SS7 is a protocol responsible to move the signaling messages between telecom network nodes. It is using the Service Switching Points (SSPs), and Signal Transfer Points (STPs).
The SSPs are the end points that need to transfer messages to another node. While the STPs are the routing devices that route those messages to reach the required destination.
Later, we will go deeper in Gr, Gs, and Iu interfaces. And those interfaces use SS7 protocol.
TCP/IP protocol stack
To understand SS7 protocol well, let's take a look on TCP/IP protocol stack that supports IP based networks,
The TCP/IP protocol stack is used to exchange messages in IP based networks. The layers of this protocol stack are,
- Physical, it supports the physical transmission of data between nodes in the form of electrical signal (Wire cables), or optical pulses (Fiber optics).
- MAC, it is the medium between the physical layer and the upper layers.
- Network, responsible for packet routing between the nodes, it is using the IP protocol.
- Transport, it provides end-to- end routing, there may be more than one application on the node. Therefore, it differentiate between those applications by assigning a 'port number' for each application. It is using TCP or UDP protocol.
- Application, it supports the communication between applications installed on the nodes.
SIGTRAN (SS7 over IP)
The SS7 protocol stack is based on the MTP, which refers to 'Message Transfer Part'. The MTP1, MTP2, and MTP3 are making the functions of Layer1, Layer2, and Layer3 in TCP/IP protocol stack.
But today’s networks are based on IP addressing. Therefore, the SS7 protocol is enhanced to be able to support IP based networks. As a result, a new protocol stack is developed which is the 'SS7 over IP', which is the 'SIGTRAN'.
The protocols exist in SIGTRAN are,
IP (Internet Protocol)
The routing protocol that uses IP addresses to route traffic between different nodes.
SCTP (Stream Control Transmission Protocol)
Establishes a connection between two endpoints, called an association, for transmission of messages. It operates at the same layer as TCP or UDP in TCP/IP protocol stack.
M3UA (MTP Level 3 User Adaptation Layer)
It transports SS7 MTP3 user part signaling messages over IP using SCTP.