Following an ISUP Call from IAM to Release Guard
An ISUP signalling connection is a controlled exchange of messages that carries a circuit-switched call from route selection to circuit clearance. Although many voice services now cross IP networks through SIGTRAN, the logic remains familiar: reserve a bearer circuit, report progress, connect the parties, supervise the call and return the circuit to service.
For engineers examining SS7 traces, the important skill is reading the messages as a sequence rather than treating each packet as an isolated event. The Initial Address Message (IAM), Address Complete Message (ACM), Answer Message (ANM), Release (REL) and Release Complete (RLC) form the central path, while timers, cause indicators and network roles explain why a call succeeds or fails.
| Signalling stage | Main message | Primary purpose | Useful trace detail |
|---|---|---|---|
| Call request | IAM | Reserves a circuit and sends called-party data | CIC, called number, nature of connection |
| Progress | ACM | Confirms that routing and alerting can continue | Backward indicators, optional progress |
| Answer | ANM | Confirms that the called party has answered | Billing supervision normally begins |
| Clearing | REL | Requests release and states the cause | Cause value, direction and timing |
| Guard period | RLC | Confirms circuit release | Circuit returns after the release guard |
Building the Call with the IAM
The originating exchange creates an IAM after analysing the dialled digits and selecting an outgoing route. It places the selected Circuit Identification Code (CIC) in the message, then sends the called number, calling number where permitted, transmission medium information and other compatibility indicators. MTP3 supplies the signalling point code and routing label that deliver the message through the SS7 network.
The IAM is therefore more than a digital version of dialling. It asks the next exchange to seize a specific voice path and provides the information required to route it. In an IP-based deployment, SIGTRAN may carry MTP3 user parts over SCTP, yet the ISUP call model still depends on circuit identity and ordered signalling.
A mismatch between the CIC state at two exchanges can create glare, where both sides attempt to seize the same circuit. Careful circuit management, point-code configuration and route-set testing are essential in large Australian networks connecting Sydney, Melbourne, Brisbane and regional exchanges.
Progress Messages and Remote Alerting
After receiving a valid IAM, the terminating exchange checks the number, routing permissions and subscriber condition. It may return an ACM to indicate that the address is complete and the call can progress. The destination may then alert the handset or deliver a network tone, while the originating side provides ringback to the caller.
ACM does not mean that the person has answered. The distinction matters for billing, customer experience and fault diagnosis. A trace showing IAM followed by ACM but no ANM may indicate unanswered ringing, a blocked destination, an interworking problem or a timer expiry. In Australian mobile and fixed markets, calls can cross several carrier domains, so an interconnect trace may show different progress behaviour at each boundary.
Optional messages such as CPG can report call progress, while Facility or user-to-user information may appear in specialised services. Engineers should interpret these messages alongside the destination exchange’s response rather than assuming every call uses an identical sequence.
Answer Supervision and the Active Call
The terminating exchange sends ANM when the called party answers. This changes the call from alerting to conversation and generally starts charging supervision according to the commercial and technical arrangement between carriers. The voice path should now be connected in both directions, with the CIC remaining associated with the active call.
If ANM is missing, a call may appear to ring indefinitely in a simplified trace even though the user experience is different. If ANM arrives too early, the network may charge for an unanswered call or expose a media path before the destination is ready. Timer values and answer-seizure delay are therefore useful performance indicators.
Australian operators also need to account for emergency calling to 000, number portability and lawful access obligations under telecommunications legislation. These requirements influence routing, caller identity handling and trace retention. Operational teams commonly use structured signalling solutions to correlate ISUP messages with exchange, STP and SIGTRAN events.
Clearing the Circuit with REL
Call clearing begins when one side sends REL, usually with a cause value such as user busy, no answer, normal call clearing or temporary failure. The receiving exchange should disconnect the speech path, release local resources and respond with RLC. Either party may initiate the release, so the direction of the first REL is diagnostically significant.
A failed call should be assessed through its cause value, message timing and network location. A REL immediately after IAM suggests screening, routing or resource rejection. A REL after ACM may point to alerting or destination conditions. A REL after ANM is more consistent with a user ending the call, a bearer fault or a supervisory event.
Cause values are not always a perfect description of the customer’s experience. Interworking gateways may translate causes between ISUP, SIP and another national signalling variant. That is particularly relevant where Australian PSTN services coexist with VoIP platforms, NBN-connected voice products and mobile interconnects.
RLC and the Release Guard
RLC confirms that the remote side has processed the release and that the circuit can be considered idle. The release guard is the protection interval around this transition: equipment prevents premature reuse until the expected release exchange has completed and the CIC state is stable. Exact implementation depends on the ISUP variant and switch design, but the principle is consistent—avoid assigning a circuit while the previous call’s signalling is still settling.
A missing RLC, repeated REL or unexpectedly long guard period can leave a circuit blocked, create apparent capacity loss or produce inconsistent states between exchanges. Monitoring should therefore track IAM-to-ACM delay, ACM-to-ANM delay, REL-to-RLC delay and the number of circuits held in transient states.
When investigating a trace, follow the complete lifecycle: confirm that IAM seized the intended CIC, verify that progress messages match the destination state, check that ANM reflects genuine answer supervision, then inspect the cause and timing of REL and RLC. The key point to remember is that a healthy ISUP connection ends only when signalling state, bearer state and the release guard all agree that the circuit is free.
