In GSM system MS is always within the cell offering the best coverage. In dedicated mode this is handled by handovers, but in idle mode it is done by a process called “Cell Selection”, based on C1 and C2 comparison. Mobile compares field strength levels coming from different cells with each other and chooses the best one.
C1={A-max(B,0)}
where A=average recieved level-p1
& B=p2-Max RF power of MS
p1=Rxlevel access min
p2=MSTX powqer max CCH
more over
Rxlevel access min is broadcast on the BCCH, and is related to the minimum signal that the operator wants the network to receive when being initially accessed by an MS.
msTxPwrMaxCCH, which is also broadcast on the BCCH, and is the maximum power that an MS may use when initially accessing the network