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EDCF in comparison with DCF, has some important
differences, when detecting that the channel is busy in
AIFS period, the back-off counter reduce 1 at the last
time slot of AIFS. However in the DCF, the back-off
counter reduces 1 at the first time slot after DIFS. When
the transmission fails, EDCF makes use of the PF
(Persistent Factor) to control the selection of CW,
CWnew [TC]=PF×CWold [TC]. However in the DCF,
after the failing transmission, the CW always doubles the
size, that is PF = 2.
In summary, EDCF provides different access
priorities by modifying three control parameters of DCF
competitive mechanism: 1) InterFrame Space; 2)
Minimum contention window; 3) Persistent Factor.
III. PERFORMANCE ENHANCEMENT SCHEME
EASDCF
A. Improved back-off mechanism
Figure 3Improved back-off mechanism
The improved back-off mechanism as shown in figure
3, we set a variable whose stage is stage_k in our scheme,
to some extent, the stage_k could reflect the system state.
While the stage number is high, it is indicated that the
number of stations is many and the congestion of the
network is serious. While in a lower stage shows a better
network condition. Because the stage of each station is
different before a successful transmission, so we can use
the different stages of stations to control different backoff
methods.
Different schemes can be chosen through controlling
the stage_k. With fewer stations, adopting the DCF backoff
mechanism because of it’s simple and easy to achieve.
If there are many stations, we adopt the Slow CW
Decrease scheme [6] . For the successful transmission,
don’t return to the CWmin to select back-off counter,
otherwise results in unfairness and collision problems.
Setting a threshold value K, when stage_k K. If previous back-off stage is big
enough, stage_k>K, it adopts stage_k-g back-off window
before the next transmission (g is a constant, the value is
1 or 2), the window descends after each successful
transmission. When stage_k CWjmin j>i
Each grade has the same size of maximal back-off
stage, that is: mi= mj=m=5 , i, j∈[0,M]
According to the above principles, the minimal
contention window of each grade is divided in table 1 as
following:
Table 1: The sizes of contention window of each grade
Level 1 2 3 4 5 6 7 8
102 204
CWmin 32 64 128 256 512
4096
4 8
102 204 409 819 163 327 655 1310
CWmax
4 8 6 2 84 68 36 72
Besides, packet drop rate also take into
consideration, we divide the frame into N grades. The
relationship of maximal retry limit among each grade is:
mq> mp , q>p , p, q∈[0,N].The division method is as
following in table 2:
Table 2: Maximal retry limit of each grade
leveli 1 2 3 4 5 6 7 8
Shot
frame
4 6 8 10 12 14 16 18
Long
frame
7 9 11 13 15 17 19 21
Delay and packet drop rate belong to two aspects of
evaluation network QoS performance. Through the
combination of them, from Table 1 and Table 2, we know
that EASDCF obtain 64 different QoS classification
services in all.
IV.
SIMULATION AUTHENTICATION
NS2 is a simulation tool for optimizing real network
traffic transmission by establishing network devices and
links [10] . In the process of simulation, adopting right
scale to evaluate the QoS of the WLAN is a key point. It
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