We the human beings …belongs to a world which uses a watch(wrist watch) for synchronizing our life our activities.
Lets us say if it is 6 PM in my watch then it will be 6 PM in your watch too if we are in the same part of world.
The MS belongs to a different world…… which is termed as GSM.
All the MS synchronizes there watch(s)from BTS. Just see the term watch replaced with clock in this case.
With the above background …
just read this…
It is all there in the name itself. Its “Timing Advance”.
Let us say it like this:
A MS will use SCH initially to know the time(TDMA) of network/BTS first.
But before entering into the dedicated mode the MS will be told by the BTS to correct its watch(the one which is to be used for Tx)using the given TA.
Also for Rx the MS need not to do anything as BTS is taking care of adjusting its watch attached to the particular TS of MS under question.
To make the circuitry inside MS somewhat simpler (As MS need not have to send and receive simultaneously), The concept of difference of 3 TS between UL/DL or Tx and Rx is there in GSM.
MS adjusts its clock as per the clock(1) of BTS. Under hypothetical condition(assuming no delay)….MS will use clock(1) for listening(Rx) and speaking(Tx)…….MS will apply a difference of 3TS between Tx and RX. Everything is fine here and happy ending.