This document presents the Contention Window Multiplicative Increase Decrease Backoff (CWMIDB) algorithm for reducing collisions in mobile ad hoc networks (MANETs). CWMIDB dynamically controls the contention window size of nodes experiencing collisions. During collisions, the window size increases by 4x, and decreases to the minimum size after a successful transmission. The algorithm is simulated and shown to improve transmission probability and throughput compared to the existing binary exponential backoff algorithm.

1. AN EFFECTIVE BACKOFF ALGORITHM FOR
COLLISION REDUCTION IN MANET
R.K. Shanmugasundaram, S. Vaidheeswaran
BTECH, B.S Abdur Rahman University,
Vandalur, Tamilnadu, (India)
Professor, Dr.N. Sabiyath Fatima, Dept of CSE, B.S Abdur Rahman University,
Vandalur, Tamilnadu, (India)
ABSTRACT
In Mobile Ad-hoc Networks (MANETS), every node performs both as transmitter and receiver. The existing
backoff models do not accurately predict the performance of the wireless network. Also, the existing models
suffer with high packet collisions. Whenever a collision occurs, the contention window (CW) of the station is
doubled until it reaches the maximum value. The main objective of this project is to reduce collision using
contention window Multiplicative Increase Decrease Backoff (CWMIDB) scheme. The purpose of increasing
CW is to reduce the collision probability by distributing the traffic into a larger time space. In wireless Ad hoc
networks, the CWMIDB algorithm dynamically controls the contention window of the nodes experiencing
collisions. During packet transmission, the backoff counter is uniformly selected from the given range of [0,
CW-1]. Here, CW is known as contention window and its value depends on the number of failed transmissions
for the packet. At the first transmission attempt, CW is set to minimum value (Cmin), if transmission attempt fails
then its value gets doubled, and again set to minimum value on successful transmission. CWMIDB is simulated
in NS2 environment and its performance is compared with Binary Exponential Backoff Algorithm. The
simulation results shows improvement in transmission probability compared to that of the existing backoff
algorithm.
Keywords— backoff, MANET, contention window,CWMIDB.
I. INTRODUCTİON
Mobile Ad Network (MANET) is an infrastructure-less dynamic network without fixed routers. All nodes are
capable of moving and can be connected dynamically. The responsibilities for organizing and controlling the
network are distributed among the terminals themselves. The entire network is mobile, in this type of network,
some pairs of terminals may not be able to communicate directly with each other and relaying of some messages
is required so that they are delivered to their destinations. The main drawback of MANET is energy
conservation. Energy consumption increases due to packet loss. This packet loss occurs due to collision between
the nodes during transmission of data. For wireless networks, the devices operating on battery try to pursue the
energy efficiently by reducing the energy they consume, while data transmission. The features of MANET
introduce several challenges. These include, collision due to transmission errors. This system breaks down when
two computers attempt to transmit at the same time. This is a case of collision. To avoid collision, carrier
sensing mechanism is used. Here each computer listens to the network before attempting to transmit. If the
network is busy, it waits until network quiets down. In carrier detection, computers continue to listen to the
network as they transmit. If computer detects another signal that interferes with the signal it is sending, it stops

2. transmitting. Both computers then wait for random amount of time and attempt to transmit. A collision can be
detected by listening to the shared medium immediately after transmitting and identifying collision
characteristics; or by capturing data from the medium and performing error detection.An alternative method to
handle collisions in a contention based system is to attempt to avoid them. Some systems may utilize a strict
scheduling guideline to identify who may use which resources another system may have the senders listen to the
channel immediately prior to transmitting and determine suitable times to transmit. The purpose of using
CWMIDB algorithm in IEEE 802.11 is to minimize collisions during contention between multiple nodes and
also in the presence of hidden nodes. This is a very simple backoff algorithm in which the size of the contention
window increases and decreases rather than increasing exponentially. The basic operation of the MAC protocol
is by successive Frame Exchange Sequences (FES) RTS-CTS-DATA-ACK as in 802.11 DCF. Each transmitter
before attempting to transmit must find the carrier to be idle for a time period of Distributed Inter-Frame
Spacing (DIFS) seconds. After deferring for the DIFS period the station selects a back off value for an
additional deferral time before transmitting. This back off period corresponds to an integer number of time slots
of the protocol and is selected based on Contention Window Multiplicative Increase Decrease Backoff
Algorithm. When some other transmission is detected the back off procedure pauses and starts over when the
medium is found to be idle for one DIFS period. If collision occurs, then the stations that were involved have to
restart the access procedure with DIFS period and new back off value.
II. AN EFFECTIVE BACKOFF ALGORITHM FOR COLLISION REDUCTION IN
MANET
The major problem in MANET is degradation in throughput because of node mobility, unreliable medium,
interference and route failure. The degradation in throughput is because MANET is unable to distinguish packet
loss. The packet loss occurs due to hidden node and congestion. This problem can be solved by using certain
backoff scheme. The backoff algorithm is a part of Media Access Control (MAC) protocol which is used to
avoid collision in Mobile Ad hoc Network (MANET). When the nodes in the network try to access the channel,
one of these nodes gains access the channel while the other nodes still contend for a time period. Collision
occurs in mobile ad-hoc networks when the node chooses the same value for data transmission. In Figure 3.1, A
and B are two nodes. During the transmission of data between these two nodes, collision occurs. After receiving
the collision signal, transmission is stopped from A and B. They will try to retransmit, since there is no
restriction.
Figure 2.1 Collision Problem

3. To overcome from the problem of collision, a new Contention Window Multiplicative Increase Decrease
Backoff scheme is proposed. CWMIDB provides minimum transmission delay and maximum throughput by
improving collision. This collision can be improved by certain mechanism. In the existing Binary Exponential
Backoff algorithm, for each packet transmission, the backoff time is uniformly chosen in the range (0, W-1)
where W is the initial contention window size. When the packet transmission is not successful the node
increases the contention window size, so the node has to sense the channel for long period of time. Due to this
throughput of the wireless network decreases automatically. The average delay required for the packet delivery
increases.
Figure 2.2 Backoff Concept
The Contention Window Multiplicative Increase Decrease Backoff Algorithm (CWMIDB) consist of three
phases, they are Identification of channel idleness, Setting up Backoff Counter, Performance comparison
of CWMIDB & Binary Exponential Backoff Algorithm. The steps for Contention Window
Multiplicative Increase Decrease Backoff Algorithm is implemented as follows,
CW = min [4*CW, CWmax] --------- (2.1)
The equation 2.2 describes that size of the contention window increase by four times during collision
CW = CWmin ------------------ (2.2)
The equation 2.2 describes that size of the contention window decrease during successful transmission
The CWMIDB algorithm steps are discussed
Step 1: Initially source node sends data when channel is in idle state until the destination is reached. The data
transmission process consists of two states namely idle and non-idle state.
Step 2: In Figure 2.3 S,A,B,D are nodes where S is source node and D is destination node, This node check for
idleness, if channel is in idle state, then it transfers the data to destination. If channel is not idle then it
waits for random backoff time.
Figure 2.3 A Scenario of Collision Detection

4. Step 3: When random backoff time value reaches zero it will transmit packet again to the destination.
Step 4: If the frame is correctly received, the receiving node sends an acknowledgment (ACK) frame to sender.
If ACK is not received, the size of the contention window size increases and retransmit packet again to
the destination.
Step 4.1: During successful transmission, the size of the contention window decreases to minimum.
Step 4.2: During the second transmission attempt, the size of contention window increases exponentially by CW
= CW x 4.
Step 5: During third transmission attempt, the size of the contention window decreases by CW = current value
of CW / 2
Step 6: If acknowledgement is received, go to step1.
Step 7: Stop
III Pseudocode for CWMIDB algorithm during successful transmission
Step 1: Set BackOff Timer to initial value
Step 2: While BackOff Timer ≠ 0 do
For each Time Slot
If channel is idle then BackOff Timer = BackOff Timer – 1
If channel is idle for more than DIFS then
Send
If (Send Failure) then
If (NumberOfBackoffs<=W) then
CW= CW * K
Else if (NumberOfBackoffs>=W)
CW= CW / K
BackOffTimer = Random x; 1 ≤ x ≤ CW -1
Else
CW = CW - 1
BackOffTimer = 0
Go to Step 1
Step 3: Stop
[Where
DIFS – Distributed Inter-Frame Spacing
CW-Contention Window]
/* Channel idleness process*/
When idle
MinCW=32;
MaxCW=256;
Backoff range = [0,CW];
/*During Successful Transmission*/
MinCW=32;
MaxCW=256, 1024;
Backoff range = [0,CW];

5. if (success) node's CW = MinCW;
node's BC = randomly select (0, MinCW);
end if
/*During Packet Collision*/
MinCW=32;
MaxCW=256, 1024;
Backoff range = [0,CW];
if (collision),
node's CW =
min(4*currentCW, MaxCW);
node's BC = randomly
select (0, newCW);
end
3.1 Channel Idleness phase
In channel idleness phase every node must sense the medium, in order to check the state of the channel (idle or
busy). If a node has data to send, but it sees that the channel is busy, then it waits for the end of transmission. At
the end of transmission, it must wait for a DIFS. The back off time counter continuously gets decremented until
the channel is sensed in an “Idle” state. It goes in “Frozen” State when a transmission is detected on the channel
and it goes to the reactivated state when the channel is sensed idle again for more than a DFIS. As soon as the
back-off time reaches zero the station starts the transmission. If two or more wireless nodes finish their
countdowns at the same time-slot, there occurs a collision between RTS (ready to send) packets if the
CSMA/CA (carrier sense multiple access with collision avoidance) is implemented, otherwise two data packets
collide with each other. If there is a collision, every node which participated in the collision multiplies its
contention window by the multiplicative factor m.
Figure 3.1 IDLE Procedure Finite State Representation
In Figure 3.1, the contention period can be determined by the CWMIDB algorithm. It increments the
appropriate retry counter associated with the frame. The CWMID Backoff mechanism chooses a random
number which represents the amount of time must elapse while there are not any transmissions, i.e., the medium
is idle before the listening station may attempt to begin its transmission again. The random number resulting
from this algorithm is uniformly distributed in a range, called the contention window, the size of which
increases with every attempt to transmit that is deferred, until a maximum size is reached for the range. Once a
transmission is successfully completed, the range is reduced to its minimum value for the Next transmission.

6. 3.2 Backoff Counter Phase
In backoff counter phase, it attempts to double the contention window size during collision .Set the backoff counter
(BOC) between 0 and CWlow. Decrement the BOC by 1 if the channel is idle. If BOC reaches zero, transmit the data
frame. If acknowledgment is received then, transmit the next data frame. If the acknowledgement is not received
select the BOC between 0 and CW in the next backoff stage as
For second transmission attempt, CW = CW x 4. For third transmission attempt, CW = current value of CW / 2. For
fourth transmission attempt, CW = current value of CW X 4. For fifth transmission attempt, CW = current value of
CW / 2 and so on. If contention window is high, it will select the BOC between 0 and CW in the current backoff
stage.
Figure 3.2 BACKOFF Procedure Finite State Representation.
The Figure 3.2 depicts the backoff procedure for finite state representation.To prevent all nodes sensing the
channel from beginning of the transmission at the same time, each node chooses a random waiting time called
Backoff before starting transmission. The range from which the Backoff value is chosen is called the Contention
Window (CW). If the channel becomes busy during Backoff, Backoff counter decrementing is stopped until the
channel becomes idle again. For example, let us assume that the current backoff stage is ‘i’ with contention
windowCW( i ) = 4i * CWmin , and there is a successful transmission, the next backoff stage will be stage 0
with contention window CW( 0 ) = 31 according to the specification. But if the number of competing nodes is
large enough (>>31), the new collision will likely occur at the backoff stage 0. The main argument is that since
the current backoff stage is ‘i’ some collision must have occurred recently at the previous stage. Now if the
number of current competing nodes is larger than or close to CW( i ), and if the backoff stage is set to 0, there is
a high probability that new Collision will happen. So resetting the contention window after every successful
transmission is an inefficient approach if the number of nodes is large.
3.3 PERFORMANCE COMPARISON OF CWMIDB AND BEB
Here in this project performance comparison is done with CWMIDB and Binary Exponential Backoff (BEB) .
This performance comparison is done by taking into the account the performance metrics like packet delivery
ratio (PDR), Energy, Throughput. While making comparison CWMIDB is proven to be highly energy
conservation and efficient one than other.

7. The characteristics of NS2 parameter like throughput, packet information, etc can be plotted using trace graph.
The following Table 7.2 illustrates the comparison of various performance metrices like Packet Delivery Ratio
(PDR), Energy and throughput. These metrices are calculated by taking in to the account the number of sending
packets, number of receiving packets at destination and considerably the number of routing packets which is
generated in the trace file. The packet delivery ratio (PDR) of CWMIDB gradually increases from 10.28 percent
to 91.92 percent. The PDR ratio is high because CWMIDB send the packets by increasing and decreasing the
contention window size. CWMIDB initially increases and then decreases. The simulation analysis of CWMIDB
algorithm is obtained by considering the following performance metrics like, Packet delivery ratio and
Throughput
IV Packet Delivery Ratio (PDR)
Packet Delivery Ratio (PDR) is the ratio between the number of packets transmitted by a traffic source and the
number of packets received by a traffic sink. It measures the loss rate as seen by transport protocols and as such,
it characterizes both the correctness and efficiency of ad hoc routing protocols. In CWMIDB the packet
Delivery Ratio is high as 97.94%.
PDR is defined as the total amount of packets received by the receiver and amount of data packet sent by
source. The formula to calculate the packet delivery ratio is defined as follows,
PDR=[ ]* 100
Then by taking into the account number of sending packets, receiving packets and routing packets the graph is
plotted. Here, the packet delivery ratio gradually increases with time in CWMIDB. Here, the PDR of CWMIDB
is 97.94%.The Figure 4.1 shows the packet delivery ratio between CWMIDB and BEB.
Figure 4.1 Packet delivery ratio between CWMIDB and BEB
V Throughput analysis
It is the ratio of the total amount of data that reaches a receiver from a sender to the time it takes for the
receiver to get the last packet. When comparing the routing throughput by each of the node, AODV has the high
throughput. It measures of effectiveness of a routing protocol. The throughput value of CWMIDB algorithm is
high when compared to BEB. Here, the throughput ratio increases with increase in number of packets received.

8. Figure 4.2 Throughput comparison between CWMIDB and BEB
The Figure 7.2 shows the throughput comparison between CWMIDB and BEB. It shows that
CWMIDB has high throughput than BEB. The throughput percentage of CWMIDB is 97.14%
VI CONCLUSION AND FUTURE WORK
In this project, Contention Window based Multiplicative Increase Decrease backoff (CWMIDB) algorithm for
IEEE 802.11 based wireless networks is proposed. The main feature of this algorithm is to avoid channel
capture and to decrease the number of collisions. In this algorithm, the size of the contention window will not be
doubled immediately following a collision. Rather it increases by four times for first collision and decreases by
half for second collision and so on. The performance of the CWMIDB algorithm is very efficient especially
when basic access mechanism is employed under high congested environments as well as in Ad hoc networks.
The simulation result and analysis shows that Contention Window Multiplicative Increase Decrease Backoff
algorithm has higher efficiency with high packet delivery ratio and throughput than the Binary Exponential
Backoff algorithm.
CWMIDB can be extended further by taking into account the data transmission among number of nodes. If
number of node increases there is a slight deflection in the performance. The future work concentrates on the
reduction of collision by increasing the number of nodes. Integration of modified BEB and CWMIDB may
produce high throughput which may help in increasing the efficiency of CWMIDB.
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