19-12-2012, 02:12 PM
What is telephone?
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Telephone
A basic telephone or telephone set is an analog base-band transceiver. It has a handset which contains a microphone and a speaker, better known as a transmitter and receiver. It has also a ringer and dialing mechanism.
Telephone Functions
Overall the telephone set fulfills the following basic functions
An incoming signal that rings a bell or produces an audio tone, indicating that a call is being received.
A signal to the telephone system indicating that the signal has been answered.
Transducers to convert voice into electrical signals and electrical signals into voice.
Indicates to the telephone system that a call is to be made when the handset is lifted.
Transmit mode indicates that the telephone system is ready to use by receiving a signal called the dial tone.
It provides a way of transmitting the telephone number to be called to the telephone system
It receives an indication that the call is being made by receiving a ringing tone.
Transmit mode of telephone set provides a means of receiving a special tone indicating that the called line is busy.
This mode provides a means of signaling the telephone system that the call is complete.
The Transmit Mode Functions Functions of Advance Telephone Sets
All the telephones sets provides the above mentioned basic functions of operation but some of the more advanced electronic telephone systems have other features such as multiple line selection, hold, and speaker phone.
Below figure shows the block diagram of a telephone set. The function of each block is described below.
Ringer:
The ringer is either a bell or an electronic oscillator connected to a speaker. It is continuously connected to the twisted pair of the local loop back to the central office. When central office will cause the bell or ringer to produce a tone
Hook Switch:
A switch hook is a double pole mechanical switch that is usually controlled by a mechanism actuated by the telephone hand set. When the hand set is on the hook, the hook switch is open, thereby isolating all the telephone circuitry from the central office. Local loop, when a call is to be made or to be received, the handset is taken off the hook. This closes the switch and connects the telephone circuitry to the local loop. The direct current from the central office is then connected to the telephone, closing its circuits to operate
Dialing Circuit:
The dialing circuit of telephone set provides a way for entering the telephone number to be called. In older telephones, a pulse dialing system was used. A rotary dial connected to a switch produced a number of ON-OFF pulses corresponding to the digit dialed. These ON-OFF pulses formed a simple binary code for signaling the central office.
In most modern telephones, a tone dialing system is used. Known as the dual-tone multifrequency (DTMF) system, this dialing method uses a number of push-buttons that generate pairs of audio tones that indicate the digits selected.
Whether pulse dialing or tone dialing is used, circuits in the central office recognize both types of signals and make the proper connections to the dialed telephone.
The Local Loop or Subscriber Loop in Telephone System:
A telephone call originating at telephone will pass through the central office and then into the main system, where it is transmitted via one of many different routers to the central office connected to the desired location.
The two wires, twisted pair connection between the telephone and the central office is referred to as the local loop or subscriber loop.
The circuits in the telephone and at the central office form a complete electric circuit, or loop. This single circuit is analog in nature and carries both dc and ac signals.