Frame structure is the division of defined length of digital information into different fields (information) parts. Frame structure fields typically include a preamble for synchronization, control header (e.g. address information), user data, and error detection. A frame may be divided into multiple time slots. The GSM system

A GSM frame is 4.615 msec and it is composed of 8 time slots (numbered 0 through 7). During voice communication, one user is typically assigned to each time slot within a frame.

Between the downlink channel and uplink channel, the time slot numbers are offset by 3 slots. This allows the mobile telephone to transmit at different times than it receives. This allows the design of the mobile device to be simplified by replacing a frequency filter (duplexer) with a more efficient transmit/receive (T/R) switch.