Another Interframe Space: RIFS

Another Interframe Space: RIFS

By CWNP On 08/15/2007 - 9 Comments

RIFS (Reduced Interframe Spacing) is a means of reducing overhead and thereby increasing network efficiency.  RIFS may be used in place of SIFS to separate multiple HT format transmissions from a single transmitter when no SIFS-separated response transmission (like an ACK) is expected.  The RIFS is the time from the end of the last symbol of the previous frame to the beginning of the first symbol of the preamble of the subsequent frame as seen at the air interface.  The value of RIFS for the MIMO PHY is 2us.  

 

An AP must set the RIFS mode field of the HT information element to 0 (prohibited) if the Operating Mode is set to 3 (HT mixed mode).  If the Operating Mode is set to 1 (HT non-member protection mode), the AP may set the RIFS mode field to 0 according to implementation-specific criteria (i.e. such as to protect legacy overlapping BSSs in the primary or secondary channels).  Otherwise the RIFS mode field is set to 1 (permitted).  

A station that is associated with a BSS may protect RIFS sequences when the Operating Mode field of the HT Information element transmitted by its AP is set to 1 (there may be non-HT stations in either the primary or secondary channel or both).  A STA that is associated with a BSS must protect RIFS sequences when the Operating Mode field of the HT Information element transmitted by its AP is set to 3 (HT Mixed).  RIFS are only to be used when the RIFS Mode field of the HT Information element is set to 1.

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