ISUP subsequent address messages in overlap signalling
The ISUP Subsequent Address Message, commonly called SAM, carries additional called-party digits after the initial address information has already been sent. It belongs to the overlap signalling method used in SS7 voice networks, where a complete destination number is delivered in stages rather than as one finished block.
This behaviour matters when analysing call setup, inter-carrier routing and signalling traces. A receiving exchange may obtain the first digits in an Initial Address Message (IAM), then use one or more SAMs to complete the called number before selecting the outgoing route.
Overlap signalling is especially useful when a caller dials gradually through a traditional PSTN service. It is less central in many modern SIP environments, where the full address is commonly available before the INVITE is transmitted, but it remains important for legacy interworking and SS7-to-IP gateways.
For engineers working with Australian networks, the subject connects older fixed-line infrastructure with current voice services over the NBN. Number portability, ACMA numbering rules, carrier interconnects and emergency calling to 000 can all affect how incomplete or slowly delivered address information is handled.
What the SAM message does
The IAM starts the call and includes the initial portion of the called-party number when overlap sending is in use. If the originating exchange has more digits to send, it transmits a SAM containing the next part of the address. Several SAMs may follow the IAM until the destination exchange has enough information to determine the called subscriber or route.
SAM is therefore a message for digit continuation, not a new call attempt. It uses the same circuit context identified by the signalling link and circuit identification code. The receiving exchange associates each subsequent address message with the call already established by the IAM.
A useful SS7 learning resource should distinguish SAM from messages such as Address Complete Message (ACM), Answer Message (ANM), Release (REL) and Continuity Check messages. Those messages control call progress or circuit supervision; SAM extends the destination address.
Why overlap signalling is used
Overlap sending can reduce the delay before a call begins when a network receives digits in real time from a subscriber or downstream exchange. The originating side does not need to wait for every digit before forwarding the first available portion. This was valuable in electromechanical and early digital telephone networks, where digit collection and routing occurred progressively.
The trade-off is increased state management. The destination exchange must retain partial address information, wait for more digits and decide when the number is complete. A missing SAM, a duplicated digit or an unexpectedly long inter-message gap can leave a call suspended until a timer expires.
Australian networks have historically combined different generations of switching equipment. A Telstra or Optus interconnect, a regional exchange and an IP voice gateway may apply different national profiles or conversion rules. Engineers should therefore confirm whether a particular interface supports overlap, en bloc signalling or a gateway-specific adaptation.
How an ISUP call progresses
A typical sequence begins with an IAM containing the circuit identification, called-party information and other call parameters. The destination exchange examines the available digits. If they are insufficient, it waits for a SAM rather than immediately returning an address error.
Each SAM adds the next digit segment in order. Once the full number is recognised, the terminating exchange can send ACM to indicate that call setup is progressing. When the called party answers, ANM confirms answer supervision. If the address is invalid, unavailable or timed out, an appropriate release or failure response follows.
The sending exchange must preserve ordering and maintain the call’s signalling context. A trace showing an IAM followed by SAM, SAM and ACM is materially different from a trace with one IAM containing the complete number. That distinction helps identify whether a fault lies in digit collection, routing translation or message transport.
Message content and timer behaviour
The important payload in a SAM is the subsequent portion of the called address. The exact parameter names and permitted formats depend on the ISUP variant, national specifications and vendor implementation. MTP3 provides network-layer delivery, while the CIC links the message to the bearer circuit reserved for the call.
Timers are central to overlap operation. A receiver may start an inter-digit or address-completion timer after the IAM or after each SAM. If the next message does not arrive within the permitted interval, the exchange can clear the call or return an address-related cause. Values and actions should be checked against the applicable carrier specification rather than assumed from a generic textbook.
| Signalling method | Address delivery | Main operational benefit | Common diagnostic risk |
|---|---|---|---|
| En bloc ISUP | Complete number in IAM | Simpler state handling | Incorrect full-number translation |
| Overlap ISUP | IAM followed by one or more SAMs | Early forwarding of received digits | Timer expiry or missing digit segment |
| SIP-style setup | Usually complete destination in INVITE | Flexible IP service integration | Gateway mapping and normalisation errors |
Reading traces in Australian deployments
When inspecting a trace, first verify the CIC, point codes, signalling link and message sequence. Confirm that every SAM belongs to the same call and that the digit segments concatenate in the expected order. Compare the received number with the national or carrier dialling format, including leading trunk prefixes and translated mobile or geographic ranges.
An Australian call may cross a legacy SS7 interconnect before reaching an NBN voice service. A gateway could convert overlap ISUP into a SIP request, buffer digits and then send a complete address. If the gateway waits indefinitely, the user may hear silence even though the SS7 link remains available.
Emergency calls require special care. Calls to 000 must be routed and presented correctly, and any delay caused by digit collection or protocol conversion can have serious consequences. Testing should use approved procedures and avoid generating uncontrolled emergency traffic.
Checks for fault isolation
Use a disciplined sequence when a call appears to stall after the IAM:
- Confirm that the originating exchange actually sends the expected SAM.
- Check digit order, number length and translation rules at every gateway.
- Compare inter-message delays with the configured overlap timers.
- Verify that the destination exchange accepts SAM for that route and service.
- Correlate ISUP events with MTP3 alarms, CIC state and bearer availability.
Australian operators also need to consider privacy and operational controls when capturing signalling traces. Call records can contain identifying information, so access, retention and handling should align with internal policy and obligations under the Privacy Act 1988.
Applying SAM knowledge in network design
Overlap signalling should be enabled only where the adjacent network and service profile support it consistently. If one side expects en bloc delivery while the other sends SAM, the interworking gateway must buffer and reconstruct the address safely. The reverse conversion must avoid losing digits or releasing the call before address completion.
Documentation should record the ISUP variant, supported message types, numbering plan, timer values, gateway behaviour and release causes. This is particularly useful when a service spans Sydney, Melbourne and regional locations, where different access technologies and inherited switch configurations may coexist.
The practical takeaway is simple: read SAM as a continuation of the IAM, verify every digit segment and timer against the live interface specification, and trace the complete path through SS7, gateways and the final voice service before changing routing logic.
