Calculating an Interframe Space for 80211n

The 802.11-2007 specification provides the information necessary for us to calculate the durations for each IFS.
As noted previously, SIFS, PIFS, and DIFS are fixed values for each PHY, while AIFS will vary in accordance with the AC in use.
EIFS are fixed per PHY in DCF networks, but vary when used with EDCA.
The formulas and components used for SIFS, PIFS, DIFS, EIFS, and AIFS calculations are as follows:

aSIFSTime = aRxRFDelay + aRxPLCPDelay + aMACProcessingDelay + aRxTxTurnaroundTime
aSlotTime = aCCATime + aRxTxTurnaroundTime + aAirPropagationTime+ aMACProcessingDelay.

The “aSIFSTime” is the same as a SIFS, measured in microseconds (µs). Similarly, the “aSlotTime” is the same as a slot time. Both of these values are provided for each PHY in the 802.11 specification.

PIFS = aSIFSTime + aSlotTime
DIFS = aSIFSTime + 2 × aSlotTime

Given that the SIFS and slot time values are provided for us in the standard, these calculations are pretty simple.

See below IFS calculations

EIFS (DCF) = aSIFSTime + DIFS + ACKTxTime

In this formula, the “ACKTxTime” is the amount of time it takes to transmit an ACK frame at the lowest mandatory rate in the BSS.

EIFS (EDCA) = aSIFSTime + AIFS[AC] + ACKTxTime

The EIFS (EDCA) formula mirrors the same for DCF, but replaces the DIFS with the appropriate AIFS[AC].

An AIFSN is a number (AIFS Number) value that is user-configurable and determines the brevity (or length) of an AIFS interval. AIFSN values are set for each access category, giving the AIFS[AC] a shorter or longer duration, in accordance with the desired priority.

This is demonstrated by the AIFS[AC] formula:
AIFS[AC] = AIFSN[AC] × aSlotTime + aSIFSTime