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Psion Teklogix 9160 G2 Wireless Gateway User Manual 187
Chapter 19: Quality of Service (QoS)
QoS Queues And Parameters To Coordinate Traffic Flow
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 9160 G2 Wireless Gateway supports the Enhanced Distribution Coordination
Function (EDCF) as defined by the 802.11e standard. EDCF, which is an enhance-
ment 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.
This parameter is configurable.
Note: Sending data frames in AIFS allows higher priority management and con-
trol frames to be sent in SIFs first.
The AIFS ensures that multiple access points do not try sending data at the same
time but instead wait until a channel is free.
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) 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 19.1 DCF Random Backoff Timer
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.