802.11 Protocol Architecture
Frame Transmission
The purpose of 802.11 is to move packets from higher layers across the
wireless interface. This involves encapsulating these packets into
802.11 data frames. In addition, a host of other frames, such as
beacons, association requests, and probe requests, are necessary to
manage and control the wireless link.
Access Timing
The 802.11 standard defines four spacing intervals – SIFS, PIFS, DIFS,
and EIFS – that defer a station's access to the medium and provide
various levels of priority. Figure 2.7 below illustrates three of these
intervals. Each interval defines the time between the end of the last
symbol of the previous frame and the beginning of the first symbol of
the next frame.¹
FIGURE 2.7
Basic Medium Access Procedure
Short IFS (SIFS)
The SIFS is the shortest of the interframe spaces, providing the highest
priority level by allowing some frames to access the medium before
others. The following frames use the SIFS interval because they
require expedient access to the network:
1
IEEE 802.11 – 1999 (R2003) – Section 9.2.3
