Quality of Service Overview
February 22, 2008
Page 5 of 29
Figure 2 Strict Priority Queuing Packet Behavior
Weighted Fair Queuing
With
weighted
fair
queuing,
queue
access
to
bandwidth
is
divided
up
by
percentages
of
the
time
slices
available.
For
example,
if
100
percent
is
divided
into
64
time
slices,
and
each
queue
is
configured
for
25
percent,
each
queue
will
get
16
time
slices,
after
which
the
next
lowest
priority
queue
will
get
the
next
16,
and
so
on.
Should
a
queue
empty
before
using
its
current
share
of
time
slices,
the
next
priority
queue
inherits
the
time
slices
that
remain.
Figure 3
depicts
how
weighted
fair
queuing
works.
Inbound
packets
enter
on
the
upper
left
of
the
box
and
proceed
to
the
appropriate
priority
queue.
Outbound
packets
exit
the
queues
on
the
lower
right.
Queue
3
has
access
to
its
percentage
of
time
slices
so
long
as
there
are
packets
in
the
queue.
Then
queue
2
has
access
to
its
percentage
of
time
slices,
and
so
on
round
robin.
Weighted
fair
queueing
assures
that
each
queue
will
get
at
least
the
configured
percentage
of
bandwidth
time
slices.
The
value
of
weighted
fair
queueing
is
in
its
assurance
that
no
queue
is
starved
for
bandwidth.
The
downside
of
weighted
fair
queueing
is
that
packets
in
a
high
priority
queue,
with
low
tolerance
for
delay,
will
wait
until
all
other
queues
have
used
the
time
slices
available
to
them
before
forwarding.
So
weighted
fair
queuing
would
not
be
appropriate
for
applications
with
high
sensitivity
to
delay
or
jitter,
such
as
VoIP.
