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The ISDN (Integrated Services Digital Network) User Part or ISUP is part of the 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.
Contents
- ISUP variants
- ITU T specification versions
- Message types
- Sample call flow
- Cause codes
- Message format
- References
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 E1 or T1 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.
ISUP variants
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.
ITU-T specification versions
According to ITU-T Q.761 section 2.4.1 ISUP interworking ISUP'92 is backwards compatible with ISUP Blue Book and Q.767 for basic call procedures and supplementary services except for some procedures (e.g. number portability). Additionally the compatibility features introduced in this version ensure forward compatibility with newer versions.
Message types
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:
Sample call flow
This is a very basic call flow involving only two telecom switches which exchange the ISUP messages. The subscriber interfaces are not covered here and are only listed for a better understanding.
A subscriber telco switch A telco switch B B subscriber Off hook Dial digits ---> -- IAM --> -Ringing -> <-- ACM—Off hook <-- ANM -- ----------------------- Conversation ----------------------- On hook—REL --> On hook <-- RLC—Detailed call flows are provided in ITU-T Recommendation Q.784.1.
Below is a detailed exchange of ISUP messages involving two ISDN telecom switches. The report was from an Alcatel S12 digital switch.
Cause codes
Release cause codes are used to identify and debug any events occurring in ISDN User Part signaling. Every event in ISUP signaling generates a cause code number. Even for a normal ISUP call, a cause code is generated. There are lot of applications developed based on the cause code from ISUP signaling. Similarly Telecom operators trace for Causecodes to debug any call failures.
Following are the list of cause codes used. Cause codes only defined by number are effectively undefined, and may be used for proprietary solutions.
Message format
The Signalling Information Field (SIF) for all ISUP Message Signal Units (MSU) contain the following components:
The Routing Label indicates the Point Codes of the originating and destination nodes in the network; it also includes the Signalling Link Selection field that is used to select between the multiple routes an MSU could take between two nodes.
The Circuit Identification Code is used to specify which trunk between two switches is used to carry a particular call. Note that some versions of ANSI ISUP permit a CIC with 14 significant bits instead of the 12 that are shown.
- The Message Type indicates the ISUP message type. The presence and form of the remaining 3 components are determined by this message type.
- The Mandatory fixed part, when present, contains the mandatory, fixed-length parameters associated with the message type.
- The Mandatory variable part, when present, contains the mandatory, variable-length parameters associated with the message type.
- The Optional part, when present, contains the optional parameters permitted to be included in the message type.
When sent using the services of the Signalling Connection Control Part, ISUP messages passed to SCCP in the User Data parameter (NSDU) consist of only the last 4 components (Message Type, Mandatory fixed part, Mandatory variable part, Optional part). The routing label and circuit identification code are not included in the user data passed to SCCP.