Guard Interval (GI)
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Guard Interval (GI)
 For digital signals, data is modulated onto the carrier signal in bits orcollections of bits called symbols.
 When 802.11a/g radios transmit at 54 Mbps, each OFDM symbol contains 288 bits; 216 of these bits are data, and 72 of the bits are errorcorrection bits.
 All the data bits of an OFDM symbol are transmitted across the 48 data subcarriers of a 20 MHz nonHT channel.
 802.11a/g radios use an 800nanosecond guard interval (GI) between OFDM symbols.
 The guard interval is a period of time between symbols that accommodates the late arrival of symbols over long paths.
 In a multipath environment, symbols travel different paths, and therefore some symbols arrive later.
 A “new” symbol may arrive at a receiver before a “late” symbol has been completely received. This is known as intersymbol interference (ISI) and often results in data corruption.
 Delay spread is the time differential between multiple paths of the same
signal.  Normal delay spread is 50 nanoseconds to 100 nanoseconds, and a maximum delay spread is about 200 nanoseconds.
 The guard interval should be two to four times the length of the delay spread.
 Think of the guard interval as a buffer for the delay spread.
 The normal guard interval is an 800nanosecond buffer between symbol transmissions.
 As pictured in Figure 18.12, a guard interval will compensate for the delay spread and help prevent intersymbol interference.
 If the guard interval is too short, intersymbol interference can still occur.
 802.11n also uses an 800nanosecond guard interval; however, a shorter 400nanosecond guard interval is optional.
 A shorter guard interval results in a shorter symbol time, which has the effect of
increasing data rates by about 10 percent.  If the optional, shorter 400nanosecond guard interval is used with an 802.11n radio, throughput will increase; however, the odds of an intersymbol interference
occurrence increases.  If intersymbol interference does indeed occur because of the shorter GI, the result is data corruption.
 If data corruption occurs, layer 2 retransmissions will increase and the throughput will be adversely affected.
 Therefore, a 400nanosecond guard interval should be used in RF environments without excessive multipath which could cause late symbols to overlap with new symbols.
 If throughput goes down because of a shorter GI setting, the default guard interval setting of 800 nanoseconds should be used instead.
Reference:
Coleman, David D.,Westcott, David A. CWNA: Certified Wireless Network Administrator Official Study Guide: Exam CWNA106 Wiley.
Coleman, David D.,Westcott, David A. CWNA: Certified Wireless Network Administrator Official Study Guide: Exam CWNA106 Wiley.
