Making a Call

Making a Call

When a mobile is idle, i.e., it is not experiencing the process of a call, then it searches
all the FCCs to determine the one with the highest signal strength. The mobile
then monitors this particular FCC. However, when the signal strength falls below
a particular threshold that is insucient for a call to take place, the mobile again
searches all the FCCs for the one with the highest signal strength. For a particular
country or continent, the control channels will be the same. So all mobiles in that
country or continent will search among the same set of control channels. However,
when a mobile moves to a dierent country or continent, then the control channels
for that particular location will be dierent and hence the mobile will not work.
Each mobile has a mobile identication number (MIN). When a user wants to
make a call, he sends a call request to the MSC on the reverse control channel. He

also sends the MIN of the person to whom the call has to be made. The MSC then
sends this MIN to all the base stations. The base station transmits this MIN and all
the mobiles within the coverage area of that base station receive the MIN and match
it with their own. If the MIN matches with a particular MS, that mobile sends an
acknowledgment to the BS. The BS then informs the MSC that the mobile is within
its coverage area. The MSC then instructs the base station to access specic unused
voice channel pair. The base station then sends a message to the mobile to move to
the particular channels and it also sends a signal to the mobile for ringing.
In order to maintain the quality of the call, the MSC adjusts the transmitted
power of the mobile which is usually expressed in dB or dBm. When a mobile moves
from the coverage area of one base station to the coverage area of another base station
i.e., from one cell to another cell, then the signal strength of the initial base
station may not be sucient to continue the call in progress. So the call has to be
transferred to the other base station. This is called hando. In such cases, in order
to maintain the call, the MSC transfers the call to one of the unused voice channels
of the new base station or it transfers the control of the current voice channels to
the new base station.