empirical study of mobility effect on ieee 802.11 mac...
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Empirical Study of Mobility effect on IEEE 802.11 MAC protocol for Mobile Ad-
Hoc Networks
Mojtaba Razfar and Jane Dong
mrazfar, [email protected]
Department of Electrical and computer Engineering
California State University Los Angeles
ABSTRACT
To design an efficient and effective MAC layer
protocol for Mobile Ad-Hoc networks is a challenging
task. IEEE 802.11 MAC protocol, which supports ad hoc
network mode, provides a good reference for research
work in this area. In the recent years, many researchers
have investigated the performance of IEEE 802.11 on
MANET both theoretically and empirically. However,
the impact of the mobility on MAC layer design has not
been evaluated thoroughly. In our research, we used
OPNET simulator to analyze the performance of IEEE
802.11 MAC protocol under various mobility patterns
for different network topologies. The findings revealed
interesting correlation between the speed of movement/
the mobility pattern and key network performance
parameters including delay and throughput. We also
investigated the impact of mobility on fairness issues in
media access. The empirical study presented in this
paper will be useful to enhance the MAC design of
MANET with median or high mobility nodes.
Keywords: MANET (Mobile Ad-Hoc Networks), Mobility,
(Medium Access Control) MAC, IEEE 802.11, OPNET
1. INTRODUCTION
Mobile Ad Hoc Networks (MANET) are becoming more
and more popular due to its ability to offer convenient,
flexible and low cost network service for many non-
traditional applications. Unlike the widely used Wi-Fi
network which relies on the access point to attach to the
existing networking infrastructure, MANET is
infrastructure-less, where each node acts as a sender,
receiver, and router. While the freedom to deploy a mobile
ad hoc network at anytime anywhere is very attractive, to
make such network function properly presents a lot of
technical challenges. For MAC layer protocols, the well-
known challenges are imposed by hidden and exposed
terminal problems, fairness access issues, limited
bandwidth, limited power supply, as well as limited
transmission range and mobility.
IEEE802.11was primarily designed for WLAN, but it
also supports ad hoc network mode. The MAC layer
protocol in IEEE 802.11 laid the foundation for many
proposed MAC layer protocols for MANET. Therefore, it is
worthwhile to evaluate the performance of IEEE 802.11 on
MANET to see how to improve the design. Many existing
research [1-3] focused on the effectiveness on handling
hidden and exposed terminal problem, and some addressed
fairness issues. Mobility, although an important design
factor of MANET, its impact on MAC layer performance
has not been fully analyzed yet. Most of the current
researches that investigated the mobility effect are focused
on the network layer since it is a major concern in routing
[4-5]. In [6], the authors briefly compared the performance
IEEE 802.11 and other MAC protocols under network
scenarios with mobility. However, to develop a full
understanding of the mobility effect on MAC layer
performance including delay and throughput, fairness,
collision probability, a more comprehensive and in-depth
study is necessary. The objective of our research is to
conduct such study using OPNET [7] to show how mobility
impacts the key MAC layer performance parameters.
In this paper, we will present our findings of the empirical
study using OPNET simulation. Due to the nice property of
OPNET, it is possible to set up different network scenarios
with different mobility patterns, which allowed us to better
study the impact of various factors including speed,
transmission range, and moving trajectory. The network
parameters that were taken into account in our study
includes delay, throughput, collision count, overhead of
control traffic, and backoff time. The documented results
will be useful to enhance the MAC design of MANET with
different mobility.
The paper is organized as follows. Section 2 provides a
brief overview of IEEE 802.11 and highlights the important
design issues. The empirical study exploring the mobility
effect using the OPNET software is presented in section 3.
Experimental results are described in this section as well.
Finally, we will conclude our findings in Section 4.
2. OVERVIEW OF IEEE 802.11 MAC PROTOCOL
IEEE 802.11 MAC layer protocol is referred to as
Distributed Coordination function “DCF” which was based
on virtual carrier sensing and the physical carrier sensing
[8]. IEEE 802.11 DCF uses RTS/CTS/DATA/ACK when
the size of the data frame is large enough; it may just use
carrier sense or it may use both methods referred to as
CSMA/CA with RTS/CTS as a MAC protocol. The three
major issues related to MAC layer protocol over MANET
are the ability to handle hidden and exposed terminal
problems, the ability to ensure fair access of multiple
stations, and the ability to cope with mobility.
2.1. Hidden and Exposed Terminal Problems
These two problems have become a major issue in
MANET. Hidden terminal problem [3] occurs when two
stations are out of the range of each other and trying to send
to the same receiver. As a result, the effect significantly
decreases the throughput and makes the delay longer.
Exposed terminal problem, on the other hand, is when a
node is blocked from transmission to the other stations due
to the transmission of the adjacent node. This will cause
collision and bandwidth waste (less spatial reuse) and will
bring up the starvation problem of the unlucky node. IEEE
802.11 DCF proposed the RTS/CTS handshaking method in
order to alleviate the negative impact of these issues on the
whole network. In the literature, several schemes have been
proposed to solve the hidden and exposed terminal problems
using different mechanism. In [9], the authors explored the
IEEE 802.11 MAC protocol with and without using
RTS/CTS handshaking method. The total WLAN
retransmissions, data traffic sent/received, WLAN Delay
factors of the whole network was investigated using both
methods. They demonstrated that in the scenarios that the
Hidden terminal problem exists, it will be a good idea to use
this option as it decreases the delay of the network
dramatically. They also mentioned that this handshaking
method is not necessary to be used where the hidden nodes
are not present due to the overhead that it adds to the
network. The mobility factor was investigated when the
hidden nodes exist. However, the speed of the nodes and the
location of them have not been studied in this paper.
MACA [10] on the other hand, did not use the carrier
sensing option and instead, it used the RTS/CTS/DATA
handshake to reserve and use the channel. Although this
protocol was a simple design, the control channel collisions
made the scheme not effective in the MAC layer. Moreover,
in these papers, they never spoke of the effect of the
Mobility of the performance of the whole network.
2.2. Fairness issue
Another important factor that should be considered in
designing MAC protocols is to make sure that all nodes
have fair access to the channel in order to transmit their
data. So far most of the single channel MAC schemes rely
on the back-off procedure. Upon collision, the mobile nodes
will go through the back-off procedure and will try to
retransmit after a certain amount of time. Because the
backoff time is different for different nodes, some nodes
may have more chance to transmit than the others and they
are favored in data transmission. This will result in starving
problem of the unlucky nodes with long contention window
size. Therefore, designing good strategies for back-off
procedures and providing fair chances among nodes to
access the channel is one of the important aspects in
MANET. MACAW [11] for wireless LANs is another
single channel schemes which tried to improve the
performance of MACA protocol. A five handshake
RTS/CTS/DS/DATA/ACK has been used in this protocol
which leads to alleviation of the hidden and exposed
terminal problem and better fairness among nodes. By using
a different back-off approach (MILD), this protocol allowed
the nodes to access the channel in a fair manner which is
more desirable in ad hoc networks. However, the effect of
mobility on fairness issue using this protocol has not been
investigated.
2.3. Mobility issue
For the infrastructure-based networks, the access point has
the major influence on the delivery of the data to the
destination. Within a Basic Service Set (BSS), the stations
have to share information using the access point and
therefore their position towards each other is not that
important. Hence, the mobility of nodes does not have a
major effect on the MAC layer protocol [9]. For
infrastructure-less network as MANET, the mobile nodes
are in direct contact with each other. Since they can be
sender, receiver and router, mobility has a significant impact
on the performance of their data delivery. One may wonder
what influence may the mobility of the nodes cause on the
performance of the network. Will mobile nodes be treated
the same way as they move? Will the efficiency of the
transmission stays the same as the mobility varies? How
will the delay and overall throughput be affected via
different mobility pattern? Can we enhance the network
performance using the handshaking RTS/CTS method under
high mobility? Most of the questions do not have a solid
answer yet. In our research, we will explore the relationship
between mobility and all these factors using OPENT
simulation. The results presented in this paper will shed
some light to answer some of the questions related to the
impact of the mobility on MANET networks.
3. EMPIRICAL STUDY USING OPNET
3.1. OPNET Simulator
OPNET modeler is one of the powerful simulation
software allowing the users to implement different network
topologies using a friendly graphic user interface. As lots of
research papers in networking field used NS-2 simulator
[12], OPNET makes it easier to use as it provides ready-to-
use components without the need of writing codes to create
real time network simulations. It also provides the flexibility
for advanced users to create their own network node and
link by hard coding. For our research, OPNET is selected
since its Wireless Modeler includes a rich library of detailed
mobile protocols and application models that can be utilized
to create MANET with various mobility patterns.
3.1.1. MANET and Mobility in OPNET
OPNET [1] uses the IEEE 802.11 MAC protocol with
DCF for Mobile Ad-Hoc Networks. RTS/CTS handshaking
option is also included in case a user decides to implement
it. The software has different objects for MANET networks
such as the MANET station, MANET work station, and
Mobility configuration options in order to set up the
movement of the nodes. In fact, Mobility is one of the most
valuable options that are included in the simulator so that
the users can easily define the way the stations move. The
speed of the stations can also be easily defined for various
applications. This option makes it simpler in real time
simulations in comparison to the other simulators where the
mobility is a difficult task to define and implement. Figure 1
illustrates a MANET scenario with pre-defined node
mobility. The statistics that are related to this work are
explained briefly as follows:
1) MANET delay: the end to end delay of MANET
packets for the whole network (seconds).
2) Throughput: the total number MANET traffic
which is received in bits per second by all the
MANET receivers.
3) Media Access delay: The global statistic for the
total of queuing and contention delays of the data,
management, delayed block-ACK and Block-ACK
frames transmitted by all WLAN MACs in the
network (seconds).
4) Back off slots: the number of slots that a stations
needs to back off before transmission while
contenting for the medium, and the number of slots
in the contention window after the successful
transmission of the station.
5) Retransmission attempts: the total number of
retransmissions by all the WLAN MACs in the
whole network until the delivery of the packet or
being discarded as a result of reaching the short or
long retry limits. We used this factor to study the
impact of mobility on the collision counts as well
as the effectiveness of RTS/CTS handshake.
Fig.1. Mobility of the Mobile Ad-Hoc Networks
3.1.2. Simulation environment
In our study, two different network topologies were
created and analyzed to evaluate the mobility effect on the
node’s behavior and the network performance. Different
settings have been applied to the two topologies based on
the needs of the network simulations.
In the first topology, the relationship between mobility,
transmission range and the overall throughput and delay of
the network is investigated. As Figure 2 illustrates, one
subnet consists of eight nodes around each other. Another
single node is approaching this sub network with a constant
speed. To study the impact of mobility among the nodes
inside the sub network, different scenarios were created to
compare the delay and throughput where the nodes are
either static or moving randomly. We also changed the
speed of the nodes in different steps to see the influence of
this factor on the network. To evaluate the effect of the
transmission range, we also varied the transmission range in
different scenarios according to the distance and the area
that were used in the simulation. In addition, the mobility
impact on this network with different traffic loads was also
studied.
Table 1 shows the setting used for the first topology.
Fig.2. First topology where a node is approaching a static network
Attribute Value
Transmission power (W) Varies per scenario
Data Rate (bps) 11 Mbps
Physical Layer Method Direct Sequence
Buffer Size (bits) 256000
Packet Size (bits) Exponential (1024)
Traffic generated per node Varies per scenario
Node’s Speed Varies per scenario
Nodes movement method Defined/Vector trajectory
Simulation time (min) 60
Table.1. Topology 1 configurations
For the second topology, two similar subnets are created
and each consists of 7 nodes. One of the nodes is static and
it transmits to the other static node in the second subnet. The
other nodes inside the subnet are either static or moving
while trying to transmit data to the static station inside their
subnet. The two subnets are moving towards each other with
a constant speed. The internal nodes are located with
different distances from the static receiver in order to study
the effect of different movement trajectories on the fairness
among nodes. Note that the internal nodes are in the
transmission range of each other. That is, each subnet allows
their nodes to transmit inside the region of the subnet. The
transmission range for the static receiver is higher than the
others due to the fact that the receiver will need to transmit
to the other static receiver located at the second subnet. In
this topology, we not only look into the delay and
throughput factors, but also check the fairness among nodes
and the effect of RTS/CTS. Table 2 shows the setting we
used for the second topology.
Fig.3. Second topology where the mobile nodes are moving around the
static receiver inside the two subnets
Attribute Value
Transmission power of the
static receiver (W)
0.001
Mobile Nodes Transmission
power (W)
0.0003
Data Rate (bps) 11 Mbps
Physical Layer Method Direct Sequence
Buffer Size (bits) 256000
Packet Size (bits) Varies per scenario
Traffic generated per node Varies per scenario
Internal Node’s Speed (m/s) 0.2
Subnet speed (m/s) 1
Nodes movement method Defined trajectory
Simulation time (min) 30
Table.2. Topology 2 configurations
3.2. Experimental results
3.2.1. Impact of mobility on delay and throughput
A) The impact of mobility with lower transmission
range
To evaluate the impact of mobility with lower
transmission range, three scenarios were created under the
first topology (figure 2). The transmission range and traffic
load for these scenarios are the same, while the mobility
inside the subnet is different:
1) Scenario 1: inside nodes are static
2) Scenario 2: inside nodes move with speed 0.2m/s
3) Scenario 3: inside nodes move with speed 1m/s
For all these scenarios, the single node in approaching the
sub-network with a constant speed 0.2m/s. Table 3 shows
the configuration of transmission power and traffic load of
these scenarios. The Domain which covers an area of 120 ×
120 square meter allows the nodes to move inside this
region. A lower transmission range is defined so that the
nodes can sense each other at a maximum of 80 meters
distance. That is, the nodes will not be able to sense each
other at some parts of the Domain. This will allow us to see
the effect of lower transmission region on the networks
using the mobility feature.
Attribute Value
Transmission power (W) 3E-005
Packet Size (bits) Exponential (1024)
External Node’s Speed (m/s) 0.2
Traffic generated per node Exponential (0.1)
Table 3: Common parameters
Fig.4. Comparison of average Delay and Throughput for static and moving
inside nodes with speed 0.2m/s with lower transmission range
Figure 4 compares the results for scenario 1(static) and 2
(node moving with low speed 0.2/m). Results show that
when the nodes are not moving inside the domain, the
network has higher throughput and lower delay. This seems
be to due to the fact that when the nodes move around, the
transmission range decreases and the connection
establishment among nodes becomes weaker. Hence, the
overall throughput decreases coming up with higher delay.
Figure 5 compares the results for scenarios 2 (low
movement speed) and 3 (high movement speed. Results
demonstrate a higher delay and lower traffic being received
as a result of an increase in the speed of the mobile nodes.
Higher speed will make the node to move further from the
receiver in a shorter amount of time and therefore, less
chance to deliver their data to the destination. Higher delay
is due to the fact the nodes are having more problem in
delivering their data to the receiver and experiencing a
higher back off time and retransmissions of the data. It also
contributes to the internal collision or packet loss which
prevents the delivery of the data.
Fig.5. Comparison of average Delay and Throughput for the networks with
low movement speed (0.2m/s) and high movement speed (1 m/s) with low
transmission range.
B) The impact of mobility with higher transmission
range
Attribute Value
Transmission power (W) 0.0001
Packet Size (bits) Exponential (1024)
Internal Node’s Speed (m/s) 1
Traffic generated per node Exponential (0.005)
Table 4: Common parameters
In this case, we increased the transmission range (0.0001
W) so that the range covers the whole area of the movement.
We also increased the Traffic generated by each node to see
how the mobile stations behave while generating more
traffic. Figure 6 shows the comparison results for the two
network scenario with static inside nodes and moving inside
notes (speed 0.2m/s). It is interesting to see that in this case,
the mobility will have a positive impact which leads to a
little higher throughput and lower delay for the entire
network.
Fig.6. Comparison of average Delay and Throughput for static and moving
inside nodes with speed 0.2m/s with higher transmission range
We also found out that increasing the speed of a network
with high transmission range will slightly improve the
performance of the network in case of throughput and delay.
The reason may be due to the fact that all nodes are within
the transmission range of each other no matter how they
move. Therefore, the random movement pattern of the
inside nodes may lead to a more even distribution of the
nodes that helps with channel access.
C) Impact of group mobility on delay and
throughput
Starting from this subsection, we will describe our
findings on the impact of group mobility pattern. The
simulations were created using the second topology as
discussed earlier (Figure 3). The two subnets are moving
towards each other with a speed of 1 m/s. The internal nodes
inside each subnet are moving with the speed of 0.2 m/s.
The nodes have different distances to the fixed receiver. In
the case where the nodes are moving, they move around the
receiver based on their location and distance with regards to
the receiver. Besides the delay and throughput factors, the
fairness among nodes and the effect of RTS/CST method is
investigated in the following section.
Attribute Value
Transmission power (W) 0.0003
Packet Size (bits) Exponential (8192)
Internal Node’s Speed (m/s) 0.2
Traffic generated per node Exponential (0.0008)
Table 5: Common parameters
In this scenario, we investigated the effect of mobility on
a network with stations generating a relatively larger traffic
inside the network. We also increased the packet size per
station. Similar to the first topology, results demonstrate a
better performance of the network with mobility in
comparison to the static one when the transmission ranges
covers the movement paths.
Fig.7. Comparison of average Delay and Throughput for static and moving
inside nodes with speed 0.2m/s for group mobility
3.2.2. Impact of mobility on Fairness
A) The fairness issue without RTS/CTS
To evaluate the fairness of IEEE 802.11 MAC layer
protocol for mobile network, we used the back off slot time
which is the number of slots that a stations needs to back off
before transmission while contenting for the medium, and
the contention window size after the successful transmission
of the station as the measurements. . Moreover, the
retransmission attempt is a good factor to analyze the
delivery efficiency of the packets per node when the stations
are moving around the receiver. This factor reflects the
impact of both the internal collision and the transmission
errors including the loss of acknowledgment or an error
occurred in the packet. The effect of the amount of traffic
generated by each node on the fairness issues has also been
investigated in this part. Node 3 and Node 6 are selected for
our results. As mentioned before, the reason for this
selection is the distance difference between the nodes and
the receiver on their moving paths. Therefore, we can
clearly see the effect of the different distances caused by
mobility on the fairness among these nodes.
Fig.8. Average Back off slot time and retransmission attempts for the two
selected nodes with low traffic load
Figures 8 and 9 present our simulation results for
CSMA/CA without RTS/CTS. For the low load generated
by each station, the two nodes have almost the same back
off slot time (as shown in Figure 8) demonstrating that they
have the same chance to access the channel. On the other
hand, the retransmission attempts per node are much more
less for the closer node to the receiver (Node 3). This might
be due to the internal collision or the error inside the packets
resulting in the failure of the delivery of the packets.
We repeated the same procedure but changing the amount
of traffic generated by each station to a higher level
(exponential (0.0008)). We also increase the packet size up
to eight times (exponential (8192)).
Fig.9. Average Back off slot time and retransmission attempts for the two
selected nodes with higher traffic
As shown in Figure 9, the results illustrate that the back
off slot time has increased dramatically for both nodes and
that the difference becomes obvious as the distance to the
receiver increases. This can be due to higher collision and
more competition for accessing the channel resulting in
higher back off’s and retransmission attempts for both
nodes. The closer the node is to the receiver, the higher
chance it has to access the channel while moving around the
receiver.
B) The fairness issue using RTS/CTS
Attribute Value
Transmission power (W) 0.0003
Packet Size (bits) Exponential (1024)
Internal Node’s Speed (m/s) 0.2
Traffic generated per node Exponential (0.1)
RTS threshold (bytes) 256
Internal Node’s Speed (m/s) 1
Subnet speed (m/s) 1
Table 6: Common parameters
In this case, we added the RTS/CTS option to each node
to see the efficiency of this method on the network. Low
traffic has been used in this scenario. From Figure 10, we
can see that the retransmission attempts have been decreased
for Node 3, which demonstrated the effectiveness of the
handshaking method. Same results occurred for other nodes
inside the network showing the good efficiency of the
handshaking method.
Fig.10. Comparison of the average retransmission attempts for
Node 3 using CSMA/CA only (RED); and using CSMA/CA
with RTS/CTS (BLUE)
C) The network performance using RTS/CTS
We also studied the performance of whole network using
the RTS/CTS access mechanism. The same configurations
were used as the above simulation. Results depict that the
delay of the whole network decreases due to the prevention
of the collisions and allowing the nodes to have their data
delivered in a shorter amount of time.
Fig.11. Comparison of average network delay: Red plot—
without RTS/CTS; Blue plot-- with RTS/CTS
4. CONCLUSION
In this paper, the performance of the Mobile Ad-hoc
networks is investigated using the IEEE 802.11 MAC
protocol. We have shown that Mobility can affect the
network based on different factors. We studied the effect of
varying the speed of the nodes, and their location on the
network. We have also studied the fairness and the effect of
the RTS/CTS handshaking process on the performance of
the nodes inside the network. Our results show that the
performance of the network varies as the mobile nodes
move inside the network. We illustrated that the
performance of the network improves as the traffic increases
when a sufficient transmission ranges of nodes is provided.
We have also shown that Mobility will cause the nodes to
have longer back off times and retransmission attempts in
order to deliver their information to the destination. The
RTS/CTS handshaking method demonstrated its efficiency
on the mobile nodes when the number of collisions becomes
more and more.
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