SS7 protocol technical overview

     In the previous post we have seen what SS7 can do and what types of services that we use everyday are based on it. We will turn now a bit more technical and see in a nutshell how SS7 works (more details can be found in various tutorials over the web). In many of its aspects, the SS7 architecture is similar to the OSI network layer architecture.

Signaling points and their types

     SS7 nodes are called signaling points (SP) and they are usually identified by an integer called point code (PC).  The international SS7 network uses its own unique PC numbering and each national network and each operator use their own numbering schemes internally.  There are three different types of SPs in an SS7 network, categorized based on their functionality:

     A Service Control Point (SCP) is an interface between SS7 networks and databases. A Service Switching Point (SSP) is a voice switch with integrated SS7 functionality. And a Signal Transfer Point (STP) is responsible for transferring information between other SPs. Many SPs in the network typically play multiple roles (for example an SSP can also support STP functionality).

Signaling links and their types

     Signaling points are connected to each other with signaling links, over which signaling information is exchanged. More than two links (up to a maximum of 16) can be used to connect two signaling points, both for increased capacity as well as redundancy. These links bundled together are called combined linksets.  There are several types of logical links that can be used in SS7 networks, categorized depending on their functionality and what types of SPs they connect (the physical link remains the same for all these cases). These are A(access)- links, C(crossover)- links, B(bridge)- links, D-(diagonal) links E(extended)- links and F(fully associated)- links.

Routing

     Routing between nodes is configured statically. Each SP maintains a routing table with all the available information needed for routing. The group of routes that can be used to reach a particular destination is called routeset.

The SS7 protocol stack

     SS7 is a work in progress since the early 1970s. During this period, a stack of protocols was developed to provide the functionality required. The architecture of these protocols is layered and it is pretty much analogous to the OSI 7-layer model which is used on data networks. The lower layer protocols are common for all SS7 implementations. The higher level of the stack differentiates depending on the applications to be used by the SS7 infrastructure. The schematic below shows the most commonly used protocols in the SS7 stack: