Nowadays cellular or mobile communication has a strong impact on our life. But do you ever think how a mobile call is established? Today, I am here to answer your this question.
When a cellular phone is switched on, but is not yet on call, it first scans the group of forward control channels to determine the one with the powerful signal, and then monitors that control channel until the signal drops below operable level. After this it again scans channels to search the strongest base station signal. For each mobile system, the control channels are defined over the entire geographic area covered and typically make up about 5-6% of the total number of channels available in the system(the remaining are dedicated to voice and data traffic for the end-users). When a call is made to a mobile user, the MSC carry off the request to all base stations in the system. The subscriber’s telephone number is then broadcast as a paging message over all of the forward control channels through out the mobile system. The mobile receives the paging message sent by the base station which it scans, and reacts 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 orders the base station to move the call to a vacant voice channel within the cell. At this point, the base station signals the mobile to change frequencies to a vacant forward/ reverse voice channel pair, at which point another data message is transmitted over the forward voice channel to order the mobile telephone to beep, thereby asking mobile user to answer the phone. Once a call is started, the MSC adjusts the transmitted power of the mobile and varies the channel of the mobile equipment and base stations in order to retain call quality as the user moves in and out of the range of base station. This phenomenon is called as handoff. Special control signaling is enforced to the voice channels so that the mobile unit may be managed by the base station and the MSC while a call is ongoing.
When a mobile starts a call, a call startup request is sent on the reverse control channel. With this request the mobile unit forwards its Mobile identification number (MIN), ESN, and the phone number of the called party. The mobile also transfuses a station class mark (SCM) which shows what the maximum transmitter power level is for the particular subscriber. The cell base station acquires this data and sends it to the MSC. The MSC authenticates the request, makes connection to the call party through the PSTN, and orders the base station and mobile user to move to a vacant forward and reverse voice channel pair to permit the conversation to begin.
All mobile systems give a service usually called as roaming. This permits users to react in service areas other than the one from which service is undersigned. When a mobile enters a city or area that is different from its home service area, it is registered as a roamer in the new service area. This is executed over the FCC, since each roamer is camped on to an FCC at all times. Every several minutes, the MSC prints a global command over each FCC in the system, asking for all mobiles which are previously not enrolled to report their MIN and ESN over the RCC. New unregistered mobiles in the system regularly report back their user information upon receiving the registration request, and the MSC than uses MIN/ESN information to request billing status from home location register (HLR) for each roaming mobile equipment. If a specific roamer has roaming permission for billing purposes, the MSC registers the user as a valid roamer. Once enrolled, roaming mobiles are allowed to receive and make calls from that location, and billing is routed automatically to the user’s home service provider.
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