Reading FISU, LSSU, and MSU on an MTP2 Link
MTP2 is the data-link layer of Signalling System No. 7. It provides reliable transfer between signalling points by numbering messages, checking error conditions, controlling flow, and reporting whether a link is ready for service. Its three visible frame types—FISU, LSSU, and MSU—offer a practical view of what the link is doing at any moment.
For engineers working with legacy PSTN interconnects, mobile signalling, or SIGTRAN gateways, interpreting these units helps separate a failed signalling route from a healthy link carrying little traffic. The same reasoning applies in Australian networks, whether the circuit terminates in a Sydney exchange, a Melbourne mobile core, or a remote regional site with limited transmission diversity.
The Three Signal Unit Families
A FISU, or Fill-In Signal Unit, carries no user signalling message. It is transmitted when there is no MSU waiting, allowing both ends to maintain link supervision and sequence activity. A steady stream of FISUs generally indicates an idle but operational link, provided acknowledgements, error counters, and alignment state remain normal.
An LSSU, or Link Status Signal Unit, carries status information rather than call-control data. Its status field reports conditions such as alignment, out-of-service state, or processor outage. Operators often see LSSUs during start-up, recovery, testing, or a transition between available and unavailable states.
An MSU, or Message Signal Unit, contains signalling information for higher layers. ISUP call setup, release, and address-complete messages are common examples, while SCCP traffic may support services such as number portability or SMS-related applications. The presence of MSUs confirms traffic, but it does not by itself prove that the link is stable.
Reading Sequence And Control Fields
The backward sequence number and backward indicator bit acknowledge received signal units. The forward sequence number and forward indicator bit identify outgoing units and help detect retransmission or loss. A mismatch in these values can point to transmission errors, a reset condition, or a peer that has moved into a different link state.
The length indicator distinguishes the useful content that follows the control fields. An MSU normally includes a service information octet and signalling information, while an LSSU contains a status field and a FISU has no equivalent payload. The flag and check bits frame each unit and allow the receiver to identify boundaries and reject corrupted data.
A packet capture should therefore be read as a sequence, not as isolated hex values. For example, repeated FISUs with valid checks and advancing sequence activity suggest an idle link. Repeated LSSUs with alignment codes suggest that the endpoints are still negotiating service, while MSUs followed by retransmissions may indicate a degraded physical or transport path.
Link Alignment And State Changes
MTP2 uses LSSUs to control the path through alignment and proving procedures. Normal alignment indicates that the endpoints are attempting to establish service under ordinary conditions; emergency alignment is used when faster recovery is required under defined network conditions. Once proving succeeds, the link can enter service and carry MSUs.
Processor outage is different from a transmission fault. An endpoint may have working physical connectivity while its signalling processor is unable to accept traffic. The associated status indication helps the remote side stop sending unsuitable traffic and begin recovery procedures rather than treating every failure as a bit error.
Timers are essential to interpreting these transitions. A link that remains in alignment too long, repeatedly returns to out-of-service, or fails to receive expected acknowledgements should be correlated with configured timer values and implementation logs. The guide to SS7 timers is useful when checking whether a delay is protocol behaviour or a local configuration problem.
Diagnosing Faults In Live Networks
Start with the physical and framing evidence: CRC errors, lost flags, slips, abnormal signal-unit lengths, and link utilisation. Then compare FISU, LSSU, and MSU counts at both ends. A sudden rise in LSSUs paired with falling MSUs usually warrants investigation of alignment, hardware, clocking, or a remote processor condition.
In IP-based deployments, MTP2 may be emulated or mapped through a SIGTRAN stack rather than transported over a traditional 64 kbit/s timeslot. SCTP association state, heartbeat results, retransmission counts, and path selection then become part of the diagnostic picture. The material on SCTP path detection explains why an apparently available IP route can still fail signalling delivery.
Australian operators also need to account for long backhaul paths between metropolitan cores and regional facilities. A fault affecting a Brisbane–Perth route, a carrier Ethernet service near Adelaide, or a constrained link serving a remote Northern Territory site may produce different timing and recovery symptoms. Check maintenance windows, carrier alarms, and ACMA-related service obligations alongside protocol traces.
Practical Interpretation At A Glance
A disciplined review keeps protocol meaning separate from operational assumptions. An idle link is not automatically faulty, and a busy link is not automatically healthy. The following indicators provide a compact first pass:
- FISUs with valid checks: idle supervision is likely active
- LSSUs during start-up: alignment or proving is in progress
- LSSUs during established traffic: investigate a state change
- MSUs with retransmissions: examine errors, acknowledgements, and timing
When documenting an incident for a Sydney or Melbourne network operations centre, record timestamps in the relevant local time zone and preserve captures from both signalling points. Operational teams can also use electronic Kanban comparisons when managing replacement cards, test equipment, and spares across dispersed Australian sites.
Useful evidence to collect includes:
- Signal-unit counters before and after the event
- LSSU status codes and link-state transitions
- CRC, alignment, and retransmission statistics
- SCTP, M3UA, carrier, and power alarms
| Signal unit | Main purpose | Typical content | What repeated instances may indicate |
|---|---|---|---|
| FISU | Maintain link activity when no message is waiting | Control fields, sequence information, check value | Idle but supervised link |
| LSSU | Report link condition and alignment state | Status indication | Start-up, recovery, outage, or processor condition |
| MSU | Carry higher-layer signalling | Service information and signalling message | Active call, mobility, SCCP, or other SS7 traffic |
The key diagnostic habit is to correlate frame type, sequence behaviour, status codes, timers, and transport alarms before declaring a fault. Begin with a short capture from both ends of the affected link and classify every unit as FISU, LSSU, or MSU.
