Converting ISUP Messages Between TDM And IP Networks
ISUP, the ISDN User Part, carries the call-control information used to establish, supervise and release telephone calls. In traditional public switched telephone networks, these messages travel over SS7 links alongside TDM voice channels. In an IP-based core, the same signalling is transported through SIGTRAN, usually with MTP3 User Adaptation (M3UA) over SCTP.
The conversion is therefore less about rewriting every message and more about preserving meaning across two transport environments. A signalling gateway or integrated media gateway must maintain circuit identity, message order, point-code information, timers and call state while voice moves between a circuit-switched trunk and an RTP-based bearer.
For Australian operators, this can arise when an older PSTN interconnect, a mobile switching environment or a regional service is connected to an IMS or softswitch platform. A carrier in Sydney may still exchange SS7 with one network while delivering calls through SIP and IP internally. The same practical issue appears in Melbourne enterprise trunks and in long-distance services linking metropolitan and regional exchanges.
Australians may simply call this a “TDM-to-IP interworking” job, but the engineering detail matters. A call that sounds straightforward can fail because of an incorrect CIC, a missing Nature of Connection indicator, a mismatched answer condition or a timer that expires before the far-end network responds.
| Area | TDM SS7 Domain | IP Signalling Domain |
|---|---|---|
| Signalling transport | MTP1, MTP2 and MTP3 over dedicated links | SCTP with SIGTRAN adaptation layers |
| Voice bearer | DS0 timeslots on E1 or similar circuits | RTP streams, commonly controlled by SIP or gateway logic |
| Circuit identity | CIC tied to a physical timeslot | Logical circuit or gateway resource mapping |
| Main risks | Link failure, slip, congestion and physical alarms | SCTP association loss, routing-key errors and packet delay |
| Core conversion task | Preserve ISUP semantics | Preserve ISUP semantics while changing transport context |
Separate Signalling From Media
ISUP signalling and the speech path are related but separate. An Initial Address Message reserves or requests a circuit, while the actual audio may use a TDM timeslot on one side and an RTP port pair on the other. The gateway must correlate both legs throughout setup, answer, clearing and release.
A media gateway controller generally owns call logic, while the media gateway handles bearer conversion. In some platforms these functions are combined. Either way, the design needs a stable association between the ISUP Circuit Identification Code and the IP-side call or media resource.
Map The Circuit Identity Carefully
The CIC identifies a bearer circuit within a signalling linkset and trunk context. It is not a universal global identifier. During interworking, the gateway may map an incoming CIC to a local timeslot, an RTP resource, a SIP dialog or an internal connection record.
That mapping must remain consistent in both directions. An Answer Message, Release Message or circuit blocking event needs to refer to the same logical call leg created during setup. Incorrect CIC translation can produce stuck calls, false release events or a circuit that appears busy after the caller has hung up.
Preserve Message Semantics
The common call sequence includes IAM, ACM, ANM and REL, with RLC completing release. Address Complete Message indicates that the destination has received enough digits or that more information may be expected. Answer Message confirms that the call has been answered and usually starts charging logic.
Early media creates an important variation. Some networks use an early ACM to provide progress tones or announcements before the destination answers, while others wait for a fuller call-progress indication. The distinction is covered in this guide to ACM call progress, and it should be reflected in the IP-side state machine rather than treated as a cosmetic message difference.
Handle Number And Parameter Conversion
ISUP parameters such as Called Party Number, Calling Party Number, Transit Network Selection and Generic Number need to survive the domain boundary with their meaning intact. Digits, numbering plan, address nature, screening and presentation indicators are particularly important when calls cross carrier networks.
A gateway may also translate between national and international formats. For example, an Australian number presented as an E.164 address may need to match local routing rules before it is passed to a domestic destination. Stripping or adding “+61” without checking the numbering plan can send a call to the wrong route or cause rejection.
Align Timers And Failure Handling
ISUP timers govern how long a node waits for progress, answer or release. IP transport introduces different failure modes, including delayed packets, SCTP association failure and an unavailable application server process. A gateway should avoid allowing an IP timeout to contradict an active SS7 transaction.
Failure handling must cover both directions. If an IP leg disappears, the gateway may need to send REL toward the TDM network. If an SS7 link fails, active IP calls require a defined treatment, such as immediate clearing or controlled resource recovery. Alarm correlation is essential when a fault affects a busy exchange in Brisbane or a remote regional service.
Use SIGTRAN Routing Correctly
M3UA carries MTP3-user traffic over SCTP, but it does not remove the need for sound signalling architecture. Point codes, routing contexts, application servers and traffic modes must be planned so that messages reach the correct ISUP process.
An IP signalling gateway may use redundant SCTP associations across separate paths and route traffic using an originating or destination point code. The configuration should prevent duplicate delivery, preserve sequencing and support controlled failover. Packet capture, M3UA traces and ISUP decoding are valuable together; inspecting only the IP layer can hide a CIC or parameter problem.
Validate The Interworking In A Lab
Testing should begin with basic call establishment, answer and release, then expand to busy, no-answer, number-unobtainable, congestion and abnormal clearing cases. Include continuity checks, circuit blocking and unblocking, reset procedures, dual seizure and calls with early media.
A useful Australian test set includes local, national and international number formats, calls between an E1-connected service and an IP trunk, and failover during peak traffic. Record the IAM, ACM, ANM, REL and RLC on both sides, then compare timestamps, CIC values, number indicators and timer behaviour.
The next practical step is to capture one successful TDM-to-IP call and one failed call, then build a side-by-side trace showing every ISUP message, CIC mapping and corresponding media event.
