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where
t
pre
and
t
denote, respectively, the previous polled time for
QST A
i
and
the present time. SETT-EDD chooses the QSTA which has the highest deadline
calculated using the following equation.
Deadline
i
=
t
pre
+
MSI
i
(6)
2.4 ARROW Scheduler
SETT-EDD can reduce packet loss and jitter in VBR because of adaptive SI
and TXOP. However, as the trac becomes more bursty, the number of packets
waiting in the queue increases. This is because SETT-EDD assigns based on the
average rate of QSTA. ARROW allocates TXOP considering the exact queue
length of QSTA [9]. ARROW assigns TXOP based on the average rate of QSTA,
but it adjust TXOP using the queue length information carried in the packet
transmitted in the previous transmission. And the basic operation of SI is based
on EDD algorithms like SETT-EDD. The operation of ARROW scheduler can
also be divided into two steps as follows.
1st Step
:
TXOP allocation
TXOP assignment of ARROW is illustrated in figure 3. For simplicity only one
QSTA is assumed. For
x
th
transmission of
QST A
i
,
TXOP
i
(
x
)of
QST A
i
is
allocated at time
t
i
(
x
) according to TXOP duation requested field(
TDr
)ofthe
QoS control field. Here, (
TDr
) means the required time to transmit the packet
remaining in queue of QSTA if the 4th bit of the QoS Control field is 0 when
QSTA transmits the QoS data frame. Hence, if TXOP duration requested field
is used when transmitting final QoS data frame in the present SI, the additional
TXOP can be allocated to the next SI. Hence, QSTA can transmit the packet
which is not transmitted during the current SI during the next SI.
Hence, in figure 3, ARROW first allocates
TXOP
i
(
x
)attime
t
i
(
x
)andthe
queue length information(
QS
i
(
x
)), Queue Size of
QST A
i
,isprovidedtoHC
with it own packet at the end of the data transmission. And next
TXOP
i
(
x
+1)
is assigned to
QST A
i
as much as
QS
i
(
x
)and
QS
i
(
x
) is transmitted at the end of
data transmission. In the same manner, the operation is repeated. As illustrated,
Fig. 3.
The TXOP allocation method of ARROW algorithm
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