A Basic Call

    Everyone knows how to make a call. Lift the handset, wait for the dial tone, dial the number
    and wait for an answer. At this point either a congestion tone or a ringing tone will be received.
    It seems simple but there are a lot of functions that are involved in the call set up.

    The telephone cannot of course manage to set up the call on its own. Most of the functions
    occur in dialogue and cooperation with the exchange. In general terms, this is what happens.....

   All the subscriber lines are connected directly to the exchange and in the exchange  there
         is an equipment which constantly monitors all the subscriber lines. The subscriber lines are
         scanned at all time and when the handset are lifted up, the exchange detect this as ' hook
         off ' signal.
   In response to the ' hook off ' the exchange first check the subscriber category to find out
         whether the subscriber is allowed to make a call or not for example, the subscribers
         telephone is not blocked due to unpaid telephone bills. If the subscriber is not blocked
         then, the exchange sends a dialing tone , to the telephone. The tone can be heard in the
         receiver, and this indicates that the exchange is ready to receive the  required telephone
         number. The duration of the dialing tone is monitored, and it lasts for about 45 seconds
         (this time may vary). This time supervision is required so as to avoid exchange equipment
         being tied up unnecessarily if the handset is left off accidentally.

    There are two ways in which the number are sent by push-buttons or dial. The push-buttons
    send the numbers as a combination of two tones at different frequencies. Each number has its
    own special tone combination as shown in table :
 

DTMF Frequency Table
                           Frequency in Hz
1209
1336
1477
697
1
2
3
770
4
5
6
852
7
8
9
941
*
0
#
 
Example = > Digit '1' = 697 Hz. + 1209 Hz.
    In the case of dial phone, pulses are used instead. The number of pulses indicates the digit,
    one pulse for the digit 'one' , two for the digit 'two' etc.

    The numbers having received by the exchange, are stored in a register memory for analysis.
    The main aim of the analysis is to determine the route by which the call is to be switched.
    The exchange also checks that the called subscriber is not engaged. If the called subscriber
    is free, the exchange sends the ringing signal to him and the ringing tone to the calling subscriber.
    If the called subscriber is engaged, the engage tone is sent to the calling subscriber. Figure 24
    shows the signals needed for setting up the call.
 

 
 Figure. 24
 
 
 
 
 
 
 
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