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
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.
9 microseconds - a/n (5GHz) and g/n (2.4GHz, HT or ERP with short
20 microseconds - b/g/n (2.4GHz, DSSS) and HT or ERP with long
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)