– 42 –
Chapter
4
|
Wireless
Settings
Radio
Settings
WMM
Operation
—
WMM
uses
traffic
priority
based
on
the
four
ACs;
Voice,
Video,
Best
Effort,
and
Background.
The
higher
the
AC
priority,
the
higher
the
probability
that
data
is
transmitted.
When
the
access
point
forwards
traffic,
WMM
adds
data
packets
to
four
independent
transmit
queues,
one
for
each
AC,
depending
on
the
802.1D
priority
tag
of
the
packet.
Data
packets
without
a
priority
tag
are
always
added
to
the
Best
Effort
AC
queue.
From
the
four
queues,
an
internal
“virtual”
collision
resolution
mechanism
first
selects
data
with
the
highest
priority
to
be
granted
a
transmit
opportunity.
Then
the
same
collision
resolution
mechanism
is
used
externally
to
determine
which
device
has
access
to
the
wireless
medium.
For
each
AC
queue,
the
collision
resolution
mechanism
is
dependent
on
two
timing
parameters:
◆
AIFSN
(Arbitration
Inter
‐
Frame
Space
Number),
a
number
used
to
calculate
the
minimum
time
between
data
frames
◆
CW
(Contention
Window),
a
number
used
to
calculate
a
random
backoff
time
After
a
collision
detection,
a
backoff
wait
time
is
calculated.
The
total
wait
time
is
the
sum
of
a
minimum
wait
time
(Arbitration
Inter
‐
Frame
Space,
or
AIFS)
determined
from
the
AIFSN,
and
a
random
backoff
time
calculated
from
a
value
selected
from
zero
to
the
CW.
The
CW
value
varies
within
a
configurable
range.
It
starts
at
CWMin
and
doubles
after
every
collision
up
to
a
maximum
value,
CWMax.
After
a
successful
transmission,
the
CW
value
is
reset
to
its
CWMin
value.
Figure
30:
WMM
Backoff
Wait
Times
For
high
‐
priority
traffic,
the
AIFSN
and
CW
values
are
smaller.
The
smaller
values
equate
to
less
backoff
and
wait
time,
and
therefore
more
transmit
opportunities.
High Priority
Low Priority
Time
CWMin
CWMax
AIFS
Random Backoff
Minimum Wait Time
Random Wait Time
CWMin
CWMax
AIFS
Random Backoff
Minimum Wait Time
Random Wait Time
Summary of Contents for SkyFire AC1200
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