SS7 is the world’s largest data network. It links telcos, cellular, and long-distance networks
nationwide and worldwide. SS7 interconnects thousands of telephone company providers into one common signaling network. SS7 will continue to evolve as new features are added to the Advanced Intelligent Network.
The Evolution of SS7
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 Signaling Modes
1.Associated Signaling—Uses one dedicated path between switches as the signaling link.
2.Non-Associated Signaling—Uses separate logical paths and multiple nodes.
3.Quasi-Associated Signaling—Uses a minimal number of nodes (preferred for SS7, causes less delay).
Features of SS7
➦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.
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.
Local Number Portability (LNP)
Prior to SS7, 800 numbers were not portable. If a company moved, they had to get a new number. The Telecom Act of 1996 mandated that personal phone numbers should also be portable. Telcos are required to support the porting of telephone numbers within a geographic area, increasing the demands on the SS7 network.
Seamless Roving
Seamless roving in cellular networks uses SS7 to share subscriber information from Home Location Registers (HLRs) so users do not have to register their cell phones with other providers when they travel. All cellular providers can access each others databases via SS7, enabling their subscribers to roam seamlessly from one network to another, while still allowing the home network to track and bill for all calls.
Summary
nationwide and worldwide. SS7 interconnects thousands of telephone company providers into one common signaling network. SS7 will continue to evolve as new features are added to the Advanced Intelligent Network.
While similar to ISDN-PRI, Signaling System Number Seven (SS7) uses different messaging for call setup and teardown. SS7 lets any SS7-enabled node to talk to any other, regardless of whether they have direct trunk connections between them.
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| SS7 in Telecommunication Network |
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.
1.Associated Signaling—Uses one dedicated path between switches as the signaling link.
2.Non-Associated Signaling—Uses separate logical paths and multiple nodes.
3.Quasi-Associated Signaling—Uses a minimal number of nodes (preferred for SS7, causes less delay).
1.Associated Signaling
With this type of signaling, the signaling link directly parallels associated voice trunks. Thus, dedicated links must be provisioned between every interconnected switch. |
| Signaling Mode |
2.Non-Associated Signaling
With this type of signaling, voice/data and signaling are carried on separate, logical paths. Multiple nodes in the signaling path to the final destination can causes delays. Although used in the SS7 network, it is not preferred.
With this type of signaling, voice/data and signaling are carried on separate, logical paths. Multiple nodes in the signaling path to the final destination can causes delays. Although used in the SS7 network, it is not preferred.
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| Signaling System |
3.Quasi-Associated Signaling
This type of signaling employs a minimal number of nodes, thus minimizing delays. Quasi-associated signaling is the preferred signaling mode for SS7. |
| Signaling System |
➦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.
Uses of SS7
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.
➦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.
Expansion of SS7
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 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.
Seamless roving in cellular networks uses SS7 to share subscriber information from Home Location Registers (HLRs) so users do not have to register their cell phones with other providers when they travel. All cellular providers can access each others databases via SS7, enabling their subscribers to roam seamlessly from one network to another, while still allowing the home network to track and bill for all calls.
Above in starting of this page I have already mentioned the importance and use of SS7 that SS7 is the world largest data network. It links the telecommunication network together nationwide and worldwide. It provide interconnection thousands of telephone into one common signaling network.
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