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B. Simulation results and analysis of delay
performance
C. Simulation results and analysis of packet drop
rate
Figure 7 Overall delay without services differentiation
Figure 9 Simulation of packet drop rate
Figure 8 Delay of TC1 and TC2
Delay simulation curves as shown in figure 7, totally
speaking, the delay of DCF is the largest, especially as
the number of station increases it becomes more and
more evident. Because DCF always minimizes the
window after a successful transmission, when there are
many stations, the time spent on back-off may obviously
increase. But there aren’t great differences in EDCF and
EASDCF.
Figure 8 depicts the relationship between the station
numbers in the network and the frames of TC1 and TC2.
The frame of TC1 in EDCF and EASDCF have less delay
than TC2, it is mainly caused by the small contention
window of TC1. Besides, in EASDCF the maximal retry
limit of TC2 is 9, so its delay is a little larger than EDCF.
Meanwhile, due to adopting the adaptive back-off
strategy, the probability of collision reduces and the time
spent on back-off waiting shortens. Generally speaking,
TC2 frame of EASDCF has few differences with that of
EDCF. Namely, the adoption of adaptive back-off
strategy compensates the delay to some extent which is
caused by the increment of the maximal retry limit.
From the curves of figure 8, we can know, EASDCF
and EDCF make a classification of delay. The delay
curves of TC1 and TC2 have evident difference. It offers
a guarantee of QoS for services with different delay
requirement.
Figure10 Packet drop rate of TC1 and TC2
Figure 9 shows the packet drop rate of the three modes.
Conspicuously, from the curves DCF has the biggest
packet drop rate. When the number of station in the
network surpasses 40, the packet drop rate of EASDCF is
lower than DCF and EDCF evidently.
Likewise, we now analyze the packet drop rate of
each TC. As shown in figure 10, the frame of TC2 in
EASDCF has larger retry limit than that of TC1, so the
packet drop rate of TC2 is fewer than TC1. EASDCF has
the lowest packet loss rate, there are two reasons. One is
that TC2 frame of EASDCF has maximal retry limit,
which increases the retry limit leading to the reduction of
packet drop rate. Another is that the property of adaptive
back-off, it will select a back-off way based on the
working condition of the network, thus it will reduce the
possibility of collision. In all frames, the TC2 of
EASDCF has the least packet drop rate, thus it can offer a
reliable guarantee for the date services.
VI. CONCLUSION
This paper proposed a new algorithm for the MAC
access, and it synthesized the two methods of slow
decrease back-off and services classification. Compared
with the proposed back-off algorithm in WLAN, its
advantage is that: (1) EASDCF mechanism can combine
with the existing IEEE802.11 access protocol without
need to introduce any additional expenses and other
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