802.11 Protocol Architecture
frames. This spacing makes the transmission of these frames lower
priority than PCF-based transmissions.
FIGURE 2.8c
Use of DCF Interframe Spaces (DIFS)
Extended IFS (EIFS)
The EIFS is used by DCF stations whenever the PHY has indicated to
the MAC that a frame transmission began but did not result in the
correct reception of a complete MAC frame with a correct Frame
Check Sequence (FCS) value.¹
Calculations
The SIFS and Slot Time values are defined in the relevant physical
layer (PHY) specification. The PIFS and DIFS are calculated based on
SIFS and Slot Time for a given PHY as shown below:
PIFS = SIFS + Slot Time
DIFS = SIFS + Slot Time + Slot Time
The EIFS is derived from a number of factors as shown in the formulas
below:
EIFS (µs) = aSIFSTime + (8 x ACKSize) + aPreambleLength
+ aPLCPHeaderLngth + aSIFSTime + (2 x aSlotTime)
1
IEEE 802.11 – 1999 (R2003) – Section 9.2.3.4
