Information Technology Reference
In-Depth Information
30ms, SI is set to 25ms since the greatest divisor for the beacon interval, which
is smaller than 30ms is 25ms as illustrated in figure 2-(c).
2nd Step:
TXOP allocation
HC allocates TXOP to each station in order to transmit only the average accu-
mulated data during SI. TXOP of QSTA is calculated as follows.
TXOP
i
=max(
N
i
× L
i
R
i
+
O,
M
i
R
i
+
O
)
(1)
where
L
i
,
R
i
,
N
i
and
M
i
represent, respectively, Nominal MSDU size, Physical
Transmission Rate, the number of packet arrivals in SI and the maximum MSDU
size of
QST A
i
.
O
represents the overhead time to transmit a QoS-Poll frame, a
QoS-ACK frame and IFS(Inter frame Space).
N
i
is calculated as follows.
SI × ρ
i
L
i
N
i
=
(2)
where
ρ
i
denotes Mean Data Rate of
QST A
i
.
3rd Step
:
Admission control algorithm
QSTA is admitted if the following inequality holds.
+
k
k
=1
TXOP
k
+1
SI
TXOP
i
SI
T − T
CP
T
≤
(3)
and
T
CP
represent the time interval for beacon and the length of
contention period in the superframe, respectively.
The reference scheduler satisfies the minimum service requirements and per-
forms well for CBR trac. However, for VBR trac the delay may increase and
fairness can not be achieved because it uses simple round-robin polling mecha-
nism and guarantee only the average transmission [9].
T
where
2.3 SETT-EDD Scheduler
As aforementioned, the reference scheduler guarantees the minimum perfor-
mance requirement and provides satisfactory service for CBR but perform poorly
for VBR trac. SETT-EDD is a one of the representative schedulers which
show improved performacnce for VBR trac. The scheduler provides variable
SI, adaptive TXOP, and earliest deadline polling mechanism [7].
The operation of SETT-EDD can be divided into two steps, i.e., TXOP allo-
cation and SI allocation.
1st Step
:
TXOP allocation
SETT-EDD allocates TXOP to each station by using single token bucket. At this
time, TXOP Timer replaces TXOP to give tokens to each station. Each station
which received TXOP Timer reduces TXOP Timer as much as the time it used
to transmit its data. If TXOP Timer remains after the transmission, it is saved
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