Edimax EW-7303APn v2 Quick Installation Manual page 70

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A GI is a period of time between symbol transmission that allows reflections (from multipath) from
the
previous data transmission to settle before transmitting a new symbol. The 802.11n draft specifies two
guard intervals: 400ns (short) and 800ns (long). Support of the 400ns GI is optional for transmit and
receive. The purpose of a guard interval is to introduce immunity to propagation delays, echoes, and
reflections to which digital data is normally very sensitive.