In industrial countries, most companies (with more than around 10 employes) need its employes can call each other, call to outside, and receive call from outside. By outside we speak about the public telephone network (PSTN). Big PBX are sometimes interconnected by so called trunk lines.
An aternative is to connect all the telephone sets to the PSTN, but the major disandvantage is that even internal calls would have to be payed.
Functionnaly, the PBX performs three main duties: - Establishing connections (circuits) between the telephone sets of two users. Note that fax, modems and many communication devices can often be connected to the PBX. Therefore we speak about extensions. - Maintaining such connections as long as the users require this - Providing information for the Accouting Department (e.g. metering calls)
The are many PBX manufacturers. All offer many facilities, but their are no real standard in order to name these facilities (one facility can have a name by one manufacter and another by a second one). Here is a short list of common facilities: - Direct Dialing (DDI), also called Direct Inward Dialing (DID) - customised Abreviated dialing - follow-me - call forwarding on absence - call forwarding on busy - call transfer - music on hold - automatic ring back - night service - call disribution (ACD,fixed sequences,...)
The extension interface can be: - proprietary: the manufacter has defined a protocol. One can only connect the manufacturer's sets on the PBX - standard interfaces: any device supporting the standard can be connected
The most common digital standard for fixed devices is ISDN.
Cordless phones can also be used on sofisticated PBX: DECT devices become a standard.
Potential links between PBX (trunk lines) can also use proprietary protocols, but if severals manufacters are on site, the use of a standard protocol is required. Most used standard protocols are QSIG? and DPNSS.
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