Overview
A PBX, or private branch exchange is a telephone system that is usually installed in a business. It provides service among many extensions within the business as well as outside lines. Typically, PBXs are used when a large number of extensions are needed. A PBX can be thought of as a mini version of a telephone company's central office (CO) switch. Key advantages to owning a PBX are:
Grouped with PBXs are key telephone systems (KTSs). A KTS is generally a smaller version of a PBX that also provides direct access to outside telephone lines (trunks). When you press a "line" key on a KTS you immediately hear a dial tone from the central office. In contrast, on a PBX system, you have to dial a digit, usually "9", to get the dial tone from the central office. Typically, KTSs are used when less than 50 extensions are needed. Advantages of having a KTS are that anyone in your office can answer an incoming call simply by pressing the correct line button and KTSs usually cost less than PBXs.
Systems have been developed that combine PBX and KTS features. These hybrid systems typically serve up to 100 users and contain some features found only in PBXs (the ability to use single line phones) and features typically found in KTSs (hands free announcing and answerback). An example of a hybrid system is the NEC Electra Professional which can connect to a maximum of 64 outside lines and 96 extensions. Some features include least cost routing, call forwarding, call hold, automated attendant, and caller ID.
For simplicity, throughout this manual the term PBX will be used to denote a PBX, KTS, or hybrid system.
Most PBX systems are digital. In a digital system, both the voice signals and control information transmitted between station sets within the PBX are sent as binary data. Analog voice signals received from outside the PBX (usually a CO) are converted to digital voice data and sent through the PBX. Digital voice data may be sent outside the PBX if outside networks also use digital circuits, however, they are usually converted back to analog voice signals.
PBXs use control information to instruct their station sets to perform specific functions such as setting the message waiting indicator and call transfer. This control information is sent using proprietary digital protocols. A protocol is a set of rules relating to the format and timing of data transmissions. These protocols not only contain control information, but also "message" data that can be used to significantly enhance computer telephony (CT) applications that use PBX call control elements such as called/calling number ID.
The term "computer telephony" refers to the ability to interact with computer databases or applications from a telephone. Computer telephony technology supports applications such as:
PBXs can communicate with their station sets using in-band or out-of-band signaling. In-band signaling is a method used by analog (2500) telephones (e.g., calling into a PBX and using DTMF to respond to voice prompts). In-band signals use the same band of frequencies as the voice signal. This method provides limited integration because there are no standards and different PBXs provide varying levels of control.
Out-of-band signaling is used by PBXs to send and receive data from station sets or a CT computer. This data that can include information such as called/calling number ID. Out-of-band signals do not use the band of frequencies use by the voice signals. They can be transmitted using the same wires as the telephone set or separate wires (e.g., RS-232). Because of its versatility, out-of-band signaling is the preferred method.
CT equipment comprises a PC containing a Dialogic D/42 Series PBX integration board (also referred to as D/42-xx) and a software application. D/42-xx boards, together with Dialogic's Unified APITM, make it easier to create applications that are tightly integrated with a PBX and take advantage of call control elements.
Below is a list of popular PBX features and functions supported by Dialogic's D/42 Series of PBX integration boards. KTSs and hybrid systems may support only some of these features.
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