Introduction to Telephony systems

The telephony communication principle :-

 
There are 3 main parts as

1. Transmitting transducer
2. Recieving transducer
3. One pair wire system 

Telephone bandwidth

In order to allow more long-distance call to be transmitted the frequencies transmitted are limited
 to a bandwidth of about 3000 hertz. All of the frequencies in our voice below 300 hertz and above 3,400 hertz are eliminnated. That's why someone's voice on a phone has distinctive sound.

                                                                   Figure 1.1   Telephone

Figure 1.2  Parts of the telephone set

  

Digital telephone

The digital button is the latest technique of dialing. It uses the button to give signal for every one digit. The diagram in figure 1.3 shows the dual tone multi frequensy ( DTMF) type of dialing. The button on the phone is being connected to a set of oscillators which produces a pair of tone on the local line whenever a button is being pressed. The tone will be detected at the main distributor will comfirm the tone within 33 ms.

                                                             Figure 1.3  Dual tone multi frequensy

 Progress tone 

The various types of tone generated by the exchange guide the users are Dial tone (DT). This is a 33c/s continuous note and is applied to the line after the subcriber has lifted his handset and the switching equipment has allocated him an available outlet for this call to proceed. There  would have been a physical limit on the number of calls an exchange could handle so if all equipment was already in use , the subscriber would not get a dial tone.

Busy tone 


A higher pither note of 400 c/s interrupts to give a cadece of 0.75 seconds, 0.75 seconds off. Busy tone indicates either that the called subscriber is already off-hook (busy) or that the route to the called subscriber is congested. In later systems, a sligtly different cadence was introduced in order to distinguish between these to scenarios. A busy tone is made up of a 480 hertz and 620 hertz tone, with a cycle of one and a half second on and one a half second off.

Number unobtainable tone(NUT)

Indentical pitch to the busy tone but continous. This tone is used to indicate that a number is out of service faulty or that a spare line has beendialed.


 Ring tone (RT)

A tone of 133 c/s which interrupts in the same cadence as the ring current which rings the telephone bell at the called party's end 0.4 seconds on ,0.2 second off.