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PSTN

Acronym for Public Switched Telephony Network. This term refers to the sum of the worlds public circuit-switched telephony networks, regardless of the subscribers access technology (i.e. ISDN or POTS).

From an architectural point of view, the

PSTN consists of a number of exchanges which are connected by SS7 signaling links. As their name suggests, SS7 links are used for signaling. Voice links are separate from this and signaling and voice data may (and usually does) take different routes. This is known a outband signaling. Some of the exchanges (so-called local exchanges or class 5 switch) will have links to subscribers, commonly known as local loop or last mile. These links may be simple 2-wire lines (such as for POTS lines or ISDN). However, subscriber links may be implemented as

sophisticated networks on their own (e.g. in case of mobile phones). Such networks then are called access networks.

While SS7's major task is to perform call signaling, it used in many other ways too. For example, mobile phones (o more precisely, the base station they are logged-in to) will report their current location to the HRA using SS7 so that incoming calls can be routed correctly. Also, the evaluation of 0700 or 0800 numbers is based on SS7.

Today, VoIP technologies actually are used in all PSTN areas. SS7 may be carried over IP by virtue of the SIGTRAN protocol, long distance carries implement voice channels using SIP or H.323, nextgen telephony companies that have no class 5 switch on their own to implement a SIP based access network and finally enterprises create their own private telephony network based on VoIP PBXs.

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PSTN