This means that the TA value changes for each 550-metre change in the range between a mobile and the base station. With radio waves travelling at about 300,000,000 metres per second (that is 300 metres per microsecond), one TA step then represents a change in round-trip distance (twice the propagation range) of about 1,100 metres. The TA value is normally between 0 and 63, with each step representing an advance of one bit period (approximately 3.69 microseconds). Technical Specifications 3GPP TS 05.10 and TS 45.010 describe the TA value adjustment procedures. Timing Advance (TA) is the variable controlling this adjustment. The time at which the phone is allowed to transmit a burst of traffic within a timeslot must be adjusted accordingly to prevent collisions with adjacent users. Since the users are at various distances from the base station and radio waves travel at the finite speed of light, the precise arrival-time within the slot can be used by the base station to determine the distance to the mobile phone. Each user transmits periodically for less than one-eighth of the time within one of the eight timeslots.
GSM uses TDMA technology in the radio interface to share a single frequency between several users, assigning sequential timeslots to the individual users sharing a frequency. In the GSM cellular mobile phone standard, timing advance value corresponds to the length of time a signal takes to reach the base station from a mobile phone.
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