The Phone Network's Hidden Brain

Imagine the traditional phone network (the network that carries your voice) as the body's circulatory system. For it to work intelligently, it needs a nervous system to carry control signals. Signaling System No. 7 (SS7) is that nervous system. It's a separate, high-speed packet-switched network dedicated entirely to carrying control information, known as signaling messages.

This separation is key: the voice traffic travels on one set of paths, while the "call setup" and other intelligent service information travels on the SS7 network. This architecture is what enabled advanced features like toll-free (800) numbers, call forwarding, Caller ID, and mobile phone roaming.

The Core Components of an SS7 Network

The SS7 network is built from a few types of specialized nodes, each with a distinct role.

• SP (Signaling Point)

  An SP is any node that can originate or terminate SS7 messages. Think of it as an entry or exit point for signaling information.

  • Service Switching Point (SSP): This is the most common type of SP. It's typically a telephone exchange (switch) that serves end-users. When you pick up the phone and dial a number, the SSP generates SS7 messages to set up the call.
  • Service Control Point (SCP): This is a centralized database that contains service logic. For example, when you dial a toll-free number, the SSP sends a query to an SCP. The SCP looks up the real, physical number associated with that toll-free number and sends it back to the SSP to complete the call routing.
• STP (Signal Transfer Point)

  An STP is a specialized packet switch that acts as a router for SS7 messages. It doesn't originate messages itself but reads the destination address of incoming messages and routes them to the appropriate SP or next STP. They form the backbone of the SS7 network, ensuring messages get where they need to go efficiently and reliably.

Essentially, SSPs create the questions (e.g., "how do I connect this call?"), SCPs provide the answers for special services, and STPs ensure the questions and answers are delivered to the right places.

Visualizing the Architecture

The relationship between the telephone network and the SS7 signaling network can be clearly visualized. They are two distinct, yet interconnected, networks working in tandem.

In this diagram, the "Sieć telefoniczna" (Telephone Network) represents the voice channels, while the "Sieć sygnalizacyjna" (Signaling Network) is the SS7 network. The SSPs (located within the telephone exchanges) are the bridge between these two worlds. The STP acts as a central router for signaling messages, and the SCP is the service database queried for advanced call handling.

Connections, Links, and Routes

To formally describe the SS7 network's structure, a specific terminology is used for connections and paths.

• Signaling Relation: This is a logical concept. A signaling relation exists between two signaling points (SPs) if they need to exchange signaling traffic with each other. For example, two telephone exchanges that can connect calls between their subscribers have a signaling relation.
• Signaling Link: This is the physical (or logical) transmission path carrying SS7 messages between two adjacent signaling points.
• Signaling Linkset: For capacity and reliability, it is common to have multiple parallel signaling links connecting the same two points. This bundle of parallel links is called a linkset. Traffic is shared across the links, and if one fails, the others can take over.
• Signaling Route: This is the end-to-end path that a signaling message takes from its Originating Point (OP) to its Destination Point (DP). A route can consist of one or more linksets and may pass through one or more STPs.
• Signaling Routeset: The complete collection of all available signaling routes between an Originating Point and a Destination Point.

Network Addressing: The Signaling Point Code (SPC)

Just like every house needs a unique street address, every node in the SS7 network needs a unique identifier. This address is called the Signaling Point Code (SPC).

• Unique Identifier: Each SP, STP, and SCP in a national or international network is assigned a unique SPC.
• Routing Information: The SPC is included in the header of every SS7 message. The STPs use the Destination Point Code (DPC) within the message to make routing decisions, looking up the DPC in their routing tables to determine the outgoing linkset.
• Structure: SPCs are typically 14-bit numbers (in North America) or a structured number, often represented in a 3-8-3 format (in some international standards), which allows for hierarchical network identification.