What is SS7?

Signaling System No. 7 (SS7) is an international telecommunication standard that defines how public telephone networks (PSTN, ISDN, GSM) exchange control information. It is a Common Channel Signaling system, which means it creates a logically separate, packet-switched network dedicated solely to signaling messages.

This separation of the signaling plane from the user traffic plane was a revolution in telephony. Instead of slowly sending tones down the same line as the voice call, SS7 sends high-speed digital messages over its own network. This enabled fast call setup, high efficiency, and the creation of countless modern services like 800-numbers, caller ID, text messaging (SMS), and mobile roaming.

The Architecture of an SS7 Network

The SS7 network is composed of distinct types of signaling nodes connected by signaling links.

Key Network Elements

• SP (Signaling Point): Any node in the network that can originate or terminate signaling messages. This includes telephone exchanges and databases. Each SP is identified by a unique address called a Signaling Point Code (SPC).
• STP (Signal Transfer Point): A packet switch that routes SS7 messages between SPs. STPs are the routers of the signaling network, ensuring messages get to their destination. They do not originate messages themselves, only transit them.
• SCP (Service Control Point): A centralized database that provides information for advanced "Intelligent Network" services. When a call needs special handling (like an 800-number call), the local exchange (SSP) queries the SCP to get instructions on how to route the call. It contains logic and data for services like number translation or roaming subscriber information (HLR).
• SSP (Service Switching Point): A telephone exchange (switch) that is equipped to handle SS7 signaling. An SSP can originate, terminate, or transit calls by sending SS7 messages to other SSPs or querying an SCP for instructions.

Signaling Modes of Operation

The relationship between the signaling path and the traffic path it controls defines the mode of operation.

• Associated Mode: The signaling messages are sent over a direct signaling link between two signaling points that are also connected by the traffic channels being controlled. This is the simplest mode but requires a direct signaling link for every traffic route, which can be inefficient for small routes.
  
• Quasi-Associated Mode: Signaling messages are not sent over a direct link but follow a separate, predetermined path through one or more STPs. This is the most common mode in large networks as it allows for a more flexible and hierarchical network design, avoiding the need for a fully meshed signaling network.
  

The SS7 Protocol Stack

SS7 is a layered protocol suite. While conceptually similar to the OSI model, it defines its own four-level architecture. Each level performs a well-delimited role that together provides reliable, scalable and secure signaling for voice calls, intelligent-network services and mobile data across fixed and cellular networks.

• MTP Level 1 (Signaling Data Link): The physical layer responsible for the transmission medium and electrical/optical interface. In practice it uses 64 kbit/s timeslots in an E1/T1 stream, but it can also operate over optical fiber, microwave PDH/SDH or ATM (AAL1) bearers. It specifies line coding, framing and signal levels.
• MTP Level 2 (Signaling Link): The data-link layer that guarantees error-free delivery of MSU (Message Signal Units). It provides error detection and correction (CRC-16), positive acknowledgments and retransmissions, sequence preservation, flow control, and link status supervision using LSSU and FISU frames.
• MTP Level 3 (Signaling Network): The network layer responsible for end-to-end message delivery. It performs routing based on the Signaling Point Code (PC), link selection and load sharing, automatic rerouting during failures (changeover/changeback), overload control and overall Signaling Network Management.
• User Parts (Level 4): A collection of application protocols that leverage the transport service of MTP/SCCP. Principal user parts include:
  • ISUP (ISDN User Part): Provides call control in PSTN/ISDN networks — call setup, modification and release for voice and circuit-switched data.
  • B-ISUP (Broadband ISUP): An extension of ISUP for broadband networks (ATM/SDH) supporting bandwidths above 2 Mbit/s.
  • TUP (Telephone User Part): An earlier user part for analog networks, gradually replaced by ISUP but still encountered in some regions.
  • SCCP (Signaling Connection Control Part): Adds application-level addressing (subsystem numbers) and offers connectionless and connection-oriented services above MTP-3.
  • TCAP (Transaction Capabilities Application Part): Enables query/response transactions without establishing a dedicated circuit. Used by Intelligent Network (IN) and databases such as HLR/VLR.
  • MAP (Mobile Application Part): Specific to GSM/UMTS networks. Handles roaming, authentication, SMS routing and location updates, transported over TCAP.
  • BSSAP (Base Station Subsystem Application Part): Carries signaling between the radio network (BSC) and the core switch (MSC) in GSM.
  • INAP (Intelligent Network Application Part) / CAP (CAMEL Application Part): Support value-added services such as prepaid billing, number translation and call screening.
  • OMAP (Operations and Maintenance Application Part): Protocols for SS7 network operations, administration and maintenance.