For most of us,
A mobile phone is a part of the life, and we use it regularly to make or receive a number of calls. Have you ever wondered how a mobile phone establishes a call or receives a call. If you think yes, then read on. In this post I have tried to explain the steps that occur to establish a mobile phone call. Before jumping to the steps, you need to know some basic terms related to mobile phone communication system which is also given. After that, the steps involved in making and receiving a call is also described.
So here are a few terms that you will now find frequently in the comming sections.
Base Station : It is a fixed station that consist of transmitters and receivers mounted on tall Towers strategically placed at different locations.
Control Channel : Radio channel used for transmission of control signals.
Voice Channel : Radio channel used for transmission of voice signals.
Forward Channel : Radio channel used for transmission of information from base station to mobile phone. (Includes Control and Voice Channels).
Reverse Channel : Radio channel used for transmission of information from mobile phone to base station. (Includes Control and Voice Channels).
Before a mobile phone can establish a call, it first needs to establish a connection to the available sevice providers network. Whenever a mobile phone is switched on, it first scans the group of forward control channels to determine the strongest one. Then it keeps on monitoring the same channel untill the signal level drops below a minimum level after which it again starts to scan. Every mobile phone repeats this process as long as it is kept switched on. The signal level of the current channel which the phone is locked to will be displayed on the handset.
So now lets see the steps that occur when a mobile phone user dials a number to make a call. First of all, a call initiation request is sent on the reverse control channel. Along with this request, the mobile phone also sends its Mobile Identification Number (MIN), Electronic Serial Number (ESN) and the dialled telephone number. The base station receives this data and sends it to the Mobile Switching Center (MSC). The MSC validates the request by checking the MIN with the records on its database. If it is valid, a connection to the called party is made through Public switched Telephone network (PSTN). Then the MSC requests the base station to move the mobile phone to an unused voice channel so that the conversation can begin. Once a call is in progress, the MSC adjusts the power transmitted by the mobile phone as it moves in and out of the coverage area of each base station. When a mobile phone with a call in progress moves from one base station to another, the process is called handoff.
Now let us also see the steps that occur when a call is made from a land line to a mobile phone. When ever a call is made through PSTN to a mobile, the request is transfered to the MSC. The MSC dispaches the request to all the base stations. Then the MIN which is the subscribers number is transmitted as a message throught all the base stations in the system. The mobile identifies the message and responds to it. The base station then relays the acknowledgement to the MSC. Then the MSC requests the base station to move the phone to an unused voice channel. During this process,a control signal is also send which triggers the mobile phone to start ringing. The MSC connects the call to the PSTN and the conversation can begin as soon as the mobile user accepts the call.
When a call is made and there is no free voice channels available then you will probably hear a message that the network is busy. This happens when a large numbers of calls are made from within the coverage area of a single base station. All these steps described above happens within a few seconds that the user is not able to identify it. They almost start hearing the ringing tone as soon as the call is placed.