ISUP Message Flow

This section provides an introduction to the core set of ISUP messages that are used to set up and release a call. The ISUP protocol defines a large set of procedures and messages, many of which are used for supplementary services and maintenance procedures. While the ITU Q.763 ISUP standard defines nearly fifty messages, a core set of five to six messages represent the majority of the ISUP traffic on most SS7 networks. The basic message flow that is presented here provides a foundation for the remainder of the chapter. Additional messages, message content, and the actions taken at an exchange during message processing build upon the foundation presented here.

A basic call can be divided into three distinct phases:

• Setup

• Conversation (or data exchange for voice-band data calls)

• Release

ISUP is primarily involved in the set-up and release phases. Further ISUP signaling can take place if a supplementary service is invoked during the conversation phase.

In Figure 8-3, part A illustrates the ISUP message flow for a basic call. The call is considered basic because no supplementary services or protocol interworking are involved. The next section, "Call Setup," explains the figure's message timer values.

Figure 8-3. Simple ISUP Message Flow

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Call Setup

A simple basic telephone service call can be established and released using only five ISUP messages. In Figure 8-3, part A shows a call between SSP A and SSP B. The Initial Address Message (IAM) is the first message sent, which indicates an attempt to set up a call for a particular circuit. The IAM contains information that is necessary to establish the call connection—such as the call type, called party number, and information about the bearer circuit. When SSP B receives the IAM, it responds with an Address Complete Message (ACM). The ACM indicates that the call to the selected destination can be completed. For example, if the destination is a subtending line, the line has been determined to be in service and not busy. The Continuity message (COT), shown in the figure, is an optional message that is used for continuity testing of the voice path before it is cut through to the end users. This chapter's "Continuity Test" section discusses the COT message.

Once the ACM has been sent, ringing is applied to the terminator and ring back is sent to the originator. When the terminating set goes off-hook, an Answer Message (ANM) is sent to the originator. The call is now active and in the talking state. For an ordinary call that does not involve special services, no additional ISUP messages are exchanged until one of the parties signals the end of the call by going on-hook.

Call Release

In Figure 8-3, the call originator at SSP A goes on-hook to end the call. SSP A sends a Release message (REL) to SSP B. The REL message signals the far end to release the bearer channel. SSP B responds with a Release Complete message (RLC) to acknowledge the REL message. The RLC indicates that the circuit has been released.

If the terminating party goes on-hook first, the call might be suspended instead of being released. Suspending a call maintains the bearer connection for a period of time, even though the terminator has disconnected. The terminator can go off-hook to resume the call, providing that he does so before the expiration of the disconnect timer or a disconnect by the originating party. This chapter discusses suspending and resuming a connection in more detail in the section titled "Circuit Suspend and Resume."

Unsuccessful Call Attempt

In Figure 8-3, part B shows an unsuccessful call attempt between SSP A and SSP B. After receiving the IAM, SSP B checks the status of the destination line and discovers that it is busy. Instead of an ACM, a REL message with a cause value of User Busy is sent to SSP A, indicating that the call cannot be set up. While this example shows a User Busy condition, there are many reasons that a call set-up attempt might be unsuccessful. For example, call screening at the terminating exchange might reject the call and therefore prevent it from being set up. Such a rejection would result in a REL with a cause code of Call Rejected.