Cellular Telephone Call
The following section explains the procedure of telephone call.
How a telephone call is made?
- When a cellular phone is turned on, but is not yet engaged in a call, it first scans the group of forward control channels to determine the one with the strongest signal, and then monitors that control channel until the signal drops below a usable level.
- At this point it again scans the control channels in search of the strongest base station signal.
- Since the control channels are standardized and are identical throughout different markets within the country or continent, every phone, scans the same channels while idle.
- When a telephone call is placed to a mobile user, the MSC dispatches the request to all base stations in the cellular system.
The mobile identification number (MIN), is the subscriber's telephone number which is broadcast as a paging message over all of the forward control channels throughout the cellular system
- The mobile receives the paging message sent by the base station which it monitors, and responds by identifying itself over the reverse control channel.
- The base station relays the acknowledgment sent by the mobile and informs the MSC of the handshake.
- Then, the MSC instructs the base station to move the call to an unused voice channel within the cell
- At this point the base station signals the mobile to change frequencies to an unused forward and reverse voice channel pair, at which point another data message (called an alert) is transmitted over the forward voice channel to instruct the mobile telephone to ring, thereby instructing the mobile user to answer the phone.
- Once a call is in progress, the MSC adjusts the transmitted power of the mobile and changes the channel of the mobile unit and base stations in order to maintain call quality as the subscriber moves in and out of range of each base station. This is called ahandoff.
- When a mobile originates a call, a call initiation request is sent on the reverse control channel.
- With this request the mobile unit transmits its telephone number (MIN), electronic serial number (ESN), and the telephone number of the called party.
- The mobile also transmits a station class mark (SCM) which indicates what the maximum transmitter power level is for the particular user.
- The cell base station receives this data and sends it to the MSC.
- The MSC validates the request, makes connection to the called party through the PSTN, and instructs the base station and mobile user to move to an unused forward and reverse voice channel pair to allow the conversation to begin.
- This service allows subscribers to operate in service areas other than the one from which service is subscribed.
- When a mobile enters a city or geographic area that is different from its home service area, it is registered as a roamer in the new service areaaccomplished over the FCC, since each roamer is camped on to a FCC at all times
- New unregistered mobiles in the system periodically report back their subscriber information upon receiving the registration request, and the MSC then uses the MIN/ESN data to request billing status from the home location register (HLR) for each roaming mobile
- Once registered, roaming mobiles are allowed to receive and place calls from that area, and billing is routed automatically to the subscriber’s home service provider.