Time-division multiple access
2G systems
### 2G systems

Most 2G cellular systems, with the notable exception of [IS\-95](/wiki/IS-95 "IS-95"), are based on TDMA. [GSM](/wiki/GSM "GSM"), [D\-AMPS](/wiki/D-AMPS "D-AMPS"), [PDC](/wiki/Personal_Digital_Cellular "Personal Digital Cellular"), [iDEN](/wiki/IDEN "IDEN"), and [PHS](/wiki/Personal_Handy-phone_System "Personal Handy-phone System") are examples of TDMA cellular systems.

In the GSM system, the synchronization of the mobile phones is achieved by sending timing advance commands from the base station which instruct the mobile phone to transmit earlier and by how much. This compensates for the speed\-of\-light [propagation delay](/wiki/Propagation_delay "Propagation delay"). The mobile phone is not allowed to transmit for its entire time slot; there is a [guard interval](/wiki/Guard_interval "Guard interval") at the end of each time slot. As the transmission moves into the guard period, the mobile network adjusts the timing advance to synchronize the transmission.

Initial synchronization of a phone requires even more care. Before a mobile transmits there is no way to know the offset required. For this reason, an entire time slot has to be dedicated to mobiles attempting to contact the network; this is known as the [random\-access channel](/wiki/Random-access_channel "Random-access channel") (RACH) in GSM. The mobile transmits at the beginning of the time slot as received from the network. If the mobile is near the base station, the propagation delay is short and the initiation can succeed. If, however, the mobile phone is just less than 35 km from the base station, the delay will mean the mobile's transmission arrives at the end of the time slot. In this case, the mobile will be instructed to transmit its messages starting nearly a whole time slot earlier so that it can be received at the proper time. Finally, if the mobile is beyond the 35 km cell range of GSM, the transmission will arrive in a neighbouring time slot and be ignored. It is this feature, rather than limitations of power, that limits the range of a GSM cell to 35 km when no special extension techniques are used. By changing the synchronization between the uplink and downlink at the base station, however, this limitation can be overcome.


How does synchronization work in GSM system?