Tutorial – SS7 or C7: Signaling System #7


What is a SS7?

Signaling System #7 (SS7), also known as C7 in Europe, is a critical component of modern telecommunications systems. Despite the upcoming migration to IP networks and evolution to IP-centric architectures such as the IP Multimedia Subsystem (IMS), it remains the glue that binds all telecommunication networks together. It is a communications protocol that provides signaling and control for various network services and capabilities. Being a layered protocol, SS7 provides various protocol levels for connection oriented and connection less (database) signaling in fixed and mobile networks.

SS7 is a specific protocol utilized for inter-system signaling. Stated differently, SS7 defines a set of parameters for messaging between telecommunications “nodes”. This signaling process takes place over telecommunications facilities called “links”. The SS7 messages traverse the links between two or more nodes, enabling data communications between and among the system. There are different types of SS7 links and they are used for different purposes. We will discuss nodes and links more fully later in this article.

SS7 signaling is a form of packet switching. Unlike circuit switching, which utilizes dedicated data “pipes” for transmission of information, packet switching dynamically assigns “routes” based on availability and “least cost” algorithms. Another example of packet switching is TCP/IP, the protocol used for routing messages over the Internet. Unlike the Internet, which utilizes a vast public “web” of interconnecting facilities and routing equipment, SS7 networks are private and logically self-contained. The private nature of SS7 networks is critical for security and reliability.

Purpose of SS7

The major purpose of SS7 is to enable connection oriented as well as connectionless signaling. Connection oriented signaling is associated with establishing a temporary dedicated connection between two or more points. A common example is simply the establishment of an inter-office voice call between two telecommunications switches. Connectionless switching is database oriented. There is no dedicated connection established. Instead, information is requested (from a database), provided (by the database), and relayed to the point in the network where it is required.

SS7 Architecture

There are four major types of telecommunications nodes important to SS7:

Service Switching Point (SSP): A telecommunications switch that contains the control logic (software) necessary to send/receive SS7 messages to other nodes in the network. Telecommunications switches that cannot send/receive SS7 messages are referred to as a Switching Point (SP). An SP must interface directly to an SSP (on a one-to-one relationship) in order to access SS7-based services.

Signal Transfer Point (STP): This is the “heart” of the SS7 network. Think of the STP as an electronic “post office”. While all that the STP does is route messages from point A to point B in the network, the network would be lost without it. The routing of messages follows a scheme that we will discuss later in this section. STPs are ALWAYS provided in mated pairs. STPs operate in what is called “load sharing mode”. This means that, at any given time, each STP should be processing 40% of the total signal-processing load. In the event of an STP and/or link(s) failure, the network is designed to change over to the remaining STP so that it can continue to operate at 80% load.

Service Control Point (SCP): This is the “brain” of the SS7 network. The SCP is nothing more than a database. However, utilization of an SCP offers profound enhancements for service delivery and network control. Service logic may be placed in the SCP (rather than the switch), creating the impetus for many improvements such as rapid feature deployment, mass customization of features, and improved utilization of switch resources. SCPs are almost always deployed in a “mated pair” configuration. This designed redundancy makes allowance for a back-up SCP should the other go out of service for some reason. Some non-call affecting, or otherwise non-critical, functions may be served by a single SCP.

Service Node (SN): Includes database functionality of the SCP along with additional capabilities such as voice interaction and control of voice resources. Generally speaking, SCPs work well with requirements that call for voluminous data transactions. SNs, on the other hand, are typically not designed for high volume data processing. Instead, SNs are best suited for special circumstance call processing involving voice resources and/or interaction.

SS7 Links

SS7 messages are carried by a physical medium referred to as a link. While there is some variation throughout the world, the traditional facility type utilized for an SS7 link is a DS-0 circuit, with a load carrying capacity of 56 kilo-bits per second (kbps). Some SS7 manufacturers have designed equipment and interfaces for DS-1 (24 DS-0 circuits) level interconnection for purposes of SS7-to-ATM signaling, but that is a subject for a more advanced tutorial.

There are six different types of links and they all have a different role in the SS7 network. SS7 links are typically provides in either pairs or quads, sets of four. The number of links or the number of pairs/quads, actually provided between nodes is based on traffic engineering considerations which we will discuss later.

A-links: Access links connect SSPs to STPs and SCPs to STPs.

B-links: Bridge links connect STP pairs that are at the same “hierarchical level”. SS7 is required for signaling between different networks, just as it is required within a given network type.

C-links: Cross links provide the connection between a mated pair of STPs.

D-links: Diagonal links are utilized to connect STP pairs that are at different hierarchical levels.

E-links: Extended links are supplementary links providing back-up to primary link pairs in the event the latter experience a failure.

F links: Fully associated links connect an SSP to another SSP. Typically, F links utilize one channel (one DS-0) of a DS-1 between MSCs that share a “hand-off” boundary. The DS-1 is utilized for voice communications when a mobile caller “hands off” or traverses from one MSC to another during a call. This leaves 23 channels for voice and 1 channel for signaling. F links will become increasingly rare as greater dependence is placed on more network centric SS7 signaling based on STPs.

What Traverses the SS7 Link?

SS7 signal units traverse the SS7 link. There are three different types of signaling units:

  • Message Signal Unit (MSU) is the signaling unit that carries the actual message “payload”.
  • Link Status Signal Units (LSSU): As the name implies, these signaling units provide status of the links operating condition.
  • Fill-in Signal Unit (FISU) act, as their name implies, to fill in the space between MSUs and LSSUs.

Due to the nature of their work, SS7 networks require very precise timing and synchronization. There is therefore a constant stream of signal units traversing an SS7 link. A link is a bi-directional circuit. This means that (in a DS-0) there is 56 kb of information traversing each direction every second. The bi-directional nature of SS7 links allows information to be transmitted and received simultaneously over the same link.


Despite the eventual migration to IP networks and evolution to IP-centric architectures such as the IP Multimedia Subsystem (IMS), SS7 does remain an important glue that binds all telecommunication networks together. Bridging technologies such as SIGTRAN will ensure a smooth migration to all IP networks with IP-based signaling such as SIP eventually taking over in many networks. However, it is critically important for telecommunications professionals to understand SS7 as it will continue to be a core element of telecom networks for the decades to come.

How to Learn More about It


Eogogics offers a host of courses on SS7/C7 and related subjects, including CAMEL, SIP, IMS, and Diameter.