Separating User Traffic from Signaling
In a traditional telephone network, bearer channels carry voice, while a separate signaling network carries control information. Signaling System No. 7 (SS7) is a common-channel packet network for that information. It carries .
Signaling does not share the bearer channel with voice. SS7 messages set up calls, report their state, support network services, and manage traffic, while bearer channels carry user traffic. This separation supports features such as toll-free (800) numbers, call forwarding, Caller ID, and mobile phone roaming.
SS7 Network Nodes and Their Roles
An SS7 network contains specialized signaling points (SPs). Their roles depend on whether a point originates, receives, or transfers messages.
- SP (Signaling Point)
An SP is a node in the signaling network that may originate or receive SS7 messages and, in the role of an STP, transfer them. A signaling point therefore participates in message creation, delivery, or transfer according to its function.
- Service Switching Point (SSP): An SSP is an SP that provides telephone switching, typically in an exchange serving end users. It detects service requests and generates SS7 messages. When a subscriber picks up the phone and dials a number, the SSP starts call setup and exchanges the required messages.
- Service Control Point (SCP): An SCP contains service logic programs and may contain customer or service data; it is not necessarily one centralized database. When a subscriber dials a toll-free number, the SSP sends a query to an SCP. The SCP can look up the real number associated with the 800 number and return the result so the SSP can continue call handling.
- STP (Signal Transfer Point)
An STP is a specialized packet switch that transfers SS7 messages. In its transfer function, it reads the DPC in the routing label, consults routing tables, and forwards the message to the appropriate SP or the next STP. In a quasi-associated network, a pair or group of STPs can provide the intermediate layer and support efficient, reliable message delivery.
In practice, an SSP initiates signaling for call setup and sends a service query when needed, an SCP returns the service-logic result, and an STP transfers both types of messages to the appropriate points.
Two Interconnected Networks
The telephone network and the SS7 signaling network are separate networks that work together during call handling. The first carries user traffic, while the second transfers control messages.
In the diagram, the Telephone Network represents voice channels, while the Signaling Network is the SS7 network. SSPs, located in telephone exchanges, connect switching functions with signaling. STPs transfer messages between points, while SCPs provide service logic and data used by queries for call handling.
Signaling Relations, Links, and Routes
Formal descriptions of an SS7 network use specific terms for points, links, and routes because these terms cover both logical relationships and message transmission.
- Signaling Relation: This is a logical relationship. A signaling relation exists between two signaling points (SPs) when they need to exchange signaling traffic. For example, two telephone exchanges that connect calls between their subscribers have a signaling relation.
- Signaling Link: This is a physical or logical transmission path carrying SS7 messages between two adjacent signaling points.
- Signaling Linkset: A linkset is a set of links directly connecting the same two points. Multiple parallel links can increase capacity and reliability, and traffic can be shared across them. If one link fails, the others can take over its traffic.
- Signaling Route: This is a predetermined sequence of points and links used by a message from its Originating Point (OP) to its Destination Point (DP). A route can contain one or more linksets and may pass through one or more STPs.
- Signaling Routeset: This is the complete collection of available signaling routes between an Originating Point and a Destination Point.
Addressing: Signaling Point Code (SPC)
Each signaling point receives a unique identifier within a given numbering plan, called a Signaling Point Code (SPC). The code identifies a node for SS7 routing functions, and one physical node may have separate codes for different network plans.
- Unique Identifier: Each SP, STP, and SCP in a national or international numbering plan receives a unique SPC. The coding rules for national and international networks are independent.
- Routing Information: In the standard MTP routing label, the DPC and OPC identify the destination and originating points. The SLS field supports link selection and load sharing. MTP3 routing uses the DPC and SLS and, in some cases, the service indicator. In the Q.704 format, the label is 32 bits long. The DPC and OPC fields are 14 bits each, and SLS is 4 bits. National variants may use different field sizes. STPs use the and look it up in their routing tables to determine the outgoing linkset.
- Structure: In the ANSI format used in North America, an SPC has 24 bits and normally consists of three 8-bit fields named network, cluster, and member. An international ITU-T point code has 14 bits and uses a 3-8-3 arrangement for geographical zone, area or network, and signaling point. A national numbering plan may define a different field split, so 3-8-3 is not universal. These fields support hierarchical identification of SS7 network nodes.