Tuesday, 26 May 2015

Interframe Spaces (IFS)

Today I'll present Interframe Spaces (IFS). What is an IFS? The IFS is a quiet period that APs and STAs must wait before any 802.11 frame transmission.
There is several types of IFS, and starting form the shortest to longest, they are:

RIFS - Reduced Interframe Space
            Used only by 802.11n devices using MIMO; it proceeds data frames, and
            is used between frames of a Contention Free Burst, used when block
            acknowledgements are enabled. The length is always 2 microseconds.

SIFS - Short Interframe Space
            Used to determine the length of other IFSs. Commonly used IFS,
            whenever arbitration has been completed.
            Set to 10 micro seconds (b/g/n in 2.4GHz) and 16 mocroseconds (a/n/ac
            in 5GHz)

PIFS - PCF Interframe Space
            Used only with Channel Switch Announcement frame,
            which is one of the action frames from 802.11h. Equals to one slot time
            plus one SIFS.

DIFS - DCF Interframe Space
             Used to force ordinary data frames to stay quiet for enough time to
             allow higher-priority frames to have access to the channel. It's used
             before the arbitration process. Equals to a SIFS plus two slot times.
             Slot time:
             9 microseconds - a/n (5GHz) and g/n (2.4GHz, HT or ERP with short
             preamble)
             20 microseconds - b/g/n (2.4GHz, DSSS) and HT or ERP with long
             preamble
             50 microseconds - FHSS
             Used only by APs and STAs that do not support QoS.

AIFS - Arbitration Interframe Space
             Used by APs and STAs that support 802.11e QoS. Used before the
             arbitration process. It's not a static value, it's value changes based on
             the priority level of the data, as based on 802.11e QoS:
             Voice & Video - 2 slot times
             Best Effort - 3 slot times
             Background - 7 slot times

EIFS - Extended Interframe Space
             Used to give APs and STAs a chance to retransmit after a failed frame
             reception. When APs or STAs hear a corrupt frame on the channel (FCS
             fails), they stay quiet for an EIFS. Set to SIFS plus DIFS plus the time it
             takes an Ack frame to transmit:
             364 microseconds - b/g/n (2.4GHz, DSSS)
             160 microseconds - a/n (5GHz), g/n (2.4GHz, OFDM)

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