ISUP (Integrated Services Digital Network User Part)

Integrated Services Digital Network (ISDN) is a set of communication standards for the digital transmission of voice, video, and data over traditional copper telephone lines. It was developed in the 1980s and became widely adopted in the 1990s. ISDN is designed to offer a number of advantages over traditional analog telephone systems, including higher data rates, improved voice quality, and more advanced features such as call forwarding, call waiting, and conference calling.

One of the key components of the ISDN standard is the Integrated Services Digital Network User Part (ISUP). ISUP is a protocol that is used to establish, maintain, and release connections between the switches in the Public Switched Telephone Network (PSTN) that support ISDN. ISUP is responsible for managing the signaling that is used to control the establishment and release of calls, as well as for providing information about the call to the network.

ISUP is a signaling protocol that is used to control the connection of calls between different networks. It is used to set up calls, to maintain calls, and to terminate calls. ISUP defines the messages that are exchanged between the switches in the PSTN to establish and release calls, and to provide information about the call.

ISUP is used to set up a call by exchanging signaling messages between the switches that support ISDN. The call setup process begins when a user dials a phone number. The user's telephone sends a signal to the local switch, which then sends a message to the next switch in the network. This process continues until the call reaches the destination switch, which then signals the destination phone to ring.

ISUP is also used to maintain calls. This includes monitoring the quality of the call and ensuring that the connection remains active. If the quality of the call deteriorates or if the connection is lost, ISUP will attempt to restore the call or terminate it if necessary.

Finally, ISUP is used to terminate calls. This includes releasing the connection between the switches and signaling to the destination phone that the call has ended.

ISUP messages are used to exchange information about the call between the switches in the network. These messages contain information about the calling and called party, as well as information about the status of the call. The messages are used to establish and release connections, to request information about the call, and to provide information about the call.

ISUP messages are divided into three categories: call setup messages, call progress messages, and call release messages.

Call setup messages are used to establish a connection between the calling and called parties. These messages include IAM (Initial Address Message), ACM (Address Complete Message), and ANM (Answer Message).

IAM (Initial Address Message) is the first message sent by the originating switch when a call is initiated. It contains information about the calling party, the called party, and the type of service requested.

ACM (Address Complete Message) is sent by the destination switch to indicate that it has received the call setup message and is ready to connect the call.

ANM (Answer Message) is sent by the destination switch to indicate that the called party has answered the call.

Call progress messages are used to provide information about the status of the call. These messages include PRO (Progress Message), ALERT (Alert Message), and CONNECT (Connect Message).

PRO (Progress Message) is sent by the originating or intermediate switch to provide information about the progress of the call, such as when the call is ringing or when the call is in a queue.

ALERT (Alert Message) is sent by the destination switch to indicate that the called party is being alerted.

CONNECT (Connect Message) is sent by the destination switch to indicate that the call has been connected.

Call release messages are used to release the connection between the switches and to signal the destination phone that the call has ended. These messages include REL (Release Message) and RLC (Release Complete Message).

REL (Release Message) is sent by either the originating or destination switch to indicate that the call should be released. This can happen for a number of reasons, such as the call being completed or the call being terminated due to a problem with the network.

RLC (Release Complete Message) is sent by the receiving switch to indicate that it has released the call and is ready for the next call.

ISUP is an important part of the ISDN standard, as it is responsible for managing the signaling that is used to control the establishment and release of calls. ISUP messages are used to exchange information about the call between the switches in the network, including information about the calling and called party, as well as information about the status of the call. By managing the signaling for ISDN, ISUP ensures that calls are established and released correctly and that the network operates efficiently.

ISUP has been widely adopted throughout the world as part of the ISDN standard. It is used in both circuit-switched and packet-switched networks and has been updated over the years to support new features and technologies. While ISDN is no longer as widely used as it once was, ISUP remains an important protocol for managing signaling in digital telecommunications networks.