Telephone System basics tutorial
This page on telephone system tutorial covers telephone system basics,block diagram of a basic telephone, telephone ringing sequence and more.
As shown in the figure,all the landline telephones are connected with the use of twisted pair cables to the local exchange or central office(CO). Thousands of lines are connected to the central offices like this. All the COs are connected with the use of some medium either fiber optic or wireless link etc in the large telephone system.
As shown in the figure, Telephone 'A' and Telephone 'B' are connected with the central office(CO). This two wire twisted pair connection between the telephone set and the central office is referred as the local loop or subscriber loop.
Usually any telephone set will have following basic functions in the transmit and receive direction:
In the transmit mode,
• Indication to the user that call can or can not be made by way of dial tones and busy tones respectively.
• means to send the number to be dialled to the telephone CO or local exchange.
In the receive mode,
• It should have ringer so that it rings the bell indicating call is received.
• Signal to the exchange(telephone system) that the call has been answered
• Device to convert voice to the electrical signal and vice versa, such device is called as transducer.
In a telephone system all the telephones connected to the central offices are provided with
group of basic electronic circuits referred as SLIC(Subscriber Line Interface Circuits).
This SLIC has following basic functions referred as BORSCHT.
• Provision of Battery feed
• Provision of Over voltage protection mechanism
• Provision of Ringing circuit
• Provision of Supervision, which takes care of off-hook,ground start and ring trip signaling.
• Provision of Codec functionalities
• Provision of Hybrid and signal conditioning, which takes care of 2 wire to 4 wire conversion and vice versa.
• Provision of Testing the telephone system
Modern electronic telephones are designed based on single chip as shown in the block circuit diagram above.