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SS7 – Signaling System Seven – Telecommunications Protocol SS7 June 10, 2009

Posted by HubTechInsider in Fiber Optics, Telecommunications.
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SS7 software layers

SS7 software layers

In Signaling System 7 (SS7) protocol, a worldwide standard (with variations), routing intelligence is located in low cost computer-based equipment rather than in central office switches.

One of the primary benefits of SS7 is global interoperability. It has the capability to enable all carriers to cooperate with each other. It is a standard protocol approved by the ITU. Global billing, toll-free calling, 900-number services, and international wireless call roaming are all call features that are dependent on SS7.

SS7 is used on a global basis. In North America, the ANSI version of SS7 is used. In Europe, the ETSI version is used. In other pats of the world, the ITU version of SS7 is used.

Gateways allow these international SS7 implementations to communicate with each other.

SS7 is essential to modern networking. With SS7, an overlaid packet switched network controls the underlying voice network’s operation and signaling information is carried on a separate channel from voice and data traffic.

Because signaling is such a quick network activity, it is possible to multiplex many signaling messages over one signaling channel using a packet switching arrangement.

SS7 permits the telephone company to provide one database for several switches in order to freeup switch capability for other functions. This is the capability that makes SS7 the foundation for Intelligent Networks (INs) as well as Advanced Intelligent Networks (AINs).

As an example, in order to provide a service such as 900 number and toll-free calling, in SS7, powerful parallel processing computer systems hold massive databases with information such as routing instructions for toll-free and 900 number telephone calls. One processor with its database supports many central office switches under SS7. in this way, each central office itself is not required to host the centralized database. Without the need to share the expense of maintaining the sophisticated routing information, each central office can share in the expense of a database or feature upgrade to the centralized SS7 datastore.

MCI first implemented SS7 into its network in 1988. SS7 enabled them to halve their call setup time on calls between Philadelphia and Los Angeles. Freeing up voice channels from their previous signaling duties pre-SS7 enabled carriers to pack more voice calls on their existing network paths.

Cellular networks use SS7 technology to support roaming. Every cellular provider has a database called the home location register, or HLR, where complete information regarding each subscriber is kept. They also maintain a database called the VLR, or visitor location register, that maintains information on each caller who visits from other areas. When a cellular subscriber roams, each network they visit exchanges SS7 messages with their “home” network. The subscriber’s home system also marks its HLR so that it knows where to send calls for its customers who are roaming.

SS7 has three major components:

1. Packet switches – Signal Transfer Points that route signals between databases and central switches. STPs, or Signal Transfer Points, are responsible for translating the SS7 messages and then routing these messages amongst the various network nodes and databases. Signal Transfer points are packet switches that route signals between central offices as specialized databases. Messages are sent between points on the SS7 network in variable-length packets with the addresses attached. Signal transfer switches read only the address portion of the packets and forward the messages accordingly.

2. Service Switching Points – Software and ports in central offices that enable switches to query databases. SSPs are the switches that begin and end calls. They receive signals from the Customer Provided Equipment (CPE) and then process the calls on the behalf of the end users. The user triggers the network to provide various services by dialing particular digits. SSPs are typically implemented at access tandem offices, local exchanges or toll centers that contain the needed network signaling protocols. The SSP serves as the begining and ending point for SS7 messaging.

3. Service Control Points – DBs with customer feature and billing information. Service Control Points, or SCPs, interface with SSPs as well as STPs. The STP contains the network configuration and call-completion database – the SCP contains all the service logic that is needed to deliver the type of call and feature in the call that the user is requesting. SCPs are centralized network nodes that contain software and databases needed for call management. Functions such as digit translation, call routing and verification of credit cards are all provided by SCPs. Usually a SCP will receive traffic from a SSP via the STP and will then return responses based on those queries by way of the STP.

The SS7 signaling data link is a full duplex digital transmission channel that operates at either 56 Kbs (T-Carrier transmission systems, in North America) or 64 Kbps (E_Carrier transmission systems, Europe). SS7 also defines a number of other types of links, each with a specific use within a SS7 network.

A (access) links
B (bridge) links, D (diagonal) links, and B/D links
C (cross) links
E (extended) links
F (fully associated) links

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