Timming Advance?

In the GSM, because TDMA is adopted in the air interface, the MS must employ the TSs allocated to it only, and remain inactive in other time. Otherwise, it may affect the MSs using other TSs on the same carrier.
In the GSM, the MS requires three intervals between timeslots when receiving or transmitting signals

Suppose an MS occupies TS2 and moves away from the base station, the messages sent from the base station will be delayed further and further in reaching the MS. Meanwhile, the response returned by the MS will also be delayed further and further in reaching the base station. If nothing is done to solve the problem, the message sent by the MS from TS2 will eventually overlap with another calling message received by the base station in TS3. Therefore, it is important to monitor the time when a call reaches the base station. As the distance between the MS and the base station changes, the system issues instructions to the MS, notifying it of the time advanced. This process is the adjustment of timing advance.