CHAPTER 5

CHAPTER 5

Cross Layer Cluster Approach for Energy Efficient Data Gathering Scheme for Wireless Sensor Networks

5.1 INTRODUCTION

Research in Ad hoc wireless networks and wireless technology has

played a major role in the development of smart sensing. WSNs have a

wide variety of applications in industrial process, target detection and

tracking, healthcare monitoring, military applications, public places, traffic

surveillance and so on. Traffic congestion is a major problem in

metropolitan cities. This problem of congestion needs to be alleviated.

Some solutions to this problem include location of the specific vehicle and

then monitoring its movement, real time automatic traffic data collection

during rush hour for efficient management and soon. The work here

describes cross layer approach for optimizing data gathering from sensor

nodes and in turn increasing the lifetime of the network [89]. It is

important to design a sensor node that is power efficient as the power

source in WSNs is battery operated and has limited power capacity. The

design of the sensor node should be power efficient as the battery is

limited. Cross layer cluster approach makes use of both routing and MAC

layers information to reduce congestion, increase packet delivery ratio and

minimize energy usage. The time spent on data movement can be reduced

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by using cross layer approach for energy efficient [41, 46] MAC layer in

WSN. MAC protocol, clustering, routing and various other attributes and

parameters need to be considered for cross layer design approach. While

designing sensor networks, a trade off between those parameters and

attributes are required. Some of the important attributes include scalability,

energy efficiency, channel utilization, adaptability to network topology,

node deployment and collision avoidance. Of all the attributes mentioned,

the most important attribute is energy efficiency [71] [72]. Three important

domains of energy consumption that takes place in sensor networks are

sensing, data processing and communication. The reason MAC layer is

considered here in cross layer is because it controls directly the radio of the

nodes in the networks. Thus, the design of MAC protocols plays an

important role in node's energy consumption which influences the lifetime

of sensor networks. Packet overhead, overhearing, over emitting, idle

listening and collision are the major sources of energy waste of MAC

layer.

Single layer communication protocol for designing optimal

strategies for sensor networks has been proposed by most researchers. The

traditional approach does not include overall performance improvement.

Protocols designed for MAC layer are power efficient and are categorised

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as: on demand wakeup and schedule wakeup. On demand wakeup

protocols [48] make use of some form of out-banding signaling technique

through a separate radio to wake up nodes with adding extra cost to the

network deployment. Scheduled wake up MAC schemes can be further

classified as asynchronous schemes and synchronous schemes. Clock

synchronization is not needed in asynchronous approaches. It is easier to

implement and can ensure network connectivity even in highly dynamic

networks. Asynchronous schemes are simple to implement. Asynchronous

schemes are less efficient than synchronous schemes and cannot provide

guarantees on the worst-case delay. For synchronous scheduled wake up

protocols, TDMA and duty cycling are the most commonly used

techniques. Although TDMA protocols are usually designed to create

contention free medium access for communication, they can schedule the

wakeups of the sensor nodes, as well. If the network topology is dense and

dynamic than TDMA protocols require slot allocation and management,

which can be difficult. MAC protocols for WSN can also be broadly

classified as contention based and contention less protocols. The nodes can

transmit without having any predetermined time assigned to them in case

of contention based MAC. The nodes will compete for a shared channel.

Collisions exist if multiple nodes access the medium simultaneously. A

mechanism to avoid collisions has to be provided by the protocol. One

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such protocol is standardized IEEE 802.11 Distributed Coordination

Function (DCF). In order to reduce the energy consumption of idle

listening, various kinds of schedules and wake up techniques as on

demand, asynchronous schemes, duty cycling is being adopted. Examples

of contention based MAC protocols are S-MAC, D-MAC, T-MAC. The

MAC protocols mentioned are about wake-up schemes and listen/sleep

schedule allocation and reduce processing time in a node [22]. Contention

less MAC protocols are normally based on TDMA approaches. These

protocols have a natural advantage of energy conservation in comparison

with contention based protocols since there are no collisions. Schemes

based on the several sources of power consumption also exist. One such

example is the purpose of saving energy by minimizing the idle time and

also by optimizing the route algorithms [50] is achieved. Adjust the power

by optimizing the physical layer in order to transmit proper power. These

schemes are for each layer, and they have their own limitation. If the

network layer is being optimized then, the parameter of MAC layer may

change. Hence it is not comprehensive to optimize a signal layer. No

research exists about how to reduce energy consumption with the cross

layer design approach and also incorporating cluster formation mechanism

helpful in data gathering.

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5.2 RELATED WORK

Initially lot of research work on energy consumption was based on

single layered approach. Gradually as the efficiency of sensor network had

to be improved and to achieve the goal of saving energy, cross layered

design scheme was incorporated by researchers.

Cross layered [45] approach that combined the MAC layer and the

routing layer was started by Van H L et al. [50]. The PHY +MAC

+Routing scheme was incorporated by Kong I Y, Hwang W J [52] by

considering the power control in the physical layer, the scheduling in MAC

layer, and the overhead in routing the layer. W. Ye [61] explained about an

energy-efficient MAC protocol for WSNs. There exists some research on

the cross-layer interaction and design in developing new communication

protocols [39]. However, as discussed in [39] in detail, these works either

provide analytical results without any communication protocol design, or

perform pairwise cross-layer design within limited scope, e.g., only routing

and MAC layers [75], which do not consider all of the networking layers

involving in the communication in WSN such as transport, routing,

medium access and physical layers. S-MAC (Sensor-MAC) [49][76] is an

energy efficient protocol, but it introduces significant delivery latency and

provides poor traffic throughput. S-MAC uses three novel techniques to

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reduce energy consumption and support self-configuration. To reduce

energy consumption in listening to an idle channel, nodes periodically

sleep. Neighbouring nodes form virtual clusters to auto-synchronize on

sleep schedules. Inspired by Power Aware Multi-Access Protocol

(PAMAS), S-MAC also sets the radio to sleep during transmissions of

other nodes. Unlike PAMAS, it only uses in-channel signaling. Finally, S-

MAC applies the message passing to reduce contention latency for sensor-

network applications that require store-and-forward processing as data

move through the network. Shu Du [53] presented a new duty-cycle MAC

protocol, called as Routing enhanced MAC protocol (RMAC) [53]. RMAC

exploits cross layer routing information in order to avoid these problems

without sacrificing energy efficiency. RMAC can deliver a data packet

multiple hops in a single operational cycle. Cross layer clustering approach

would save energy [77]. In this work, it is investigated that, there is a

decrease in energy consumption among nodes in the network using cross

layer cluster approach of WSN.

5.3 PROPOSED CROSS LAYER APPROACH DESIGN

MAC protocol and routing are combined to reduce data moving

between cross layer in the proposed work. The MAC protocol S-MAC is

used in wireless sensor networks in which mobile node execute both

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receiving and sending of MAC frames at the same interface. IEEE 802.11

DCF protocol is used as MAC protocol with cross layer approach as better

results are obtained as IEEE 802.11 DCF produces RTS and CTS frames

which consume less energy. The same cross layer approach is used with S-

MAC protocol and avoiding data movement between MAC and network

layer. Every node must perform data movement between NIC (Network

Interface Card) memory and host memory twice and looking up the routing

table frame decapsulation and encapsulation. The address of the next hop

is decided by the network layer. These operations consume a lot of

resources of the node such as processing time, memory and energy. But

still packets have to be delivered to the desired destination using minimum

energy. Clustering approach is used to find the nodes nearer to the Base

Station (BS) which work in a longer time and consume much energy.

Nodes near BS will exhaust their energy in a short time in this condition.

The nodes far away from BS in one hop are considered, and the nodes

wake up randomly in a period, time-synchronized sensors form on-off

schedules that enable the sensors to be awake only when necessary.

In order to minimize processing delay between layers, packet is not

required to send data twice through same interface. MAC layer does the

packet decapsulation of the network layer header. Only a portion of

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network layer header is sent to the network layer to process and establish a

route while allowing data frames to reside in NIC memory itself. All the

nodes in the network need not have to move the data packet between the

MAC and the network layer. Whenever MAC layer gets a data frame, it

checks the MAC destination address field in the frame and finds out

whether it is intended to it or not. It stores the MAC frame and removes the

MAC header to further examine it if the data frame belongs to it. MAC

layer itself removes the network layer which is supposed to be done by the

network protocols at network layer. Instead of sending complete data

frame, only network layer header with the destination address is sent to

help in finding the next hop address and then transferring next hop MAC

address to the MAC layer. Now, MAC layer encapsulates a new frame

with next hop address and the data frame reserved in NIC memory. Thus,

the amount of data moving between layers, processing time and end-to-end

delay is reduced. Once after finding the next hop, the packets have to be

routed through nodes to the desired destination. The route must be chosen

before fransmitting the packets, and choosing the correct route is the first

phase using a cluster approach in WSNs. Sensor nodes dynamically create

on-off schedules in a way that the nodes will be awake only when needed

and asleep the rest of the time. This can be done in two distinct phases in

routing:

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1, The setup and reconfiguration phase: This phase takes place during the

initializing of the network. And it is shorter in comparison to the steady

phase. The routing failure may happen because of the impact of the

external environment. It is necessary to reconfigure the route when it fails.

The goal of this phase is to set up the schedules that will be used during the

steady state phase and the setup phase corresponds to writmg the routing

tables.

2. The steady state phase: It takes place between consecutive setup and

reconfiguration phase. The nodes will transmit the data steadily in this

phase.

WSNs are application-oriented. The amount of data needs to be

transferred is very huge in applications like audio and video monitoring.

The data field in MAC frames will be large too. These applications are

usually real time applications. Thus, this approach is helpful for these

applications. The efficient S-MAC protocol and cluster approach for

minimizing processing time and energy consumption has been considered.

5.4 SIMULATION SETUP

NS2 [40] discrete simulation is used to evaluate the performance of

cross layer cluster approach. The setup consists of a testbed of 25 nodes

randomly deployed in the area of 1500 m x 1500 m. 10 nodes are 91

considered to be one hop away from the base station out of 25 nodes. The

nodes are finally increased to 100. The MAC protocol S-MAC is

incorporated into CLD-SMAC (Cross Layer Design S-MAC) and consume

energy for reading, writing and comparing a bit. With the increase in

processing time, energy consumption also increases. Processing delay

includes header processing, data movement between layers and routing

table lookup. In addition, it is helpfiil to use cluster approach for routing

data between the nodes whose residual energy is greater. The simulation

parameters are listed in table 5.1.

Simulation Parameters Number of Nodes Simulation Time Initial Energy Transmit Power Receive Power Idle listening Power Number nodes one hop near to BS/total nodes Transmission Range

Value 25, 50, 75,100 60 seconds 1000 joules 3.0 Watts 2.0 Watts 0.04 Watts 10/25,20/50, 35/75 and 45/100 25 meters

Table 5.1 Simulation parameters used for cross layer design

5.5 RESULTS and DISCUSSIONS

Evaluation of node processing delay and end-to-end processing

delay of CLD-SMAC is the primary purpose of simulation. A UDP

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connection between the source node and the base station is estabHshed. At

1 .Os, UDP packets are sent to the base station from the source node. Packet

size considered is 50 bytes. It has been assumed that the period is 5.0s, so

the sensor network needs to work for 10 periods and the number of the

nodes closer to the base station are 10.

The simulation result is shown in Figure 5.1. Figure 5.1 shows that

the performance of cross layer approach is better than a single layer

approach. Every node in the network is capable of forwarding traffic to any

destination and always it tries to find the best route to every packet.

The cross layer cluster approach is not only useful to route in the

best path, but it also saves energy and is as depicted in Figure 5.2.

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Figure 5.1 Residual Energy of each node which is closer to Base Station

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In this scenario, a network with 25 nodes randomly distributed along

with 10 nodes placed one hop away from the Base Station. The length IP

header is 20 octets with packet length of 1156 octets that are the average

value between 0-2312. The header processing is done in the MAC layer,

and header information is sent to network layer to find the address of the

next hop. Figure 5.2 indicates the nodal processing delays of S-MAC and

CLD-SMAC. Simulation shows that CLD-SMAC outperforms S-MAC in

terms of the nodal processing delay. Thus, it is desirable that the energy

efficiency of CLD-SMAC will be better than that of other MAC protocols.

From Figure 5.3 and Figure 5.4, it is very clear that CLD-SMAC

outperforms in terms of the end-to-end processing delay for 50 and 100

nodes. The improvement when the network density changes have been

observed.

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Figure 5.2 Node Processing Delay for S-MAC/CLD-SMAC Protocol

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5.6 CONCLUSION

Node energy has become a key problem and interesting research

area in WSN. Various MAC layer protocols have been proposed for sensor

networks. Many schemes have improved the existing MAC protocols and

routing algorithms to extend the lifetime of WSNs. Depending on network

layer protocols, the routing path could be chosen. The data moving,

however, causes a portion of energy consumption that cannot be neglected

for each packet transferring. The basic idea is that a node should wake up

only when another node wants to communicate with it. An energy efficient

cross-layer cluster scheme for prolonging lifetime of WSNs by integrating

S-MAC MAC protocol is proposed. The goal to save energy is achieved

through averaging the energy of the nodes consume and reducing the idle

listening and minimizing the collision.

There is a significant improvement regarding the nodal processing

delay and the end-to-end processing delay and is as shown in simulation

results. It has also been proved that the proposed scheme is feasible for the

sensor network. Results show that there is a good progress in energy

saving through processing delay and finding out residual energy for the

entire sensor network. The performance of proposed cross layer cluster

approach shows better results than before compared with S-MAC.

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There are open research problems on optimizing energy

consumption through cross layer approach with different protocols for

WSN based on this direction. The cross layer architectures enable vital

information exchange between the layers to fine tune protocol parameters

and to set route selection criteria. The network lifetime can thus be

improved by reducing energy consumption of nodes.

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