How Call has been created between any cell Phones?

How call has been created between the cell phones?

Each AMPS-capable cellular telephone includes a numeric assignment module(NAM) in read-only memory. The NAM contains the telephone number of the phone, which is assigned by the service provider, and the serial number of the phone, which is assigned by the manufacturer. When the phone is turned on, it transmits its serial number and phone number to the Mobile Telephone Switching Office ( MTSO) the MTSO maintains a database with information about mobile units that have been reported stolen and uses serial number to lock out stolen units. The  MTSO  uses the phone number for billing purposes. If the phone is used in a remote city, the service is still billed to the user's local service provider. 

When a call is placed, the following sequence of events occurs:

1. The subscriber initiates a call by keying in the telephone number of the called party and presses the send key.

2. The MTSO verifies that the telephone number is valid and that the user is authorized

to place the call; some service providers require the user to enter a PIN (personal identification number) as well as the called number to counter theft.

3. The MTSO issues a message to the user's cell phone indicating which traffic channels to use for sending and receiving.

4. The MTSO sends out a ringing signal to the called party. All of these operations(steps 2 through 4) occur within 10 s of initiating the call.

5. When the called party answers, the MTSO establishes a circuit between the two parties and initiates billing information.

6. When one party hangs up, the MTSO releases the circuit, frees the radio channels, and completes the billing information.

Written by A.Devasena.

Associate Professor/ECE