Question

Write short notes (i)Telephone hand set and it’s working. (ii) CPU based exchange

Answer 1

(i) A standard telephone set is comprised of a transmitter, a receiver, and electrical network for equalization, associated circuitry to control side tone levels and to regulate signal power, and necessary signalling circuitry. In essence, a telephone set is an apparatus that creates an exact likeness of sound waves with an electric current. Fig 8-4 shows the functional block diagram of a telephone set. The essential components of a telephone set are the ringer circuit, on/off hook circuit, equalizer circuit, hybrid circuit, speaker, microphone, and a dialing circuit. 

Ringer Circuit: The purpose of the ringer is to alert the destination party of incoming calls. The audible tone from the ringer must be load enough to be heard from a reasonable distance and offensive enough to make a person want to answer the telephone as soon as possible. In modem telephones, the bell has been placed with an electronic oscillator connected to the speaker.. 

On/off hook circuit: The on/off hook circuit (some times called a switch hook) is nothing more than a simple single-throw, double-pole (STDP) switch placed across the tip and ring. The switch is mechanically connected to the telephone handset so that when the telephone is idle (on hook), the switch is open. When the telephone is in use (off hook), the switch is closed completing and electrical path through the microphone between the tip and ring of the local loop. 

Equalizer circuit: Equalizers are combination of passive components (resistors, capacitors and so on) that are used to regulate the amplitude and frequency response of the voice signals. 

Speaker: In essence, the speaker is the receiver for the telephone. The speaker converts electrical signals received from the local loop to acoustical signals(sound waves) that can be heard and understood by a human being. The speaker is connected to the local loop through the hybrid network. The speaker is typically enclosed in the handset of the telephone along with the microphone. 

Microphone: For all practical purposes, the microphone is the transmitter for the telephone. The microphone converts acoustical signals in the form of sound pressure waves from the caller to electrical signals that are transmitted into telephone net-work through the hybrid network. Both the microphone and the speaker are transducers, as they convert one form of energy into another form of energy. A microphone converts acoustical energy first to mechanical energy and then to electrical energy.

Hybrid network : The hybrid network (sometimes called a hybrid coil or duplex coil) in a telephone set is a special balanced transformer used to convert a two-wire circuit(the local loop) in to a four wire circuit(the telephone set) and the vice-versa, thus enabling full duplex operation over a two wire circuit. In essence, the hybrid network separates the transmitted signals from the received signals. Outgoing voice signals are typically in the 1-V to 2-V range, while incoming voice signals are typically half that value. Another function of the hybrid network is to allow a small portion of the transmit signal to be returned to the receiver in the form of a sidetone. In sufficient sidetone causes the speaker to raise his voice, making the telephone conversation seem unnatural. Too much sidetone causes the speaker to talk too softly, thereby reducing the volume that the listener receives. 

 (ii) CPU Based Exchange : In centralized control, all the control equipment is replaced by a single processor which must be quite powerful. It must be capable of processing 10 to 100 calls per second, depending on the load on the system, and simultaneously performing many other ancillary tasks. A typical control configuration of an ESS using centralized SPC is shown in Fig. A centralized SPC configuration may use more than one processor for redundancy purposes. Although there are a large number of services which can be offered by CPU based exchange, they may be grouped under four broad categories. 

1. Services associated with the calling subscribers and designed to reduce the time

spent on dialing and the number of dialing errors.

2. Services associated with the called subscriber and designed to increase the call

completion rate.

3. Services involving more than two parties.

4. Miscellaneous services.

These new services are known as supplementary services and some of the prominent

ones are as follows: 

Category 1:

• Abbreviated dialing

• Recorded number calls or no dialing calls

• Call back when free.

Category 2:

• Call forwarding

• Operator answer.

 Category 3:

• Calling number record

• Call waiting

• Consultation hold

• Conference calls.

Category 4:

• Automatic alarm

• STD barring

• Malicious call tracing

A subscriber issues commands to an exchange to activate or deactivate a service, record or clear data in the subscriber line data area or solicit an acknowledgement from the exchange. As an example, a user may enable or disable STD facility on his line by using a command. A command may or may not have data associated with it. The number of digits in the data, when present, may vary depending upon the command. As a result, subscriber commands are designed to be of variable length necessitating the use of an end-of-command symbol