SS7 Protocol, Basic Overview

SS7 which stands for Signaling System No. 7 is a protocol for telecommunication systems. This is used as a standard for call initiation and controlling in any network like GSM or PSTN.

SS7 is implemented to establish a voice connections among the calling and called party entities. SS7 is also the medium for advanced capabilities and applications including mobile networking and services as well as wire line applications such as toll-free calling and automatic calling card identification. SS7 is treated as a host for facilitating mobile networking systems, calling card identifications etc.

SS7 is actually can be treated as packet switching. Routes are dynamically assigned on the basis of availability and minimum cost. Unlike TCP/IP SS& networks are private which ensures security and liability.

The SS7 protocol stack  draws similarity with the popular OSI Model . OSI layers 1 to 3 are provided by The Message Transfer Part (MTP) and the Signaling Connection Control Part (SCCP) does the job for OSI layer 1 to 3. Telephone User Part (TUP) or the ISDN User Part (ISUP), provides layer 7. OSI layer 4 to 6 are not implemented so far.[1] The Transaction Capabilities Application Part (TCAP) is the primary SCCP User .It works in the Core Network. TCAP facilitates the Mobile Application Part, the Intelligent Network Application Part and the CAMEL Application Part. The Message Transfer Part (MTP) provides network interface, information transfer, message handling and routing functionalities. Signaling Connection Control Part (SCCP) and MTP Level 3 constitute Network Service Part (NSP). Telephone User Part (TUP) used to connect calls. ISDN User Part (ISUP) actually establishes, maintains, and ends the connections. Transaction Capabilities Application Part (TCAP) makes database queries. and connects to Intelligent Network Application Part (INAP) for intelligent networks, or Mobile Application Part (MAP) for mobile.

SS7 actually two types of signaling: a. connection oriented , b. connectionless oriented signaling. Connection oriented signaling means switch-to-switch that uses inter-office trunks. These trunks are the carrier of voice data. The ISDN User Part (ISUP) part is responsible to establish trunks . Transaction Capability Application Part (TCAP) is responsible for connectionless signaling. This contains the functionality of switch-to-database or database-to-database communications.

I worked on an Interactive Voice Response System which runs on the MAP and ISDN layer of SS7 stack. Here I implemented following call control functions,

 Set-up to initiate a call
 Call Accepted to acknowledge an incoming call and set it to the Alerting state
 Call Connected to answer an incoming call
 Clear to drop a call
 Channel Free to indicate that a call control channel has been idled.

In my Research & Development  team I used the ISDN call Control Protocol based on a telephony card. Our system worked on a PSTN network. The system was developed so that the IVR server works as a switching center to our PSTN controlling center. The server was given a dedicated routing path with separate trunk ID. The system supports multiple channel, and 230 users can use the server simultaneously. I have developed the C program which constantly poll the data line and manages free and busy channels through the implementation of various service handlers which are in fact asynchronous Linux routines written in C.