USRobotics Instant802 APSDK - Manual (Page 146)

Professional Access Point
Administrator Guide
Quality of Service - 146
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 into 802.11 as infrastructure support and are not
configurable.
The Professional Access Point 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. The AIFS 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 to send 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, 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 a
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 greater the number of active users
on a network, the more significant the performance gains of the backoff timer will be due to the reduction in
the number of collisions and retransmissions.
The random backoff used by the access point is a configurable parameter. To describe the random delay,
a Minimum Contention Window (
cwMin ) and a Maximum Contention Window ( cwMax ) is defined.
• The value specified for the Minimum Contention Window is the upper limit of a range for the initial ran-
dom 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.
1 5 10 15 20
Backoff time
in milliseconds
Backoff 2 = MinCW doubled
Initial Backoff = random number in
Backoff 4 = re-doubled
range of MinCW
25
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