100
100
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. 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 D-Link DWL-2210AP supports the
Distribution Coordination Function
(
DCF
) as
defined by the
802.11e
standard. DCF, which 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
DCF interframe space
(DIF) before transmitting.
This parameter is configurable.
(Note that sending data frames in DIFs allows higher priority management and control
frames to be sent in SIFs first.)
The DCF 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 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.
Configuring Queues for Qualty of Service (QoS)
Summary of Contents for AirPremier DWL-2210AP
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