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The PABX

What is a PABX?

A Private Branch automatic switching or simply PABX, is a central digital telephone exchange which links telephone networks to VoIP networks.What is a PABX?

A Private Branch automatic switching or simply PABX, is a central digital telephone exchange which links telephone networks to VoIP networks.

At the beginning of the 1990s there were available resources and means to actually change the inner working of a PABX and have also opened business opportunities for newcomers to the market of mid-size PABX, since they have lowered the entry barrier for new manufacturers.

Functionally, the PABX performs four main call processing duties:

  • Establishing connections (circuits) between the telephone sets of two users (e.g. mapping a dialed number to a physical phone);
  • Maintaining such connections as long as the users require them (i.e. channeling voice signals between the users);
  • Disconnecting those connections as per the user’s requirement;
  • Providing information for accounting purposes (e.g. metering calls).

In addition to these basic functions, PABX offer many other calling features and capabilities, with different manufacturers providing different and more features in an effort to differentiate their products.

Common capabilities include (manufacturers may have a different name for each capability):

Interfaces for connecting extensions to a PABX include:

  • Analogue interface Is the common two-wire interface Used in most cases, it’s cheap and effective and allows almost any standard phone to be used as an extension;
  • Proprietary interface When the manufacturer has defined a protocol. One can only connect the manufacturer’s sets to their PABX (such as digital phones), but the benefit is more visible information displayed and/or specific function buttons;
  • DECT interface It’s a standard for connecting cordless phones, allowing to build a private wireless phones’ network;
  • IP interface Allowing to connect IP/SIP phones/ softphones using, for example, H.323 and SIP protocols.

Protocols for connecting two or more PABX to each other include:

  • Proprietary protocols However, if equipment from several manufacturers is on site, the use of a standard protocol is required;
  • QSIG Protocol For connecting PABX to each other, usually runs over T1 (T-carrier) or E1 (E-carrier) physical circuits;
  • DPNSS Protocol For connecting PBXs to trunk lines, this usually runs over E1 (E-carrier) physical circuits;
  • Internet Protocol H.323, SIP and IAX protocols are IP based solutions which can handle voice and multimedia (e.g. video) calls.

Interfaces for connecting PABX to trunk lines include:

  • Standard analogue lines The common two-wire interface used in many cases. This is adequate only for smaller systems and can suffer from not being able to detect incoming calls when trying to make an outbound call;
  • ISDN lines The most common digital standard for fixed telephony devices. This can be supplied in either Basic (2 circuit capacity) or Primary (15 or 30 circuit capacity) versions. Most medium to large companies would use Primary ISDN circuits carried on T1 or E1 physical connections;
  • Internet Protocol H.323 and SIP protocols operate over IP and are supported by some network providers.

Interfaces for collecting data from the PBX include:

  • Serial interface Historically used to print every call record to a serial printer or to an application via serial cable;
  • Network Port It’s a port to connect a PABX to a data network allowing external applications to connects to the TCP or UDP port or the PABX to connects to another application or buffer;
  • File The PABX generates a file containing the call records.
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