SGI stands for Short Guard Interval. Guard Interval is intended to avoid signal loss from multipath effect. In wireless transmission, RF signals reach the receiving antenna by two or more paths, if the latter information symbol reaches the antenna too fast, signal may interfere with the earlier symbol (means signal degradation).

Multipath propagation can be caused by many factors. RF signal during transmission may encounter obstacles which will alter the original signal or create new signals. One or more of the original RF may continue straight traveling to the receiving antenna, others may diffract, scatter, or reflect off of obstacles.
Diffraction: Signal is bent around sharp objects creating a new signal.
Scattering: Is where RF signal is reflected off of a non-uniform surface in multiple directions.
Reflection: Signal contacts a uniformly flat surface (e.g.. metal) and reflected at a predictable angle.
802.11 a/b/g requires guard interval (GI) to be 800 ns. From 802.11n and the next 802.11ac, short guard interval (400 ns) is introduced while the default value of GI is still set as 800 ns.
When to use short GI?
1. When intending to improve the throughput, enable short GI can improve the throughput about 10%.
2. If the multipath effect is not too serious (not too many metals or other reflecting materials), you can enable short GI.
3. If you are using 802.11n or 802.11ac only, you can enable short GI. In another word, when using mixed mode, please disable the short GI, which may cause issues.

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