Wireless - Difference between Guard Interval and Interframe Space Period

[Julian Ortiz]

Hi guys,

Studing for the CCNA Wireless certification. According to the CCNA Wireless 200-355 Official Cert Guide:

Guard interval—As OFDM symbols are transmitted, they can take different paths to

reach the receiver. If two symbols somehow arrive too close together, they can interfere

with each other and corrupt the received data. This is known as intersymbol interference

(ISI). The 802.11 standard requires a guard interval (GI), a period of 800 nanoseconds,

between each OFDM symbol that is transmitted to protect against ISI.

The 802.11 standard defines a few different interframe space periods that provide a safety

cushion between frames. These periods of silence give the channel enough time for signals

to dampen out—especially when multipath is involved and some reflected copies take longer

to propagate than others. For example, DIFS is the default period used after most standard

priority frame types.

Is this talking about the same interval or period? Are they related to each one or are they totally independent? Please some clarification.

Regards,

Julián

[pieterh]

a frame is coded into multiple OFDM symbols

so the intervals are not the same

[Julian Ortiz]

Hi pieterh,

Then the point is a frame is coded into multiple OFDM symbols, and the symbols are transmitted over different subcarriers, each carrier would arrive at the receiver at different times due to multipath, to ease this the Guard interval is used.

Once all the symbols reach the receiver the frames are rebuilded, but again due to multipath the frames could be mixed, and to ease this the interframe space periods are used.

Am I correct? Am I missing something? If so correct me please.

Regards,

Julián

[pieterh]

take a look at some images

OFDM WLAN
InterFrame Space

hope this helps?