20-09-2012, 05:27 PM
Private Branch Exchange
PBX(PART1).pptx (Size: 1.13 MB / Downloads: 26)
Introduction
PBX stands for Private Branch Exchange, and each word has a very special meaning. "Private" and "branch" are fairly straightforward; private implies non-public, or something used by a single organization or group, while branch suggests some sort of remote subsidiary. However, both modify "exchange," and that is the most interesting word of all.
an "exchange area" is usually defined as a geographical region in which telephone charges are uniform. In common usage, an exchange is sometimes taken to mean a telephone "central office" or CO, a telephone company building to which local customers are connected, although there may be several COs in an exchange area.
In any event, a PBX, or Private Branch Exchange, is a (usually) small switching system on a customer’s premises (remote from the telephone company’s central office) and dedicated to the private use of that one business or organization. In recent years, "tenant service" in an office building or industrial park has permitted several organizations to share a PBX
Extension-to-extension calling
Many modern PBXs still detect the flow of power when the telephone user picks up the handset. The calling line is then identified and its Class of Service (COS) for originating calls is checked. The PBX then establishes a connection automatically to a digit receiver (dial pulse, DTMF, digital, as identified by COS), which returns dial tone. The caller then dials or keys the called number into the digit receiver (sometimes called a register, a decoder or something similar). The system makes a busy test on the called line and, if it is free and the call permitted, rings the called line. Upon detection of answer, the calling and called parties are interconnected via the switching matrix for conversation; the system monitors for switchhook flashes that indicate the need for some additional service, or hangs up to free the portions of the switching matrix used on this call for future calls. The several parts of Figure 2a and Figure 2b suggest the detailed operation of an extension-to-extension call.
Incoming CO calls
Traditional incoming calls from the CO are quite different from the intra-PBX and outgoing calls we have examined so far (see Figure 4a and Figure 4b). It must be recalled that a PBX trunk is, as far as the CO is concerned, a station line just like the one that goes to a telephone. Thus the CO, when it wants to complete a call to somebody served by a PBX, will send ringing down the "line" (viewed from its end) or into the "trunk" (when viewed from the PBX end). Ringing (which is a large, high power signal at 20 Hz and 86 volts) operates a "ring-up relay" or similar device in the PBX's trunk circuit to make some kind of indication to the system that a CO call is coming in. This indication is forwarded to the console position where the attendant can see and respond to it. When the attendant signals the system control that the call is to be accepted, ringing is tripped and the call is connected to the console. Tripping ringing causes the CO to put its equipment into the talking state and to start charging the caller. Thus, the calling party pays for the call if it reaches the PBX attendant.