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ISUP (ISDN user part)

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Introduction:

The ISDN User Part (ISUP) is a protocol used to establish and maintain voice calls over ISDN (Integrated Services Digital Network) networks. It is part of the Signaling System 7 (SS7) protocol suite, which is used for the exchange of signaling messages between network elements. In this article, we will discuss ISUP in detail, including its functionality, message types, and call setup procedures.

ISUP Functionality:

The primary function of ISUP is to establish and maintain voice calls over ISDN networks. It is responsible for the signaling between the switch at the calling party's end and the switch at the called party's end. ISUP also handles call control messages such as call setup, call release, and call progress.

ISUP Message Types:

ISUP messages can be divided into two categories: call control messages and call information messages.

Call Control Messages: Call control messages are used to establish and tear down a call. They include:

  • IAM (Initial Address Message): The IAM is used to initiate a call by specifying the called party number and other relevant information.
  • ACM (Address Complete Message): The ACM is sent by the called party's switch to indicate that the called party's address has been completed.
  • ANM (Answer Message): The ANM is sent by the called party's switch to indicate that the called party has answered the call.
  • REL (Release Message): The REL is sent by either party to terminate the call.
  • RLC (Release Complete Message): The RLC is sent by the switch that receives the REL message to acknowledge the termination of the call.

Call Information Messages: Call information messages are used to convey additional information about the call. They include:

  • INF (Information Message): The INF message is used to convey information such as call progress or user-defined information.
  • INR (Information Request Message): The INR message is used to request information from the other end of the call.
  • CON (Connect Message): The CON message is sent by the called party's switch to indicate that the call has been connected.
  • SUS (Suspend Message): The SUS message is used to temporarily suspend a call.
  • RES (Resume Message): The RES message is used to resume a call that has been suspended.

ISUP Call Setup Procedure:

The following steps outline the call setup procedure using ISUP:

  1. The calling party initiates the call by dialing the destination number.
  2. The calling party's switch sends an IAM message to the destination switch, which includes the destination number and other relevant information.
  3. The destination switch sends an ACM message back to the calling party's switch to indicate that the destination address has been completed.
  4. The destination switch sends a ringing signal to the called party.
  5. The called party answers the call.
  6. The destination switch sends an ANM message to the calling party's switch to indicate that the called party has answered the call.
  7. The calling party's switch sends a CON message to the destination switch to indicate that the call has been connected.
  8. The parties can then communicate over the call.
  9. When the call is complete, either party can send a REL message to terminate the call.
  10. The switch that receives the REL message sends an RLC message to acknowledge the termination of the call.

Conclusion:

ISUP is a protocol used to establish and maintain voice calls over ISDN networks. It is part of the SS7 protocol suite, which is used for the exchange of signaling messages between network elements. ISUP handles call control messages such as call setup, call release, and call progress, as well as call information messages that convey additional information about the call. The call setup procedure using ISUP involves several steps, including the initiation of the call by the calling party, sending an IAM message to the destination switch, the destination switch sending an ACM message to indicate that the destination address has been completed, the called party answering the call, the destination switch sending an ANM message to indicate that the call has been answered, and the calling party's switch sending a CON message to indicate that the call has been connected. When the call is complete, either party can send a REL message to terminate the call, and the switch that receives the REL message sends an RLC message to acknowledge the termination of the call.