The ISDN User Part or ISUP is part of the Signaling System #7 which is used to set up telephone calls in Public Switched Telecom Networks. It is specified by the ITU-T as part of the Q.7xx series. Signalling System #7 is a set of protocols defined by ITU-T, specifically in the Q.7* set of documents, used to set up telephone calls. ... A telephone call is a connection over a telephone network between the calling party and the called party. ... The public switched telephone network (PSTN) is the concatenation of the worlds public circuit-switched telephone networks, in much the same way that the Internet is the concatenation of the worlds public IP-based packet-switched networks. ... The ITU Telecommunication Standardization Sector (ITU-T) coordinates standards for telecommunications on behalf of the International Telecommunication Union (ITU) and is based in Geneva, Switzerland. ...

When a telephone call is set up from one subscriber to another, many telephone exchanges will be involved, possibly across international boundaries. To allow the call to be setup correctly, the switches signal call related information like called or calling party number to the next switch in the network using ISUP messages. In the field of telecommunications, a telephone exchange (US: telephone switch) is a piece of equipment that connects phone calls. ...

The telephone exchanges are 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. Each timeslot between two switches is uniquely identified by a Circuit Identification Code (CIC) which is included in the ISUP messages. The exchange uses this information to determine which inbound CICs and outbound CICs should be connected together to provide an end to end speech path. In the field of telecommunications, a telephone exchange (US: telephone switch) is a piece of equipment that connects phone calls. ... In digital telecommunications, where a single physical wire can be used to carry many simultaneous voice conversations, worldwide standards have been created and deployed. ... Two Network Interface Units, one with a single card, the other with two In telecommunications, T-carrier is the generic designator for any of several digitally multiplexed telecommunications carrier systems originally developed by Bell Labs and used in North America and Japan. ...

In addition to call related information, ISUP is also used to exchange status information of the available timeslots. In the case of no outbound CIC being available on a particular exchange, a blocking messages is sent back to the previous switch in the chain so a new route can be tried.

Different ISUP variants exist. The main specification task is performed by the ITU-T. In Europe ETSI releases an own ISUP specification which is very close to the ITU-T ISUP. The ITU-T and ETSI ISUP are used for international connections and they are the base for national ISUP variants. Most of the countries have their own ISUP variant to cover national specific requirements. In the USA ANSI specifies the North American ISUP variant which is quite different from the ITU-T ISUP. The European Telecommunications Standards Institute (ETSI) is a standardization organization of the telecommunications industry (equipment makers and network operators) in Europe, with worldwide projection. ... The American National Standards Institute (ANSI) is a private, non-profit standards organization that produces industrial standards in the United States. ...

Message types

After the mandatory fixed-length Signaling Information Field, a ISUP message contains a variable-length part that is dependent on the type of message being sent. These messages are transmitted in various stages of call setup and teardown. 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 and calling number, type of service (speech or data) and many more optional parameters.

• Address Complete Message (ACM) — Message returned from the terminating switch when the subscriber is reached and the phone starts ringing.

• Answer Message (ANM) — Sent when the subscriber picks up the phone. Normally charging starts at this moment.

• Release (REL) — Sent to clear the call when a subscriber goes on hook.

• Release complete (RLC) — Acknowledgement of the release – the timeslot is idle afterwards and can be used again. This is also sent (without a preceding Release message) if the terminating switch determines that the call cannot be completed. The terminating switch also sends a Cause Value to explain the reason for the failure, e.g., "User busy".

Release (ISUP) - REL ISDN User Part (ISUP) message with release code. ...

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 -- 

Signaling Information Field (ANSI)

The Signaling Information Field is required on all ISUP Message Signal Units. It indicates the Point Codes of the originating and destination nodes in the network, along with the circuit identification code (CIC). The CIC is used to specify which trunk between two switches is used to carry a particular call. The Signalling Link Selection field is used when there are multiple routes an MSU could take between two nodes. This table describes the ANSI version of the ISUP protocol, which is used in the United States. Countries in Europe and other parts of the world often use the protocol specified by the ITU, which varies considerably from the ANSI version. A SS7 point code is the equivalent of an IP address in an IP network. ... The American National Standards Institute (ANSI) is a private, non-profit standards organization that produces industrial standards in the United States. ... This article is about the location. ...

External links

• ISDN User Part (ISUP) Call Flow Diagrams