Number Portability Through Signaling Relay Function and INAP
Australia's mobile market thrives on choice. Subscribers regularly switch between Telstra, Optus, Vodafone, and TPG without losing the personal or business number printed on their stationery, advertised on their shopfront in Melbourne, or listed in a Sydney trade directory. Behind that simple act of retention sits a sophisticated signaling arrangement that few customers ever see.
When a number moves from the original donor carrier to a recipient carrier, the public switched telephone network must still locate it for every incoming call, SMS, or fax. The Signaling Relay Function (SRF) and the Intelligent Network Application Protocol (INAP) provide the mechanism that makes this lookup possible in real time, even when the number range now belongs to a different operator across state borders.
Local carriers rely on the Integrated Public Number Database (IPND) as the authoritative register of every allocated number, yet the IPND is consulted mostly for directory and emergency-service routing. For active call completion to a ported subscriber, the SRF inside the donor network answers an INAP request from the recipient network and either forwards the call or releases it with a special cause.
Engineers studying this area often encounter conflicting terminology in vendor documentation, training kits, and Australian Communications and Media Authority (ACMA) publications. Aligning those terms with the way number portability actually behaves in the Australian PSTN forms the basis of practical deployment work.
Australian Porting Architecture Compared
Three principal porting architectures appear in specifications: Query on Release, All Call Redirection, and the SRF-on-Recipient approach favoured by many Australian carriers. The following table contrasts them on dimensions that matter to network planners.
| Method | Signaling Path | Database Location | Typical Latency | Australia Adoption |
|---|---|---|---|---|
| Query on Release | ISUP release cause to originating exchange | Donor network | Medium | Legacy fixed-line |
| All Call Redirection | ISUP redirection message | Donor network | Low | Some mobile deployments |
| SRF with INAP | INAP over SCCP to recipient SCP | Recipient SCP | Lowest | Common in mobile and VoIP |
The SRF approach dominates modern Australian carrier interconnects because it shifts the routing intelligence to the recipient side, where the customer now lives.
Signaling Relay Function in Practice
The SRF lives inside the Service Switching Point of the network that originally held the number. When an incoming call from Brisbane or Perth arrives for what is now a ported mobile, the SSP recognises the dialled digits as belonging to a foreign range and triggers a relay operation.
That operation formulates an INAP message addressed to the Service Control Point of the recipient carrier. The message travels across the SS7 or SIGTRAN link, using SCCP global title translation to reach the correct destination without manual routing table changes.
Once the SCP responds with a route address, the SRF re-assembles the call setup toward the new network. The caller in Adelaide hears normal ringing, and the ported subscriber answers as if no inter-carrier handover had ever occurred.
INAP Operations Used for Porting
INAP defines a set of operations that the SCP and SSP exchange to manage the relay. Several of these appear routinely in Australian number portability logs reviewed by transmission engineers.
- InitialDP for call detection at the donor exchange
- Connect to forward the call toward the recipient network
- ReleaseCall to terminate the session after a routing decision
- EventNotificationBCSM for mid-call triggers during handover
The choice of operation depends on whether the recipient carrier wants to forward the call, redirect it to voicemail, or play a network announcement.
Call Flow for a Ported Mobile Subscriber
A caller in Hobart dials a mobile number originally allocated by Telstra but now belonging to a customer of Optus. The call enters the public network and reaches the donor SSP, where the SRF recognises the range as ported.
The SRF sends an INAP query to the Optus SCP, which replies with the current routing prefix. The donor network then either completes the call directly to Optus or hands it off at an interconnect point in Melbourne or Sydney. From the caller's perspective, the call is seamless, and the called party sees the original Optus number on their handset.
Regulatory Framework Inside Australia
ACMA administers the numbering plan under the Telecommunications Act 1997 and the Telecommunications Numbering Plan 2016. Porting between carriers follows the industry code registered by Communications Alliance, which sets out timeframes, validation steps, and customer authorisation rules.
Failure to comply attracts regulatory scrutiny, so carriers document every SRF response and INAP transaction for audit. Network operations centres in capital cities monitor porting performance during business hours and report anomalies to the regulator when service levels fall outside agreed thresholds.
Common Operational Pitfalls
Engineers reviewing porting faults across Australian operators encounter a recurring set of issues that trace to signaling configuration rather than customer administration.
- Stale entries in the donor SCP after a customer moves again
- Misrouted global title translations across state boundaries
- Inconsistent handling of ported numbers during number range consolidation
- Timing mismatches between M3UA and SUA paths during SIGTRAN cutover
Resolving these issues requires coordinated change windows between the donor and recipient carriers, often scheduled outside business hours in AEST to minimise customer impact.
Migration to IP-Based Signaling
Although INAP originated in the circuit-switched intelligent network, the same logical operations now travel over SIGTRAN using SCTP. Australian carriers have transitioned most SS7 traffic to IP transport, and number portability is no exception.
The message structure remains unchanged, but the underlying transport shifts from MTP3 to MTP3 User Adaptation Layer (M3UA) and SCCP User Adaptation Layer (SUA). Engineers working on this transition can review practical deployment scenarios through SRF and INAP solutions tailored to Australian operators.
A reliable way to confirm any planned porting change behaves as expected is to set up a controlled SRF test bed, trigger an INAP query against a known ported range, and capture the SCCP and SCTP exchange with a protocol analyser. Run the exercise twice, once during peak weekday traffic and once after midnight AEST, to observe how global title translation behaves under different loads before authorising the cutover in production.
