A point code is the network-layer address of an SS7 signalling point, used by MTP3 to route messages. ITU-T point codes are 14 bits (structured 3-8-3 as zone-region-signalling point); ANSI point codes are 24 bits (network-cluster-member). Each message's routing label carries a destination (DPC) and originating (OPC) point code.
What a point code is
Every SS7 node that MTP3 can route to — an SSP switch, an STP, or an SCP database — has a point code, the SS7 equivalent of an IP address. MTP3 routes each message purely on the destination point code in its routing label. Point codes are unique within a signalling network; international interconnect uses the ITU-T international point-code space, while national networks run their own numbering (national point codes) that need not be globally unique.
ITU-T vs ANSI point codes
The two dominant formats differ in size and structure, which is why an international gateway must translate between them. ITU-T (Q.704) uses a 14-bit point code, conventionally written as a 3-8-3 split — zone, region/area, and signalling point. ANSI (used in North America) uses a 24-bit point code written as three octets — network, cluster, member. China and Japan use 24-bit variants of their own. A point code from one plan is meaningless in another, so international traffic crosses a gateway STP that maps between them.
| Standard | Bits | Structure | Example |
|---|---|---|---|
| ITU-T | 14 | Zone-Region-SP (3-8-3) | 2-101-3 |
| ANSI | 24 | Network-Cluster-Member (8-8-8) | 245-16-9 |
| China | 24 | 24-bit (national) | structured per national plan |
| ITU-T (national) | 14/16 | Operator-defined | national numbering |
The routing label: OPC, DPC and SLS
MTP3 prepends a routing label to every message. The Destination Point Code (DPC) says where the message goes; the Originating Point Code (OPC) says where it came from; the Signalling Link Selection (SLS) selects one link within a linkset for load-sharing while preserving message sequence within a flow. Together with the Service Indicator in the Service Information Octet (which user part — ISUP, SCCP, management), this label is all MTP3 needs to route.
SIO: Service Indicator (e.g. 3=SCCP, 5=ISUP) + Network Indicator
DPC: 14-bit destination point code (e.g. 2-101-3)
OPC: 14-bit originating point code (e.g. 2-050-1)
SLS: 4-bit signalling link selection (load-share within linkset)
----- then the user payload (SCCP / ISUP message) -----Point codes vs global titles
Point-code routing is fast but rigid: it requires the sender to know the exact destination node, and every node to hold routes to every point code. SCCP global-title addressing decouples this — a message addressed to an E.164 global title is resolved to a point code by Global Title Translation at an STP. In practice, inter-operator SS7 (roaming, SMS) is addressed by global title and translated to point codes hop by hop, while intra-node, circuit-related signalling (ISUP) uses point codes directly.
Frequently asked
How many bits is an SS7 point code?
It depends on the standard. ITU-T point codes are 14 bits (about 16,384 values, structured 3-8-3). ANSI point codes are 24 bits (structured 8-8-8 as network-cluster-member). China and Japan use their own 24-bit national formats.
What is the difference between OPC and DPC?
In the MTP3 routing label, the DPC (Destination Point Code) is the node the message is being routed to, and the OPC (Originating Point Code) is the node that sent it. MTP3 forwards on the DPC; the OPC lets the destination reply and lets management trace the source.
Why do international gateways translate point codes?
Because ITU-T (14-bit) and ANSI (24-bit) point-code spaces are incompatible and national point codes are only locally unique. A gateway STP translates between the international ITU-T space and each national or ANSI space so messages can cross network boundaries.
How do point codes relate to global titles?
A global title (an E.164 number) is a location-independent address that SCCP resolves to a point code via Global Title Translation at an STP. Point codes route within a network; global titles let operators address each other by number without sharing internal point-code plans.
What is the SLS field for?
Signalling Link Selection is a small field MTP3 uses to pick one link within a linkset, spreading load across parallel links while keeping messages of the same flow on the same link for in-sequence delivery. It is SS7's per-message load-balancing key.
Source specifications
The published standards this page is grounded in. Opens in a new tab.