).
Extensions to this basic protocol and exceptions that apply in the case of packet-mode connections or of circuit-mode multirate (64 kbit/s base rate) or supplementary services are described elsewhere in this Recommendation.
The call states referred to in this clause cover the states perceived by the network, states perceived by the user, and states which are common to both user and network. Unless specifically qualified, all states described in the following text should be understood as common (see 2.1.1 and 2.1.2 for user and network call states respectively). An overview diagram of call states is given in Figures A.2 and A.3 (see Annex A).
Detailed specification and description language (SDL) diagrams for the procedures specified in this clause are contained in Figures A.4 through A.6. When there is an ambiguity in the narrative text, the SDL diagrams in Figures A.4 through A.6 should be used to resolve the conflict. Where the text and the SDL are in disagreement, the text should used as the prime source.
Note 1 - This clause describes the sequences of messages associated with the control of circuit-switched connections. Optional extensions to this basic protocol and exceptions that apply in the case of packet-mode connections or supplementary services are described elsewhere in this Recommendation, in Recommendation Q.932 [4] or the Q.95x.SeriesRecommendations [83]. Annex D also contains extensions to the basic call establishment procedures defined in clause 5 for symmetric signalling.
All messages in this Recommendation may contain two types of information elements, functional and/or stimulus. Functional information elements are characterized as requiring a degree of intelligent processing by the terminal in either their generation or analysis. Stimulus information elements, on the other hand, are either generated as a result of a single event at the user/terminal interface or contain a basic instruction from the network to be executed by the terminal.
As a general principle, all the messages sent by the network to the user may contain a Display information element whose contents may be displayed by the terminal; the content of this information element shall be network dependent.
Note 2 - Keypad facility information elements shall be conveyed only in the direction user to network. Display information elements shall be conveyed only in the direction network-to-user.
In addition to the messages exchanged as described in the following subclauses, INFORMATION messages for call control may be sent by the user or by the network only after the first response to a SETUP message has been sent or received, and before the clearing of the call reference is initiated. An INFORMATION message received in the Release Request state may be ignored.
In order to accommodate the transfer of Layer 3 messages which exceeds the data link layer maximum frame length(i.e. defined in Recommendation Q.921 [3]), a method of message segmentation and reassembly may optionally be implemented as described in Annex H. Message segmentation shall only be used where all the information comprising the un segmented message is available at the time of sending the first message segment.
Note 3 - Message segmentation is not used to replace existing procedures where information is yet to be provided by call control, e.g. digit by digit sending in overlap mode, although this may be used in addition. Message segmentation shall only be used when the message length exceeds the value of the N201 parameter defined in Recommendation Q.921 [3].