What is SS7?

SS7, or Signaling System No. 7, is a series of telephone signaling protocols defined by the Telecommunication Standardization Sector (ITU-T) of the International Telecommunication Union (ITU). SS7 specifies the procedure and protocol used for signaling and control for a variety of network services and capabilities, used to set-up most of the world’s public switched telephone network. SS7 is often referred to as Common Channel Signaling System No. 7 (CSS7 or C7) in North America. Meanwhile, the same standard is referred to in the United Kingdom as C7 (CCITT number 7), number 7 and CCIS7.

What are the Uses of the SS7 Network and Protocol?

The SS7 network and protocol have the following uses:

  • Setting up and tearing down telephone calls;
  • Wireless services, such as those used in personal communications services (PCS), mobile subscriber authentication, and wireless roaming for mobile telephony;
  • Toll-free calls (800/888) and toll wireline services (900);
  • Call features, such as call forwarding, three-way calling, and calling party name/number display;
  • and Worldwide telecommunications.

SS7 Technology

SS7 technology is a type of packet switching, which dynamically assigns routes based on accessibility and “least cost” algorithms. One advantage of SS7 over the Internet, which uses a vast Web of interconnecting facilities and routing equipment, is that its networks are private and logically self-contained. This feature makes the SS7 network secure and reliable.

SS7 Signaling Modes

SS7 networks are designed to run in two modes: Associated Mode and Quasi-Associated Mode.

Associated Mode is capable of signaling moves from switch to switch through the public service telephone network (PSTN). It follows the path used by the associated facilities carrying the telephone call. The Associated Mode is suitable and more efficient for small networks. However, it is not the predominant choice for signaling mode in North America.

Quasi-Associated Mode is capable of signaling moves from the originating switch to the terminating switch. It follows a path through a separate SS7 signaling network that consists of signal transfer points (STP’s). The Quasi-Associated Mode is suitable and more efficient for large networks and is a predominant choice for signaling mode in North America.

SS7 Networks

An SS7 network is composed of several types of links (A, B, C, D, E, and F) and three signaling nodes (SSP, STP, and SCP). Each node is identified on a network by a number called a point code.

Signaling Links

The messages in the SS7 network are exchanged between signaling links, which are 56 or 64 kbps bidirectional channels. Signaling occurs out-of-band on dedicated channels, instead of in-of-band on voice channels. This attribute provides for faster call set-up time, more efficient use of voice circuits, support for intelligent network and improved control over deceptive network usage.

Signaling Types

Signaling types are classified according to their use in the SS7 signaling network:

A link

An Access link connects a signaling end point to a signaling transfer point (STP). Messages originating from or destined to the signaling end point are transmitted on this link.

B link

A Bridge connects an STP with another STP.

C link

A Cross link connects STP’s, performing the same functions into a mated pair. This link is used when an STP has no other available route to destination signaling point caused by link failure. SCP’s may also be set up in pairs to improve its reliability.

D link

A Diagonal link connects a secondary STP pair to a primary STP pair.

E link

An Extended link connects an SSp to an alternate STP. These alternate signaling paths are often used when an SSp’s STP is unreachable with an A link.

F link

A Fully Associated link connects two signaling end points.

Signaling Points

Signaling points in an SS7 network are identified by a numeric point code. These codes are transmitted via signaling messages exchanged between signaling points to identify the source and destination of each message.

There are three types of signaling points in an SS7 signaling network:

SSPService signaling point

SSP are switches that originate, terminate, or tandem calls. It sends signaling messages to other SSP’s to set up, release, and manage voice circuits necessary to complete a call. It may also determine how to establish a route for a specific call by sending a query to a central database.

STPSignal transferring point

An STP is a packet switch that routes incoming messages to an outgoing signaling link based on the information in the ss7 message. As it functions as a network hub, STP’s eliminates the need for direct links between signaling points. STP’s may perform global title translation, and act as a firewall to screen messages exchanged with other networks.

SCpService control point

SCP’s and STP’s are often set up in mated pairs in separate physical locations to ensure a system-wide service in the event of an isolated failure.