Controlling contention window to ensure QoS for multimedia data in wireless network

value will
be used to evaluate the sharing ratio of different bandwidth
data types, meaning that the closer the value is to one (1),
the more likely the ratio will be achieved.
2. Simulation models
We evaluate proposed method with different simula-
tion models. The simulation parameters are described in
Table III with NS-2 simulator. The EDCA 𝐶𝑊 used
values of three data {Voice, Video, Background} are
{17, 20, 32} as shown in Fig. 3 will ensure the ratio
Voice:Video:Background = 3:2:1. For more complex mod-
els, we use the simulations below, this ratio is only achieved
with our proposed mechanism but not with IEEE-802.11e,
as in sections: V.2.a(2 senders), V.2.b (3 senders), and V.2.c
(multiple senders with random model).
a) Three nodes model
The first model includes a chain of three nodes with
six flows corresponding with three types of data (Voice,
Video, Background) in Fig. 6. This model was known
with large-EIFS problem [24]. According to IEEE 802.11
specifications, before a station starts the transmission, it
should defer for a distributed IFS (DIFS) or an EIFS delay
time and then selects a random contention window (𝐶𝑊)
counter for backoff. Once the backoff counter decreases to
zero, the station starts its transmission. The choice between
DIFS and EIFS depends on the event of the last trans-
mission. If the last event is an unsuccessful transmission
(e.g., a collision), the station waits for an EIFS, otherwise
it waits for a DIFS. The station which waits for an EIFS
is called a retry station. The EIFS in the IEEE 802.11
standard is the largest Inter-Frame Space (IFS) and is used
to protect ongoing frame, especially acknowledge frame
(ACK) from collisions. When stations detect a transmission
but cannot decode it, they set their Network Allocation
Vectors (NAVs) for the EIFS duration. However sometimes
the EIFS duration is larger/smaller than ongoing frame
duration, leading to considerable unfairness and throughput
degradation [25].
Sender node Sender node 2 Receiver node
Voice flow
Video flow
Background flow
Voice flow
Video flow
Background flow
Figure 6. Three nodes model.
We examine network performance in this model by
letting sender nodes 1 and 2 generate traffic at the same
offered load to receiver node. The performance metrics
Fairness Index and Total Throughput are evaluated with
offered load.
Figure 7. Fairness Index in Three nodes model.
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Vol. 2020, No. 1, September
In Fig. 7 and 8, “EDCA” means the result by using
IEEE 802.11e and “Proposed Method” means our CW
size adjustment shown in the paper (Section IV). Fairness
Indexes are shown in Fig. 7. When offered load is small,
all flows get its requirement. When offered load becomes
larger, in EDCA, fixed priorities (corresponding fixed CW
size) are assigned to different data types and higher priority
flows can get more opportunity to transmit data, then
Fairness Index might be worse. In our method, flexible CW
size is controlled to different data types, abd the throughput
of each flow becomes fairer and the bandwidth allocation at
the MAC layer is also improved. Thus, our method achieves
good Fairness Index.
Figure 8. Total throughput in Three nodes model.
The Total Throughput of all flows is shown in Fig. 8.
When the offered load is small, Total Throughput of all
methods is similar. When offered load becomes large, in
EDCA, bandwidth utilization is less efficient than others
because EDCA gives some advantages to traffics of higher
priority flows. In our method, by changing the flexibility
of CW size, the lower priority flows chance to access
channel can be increased by reducing CW size (back-off
time). Therefore, the throughput of different data flows
remains at a stable level for most of simulation time. It
means, the Fairness and Throughput are often opposite.
Figures 7 and 8 show that our proposed method achieves
better fairness and there is still has a slight difference in
throughput compared with EDCA.
b) Three pairs model
Figure 9. Three pairs model.
Figure 9 shows three pairs of sender and receiver nodes.
The problem in this scenario is also known as three-pair
problem which was first investigated in [26]. In this sce-
nario, sender nodes 1-2 and 2-3 are out of the transmission
range but in the carrier sensing range. Senders nodes 1 and
3 are out of the carrier sensing range, hence the two external
pairs Sender 1 – Receiver 1 and Sender 3 – Receiver 3
are completely independent, i.e., they can send packets
simultaneously without interference with each other. Thus,
the two external pairs contend bandwidth only with the
central pair Sender 2 – Receiver 2, while the central pair
contends with both external pairs. In this topology, the
central pair cannot access the medium in the saturated state
in the original IEEE 802.11 [26].
We examine the network performance in this model by
letting the sender nodes 1, 2 and 3 generate traffic at
the same offered load to receiver nodes. The performance
metrics Fairness Index and Total Throughput are evaluated
with offered load.
Figure 10. Fairness Index in Three pairs model.
Fairness Index is shown in Fig. 10. When offered load
becomes large, EDCA cannot help the central pair access
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Research and Development on Information and Communication Technology
the medium, because scheduling queue in EDCA only
works at the link layer, so it does not have information of
flows out of the transmission range. Therefore, it cannot
improve MAC layer fairness. In our Proposed Method,
the central pair finds out that its bandwidth is less than
fair bandwidth allocation. Then Proposed Method tries to
improve its chance to access channel by decreasing CW
Size and therefore reducing the back-off time of station
Sender 2. Thus, Proposed Method can achieve a better
fairness than EDCA.
Figure 11. Total throughput in Three pairs model.
The Total Throughput of all flows is shown in Fig. 11.
When offered load becomes large, EDCA achieves larger
Total Throughput than our methods. This phenomenon is
explained as follows. In EDCA, higher priority flows such
as voice flow and video flow can use most of channel
bandwidth with its CW value, so the throughput of the
left and right pairs (numbered one and three) will take up
most of the bandwidth, and the middle pair (numbered two)
has very little bandwidth. In our method, with flexible CW
size, we can give more opportunity for lower priority flows,
so the throughput of the middle pair will increase and the
throughput of the left and right pairs will decrease, leading
to the total throughput of our method being possibly lower
than EDCA.
c) Random model
The third model is a random topology. We make a
topology with 50 stations at random positions in 1000[𝑚]×
1000[𝑚] area. Among those 50 stations, 𝑚 stations are
chosen randomly and these 𝑛 stations generate UDP traffic
flows to one destination station. Total offered load of source
stations is set equal to channel data rate 11[Mbps]. The
average of Fairness Index and total end-to-end throughput
are used as metrics to compare the throughput and fairness,
respectively. These terms of network performance are ex-
amined versus the number of flows. Each data point is the
average of over 50 simulations.
Figure 12. Fairness Index in Random model.
Figure 13. Total throughput in Random model.
These terms of network performance are examined versus
the number of flows. The simulation results also prove that
our proposed method achieves good fairness performance as
in Fig. 12. Our throughput performance is slightly reduced
by the trade-off with fairness performance as described in
Fig. 13.
VI. CONCLUSION
Inheriting from IEEE 802.11, IEEE 802.11e standard
was developed and published, partly satisfying QoS for
multimedia data, but in terms of sharing fairness, this
standard is still limited because it gives relatively fixed
values to the QoS parameters. Our research has proposed
a mechanism to improve the fair sharing of bandwidth
between flows by adapting CW in a more reasonable way.
Our algorithm is simple to implement, and it provides
flexible CW value, corresponding to the value of priority of
throughput for different data types to achieve a reasonable
level of sharing between multimedia flows. The evaluated
and simulated results by NS-2 have verified that our method
is better than original IEEE-802.11e in some simulation
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Vol. 2020, No. 1, September
models. Our future research is aimed at evaluating mixed
data UDP and TCP based on the wireless testbed system.
ACKNOWLEDGMENT
This work was financially supported by Institute of Infor-
mation Technology (IOIT), Vietnam Academy of Science
and Technology (VAST).
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Ngo Hai Anh received his M.Tech degrees
from Vietnam National University and is a
PhD student at Graduate Univeristy of Sci-
ence and Technology, Vietnam Academy of
Science and Technology. He is currently
a researcher at Telemactis Department of
Institute of Information Technology, Viet-
nam Academy of Science and Technology.
His current research interests include wireless network, network
performance and network management.
Pham Thanh Giang received his B.E.
degree from the Hanoi University of Tech-
nology, Vietnam, in 2002. He achieved
M.E. and D.E. degrees from the Nagaoka
University of Technology, Japan, in 2007
and 2010, respectively. He is currently a
researcher at Institute of Information Tech-
nology, Vietnam Academy of Science and
Technology. His interests are security, mobile ad hoc networks, the
Internet architecture in mobile environments and Internet traffic
measurement.
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