The SS7 Layered Model

Signaling System No. 7 (SS7) is the engine that drives modern telecommunication networks, handling call setup, routing, and advanced services. To manage this complexity, SS7 uses a layered protocol stack, similar in concept to the well-known OSI model. However, SS7 condenses these functions into a more pragmatic four-level structure, optimized for high-speed signaling.

This stack is divided into a robust transport foundation called the Message Transfer Part (MTP), and various "User Parts" which are the applications that use this transport to perform specific tasks, like setting up a phone call or checking a mobile subscriber's location.

The Foundation: Message Transfer Part (MTP)

The MTP is the backbone of the SS7 network, responsible for the reliable delivery of signaling messages between network nodes. It is itself divided into three levels, which correspond directly to the bottom three layers of the OSI model.

• MTP Level 1 – The Signaling Data Link (Physical Layer):

  This is the physical layer. It defines the electrical and physical characteristics of the signaling link, such as the connectors, cables (often E1/T1 links), and transmission rates. Its only job is to move bits across a physical medium.

• MTP Level 2 – The Signaling Link (Data Link Layer):

  This layer ensures reliable, error-free communication between two directly connected signaling points. It takes raw bits from MTP-1 and organizes them into packets called Signal Units (SUs). Its functions include error detection, error correction (by retransmission), and flow control. Analogy: MTP-2 is like a secure, registered mailbag sent between two neighboring post offices, ensuring nothing is lost or damaged along that specific route.

• MTP Level 3 – The Signaling Network (Network Layer):

  This layer provides the overall network functionality. It is responsible for routing signaling messages through the SS7 network from the originating point to the destination point, which may be several hops away. It manages the signaling network, re-routing traffic if a link or node fails. Analogy: MTP-3 is the entire postal service that reads the address on the mailbag and determines the best route to send it across the country.

The "Applications": SS7 User Parts

Sitting on top of the MTP are the User Parts, which are the protocols that perform the actual work of the network. They use the MTP as a reliable transport service to exchange their specific messages.

• ISUP (ISDN User Part):

  The primary user part for call control. ISUP defines the messages and procedures for setting up, managing, and tearing down voice and data calls in PSTN and ISDN networks. Messages like "Initial Address Message" (IAM) to start a call, "Answer" (ANM), and "Release" (REL) are part of ISUP. Analogy: ISUP messages are the actual contents of the letter, with instructions like "Please ring the phone at this number."

• SCCP (Signalling Connection Control Part):

  While not a user part in itself, SCCP is a crucial protocol that enhances MTP-3. It provides more advanced addressing capabilities through Global Title Translation (GTT) and allows for end-to-end connectionless and connection-oriented signaling connections. It's the foundation for database-driven services.

• TCAP (Transaction Capabilities Application Part):

  TCAP enables transaction-based communication between network nodes, typically involving database queries. It provides a standardized way for an application to invoke features on another node. TCAP itself doesn't define the services, but provides the framework for them. Analogy: TCAP is like a standardized form for a "Request for Information", which other applications fill out.

Advanced Services Using TCAP

Several critical application-level protocols rely on TCAP to provide advanced services:

• INAP (Intelligent Network Application Part):

  Used to implement intelligent network services. When you call a toll-free number (e.g., 800-number), INAP is used to query a database (SCP) to determine how the call should be routed (e.g., to the nearest call center). It also enables services like number portability and prepaid calling cards.

• MAP (Mobile Application Part):

  Essential for mobile networks (GSM, UMTS). MAP handles mobility management, allowing the network to track a subscriber 's location as they move between cells. It is also responsible for authentication, SMS (Short Message Service) delivery, and enabling seamless roaming between different operators.

Modernizing SS7: Signaling over IP (SIGTRAN)

As networks transitioned towards IP-based infrastructure, a need arose to transport SS7 signaling messages over IP networks instead of traditional dedicated links. The suite of protocols designed for this purpose is called SIGTRAN.

• SCTP (Stream Control Transmission Protocol):

  SIGTRAN uses SCTP as its transport protocol over IP. SCTP provides reliable, message-oriented transport similar to TCP, but with features better suited for signaling, such as multi-streaming (to prevent head-of-line blocking) and multi-homing (for network redundancy).

• Adaptation Layers (e.g., M2UA, M3UA):

  To allow the upper-level SS7 protocols (like ISUP and SCCP) to work seamlessly over SCTP/IP, a series of adaptation layers were created. These layers (e.g., M3UA for MTP-3 users like ISUP, or SUA for SCCP users like TCAP) "trick" the upper SS7 protocols into thinking they are still communicating with the traditional MTP, while in reality, their messages are being encapsulated for transport over the IP network.

Interactive SS7 Message Flow