ISUP

ISUP, or ISDN User Part, is a signaling protocol within Signaling System No. 7 (SS7) used to establish, manage, and release circuit-switched telephone calls in traditional public switched telephone networks. It operates between signaling points such as exchanges, mobile switching centers, and media gateways, and it can traverse signaling transfer networks to link different network domains.

The primary role of ISUP is call setup and control. It carries information required to allocate network resources, identify calling and called parties, and convey progress and service features during a call. A typical call flow starts with an Initial Address Message to begin setup, followed by subsequent messages to allocate circuits, provide call progress information, and establish the connection when the called party answers. Release messages and release-complete messages terminate calls. ISUP also supports various supplementary services through its parameter sets, including caller identification and call forwarding.

Technically, ISUP is defined within SS7 and relies on the Message Transfer Part (MTP) for transport reliability and routing. There are international (ITU-T) variants of ISUP and ANSI ISUP used in North America. In modern networks, ISUP can be transported over IP using SIGTRAN, which employs SCTP as the transport protocol, enabling interconnection between IP-based signaling nodes.

ISUP remains widely used in traditional PSTN and mobile core networks for signaling associated with inter-switch communication, interworking with legacy systems, and interconnection between different network operators. While SIP-based signaling is increasingly prevalent in all-IP networks, ISUP continues to play a critical role in many legacy and hybrid environments.