Mobile Communication using different frequencies

Hello,

I am looking for is how the mobile knows which frequency or band it is using for transmission and reception?

for e.g. : I am texting somebody. first the signal will go through triplexer then modulation of the signal. after that filtering of signal will take place. then it will go through power amplifier and in the last, mobile antenna will transmit the signal. In this whole process how my mobile knows or reads which frequency or band or 2G/3G/4G network it is using for transmitting the signal?

Thank you in advance.

The carrier RF centre is a matter of protocol i.e. the setup is done and informed by provider to all its mobiles. You don't need logic and design cost to detect RF centres which are chosen and paid by various providers.