ISUP протоколы және телефон байланысы жүйесі

The ISDN (Integrated Services Digital Network) User Part or ISUP is part of Signaling System No. 7 (SS7), which is used to set up telephone calls in the public switched telephone network (PSTN). It is specified by the ITU T as part of the Q.76x series. When a telephone call is set up from one subscriber to another, several telephone exchanges could be involved, possibly across international boundaries. To allow a call to be set up correctly, where ISUP is supported, a switch will signal call related information like called party number to the next switch in the network using ISUP messages. The telephone exchanges may be connected via T1 or E1 trunks which transport the speech from the calls. These trunks are divided into 64 kbit/s timeslots, and one timeslot can carry exactly one call. Regardless of what facilities are used to interconnect switches, each circuit between two switches is uniquely identified by a circuit identification code (CIC) that is included in the ISUP messages. The exchange uses this information along with the received signaling information (especially the called party number) to determine which inbound and outbound circuits should be connected together to provide an end to end speech path. In addition to call related information, ISUP is also used to exchange status information for, and permit management of, the available circuits. In the case of no outbound circuit being available on a particular exchange, a release message is sent back to the preceding switches in the chain.

Different ISUP variants exist. ITU T specifies the variant used in the international network. In Europe ETSI releases its own ISUP specification which is close that of the ITU T. ITU T ISUP is used for international connections and is the base for some national ISUP variants. Most countries have their own variation of ISUP to cover national requirements. ANSI specifies variations of ISUP utilized under the North American Numbering Plan; however, some countries under the NANP differ in their support of some procedures (for example, LATA is meaningless within Canada. Also, RBOCs support Telcordia procedures not fully specified by ANSI.) Some countries outside the NANP support ANSI based variants (e. g. Mexico). While these variations of ISUP differ in subtle ways, the vast majority of ISUP message type, parameter type, and parameter field code points, and related fundamental call processing procedures, agree across all variants.

An ISUP message contains a fixed header containing the circuit identification code and the ISUP message type, followed by a mandatory fixed length parameter part, a mandatory variable length parameter part, and an optional parameter part that are dependent on the type of message being sent. ISUP messages can be sent using the services of the Message Transfer Part, or, less often, the Signalling Connection Control Part. These messages are transmitted in various stages of call setup and release. The most common messages are:
Initial address message (IAM) — First message sent to inform the partner switch that a call has to be established on the CIC contained in the message. Contains the called number, type of service (speech or data) and optional parameters. Subsequent address message (SAM) — For networks that support overlap dialing procedures, and then in the case that the IAM did not contain the full called number, one or more SAMs follow containing additional digits. This message is not supported by networks that only support en bloc dialing procedures. Address complete message (ACM) — Message returned from the terminating switch when the subscriber is reached and the phone starts ringing, or when the call traverses an interworking point and the intermediate trunk is seized. Call progress (CPG) — Contains additional information about the progress of a call. Normally sent after the ACM when the status of the call changes from that reported in the ACM. Answer message (ANM) — Sent when the subscriber picks up the phone, a resource is connected or answer supervision is returned by an interworking point. Normally charging starts at this moment. It is required that the call be cut through in both directions by this point. Connect (CON) — Sent when the call is answered by an automatic terminal. This message replaces the ACM, CPG and ANM for calls that are answered by automatic terminals. Release (REL) — Sent to clear the call when a subscriber goes on hook. This is also sent (in direct response to an IAM) if the terminating switch determines that the call cannot be completed. In either case, the terminating switch provides a cause value in the message to explain the reason for the release, e. g., "User busy". Release complete (RLC) — Acknowledgment of the release – the circuit is idle afterward and can be used again.