The Message Transfer Part (MTP) is the reliable transport of the SS7 stack, defined in ITU-T Q.702–Q.704 over three levels: MTP1 (physical 64 kbit/s links), MTP2 (error-controlled data link using signal units), and MTP3 (network layer that routes messages by point code and manages the signalling network).
MTP Level 1 — the signalling link
MTP1 (Q.702) is the physical layer: a bidirectional 64 kbit/s channel, traditionally a single timeslot within an E1 (2.048 Mbit/s) or T1 (1.544 Mbit/s) carrier. It defines the electrical and physical characteristics of the signalling data link but carries no protocol logic itself — it simply moves bits between two adjacent signalling points.
MTP Level 2 — link reliability
MTP2 (Q.703) turns the raw link into a reliable one between adjacent points. It frames traffic into signal units, detects errors with a CRC-16 checksum, retransmits corrupted or lost units, and performs link alignment and flow control. Three signal-unit types carry different jobs: MSUs carry the actual signalling payload, LSSUs manage link status, and FISUs are sent continuously when idle to keep the link monitored.
| Unit | Name | Purpose |
|---|---|---|
| MSU | Message Signal Unit | Carries the SIF payload (MTP3 routing label + user data) |
| LSSU | Link Status Signal Unit | Signals link state during alignment/failure (1–2 status octets) |
| FISU | Fill-In Signal Unit | Sent when idle for continuous error monitoring and acknowledgement |
MTP Level 3 — routing and management
MTP3 (Q.704) is the network layer. Signalling Message Handling routes each MSU using the routing label — the Destination Point Code (DPC), Originating Point Code (OPC) and Signalling Link Selection (SLS) field, which spreads traffic across a linkset for load-sharing. Signalling Network Management keeps the network healthy: it reroutes around failed links (changeover/changeback), tests routes, and issues Transfer Prohibited/Allowed (TFP/TFA) messages so points stop or resume sending toward an unreachable destination.
| Field | Meaning |
|---|---|
| DPC | Destination Point Code — where the message is going |
| OPC | Originating Point Code — where it came from |
| SLS | Signalling Link Selection — chooses a link for load-sharing/sequencing |
| SIO | Service Information Octet — service indicator (which user part) + network indicator |
Link types and the STP
MTP3 routing is organised around the Signal Transfer Point (STP), a packet switch for SS7 that relays MSUs between endpoints. Links are named by the role they play — A-links connect an endpoint (SSP/SCP) to its STP, while B/C/D-links interconnect STPs — and links are grouped into linksets and combined linksets for redundancy. This mated-STP topology is what gives SS7 its carrier-grade resilience: no single link or STP failure isolates a signalling point.
| Link | Connects | Role |
|---|---|---|
| A (Access) | Signalling endpoint ↔ its home STP | Access to the network |
| B (Bridge) | Mated STP pair ↔ another mated pair (same level) | Quad interconnect between STP pairs |
| C (Cross) | The two STPs of a mated pair | Only used when other routes fail |
| D (Diagonal) | STP pairs at different hierarchy levels | Interconnect across levels |
| E / F | Endpoint ↔ remote STP / endpoint ↔ endpoint | Extended and fully-associated links |
Frequently asked
What are the three levels of MTP?
MTP1 is the physical signalling link (a 64 kbit/s channel in an E1/T1). MTP2 makes that link reliable with signal units, CRC error checking and retransmission. MTP3 routes messages across the network by point code and manages link failures and rerouting.
What is an MSU?
A Message Signal Unit is the MTP2 frame that carries actual signalling. Its Signalling Information Field holds the MTP3 routing label (DPC/OPC/SLS) plus the user-part payload — ISUP, SCCP or a management message. FISUs and LSSUs, by contrast, carry no user payload.
What does the SLS field do?
Signalling Link Selection lets MTP3 spread traffic across the links of a linkset for load-sharing while keeping messages of the same transaction on one link for in-sequence delivery. It is effectively a load-balancing key computed per message flow.
What is an STP?
A Signal Transfer Point is an SS7 packet switch that routes MSUs between signalling points by point code, without originating or terminating the application. STPs are deployed in mated pairs with redundant links so the signalling network survives individual failures.
How does MTP3 recover from a failed link?
MTP3 network management detects the failure and performs changeover, moving affected traffic to alternative links in the linkset, and issues Transfer Prohibited (TFP) messages so upstream points reroute. When the link recovers it runs changeback and sends Transfer Allowed (TFA).
Source specifications
The published standards this page is grounded in. Opens in a new tab.