CWNP Wireless Certification & Wireless Training - Another Interframe Space: RIFS

Your Account |

Search:

Free Stuff

Another Interframe Space: RIFS

Add to:

Written by Devin Akin

Wednesday, 15 August 2007

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.

Comments (1) Add Comment

Subscribe to this comment's feed

...
written by Larry, August 24, 2007

RIFS are a funny thing in 11n because the reality is that their usefullness is limited. Only data packets sent using the NoAck Policy realize the enhanced throughput which, to conjure one scenario up would be.. say.. streaming Audio.

The usage models for 11n are typically based on home uses, so streaming audio seems like a good example. However, most streaming audio sent today is UDP, but there isn't really a mechanism from my favorite internet radio station to tell my Acess Point to send this data using 802.11 NoAck policy.

Typical data transfer sequence.
1}RTS sifs 2}CTS sifs 3}DATA sifs 4}ACK

RIFS sequence.
1}RTS sifs 2}CTS sifs 3}DATA rifs 4}DATA rifs 5}DATA etc..

In the future, look for RIFS to be used in mobile devices that aren't tolerant of data retransmissions, such as a walkman or cellphone or on board video display of your car. (I just made those up and don't have any insight into those as being actual products.)

Write comment