Modified rts cts exchange mechanism for manet

International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN
0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 3, May – June (2013), © IAEME
27
MODIFIED RTS/CTS EXCHANGE MECHANISM FOR MANET
WITH A MULTI LEVEL COLLISION DOMAIN ARCHITECTURE
M ABDUL GAFUR
Dept. of Computer Science & Engineering
JNTU, Hyderabad
Andra Pradesh, India
NIRAJ UPADHAYAYA
JB Institute of Engineering & Technology, Hyderabad
SYED ABDUL SATTAR
Royal Institute of Technology & Science, Hyderabad
ABSTRACT
Efficient utilization of available capacity in mobile adhoc network is always a
challenge. Capacity is wasted due to the hidden terminal problem and exposed terminal
problem. Classic method of virtual carrier sensing make use of an RTS/CTS exchange prior
to data exchange depends merely on size of data. This method is too conservative to rely on
in adhoc network which is usually applied in emergency situations because it degrades the
throughput performance of adhoc networks. Besides, it causes the unnecessary overhead on
the nodes and delay in the networks. In this paper we propose a modified RTS/CTS method
which overcomes the limitations of the standard method. Besides the size of packet we also
consider the traffic around the node to decide for an RTS/CTS exchange. The modified
scheme monitors the traffic around every node and avoids the use of RTS/CTS exchange
wherever the chance for hidden terminal problem is less irrespective of the packet size. Our
scheme is an optimized approach rather than conservative. We analyzed standard RTS/CTS
scheme and other advanced proposals and compared with our proposal. We simulated the
networks of various numbers of nodes with different traffic rate and analyzed the
INTERNATIONAL JOURNAL OF ADVANCED RESEARCH IN
ENGINEERING AND TECHNOLOGY (IJARET)
ISSN 0976 - 6480 (Print)
ISSN 0976 - 6499 (Online)
Volume 4, Issue 4, May – June 2013, pp. 27-37
© IAEME: www.iaeme.com/ijaret.asp
Journal Impact Factor (2013): 5.8376 (Calculated by GISI)
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IJARET
© I A E M E

28
performance. Simulation result shows that our approach provides better performance in
heavily loaded network as well as in lightly loaded network. Performance enhancement of
our proposed scheme is the result of a realistic assumption of less chance for hidden terminals
in a network part of low collision rate rather than pessimistic assumption of omnipresent
hidden terminals all over the network.
Keywords: Adhoc Networks, Collision, Contention Window, CTS, RTS
1. INTRODUCTION
Mobile Ad hoc Network (MANET) is a kind of decentralized wireless network of
independent mobile nodes without having a central coordinator. MANETs are widely
applied in potential crisis management services applications in civil and military
environments, such as responses to hurricane, earthquake, tsunami, terrorism and battlefield
conditions where the entire communication infrastructure is destroyed and restoring
communication quickly is crucial [1]. As the large scale disasters very frequently happen in
these days it is important to have efficient and durable disaster emergency communication
systems like Mobile Adhoc networks.
In Mobile ad hoc networks sharing of wireless bandwidth among ad hoc nodes must
be organized in a decentralized manner as there is no central coordinator. Carrier Sense
Multiple Access with Collision Avoidance (CSMA/CA) and its' variants are widely used in
ad hoc networks. However, all these CSMA/CA based MAC protocols suffer from the well
known “hidden terminal” problem [2]. The hidden terminal problem occurs when the
simultaneous transmissions of two transmitters that lie outside carrier-sense (CS) range cause
interference at one or both receivers and prevent successful reception. An example is shown
in Fig 1. In Fig1 we have three nodes in which we assume A send data to B and C also need
to send to B. Transmission range of A and C is shown. From the figure it is clear that node A
and node C cannot hear each other. By only sensing the medium, node C will not be able to
hear transmissions by node A and start transmission and eventually leads to collision at B.
Transmission range of A Transmission range of C
Fig 1: Hidden Terminal Problem
To overcome the hidden terminal problem adhoc networks usually depends on a
virtual sensing mechanism using a pre-data control information exchange. One such virtual
sensing mechanism is the 802.11 Request to Send/Clear to Send (RTS/CTS) exchange
resulting in nodes getting exclusive access to the channel for a specific time period.
Nevertheless, this mechanism causes some nodes who heard the RTS/CTS exchange to
A B C

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refrain from sending data even though they would not have interfered with any ongoing
transmission. This problem is termed as “exposed terminal problem” and Fig 2 illustrates
such a situation.
In Fig 2 Node A wants to send to node B. Node A sends an RTS and waits for B to
send CTS. Assume node D wants to send to node C and D transmits an RTS to C just before
A sends the RTS to B. C transmits a CTS as a response to RTS from D. This CTS is also
heard by B and refrain from sending the CTS to A even though simultaneous data
transmission from A and D would not interfered each other. Exposed terminal problem
occurs when two transmitters lie within CS range and are prevented from transmitting
simultaneously, even though their transmissions do not mutually interfere [7].
We have to deal with these two issues for achieving improved capacity utilization.
Capacity is wasted because of the failed transmissions due to hidden terminals and
unexploited transmission opportunities due to exposed terminals. The existence and intensity
of hidden and exposed terminals in a given network depends on the topology and on the CS
range. In a mobile adhoc network topology cannot be predicted or controlled. We can only
control the carrier-sense range. The number of Hidden terminals can be reduced by having a
larger CS range, but results in more exposed terminals. On the other hand a smaller CS range
results in fewer exposed terminals but more hidden terminals. In short both hidden and
exposed terminals cannot be simultaneously eliminated or reduced by adjusting the CS range
alone.
Although several alternative proposals have been made to address the above
mentioned shortcomings of standard RTS/CTS scheme, many of them are not satisfactorily
address the key issues of keeping the simplicity of the protocol and avoiding the overhead on
the nodes on duty in emergency situations where usually adhoc networks are applied. In this
paper we propose a modified RTS/CTS method which overcomes the limitations of the
standard method and other related proposals. The basic principle of our scheme is to identify
the areas in the network in which chance for the presence of “active” hidden terminals are
more and areas where the packets are smoothly flowed and regulate the use of RTS/CTS
control packets accordingly. For this purpose we monitor the traffic around every node and
avoid the use of RTS/CTS exchange wherever the chance for hidden terminal problem is less
irrespective of the packet size. Our scheme is an optimized approach rather than conservative.
Transmission range of A Transmission range of D
Transmission range of B and C
Fig 2: Exposed Terminal Problem
A B C D

30
2. DISCUSSION ON RELATED WORK
In [2] authors proposed a method of delayed response to RTS from destination nodes.
If a node A wants to communicate with the destination node B, firstly A transmits an RTS to
ask for agreement. Once the destination node receives RTS, it does not reply CTS
immediately, but forwards the RTS to its neighbor nodes to inform the channel states in next
slot, then in the third slot replies CTS. Obviously this method makes too much delay in data
transmission and wastage of the channel bandwidth.
In [3] it uses an approach of calculating the number of hidden nodes around the
receiver to decide on the use of RTS/CTS prior to data transmission. But this method is
having the overhead of keeping the list of neighboring nodes and finding out number of
hidden terminals around the receiver. Since mobility is common in adhoc networks this
neighbors and hidden nodes will change frequently. Therefore this list has to be updated time
to time. This makes overhead in the network.
Another common approach is to tune the CS threshold for having an improved spatial
reuse [4, 5]. The main drawback of this approach is the poor fairness due to asymmetric CS
ranges. Another approach for better spatial reuse is to control the transmission power [6].
However, hidden and exposed terminals cannot both be eliminated by either adjusting the CS
threshold or controlling the transmission power. MACA-P [2] enhances the RTS/CTS
mechanism to increase concurrency. The RTS/CTS exchange between a pair of nodes is
followed by a control gap during which another pair of nodes can also exchange RTS/CTS
messages and synchronize its data transmission with the first node pair. RTS/CTS messages
and control gap significantly increase the overhead per data transmission. In [7] The proposed
solution consists of two phases. In the first phase, exposed links in the mesh topology are
detected through an offline training process. Coordination of simultaneous transmissions over
exposed links is then done in the second phase. But detection of exposed link is not easy tasks
and causes for extreme overhead. Moreover, as we mentioned before we have to do this
detection frequently because of the mobility of nodes.
3. PROPOSED SCHEME
The basic principal of our approach is to correctly identify the cases in which
RTS/CTS exchange is necessary prior to data transmission and cases in which direct data
transmission is desirable thereby to optimize the use of RTS/CTS exchange wherever it is
possible without having a cost of high collision of packets due to hidden terminals and hence
a performance degrade in the network. This method of optimized use of RTS/CTS yields two
benefits simultaneously. It reduces unnecessary overhead and delay in the network and
reduces the chance for exposed terminal problems. In other words performance enhancement
of our approach is the direct impact of reduced overhead and delay in packet transmission
and avoidance of the case of leaving the channel unused due to exposed terminals.
3.1. Categorizing the Network in to Multilevel Collision Domains
According to our proposed scheme the decision on whether an RTS/CTS is to be used
is taken adaptively taking in to account the current scenario of the shared medium and
collisions encountered by the nodes. Based on the result of traffic analysis network is
virtually divided into different collision domains CD1, CD2…CDn where n is the number of
collision domains in the network. Rate of packet collision in each CDi (RCDi) is less than that
of CDi+1.

31
i.e. RCD1<RCD2<RCD3…< RCDn
Initially all the nodes in the network (say N) will be in lowest collision domain.
i.e. N € CD1
Each node keeps track of its success and failure rate of data transmission. Based on
this success and failure rate the node will move from one collision domain to another. Unlike
initial case after some time of network activities total number of nodes will split into different
collision domains depends on the success and failure rate of data transmission of each node.
i.e. N1€ CD1, N2€ CD2 … Nn€CDn where Ni⊆N,
Depends on the collision intensity of each collision domains we broadly classify them
into two zones namely Red zone and Green zone. Red zone contains the entire collision
domain above a threshold called RTSthreshold by which we start using RTS/CTS prior to every
data transmission. On the other hand Green zone contains all the collision domain under the
RTSthreshold where direct data transmission without a prior RTS/CTS exchange is possible.
RTSthreshold by which we start using RTS/CTS virtual carrier sensing is defined in terms of
collision intensity experienced by the nodes and packet size rather than mere packet size.
When a data packet is ready to send, the node first check the intensity of traffic
around the node. If the intensity is below the pre-defined level the node can transmit data
immediately without a preceding virtual carrier sensing using RTS/CTS irrespective of the
size of the data. Otherwise the node should perform the RTS/CTS exchange prior to data
transmission. Flow chart in Fig 3 explains the working of our proposed scheme. As we show
in the flow chart if there is a collision experienced by the packet from the node after a direct
transmission (without preceding RTS/CTS) due to low collision intensity around the node,
collision counter is incremented. If the value of the collision counter crosses the pre-defined
Collision Threshold (Colthreshold) the node will move to Red zone of higher collision
henceforth the node start using virtual carrier sensing. On the other hand if there is no
collision even after a direct transmission collision counter will be decremented. If the
collision counter comes under the Colthreshold the node will move to Green zone hence forth no
need of RTS/CTS exchange prior to data transmission.
3.2 Value addition of Contention Window
Since behavior of node on transmission fully depends on the collision domain in
which node belongs, analyzing the collision intensity is most important in our scheme. We
can introduce a counter associated with each node to count number of collisions occurred
during data transmission from each node. Based on the value of this counter we can
correctly analyze the collision intensity around the nodes. But keeping a separate counter
with each node is highly inefficient especially in mobile devices. To solve this issue we use
the size of contention window as an indicator of the collision intensity around the node [8].
This is only possible by a reasonably varying adjustment of contention window(CW) that
correctly reflect the collision intensity of the nodes. Therefore we cannot rely on standard
contention window adjust mechanism using Binary exponential Backoff (BEB) since CW
adjustment(increment or decrement)in BEB is not in proportion of failure or success in data
transmission.

32
Fig 3. Flow chart of the Proposed Scheme
In the BEB scheme, each node doubles its contention window, CW, up to the
maximum contention window (CWmax) after a collision occurs and resets its CW to the
minimum value (CWmin) after a single successful transmission:
CW = min(2.CW;CWmax) ; upon a collision
CW = CWmin ; upon a success
Where CWmax and CWmin are the maximum and minimum value of CW respectively.
CWmax and CWmin are defined to avoid the contention window from growing too large and
shrinking too small. The values of the CWmin and CWmax are pre-determined based on the
expected range of the number of active nodes and the traffic load of the network[9,10].
Because of the exponential expansion of CW on collision and a sudden fall to the minimum
value of CW on a single transmission success current CW size cannot be taken as an indicator
of the packet collision.
To support our proposed scheme with a Contention Window which correctly reflects
the collision intensity around the nodes we make use of our recently proposed backoff
scheme called Collision Based Contention Protocol. As per this scheme if a collision
happens, the rate of expansion of CW depends on the current size of CW. If the current CW
size is CWmin then it slightly increase its size. As the size at the time of collision increases
the rate of expansion also increases. On the other hand, on a success of data transmission, as
the CW size at the time of transmission increases the rate of contraction of CW size
decreases. This method of Contention Window adjustment mechanism not only solve the
fairness issues and poor performance of standard BEB scheme but also provide a value added
Contention Window which also can be used as a perfect indicator of collisions associated
with each node. Interested readers will get more about this protocol in [8].
3.3 Handling of Hidden terminals and exposed terminals
As we discussed before two main problems in an adhoc network which degrades its
performance are hidden terminal problem (HTP) and exposed terminal problem. Exposed
terminal problem arises as a byproduct of the solution to hidden terminal problem. In other
words, exposed terminal problem is the result of RTS/CTS exchange mechanism employed in
MANETs. From this it is clear that exposed terminal problem can be mitigated by reducing
RTS/CTS exchange. But this reduction of RTS/CTS exchange causes for the frequent hidden
terminal problem. Our approach successfully overcomes this issue. When the nodes are in the
low collision domain packets from them experience less number of collisions. From this we

33
can reasonably assume that hidden terminal problem is very less for a node under low
collision domain otherwise the node would have suffered lot of transmission failure and
moved to higher collision domain due to the above mentioned inherent property of our
approach. Because of this less number of HTP or a best case of absence of HTP in low
collision domain we completely avoid the RTS/CTS exchange in this region. This way we
avoid unnecessary delay and overhead and effectively solve the issue of exposed terminal
problems. As a result better performance is obtained.
Our approach also works well in a worst case of sudden topology changes in the
network due to node movement and several hidden terminals in the previously mentioned low
collision domain. Because of the no RTS/CTS exchange surely packets will collide. But due
to this collision the respective nodes will immediately move to higher collision domain and
start sending RTS/CTS. In this way our scheme effectively deals with hidden terminal
problems and exposed terminal problem.
4. PERFORMANCE EVALUATION AND RESULT
4.1 Simulation Analysis
Evaluation and comparison of proposed scheme with standard scheme is made using
network simulator Ns-2. Ns-2 is a powerful network simulator. Ns-2 is extensively used by the
networking research community. It provides substantial support for simulation of TCP, routing,
multicast protocols over wired and wireless (local and satellite) networks, etc. The simulator suite
also includes a graphical visualizer called network animator (nam) to assist the users get more
insights about their simulation by visualizing packet trace data. AWK, a text processing utility has
been used to extract desired information from ns trace file. We have used Linux Operating System
to run our simulation code. We choose networks of varying number of nodes in an area 500m x
500m. The random way point motion pattern is adopted. We used the CBR traffic and packet size
of 1500 bytes. The performance is evaluated by adding new nodes in the network as time varies
or expedited arrival of several nodes simultaneously. Sufficient time is given for running the
simulation in order to get chances for every node to participate in the network activity of
transmission or reception.
4.2 Performance Metrics
Firstly we measured the Packet Delivery Ratio (PDR). It is the ratio of the number of
delivered data packet to the data packet actually sent to the destination.
ie . Number of packet receive/ Number of packet send
Fig 3 shows that PDR from our novel scheme is higher than the standard scheme in lightly
loaded network as well as heavy one. This performance gain is resulted from the relatively
congestion free network due to less control overhead. Then we measured throughput at
various instant in the networks of different number of nodes. This metric is helpful to
analyze the behavior of our protocol when new nodes are entering the network as time goes.
For this we made a simulation with nodes entering the network one by one rather than all the
nodes simultaneously. Fig 4 and Fig 5 show the Throughput performance of our protocol
compared to standard scheme in lightly loaded network with 20 nodes and heavily loaded
network with 80 numbers of nodes respectively.

34
Table 1
Simulation Parameters
Parameters Values
Number of nodes
Simulation time
Simulation
Area(m)
Topology
Phy
Packet size
Queue length
SIFS
DIFS
ProType
Antenna type
CWmin
CWmax
Varying
Varying
500x500
Random
wireless
1500
500
10µs
50µs
Free Space
Omni
directional
15 or 31
1023
Fig 4. Packet Delivery Ratio
Fig 5. Throughput Comparison with 20 nodes

35
Fig 6. Throughput Comparison with 80 nodes
From the above two graphs it is clear that our protocol provide better throughput in
the network of small number of nodes as well as large number of nodes. Performance
enhancement is more in lightly loaded network. In lightly loaded network chance for hidden
terminals are less compared to heavy loaded one. Therefore we can avoid or reduce the use of
RTS/CTS exchange in this case without having fear of packet collision. This is effectively
done in our protocol and hence we get a best result in lightly loaded network.
Besides, when more and more nodes enter to the network our protocol starts using
RTS/CTS exchange and avoids collisions due to hidden terminals. Therefore our protocol
also provides higher aggregate throughput both in the case of smaller and larger networks.
Fig 6 shows a comparison of aggregate throughput of our protocol with standard protocol for
networks of various numbers of nodes.
As we mentioned before this throughput gain is achieved with a controlled use of
RTS/CTS rather than a conservative use with a pessimistic assumption of hidden terminals
anywhere anytime. Because of the conservative approach against collision in the standard
scheme collisions are less compared to our scheme. But this will not result in gain in
throughput because of the bandwidth wastage due to exposed terminal issues and over usage
of control message for virtual carrier sensing. Our approach of controlled use of RTS/CTS
results in a slight increase in collision in the low collision domain as a cost for reduced use of
virtual carrier sensing. But this creates better utilization of available bandwidth by preventing
exposed terminal issues and reduced use of control message. In other words, our approach
provides a considerable gain in aggregate throughput with a cost of slight increase in collision
only on low collision domain. Fig 7 shows a comparison of gain in throughput against
increase in collision. It is clear that increment in collision is negligible to the resulted
throughput gain.
Fig 7. Aggregate Throughput Comparison for different networks

36
Fig 8. Throughput gain v/s Increase in collision
5. CONCLUSION
In this paper, we proposed a modified pre-data handshake mechanism with a
controlled exchange of RTS/CTS in Mobile Adhoc Networks. We have analyzed various
proposals presented to overcome the drawback of the standard pre-data control scheme. Our
solution presents a better approach for getting an increased throughput with an improved
spatial reuse and reduced occurrence of exposed terminal problem. According to our scheme
instead of having a conservative approach of sending RTS and CTS prior to every data
transmission an optimized approach is adopted on pre-data control messages. The proposed
scheme monitors the traffic around every node and categorizes them into different collision
domains depends on the collision rate on the packets from each node. The very purpose of
this dynamic grouping of nodes is to avoid the use of RTS/CTS exchange wherever the
chance for hidden terminal problem is less irrespective of the packet size. Performance of the
scheme is evaluated using the Ns-2.33 network simulator. The simulation result shows that
our proposed scheme outperforms the standard method of pre-data handshake mechanism in
network of small number of nodes and large number of nodes. Performance enhancement is
more in lightly loaded network because in lightly loaded network chance for hidden terminals
are less compared to heavy loaded one and reduced use of pre-data control message will not
create packet collision.
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[5] J. Zhu, B. Metzler, X. Guo, Y. Liu, “Adaptive CSMA for Scalable Network Capacity
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