Fig. 3.8. Voice signal and digital signals.

In digital transmission system, signals propagate as a discrete voltage pulses (positive voltage represents binary 1, and negative voltage represents binary 0). The digital signals are measured in bits per second (bps). In data communications, analog signals are used to transmit information over the telephone system or over radio communication systems. A MODEM (modulator/demodulator) converts digital data to analog signals and analog signals can be converted to digital information. This process involves sampling, quantizing. This process is called digitizing. Now-a-days, the transmission between central exchanges and long distance sites are done by digitizing voice cummunication. The analog transmission takes place only between the local exchange or end office and homes. In Fig. 3.9 digital to analog to digital conversion processing is shown as block diagram.

-TLTL», Modem A/WV* Repeater -► Repeater ANW* Modem -TLTLrL

Fig. 3.9. Digital to analog to digital conversion.

Voice converted to digital requires a 64 k bits/sec channel, which happens to be worldwide standard called DSO (digital signal, level zero) for transmitting voice calls.

3.3.1. Simplex, Half Duplex, and Full Duplex Transmission

Transmission media may operate in simplex, half duplex and full duplex mode. Devices involved in transmission of signals (voice or data) may be transmitter or receiver or both. If one system only transmits and other only receives, the link is called simplex. Transmission of signals by Doordarson and All India Radio (AIR) are belongs to simplex transmission, as there is no possibility of reverse transmission. In certain cases, the control signals are returned to the transmitter to indicate the reception of signals or some portion of the signals or no reception. Considering this factor into account, the international telecommunication union (ITU) defines the simplex (circuit) as "a circuit permitting the transmission of signals in either direction but not simultaneously".

If both devices (transmitter and receivers) can send and receive, but only one device at a time, the link is called half duplex. The conversation is the example of duplex mode transmission. The ITU defines half duplex (circuit) as "a circuit designed for duplex operation but which, because of the nature of the terminal equipment ; can be operated alternately only''. A full duplex line allows both systems to transmit and receive simulatneously. The terms half duplex and full duplex are normally used when referring to computer and other devices connected to an analog telephone circuit using modern.

3.3.2. Four Wire Circuits

The term four wire implies that there are two wires carrying the signals in one direction and two wires carrying them in opposite direction. In normal telephone service, the local loops are two wire circuits, on which a single telephone call can be transmitted in both directions. If the distance between the subscribers is substantial, the amplifiers (repeaters) are necessary to compensate the attenuation. As the amplifiers are unidirectional, for two-way communication, four-wire transmission is necessary. The switching equipment in the local exchange and the line from subscriber to local office (local loops) are two wire operation. The local exchange will switch the subscriber loop to a toll conncecting trunk. This is also a two-wire transmission.

The toll offices are interconnected with inter tool trunks (which connects towns and cities). These trunks are of four-wire transmission. Fig. 3.10 shows the simple arrangement of the two wire and four wire transmission.

Intermediate switching offices

Fig. 3.10. The simple arrangement of the two wire and four wire transmission.

A four-wire circuit has amplifiers in its repeaters for each direction of transmission. The four wire circuits may be physical four wire or equivalent four wire. For short distances, actual

Transmission system

Transmission system

wire line

Fig. 3.11. (a) Block diagram of two wire to four wire conversion, (b) hybrid transformer circuit.

wire line

Fig. 3.11. (a) Block diagram of two wire to four wire conversion, (b) hybrid transformer circuit.

four wires used for transmission is referred as physical four wire circuits. But for long distance trunks physical four wire is undesirable and usually equivalent four wire transmission is used, needing one pair of wires only. The two directions of transmission use different frequency bands so that they do not interfere with each other. The two directions are separated in frequency rather than space. At the toll office, the two wires are converted into four wire for long transmission. A hybrid coil accomplishes this conversion. A simple block diagram and the hybrid coil arrangement of the four wire circuit is shown in Fig. 3.11.

Hybrid transformer. While connecting the two wire circuit to the four wire circuit, a loop may be created and the signal could circulate round the loop, results in continuous oscillation known as singing. The hybrid transformer (two cross connected transformer) and balancing network together acts as a four wire/two wire terminating set and eliminates the singing problem. Hybrid circuits have been traditionally implemented with specially interconnected transformer. More recently, however, electronic hybrids have been developed.

Cross-connected transformer windings results in zero current in the line balance impedance. The power thus divides equally between the input of the send amplifier and the output of the receive amplifier, where it has no effect. The price of avoiding singing is thus 3 dB losses in each direction of transmission together with any loss in transformers.

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