Dial-tone
Why can’t we hear the dial-tone in a mobile phone but hear it in the fixed wire & wireless phones?
K. HARI KRISHNAN
Tuticorin, Tamil Nadu
In a conventional telephone network (fixed wire) the telephone is connected physically to a telephone exchange by a pair of dedicated copper wire from the customer’s home or office.
When the telephone goes off hook, the switch in telephone exchange will allow dial tone carrying voltage, to be sent on to the receiver. So you hear the dial tone.
In a mobile, communication is happening between base station and mobile by wireless.
This means the cell phone is not connected physically to the mobile exchange always. In a mobile phone handset, when you enter the number to be dialled and press the “send key”, the mobile phone sends a request to open a circuit, which means the circuit is on demand and not always on as in the case of a landline.
So you don’t require a dial tone in mobile networks. (It happens when you press the call button you may get a message stating network busy which shows the channel is unavailable.)
Fixed Wireless: To provide communication in a sparsely populated area and in extreme terrain conditions, where laying cable is not profitable or may be difficult, fixed Wireless was emerged as an alternative to the land line (fixed wire). So in this system a processor is provided in the customer premises to simulate a conventional telephone network.
The processor provides a simulated dial tone when the telephone goes off-hook. Then the dial number is analyzed by using digit analysis and transmits all the numbers to a base station. Unlike a cellular service instead of pressing a “send key”, a dial tone is provided.
In a landline a real dial tone is a method to indicate that a connection with the communication network has actually been established where as the simulated dial tone in a fixed wireless may not accurately reflect whether a connection can be made with the communication network. For example even if the communications channels are busy the fixed wireless phone will still provide a dial tone giving a false impression that the call will go through.
MADHUMOHAN HARIKUMARAN NAIR
Junior Telecom Officer (I.T.)
Bharat Sanchar Nigam Limited
Thiruvananthapuram, Kerala
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