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Chapter 19: Quality Of Service (QoS)
QoS Queues And Parameters To Coordinate Traffic Flow
Psion Teklogix 9160 G2 Wireless Gateway User Manual 191
19.1.3.3 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 ) in-
creases 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 19.1 DCF Random Backoff Timer
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 ini-
tially 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.
19.1.3.4 Packet Bursting For Better Performance
The 9160 G2 Wireless Gateway includes 802.11e based packet bursting technology that in-
creases data throughput and speed of transmission over the wireless network. Packet
bursting enables the transmission of multiple packets without the extra overhead of header
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.