ss7 (signalling system 7)

The Evolution of SS7

In the mid-1960s, the CCITT (now the ITU) developed a digital signaling standard called Signaling System #6. SS6 was based on a packet-switched, proprietary data network. SS6 used 2.4 Kbps data links to send packets of data to distant switches to request services.

--This was the first use of packet switching in the PSTN.
--SS6 packets consisted of 12 signal units of 28 bits each placed into a data block.
--SS7 began deployment in 1983, and gradually phased out SS6. It was initially used only in the interoffice network (from central office to central office), but has gradually expanded and is now deployed in local central offices as well. SS7 provides a global standard for call setup, routing, and control.

SS7 Features

• High-speed data links (56 Kbps – national; 64 Kbps – international).
• Variable length signal units with a maximum size limitation.
• Plans to increase link speed to T1 and E1 speeds to be able to handle the increased demands required of the SS7 network.


SS7 Uses

--The first use of SS7 was not for call setup and teardown, but rather for accessing databases. 800 numbers provided a problem for switches in that they could no longer route based on area code.
A second “real number” for each 800 number needed to be placed in a centralized database which multiple central offices could access.

The call flow of an 800 number is as follows:
• 800 number dialed, CO switch receives digits and routes the call to a remote database via data link.
• “Real number” is determined from the database via SS7 message packet.
• Database responds with response message packet.
• Database provides routing number for call and billing information.
• CO switch is then able to route the call in the conventional manner.

SS7 Expansion

When 800 number lookups via SS7 proved successful, the network was expanded to include the ability to do call setup, teardown, and other services. Call setup/teardown is done using the ISDN User Part (ISUP) protocol. Database lookup uses the Transactional Capabilities Application Part (TCAP) protocol.
800, 900, 911 services, custom calling features, caller ID, and enhanced services are provided by SS7 and the Advanced Intelligent Network (AIN).

SS7 Deployment Planes

SS7 is deployed on two distinct levels or planes:
• International—ITU-TS standard
• National – country specific (North America—US and Canada - uses the ANSI standard)
Bellcore is an extension of the ANSI protocol and ensures the ability to interoperate with Bell Operating Company (BOC) networks.
Gateways convert national versions of SS7 to ITU-TS versions so that the networks of all nations can interoperate with each other


for more details see the
http://rapidshare.com/files/126522388/ss7fun01.pdf.html
http://rapidshare.com/files/126522736/ss7fun02.pdf.html