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Signalling System #7 is a set of protocols defined by ITU-T, specifically in the Q.7* set of documents, used to set up telephone calls. It is usually abbreviated to SS7. In some European countries, specifically the United Kingdom, it is sometimes called C7 and is also known as number 7 and CCIS7.
SS7 was designed to replace Signalling Systems #5 and #6, also ITU standards defined by ITU-T. SS5 and earlier used in-band signalling, where the call setup information was sent by playing special tones into the telephone lines (known as bearer channels in the lingo). This led to a number of security problems when users discovered they could play these tones into the telephone handset and control the network even without the "special keys" on an operators handset. So-called phreakers experimented with fooling the telephone exchanges by sending their own user-generated signalling tones from small electronic boxes known as blue boxes.
SS7 moved to a system in which the signalling information was out-of-band, carried in a separate signalling channel. This avoided the security problems earlier systems had, as the end user had no connection to these channels. SS6 and SS7 are referred to as so-called Common Channel Interoffice Signalling Systems (CCIS) due to their hard separation of signalling and bearer channels. However it also required a separate channel dedicated solely to signalling, but due to the rapid rise in the number of available channels at the same time this was a moot point.
SS7 is loosely modelled after the OSI Model; uses Message Transfer Part MTP to provide message passing services to upper layers. The higher level protocols are usually called user parts or application parts. The most important and widely used user part is ISUP, the ISDN User Part, but other user parts are also used widely, such as MAP, SCCP/TCAP/INAP, TUP etc.
SS7 is the interface that telecoms network operators usually use to interconnect with other telecoms network operators. ISDN is the interface usually used to connect telephony customers into an operator's network. SS7 is not normally offered to regular customers due to security implications. In reality, however, SS7 ISUP is not very different from ISDN (especially when used with Non-Facility Associated Signalling, NFAS).
Intelligent Network In order to move some non-time critical functionality out of the main signalling path, and for future flexibility, the concept of a separate "service plane" was introduced by the intelligent network (IN) technology. The initial, and still the most important use of IN technology has been for number translation services, e.g. when translating toll free (U.S. 1-800) numbers to regular PSTN numbers. But much more complex services have since been built on IN, such as Caller Identification and prepaid telephone calls.
Layer 1 Information In Europe, SS7 signalling links usually reside within one or more 64 kbit/s timeslots (DS0s) within a G.703 2.048 Mbit/s E1 trunk. In contrast to the US paradigm, trunks with signalling links usually also carry bearer channels. In the US, SS7 links are usually carried over a network that is physically separated from the bearer channels.
Like the Internet the SS7 network is a packet based system which forwards the messages according to the destination address contained in the messages. In contrast to the Internet the SS7 routing information has to be administrated in each network element in the SS7 path while the Internet uses routing protocols like BGP and OSPF to select routes through the network.
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