The purpose of the guard interval is to introduce immunity to propagation delays, echoes and reflections, to which digital data is normally very sensitive.
However, in specifications for TDMA systems such as GSM, the guard period is defined as being at the end of the timeslot.
Longer guard periods allow more distant echoes to be tolerated but reduce channel efficiency.
To increase data rate, 802.11n added optional support for a 0.4 μs guard interval.
[citation needed] The shorter guard interval results in a higher packet error rate when the delay spread of the channel exceeds the guard interval or if timing synchronization between the transmitter and receiver is not precise.