D-Link DWL-3500AP - AirPremier Wireless Switching 108G Access Point Administrator's Manual page 72

D-Link Unified Access Point Administrator's Guide
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 Unified Access Point supports the Enhanced Distribution Coordination Function (EDCF)
as defined by the
is based on
frames wait for an amount of time defined as the arbitration interframe space (AIFS) before
transmitting.
This parameter is configurable.
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 would occur if multiple APs got access to the medium
at the same time and tried to transmit data simultaneously. The more active users you have on
a network, the more significant the performance gains of the backoff timer will be in reducing
the number of collisions and retransmissions.
Backoff
Initial Backoff = random number in
Backoff time
in milliseconds
1 5
The random backoff used by the access point is a configurable parameter. To describe the
random delay, a "Minimum Contention Window" (MinCW) and a "Maximum Contention
Window" (MaxCW) is defined.
• The value specified for the Minimum Contention Window is the upper limit of a range for
the initial random backoff wait time. The number used in the random backoff is initially a
random number between 0 and the number defined for the Minimum Contention Window.
• If the first random backoff time ends before successful transmission of the data frame, the
access point increments a retry counter, and doubles the value of the random backoff
window. The value specified in the Maximum Contention Window is the upper limit for
this doubling of the random backoff. This doubling continues until either the data frame is
sent or the Maximum Contention Window size is reached.
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802.11e standard. EDCF, which is an enhancement to the
CSMA/CA protocol, defines the interframe space (IFS) between data frames. Data
4
Backoff = re-doubled
2
= MinCW doubled
range of MinCW
10 15 20
Doubling continues on each try until MaxCW is reached
at which point this wait time is used on retries
until data is sent or until retries limit is reached
25
DCF standard and