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124 AT-TQ2403 - Management Software - User's Guide
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.
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 waiting 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.
Figure 47: Backoff timer Diagram
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.
Packet Bursting for Better Performance
The AT-TQ2403 Management Software includes 802.11e based packet bursting technology that increases
data throughput and speed of transmission over the wireless network. Packet bursting enables the
transmission of multiple packets without the extra overhead contention for the wireless medium. The
effect of this is to increase network speed and data throughput. The size of packet bursts allowed
(maximum burst length) is a configurable parameter.
Transmission Opportunity (TXOP) Interval for Client Stations
The Transmission Opportunity (TXOP) is an interval of time when a Wi-Fi Multimedia (WMM) client
station has the right to initiate transmissions onto the wireless medium (WM).