Manual

AT-WA7400 Management Software User’s Guide
165
Note
A frame is similar in concept to a packet. The difference is that a
packet operates on the network layer (layer 3 in the OSI model)
whereas a frame operates on the data-link layer (layer 2 in the OSI
model).
Each frame includes a source and destination MAC address, a control field
with protocol version, frame type, frame sequence number, frame body
(with the actual information to be transmitted) and frame check sequence
for error detection.
The 802.11 standard defines various frame types for management and
control of the wireless infrastructure, and for data transmission. The
802.11 frame types are (1) management frames, (2) control frames, and
(3) data frames. Management and control frames (which manage and
control the availability of the wireless infrastructure) automatically have
higher priority for transmission.
802.11e uses interframe spaces to regulate which frames get access to
available channels and to coordinate wait times for transmission of
different types of data.
Management and control frames wait a minimum amount of time for
transmission; they wait a short interframe space (SIF). These wait times
are built-in to 802.11 as infrastructure support and are not configurable.
The AT-WA7400 Management Software supports the Enhanced
Distribution Coordination Function (EDCF) as defined by the 802.11e
standard. EDCF, which is an enhancement to the DCF standard and is
based on CSMA/CA protocol, defines the interframe space (IFS) between
data frames. Data frames wait for an amount of time defined as the
arbitration interframe space (AIFs) before transmitting.
This parameter is configurable.
(Note that sending data frames in AIFs allows higher priority management
and control frames to be sent in SIFs first.)
The AIFs ensures that multiple access points do not try sending data at the
same time but instead wait until a channel is free.
Random Backoff and Minimum / Maximum Contention Windows
If an access point detects that the medium is in use (busy), it uses the DCF
random backoff timer to determine the amount of time to wait before
attempting to access a given channel again. Each access point waits
some random period of time between retries. The wait time (initially a
random value within a range specified as the Minimum Contention
Window) increases exponentially up to a specified limit (Maximum
Contention Window). The random delay avoids most of the collisions that