The user can now dial the specific digits for the desired calling party (Williams). The digits, either rotary (pulse) or tone (DTMF), are sent down the wires to the telephone system. After transmission, the telephone system generates a request through the main frame to a trunk card. The trunk card serves as the interface to the central office, (CO) requesting a dial tone. When the dial tone is received at the trunk card interface, the PBX generates the pulses or the tones across the line to the central office.
Digital Systems.
All late model systems are typically of digital design. These systems are specifically designed to process information in its digital format. A digital coder/decoder (codec) in the telephone set converts the analog voice conversation into a digital format. The digital signals are carried down the wires to the PBX, for processing. The PBX is responsible for formatting pertinent information in regards to routing all outgoing calls. A digital-analog conversion will take place whenever calls are routed across a telephone company's central office links on an analog circuit (James). This is necessary because of the fact that a digital signal at the PBX interface is a unipolar signal, opposed to the telephone company's bipolar signal. .
PBX System Benefits.
A Private Branch Exchange system can operate without the use of attendants. The PBX can function perfectly with a standard and inexpensive telephone, instead of an expensive and complex key-phone type system. Separate answering machines and additional line installation are not necessary, because of the fact that the Central Office lines are interchangeable for both fax transmissions and telephone calls. The use of a separate, Direct Inward Dialing (DID) system is not necessary. These are a few examples that compliment the area of overall cost reduction. A PBX will eliminate the inconvenience of call waiting, through a system of call recording and will also enable a system of continuous connectivity, by way of call forwarding.
