How ISUP Call Setup Messages Work From IAM To REL
ISDN User Part (ISUP) controls the establishment, supervision, and release of telephone calls across SS7 networks. Its messages carry the information needed to route a call, alert the destination, confirm answer, and clear network resources when the conversation ends.
The sequence from Initial Address Message (IAM) to Release (REL) is a practical way to understand circuit-switched signaling. Each message has a specific purpose, and the exchange depends on whether the destination is reachable, the called party answers, or the call fails before completion.
For broader background on PSTN architecture, signaling protocols, and SS7 operations, the SS7 learning resource provides useful context before examining individual ISUP parameters.
Where ISUP Fits In The Signaling Path
ISUP operates above the Message Transfer Part (MTP) in traditional SS7 networks. MTP1, MTP2, and MTP3 provide the physical, data-link, and signaling network functions, while ISUP uses the signaling connection to control voice circuits between exchanges. In IP-based deployments, M3UA and SCTP commonly transport ISUP signaling as part of SIGTRAN.
A call typically crosses an originating exchange, one or more transit switches, and a terminating exchange. The voice path may use a time-division multiplexed trunk identified by a Circuit Identification Code (CIC), while the ISUP messages travel through the signaling network. The CIC lets each switch associate signaling with the correct bearer circuit.
IAM Starts The Call
The Initial Address Message is the first major ISUP message in outgoing call setup. It reserves or requests a circuit and carries routing information toward the destination exchange. Important parameters include the CIC, Nature of Connection Indicators, Forward Call Indicators, Calling Party Number, and Called Party Number.
The called number is analyzed by each switch to determine the next route. Depending on the numbering plan and network design, the IAM may contain the full destination number or only part of it. Optional fields can communicate user-to-user information, redirection data, access transport information, or signaling capabilities.
At this stage, the originating exchange starts supervision timers, commonly including T7. T7 measures the time allowed for a response to the IAM. If the expected response does not arrive, the switch may clear the call and report a signaling failure.
ACM And CPG Show Call Progress
The Address Complete Message (ACM) indicates that the destination exchange has received enough digits to route the call. It generally means the called number is complete, but it does not mean that the called user has answered. The backward exchange can use ACM to indicate that alerting or call progress is underway.
Some networks also use the Call Progress Message (CPG) to report a significant change in call state, such as alerting, progress in an interworking environment, or a temporary condition before answer. The Event Information parameter explains the relevant event, while backward indicators provide additional status.
These messages support ringback and other tones heard by the caller. Media treatment can vary by network: tones may be generated locally, passed through the voice circuit, or controlled by an interworking gateway.
ANM Confirms The Answer
The Answer Message (ANM) is sent when the destination subscriber or service answers. It tells the originating exchange to transition the call into the connected state. Charging usually begins at answer, although billing rules depend on the operator and service.
Once ANM is received, both exchanges maintain the voice circuit for the conversation. ISUP does not carry the speech itself; it controls the associated bearer channel. The message exchange also supports accounting, supervision, and state synchronization between switches.
The main call setup messages can be summarized as follows:
| Message | Direction | Primary function | Typical effect |
|---|---|---|---|
| IAM | Forward | Requests call establishment and carries routing data | Selects or reserves a circuit |
| ACM | Backward | Confirms address completion | Indicates that setup is progressing |
| CPG | Backward | Reports a call-progress event | Supports alerting or interworking status |
| ANM | Backward | Confirms that the called party answered | Connects the call and may start charging |
| REL | Either direction | Requests release of the call | Begins circuit clearing |
| RLC | Opposite direction | Confirms release completion | Frees the circuit for reuse |
REL And RLC Clear The Circuit
Release begins when a party hangs up, a network element detects failure, or a service rejects the call. The switch sending the Release Message includes a Cause Indicators parameter explaining why the circuit is being cleared. Common causes include User Busy, No User Responding, Normal Clearing, Network Out of Order, and Unallocated Number.
REL can be sent in either direction. The receiving switch releases its side of the call and returns a Release Complete (RLC) message. RLC confirms that the circuit and related call state have been cleared. A release timer, often associated with T5 in ISUP implementations, helps detect an incomplete clearing exchange.
The cause value is essential for troubleshooting. “No route to destination,” “temporary failure,” and “normal call clearing” point to very different operational conditions. Engineers should inspect the cause location and diagnostic information rather than relying only on the message name.
Reading A Signaling Trace
A healthy answered call commonly follows IAM, ACM or CPG, ANM, and then REL followed by RLC. The exact order can differ when a call is rejected, forwarded, interworked with another protocol, or cleared before answer. Some calls may receive REL immediately after IAM, with no ACM or ANM.
Useful trace checks include:
- Match the CIC across every message and verify that it remains associated with the intended trunk.
- Confirm that IAM routing digits, numbering-plan indicators, and nature-of-address values are correct.
- Check T7, answer, and release timers for abnormal expiry or delayed responses.
- Interpret the REL cause value together with the affected switch, route, and call direction.
- Compare ISUP events with MTP3 or M3UA link status to separate call logic faults from transport failures.
Structured ISUP training materials can help connect these packet-level observations with PSTN call models, timers, routing decisions, and SIGTRAN deployment patterns.
A reliable way to learn the sequence is to capture both successful and failed calls, annotate each message parameter, and compare the signaling timeline with switch logs. Apply that method to a controlled SS7 or SIGTRAN trace, then use the cause codes and timer behavior to identify where call setup or release is breaking down.
