report rough final draft

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National Institute of Technology, Warangal

Chapter 1.Introduction

1.1 Aim

To implement an energy efficient adaptive SMAC protocol in a wireless sensor

network(WSN) and compare it with SMAC protocol.

1.2 Motivation

Energy consumption is the main constraint seen in wireless sensor networks as the

operational lifetime corresponds directly to it. The remote and mobile nature of a WSN that

results in the inability to resupply power to the energy constrained nodes. Thereforeincreasing the energy efficiency is the top priortity so that the lifetime of the WSN can be

extended.

A MAC protocol is required in sensor network to coordinate the sensor nodes’ access to the

shared medium. So many MAC protocols have been developed for wireless voice and data

communication networks.

In our study to determine a good MAC protocol for the wireless sensor networks, we have

considered the following attributes. The first is the energy efficiency. Another important

attribute is the scalability of a WSN, to the change in network size, node density and

topology. The network topology and size can change over time as well due to many reasons.

A good MAC protocol should easily accommodate such network changes. Other important

attributes include fairness, latency, throughput and bandwidth utilization.

A suitable medium access control (MAC) protocol can maximize the whole network lifetime.

This project focuses on a protocol solution that deals with MAC layer which will minimize

the energy consumption and the latency encountered while transmuting packets across the

network.




1.3 OUTLINE OF REPORT

In Chapter 2 , the fundamental concepts of Wireless sensor networks will be

explained.

In Chapter 3 , we compare the various MAC (Media Access Control) protocols and

analyze the essential components of an energy efficient MAC protocol.

In Chapter 4 , S-MAC protocol will be discussed in detail.

In Chapter 5, Adaptive S-MAC protocol and how it addresses various issues not

solved by S-MAC, will be explained.

In Chapter 6, we estimate the performance of the Adaptive SMAC protocol in

comparison with that of the S-MAC Protocol via simulation.




Chapter 2 Wireless Sensor Network

2.1 Introduction

Wireless Sensor Networks are infrastructures containing sensing, computing and

communication elements all incorporated in a sensor node with the sole aim to give the

WSN, the ability to sense, measure, collect and react to various stimulus in the monitored

environment. Because of their widespread applications, they are one of the most rapidly

developing information technologies over the last few years.

Figure 1 A Wireless Sensor Network is shown here. This figure emphasizes the range of applications in which a WSN is used. Various environments are studied and data is collected

which are then transmitted over long distance via the gateway.

Wireless Sensor Networks are networks that usually consist of a large number of

widely distributed sensing devices that are equipped to monitor physical or environmentalphenomena. These devices work autonomously and are logically linked by self-organizing

means.

The ideal wireless sensor is networked and scalable, consumes very little power, can

change its active time interval according to the traffic condition, capable of fast data

acquisition, costs little to purchase and install, and requires no real maintenance [1].

A wireless sensor network (WSN) generally consists of a base station (or “gateway”)

that can communicate with a number of wireless sensors via a radio link. Data is collected atthe wireless sensor node, compressed, and transmitted to the gateway directly or, if required,




uses other wireless sensor nodes to forward data to the gateway. The transmitted data is then

presented to the system by the gateway connection.

Figure 2 This figure shows the simple breakdown of a WSN. The blue circles represent thesensor nodes which collect data from the environment. Data is received at cluster heads

represented by green hexagons which in turn is connected to a gateway.

Some of the challenges for these systems are:

Reliability:

WSNs are wireless networks and are therefore vulnerable to problems likepacket loss. Nevertheless, they are used in areas such as forests, mountains, sea etc.

and for various applications such as temperature and precipitation detection, in which

these problems could easily lead to serious catastrophe.

Besides WSNs should be able to function seamlessly without any human intervention

as it not viable to involve human support in harsh environment.

Power Consumption:

The nodes of Wireless Sensor Networks are usually battery powered because

of their size. This limits the lifetime of a sensor node and raises the topic of energy-

efficiency in all aspects.

Node size:

Miniaturization is the keyword in many studies about WSNs. Developing

smaller nodes, with the same or even more efficiency than bigger nodes is a




challenge. Besides these nodes should be made in such a manner that chances of node

failures taking place should be minimum. They should be small in size in order to

allow large scale deployment of sensor nodes in various areas like sea and forests and

should be able to deploy the nodes from an aircraft as well.

Mobility:

Many applications urge the factor mobility into WSN challenges. For

example, commercial applications, like vehicle tracking, need networks that are able

to constantly change its routing paths and infrastructure.

Privacy and Security:

Unlike wired channels, wireless channels are accessible to both, legitimate and

illegitimate users. Therefore, several methods, like encoding the traffic, have to be

discussed.

2.2 Working

Wireless Sensor Networks forms a class of special wireless ad hoc networks. A

wireless ad hoc network is a collection of wireless nodes that communicate directly over a

common shared medium. Therefore, every node is equipped with a wireless transceiver and

has to be able to act as a router, to process packets and to transmit data to its destinations.

The main difference between common ad hoc networks and Wireless Sensor

Networks is their area of application. For WSNs, the focus is more on monitoring and

collecting data, while common ad hoc networks focus more on the communication aspects.

2.2.1 Sensor node

Figure 3 A Sensor Node and its Components are exhibited in the figure. These components

gather process and transmit data and are internally linked by an operating system.




Sensor node is a node in a wireless sensor network that is capable of performing some

processing, gathering sensory information and communicating with other connected nodes in

the network.

There are several hardware components that make up a typical sensor node:

1) Low-power embedded processor:

The computational tasks on a WSN device include the processing of both locally

sensed information as well as information communicated by other sensors. Currently,

primarily due to economic reasons, the embedded processors are often substantially

limited in terms of computational power (small MHz area). Due to the constraints of such

processors, devices typically run specialized component-based embedded operating

systems, such as TinyOS. They incorporate advanced low-power design techniques, such

as sleep modes and dynamic voltage scaling to provide energy savings.

2) Memory/storage:

In the storage, both, program memory (memory for the instruction set of the

processor) and data memory (for storing measured data and other local information, e.g.

the location of the node) are included. The size of the memory is often limited due to

economic reasons. With the ongoing price-reduction of memory devices, the quantities of

storage and memory used on sensor nodes increase over time.

3) Radio transceiver:

WSN devices include a low-rate, short-range wireless radio (10 – 100 kbps, <100m).

While currently quite limited in capability too, these radios are likely to improve in

sophistication over time – including improvements in cost, spectral efficiency, tunability,

and immunity to noise, fading, and interference. Radio communication is often the most

power-intensive operation in a WSN device, and hence the radio must incorporate

energy-efficient sleep and wake-up modes.

4) Sensors with ADC unit:

WSN devices usually support only low-data-rate sensing, because of the limitations of

energy and bandwidth. In many applications, multi-modal sensing is necessary, resulting

in the fact, that every device could have multiple sensors implemented. Which specificsensors are used, is application-based. The Analog-to-Digital Converter Unit translates




the analog signals, provided by the sensors, to digital signals, that can be processed by the

processor unit.

5) Location finding system:

To analyze the measured data, in many WSNs it is important to know in which

location, the data was monitored. But unfortunately, only a few applications allow the

designer to pre-configure the location of the sensor nodes. Particularly for randomly

deployed WSNs, which are used, for example, for outdoor operations, location finding

systems, normally based on satellite GPS, have to be implemented.

6) Power Source:

Usually, the power source is a small battery. The finite battery power is likely to be

the bottleneck in most WSN applications. However, in some applications, a couple of

nodes may be wired to continuous power source or energy harvesting techniques may

provide a small amount of renewed energy.

Figure 4 The schematic diagram of the components in a Sensor Node is shown here. All theconnections between components which were discussed earlier are shown.

2.2.2 NETWORK TOPOLOGY

The most common architecture followed in WSNs is the two-tier hierarchical cluster

architecture. In this topology, the nodes within a particular region collect data from the




environment and transmit it forward to a specific node called cluster head. This cluster head

forms a network with other cluster heads that are covering different regions. In this network,

the cluster heads of tier 2 could send their data to a new cluster head that covers these tier 2

cluster heads and forms another network with other cluster heads, and then finally send their

data to the gateway. The advantage of this hierarchical structure is that it separates a large

network into several zones of interests within which routing can be performed.

Figure 5 Two tier hierarchical architecture followed by WSN is shown in this figure. The 1 st

tier consists of sensor nodes grouped in form of clusters whereas the 2nd

consists of clusterheads.




CHAPTER 3 MAC ROUTING PROTOCOLS

3.1 INTRODUCTION

Networking unattended sensor nodes may have profound effect on the efficiency of

many military and civil applications such as target imaging, intrusion detection, weather

monitoring, security and tactical surveillance, distributed computing, detecting ambient

conditions such as temperature, movement, sound, light, or the presence of certain objects,

inventory control, and disaster management.

Wireless Sensor Networks are a class of wireless ad hoc networks that pose unique

design challenges for their developers. The sensor nodes are normally battery-powered and

therefore their lifetime is limited. Typically, these batteries also cannot be changed. Sinceenergy is a valuable resource in WSNs, energy-efficient routing is one of the most important

aspects of increasing the life span of sensors.

One of the factors, of energy-efficient routing, is the strategy of picking a route between two

nodes.

The challenges for the strategy are:

1) Number of transmissions:The number of transmissions until a packet reaches its destination should be as small

as possible due to the fact that every transmission between two nodes uses energy. If the

strategy minimizes the re-transmissions, it will also minimize the energy consumption.

2) Balanced use of nodes:

The use of the nodes for the routing has to be balanced between all the nodes. If some

nodes are used distinctly more than others, their battery-power will decrease faster and

will expire sooner. Too many dead sensor nodes could result in a partition of the network,

which would make communication impossible.

3) Delay:

For many applications of WSNs, it is important that the delay of the transmission is

not too big. It is desirable, that the strategy comes to a compromise between overhead and

delay.

4) Balancing the previous aspects:

The routing strategy has to be aware of the energy resources that are left and has to

balance all the previous aspects to produce the best possible solution.




3.2 Sources of energy wastage

With reference to our project, we made sure energy is not utilized in a manner which leads to

unnecessary wastage of power since in WSN the sensor node runs on limited power supply.

Hence it’s imperative that we know about the sources of energy wastage.

So the major sources of energy wastage are:

Collision : A collision can occur when a node receives two signals or more simultaneously

from different sources that transmit at the same time. This leads to corruption of data packets

at the receiver. Hence the transmitter got to send the packet again. Collision can be removed

in schedule-based MAC protocols however; it is a concern issue in contention-based

protocol.

Idle listening: A node doesn’t know when will be receiving a frame so it must maintain

permanently its radio in the ready to receive mode, as in the wireless network protocol (IEEE

802.11). This mode consumes a lot of energy, nearly equal to the one consumed in receipt

mode.

Overhearing: This happens when a node receives packets that are not destined for it or

when a redundant broadcast takes place.

Control packet overhead: The RTS/CTS (Request to Send /Clear to Send) used by some

protocols transport no information whereas their transmission consumes energy. Note that the

traffic generated by control frames in sensors network is far from being negligible, it could

represent until 70% of the global traffic. Hence it shows energy wastage due to control packet

overhead is not less in magnitude.

3.3 MAC protocol

A medium access control (MAC) protocol coordinates actions over a shared channel.

The most commonly used solutions are contention-based. One general contention-based

strategy is for a node which has a message to transmit to test the channel to see if it is busy, if

not busy then it transmits, else if busy it waits and tries again later. After colliding, nodes

wait random amounts of time trying to avoid re-colliding. If two or more nodes transmit at

the same time there is a collision and all the nodes colliding try again later. Many wireless

MAC protocols also have a sleep mode where nodes not involved with sending or receiving a




packet in a given timeframe go into sleep mode to save energy. Many variations exist on this

basic scheme.

In general, most MAC protocols optimize for the general case and for arbitrary

communication patterns and workloads. However, a wireless sensor network has more

focused requirements that include a local unicast or broadcast, traffic is generally from nodes

to one or a few sinks (most traffic is then in one direction), have periodic or rare

communication and must consider energy consumption as a major factor.

An effective MAC protocol for wireless sensor networks must consume little power,

avoid collisions, be implemented with a small code size and memory requirements, be

efficient for a single application, and be tolerant to changing radio frequency and networking

conditions.

3.3.1 IEEE 802.11

The IEEE 802.11 DCF protocol works in two different modes: infrastructure mode

and ad hoc mode. In infrastructure mode communication between modes passes through a

central node, while in ad hoc mode nodes communicate directly with each other.

Infrastructure mode is the mode followed by sensor network clusters, in which sensors within

a cluster communicate directly with their cluster head. On the other hand, the

communications between gateways follow the ad hoc mode.

The widely used standardized IEEE 802.11 distributed coordination function (DCF) is

an example of the contention-based protocol. While studying the IEEE 802 DCF, we can see

that the binary exponential Back off (BEB) mechanism is used for resolving packet collisions

that occur as the sensor nodes contend for the transmission channel. To ensure packet

transmission reliability and signal fidelity, MAC acknowledgment (ACK) frames are used to

indicate the correct reception of the data packets at the receiving nodes. When a node does

not receive a corresponding ACK frame, it assumes the packet has been dropped due to

collision, and invokes the BEB mechanism for retransmission [6].

The standard BEB of IEEE 802.11 degrades significantly the network performance in

noisy environments. The problem with the IEEE 802.11 is that the standards were developed

with no energy minimization mechanisms taken into consideration, which are required in a

WSN.




3.3.2 TDMA (Time Division Multiple Access) protocol

Time-division multiple access (TDMA) [11] is a scheduled-based MAC protocol in

which the channel is divided into several time slots. Each node is assigned a time slot. The

node wakes and transmits data only in their allocated time slots and remains in sleep mode

otherwise.

Conventional TDMA MAC protocols are used to prevent collisions by fixing

individual time slots for each node. It also reduces energy consumption as all nodes are in

sleep condition except for the transmitting nodes. Hence, TDMA protocol is a suitable choice

for sensor networks. However, this protocol is suitable for a network with heavy traffic load.

The energy-efficient TDMA reduces energy consumption due to idle listening. Sensor nodes

keep their radios off when there is no data to transmit. The cluster head has to keep its radio

on during all the timeslots in readiness to collect data from the nodes it serves. Hence, the

cluster head does not use its energy efficiently as it wastes energy in its idle time.

Thus, the main and most important advantage of TDMA over CSMA is low power

consumption. Also, TDMA achieves better channel utilization as the number of nodes is

higher, which is more in tune with the WSN characteristics.

TDMA shows high latency and overhead associated with synchronization between

nodes and it also introduces early sleeping problem which is a serious issue in a WSN.

TDMA doesn’t adapt well to network change , which is a quite frequent phenomenon in

sensor networks due to node failure. Synchronization problems can lead to high latency and

power consumption caused by interference from overlapping schedules.

3.3.3. PAMAS (Power Aware Media Access Sensing) Protocol

The PAMAS protocol is a combination of the original MACA protocol and the idea of

using a separate signaling channel. Here we can see that the RTS-CTS message exchange

will take place over the signaling channel that is separate from the channel used for packet

transmissions. This separate signaling channel enables nodes to determine when and for how

long they can remain in the idle state.

In order to conserve power and extend the operational lifetime of the nodes, the

PAMAS protocol requires nodes to shut them off if they encounter overhearing. A node

knows if a neighbor is transmitting because it can hear the transmission. Likewise, a node

(with a non-empty transmit queue) knows if one or more of its neighbors is receiving because

the receivers transmit a busy tone when they begin receiving a. Thus, a node can easilydecide when to power off [5].




3.3.4. TRAMA (Traffic Adaptive Medium Access Protocol) protocol

TRAMA [8] is a TDMA-based algorithm and proposed to increase the utilization of

classical TDMA in an energy efficient manner. It is similar to Node Activation Multiple

Access (NAMA), where for each time slot a distributed election algorithm is used to select

one transmitter within two-hop neighborhood. This kind of election eliminates the hidden

terminal problem and hence, ensures all nodes in the one-hop neighborhood of the transmitter

will receive data without any collision.

The Traffic Adaptive Medium Access Protocol (TRAMA Protocol) tries to minimize

power consumption by synchronizing the transmitter as well as the receiver. To be as energy

efficient as possible TRAMA let nodes go to sleep when no node is sending data, and also

when a node is not the intended receiver of the sender. Besides, if a node has no information

to send, it will give up its slot, so other nodes may send.

Looking at all the above protocols we saw that they are not completely suitable to be

implemented in a WSN. Hence for this project we did our work implementing S-MAC

(Sensor Media Access Control) protocol which will be discussed in detail in the next chapter.




CHAPTER 4 S-MAC Protocol

4.1 INTRODUCTION

The S-MAC protocol (Sensor MAC) is a protocol specifically designed for usage inWireless sensor networks and is designed by Wei Ye. S-MAC is a contention-based protocol,

designed to reduce energy consumption from the sources of energy loss: idle listening,

collision, overhearing and control overhead. Periodic sleep technique is used in this protocol

to achieve low duty cycle that reduces energy consumption significantly. It uses virtual

carrier sense technique to reduce collision avoidance and in-channel signaling to implement

overhearing avoidance. S-MAC fragments the long message into many small parts and

transmits them in burst to reduce contention and communication overhead.It is a stable MAC protocol used specifically for the WSN. S-MAC reduces energy

consumption by allowing the nodes to periodically turn off their radio receivers (and any

other resources that have no work to do) and enter a low power sleep state. The duty cycle of

a node is the ratio of the time it is active (i.e. not in the sleep state) to the total time. The

lower the duty cycle, the lower is the power consumption of a sensor node. In S-MAC the

channel access is done using a scheme similar to the IEEE802.11 distributed coordination

function. However, unlike the IEEE 802.11 MAC protocol, the intervals during which

contention can occur are scheduled. S-MAC, therefore, combines the features of both

contention based as well as time scheduled protocols. Even though the contention interval in

S-MAC is scheduled, S-MAC requires much looser time synchronization than TDMA based

protocols. Furthermore, S-MAC does not suffer from the limited scalability generally

associated with TDMA schemes [9].

4.2 Working

S-MAC carries out energy conservation by making the nodes to periodically listen for

any communication for a short interval and then allowing them to sleep; if and only if the

node is not involved in data transmission, for the rest of a pre-determined duration (a frame ).

The listen interval and the sleep interval for a node occur according to a schedule which can

of its own or it follows a schedule of another node. S-MAC follows a mechanism which

enables the nodes to learn the sleep schedules of the nodes in their neighborhood, and using




this knowledge they communicate with their neighbors. For successful communication, a

transmitting node must transmit when the intended receiver is not sleeping.

The duty cycle of the sleep schedule of a node determines the base rate of energy

consumption. The base rate is the energy consumption rate when the node is in its sleep state.

The actual power consumption depends on the data traffic at the node. In a wireless sensor

network using S-MAC, some nodes may, however, have to wake up and listen more often

than the other nodes. Since all nodes may not have the same duty cycle, some nodes may

consume their energy faster and die earlier. This reduces the average life of the WSN nodes,

and hence reduces the average useful life. This non-uniform life span of the nodes eventually

creates region within the WSN which do not have any kind of sensor coverage and this

adversely affects the network connectivity.

Basic Scheme: Periodic Listen and Sleep

Instead of having a 100% duty cycle, a WSN following SMAC protocol periodically

goes to sleep having only a timer working which maintains the schedule and switches the

nodes on and off when the timer decrements to 0

Figure 6 Basic Scheme of SMAC protocol is shown here. The entire lifetime of a WSNconsists of active and sleep time intervals which takes place one after another.

The S-MAC protocol ’s main focus is to keep the active state of a node to a minimum. It

accomplishes this in various ways:

Scheduling. Collision avoidance. Overhearing avoidance. Message passing.

4.2.1 Avoidance of collisions

In S-MAC the nodes conserve energy by sleeping (i.e. turning off their receivers and

any other hardware resources which are not required while the node is in the sleep state), and

wake up periodically to listen and check if any of its neighbors want to communicate with it.




The duration of the listen interval and the sleep interval are fixed in a system according to the

application requirements. A listen interval followed by a sleep interval comprises a frame [9].

The node which needs to send data begins the transmission procedure during the

Listen interval of the receiver, and then continue the data transfer during the time that

normally is the sleep interval. Other nodes go back to sleep turning their radios off, and thus

are able to avoid overhearing and idle listening [10].

Figure 7 SMAC Frame is shown in detail here. Here the listen interval consists of SYNCpulse used for synchronization between two nodes, RTS and CTS pulses used for contention

purposes.

The listen interval is divided into three sub-intervals. Two of these intervals are used for an

exchange of RTS and CTS control frames to avoid the hidden terminal problem in the way the

RTS-CTS exchange is used in the IEEE 802.11 MAC protocol. The protocol uses both

physical and virtual carrier sensing for collision avoidance.

In virtual carrier sensing, the RTS and CTS frames carry a time field indicating the

remaining duration of the transmission of the message. The node records this value in a

variable called the network allocation vector (NAV) and sets a timer for it. The node

decrements the NAV value until it reaches zero. When a node has data to send, it first looks

at the NAV. If its value is not zero, the node determines that the medium is busy and goes

back to sleep

4.2.2 SCHEDULING

Since the nodes periodically sleep with their radios turned off, a node must know the

listen and sleep schedule of a neighbor with which it wishes to communicate. Nodes

exchange their schedules by periodically broadcasting a special control frame, the sync frame .The protocol requires each node to broadcast a sync frame at least once in a predetermined




synchronization period. A node builds a table of the schedules of its neighbors by listening to

the sync frames [10].

4.2.2.1 Choosing a schedule

Sync frames also enable a node to choose its schedule when it starts. The procedure

outlined below is followed by the S-MAC nodes for choosing its schedule and for

constructing the table of schedules of its neighbors[9].A node first listens to the broadcasts in

its neighborhood for a pre-determined duration which is at least as long as the

synchronization period. If it does not hear a valid sync frame (containing a schedule from a

neighbor), it randomly chooses a schedule for itself and starts following it. The node also

broadcasts a sync frame to announce its schedule.

1. If the node receives a schedule from a neighbor before it has chosen or announced a

schedule on its own, it adopts the received schedule and starts following it. The node

Announces this schedule as its own schedule by broadcasting a sync frame during its next

Scheduled listen time.

2. If the node receives a different schedule after it has chosen and announced a

schedule, there are two cases to consider:

a) If the node has no other neighbor, it discards the previously chosen schedule, and

Starts following the new schedule.

b) If the node already follows a schedule with one or more neighbors, it starts waking

up at the listen interval of the newly received schedule, in addition to following the

earlier schedule.

Once a node has chosen a schedule using the procedure described above, it continues to

broadcast this schedule in a sync frame at least once in a synchronization period.




Figure 8 Synchronization of schedules between two nodes is shown here. Above figure showsthe timing relationship of three possible situations that a sender transmits to a receiver. CSstands for carrier sense. In the figure, sender 1 only sends a SYNC packet. Sender 2 only

wants to send data. Sender 3 sends a SYNC packet and a RTS packet.

4.2.2.2 Maintaining a schedule

The listen/sleep scheme requires synchronization among neighboring nodes. Although

the long listen time can tolerate fairly large clock drift, but in order to prevent long time clock

drift neighboring nodes still need to periodically update each other their schedules.

Updating schedules is accomplished by sending a SYNC packet. Receivers will adjust

their timers immediately after they receive the SYNC packet. A node will go to sleep when

the timer fires [10].

4.2.3 Message passing

S-MAC has an efficient method which it follows in order to pass large messages

around the network. Large messages that are transmitted as one large packet are inefficient as

this has a huge probability of failure in noisy environments. If packets are fragmented the

chances of failure are smaller, but this increases the control overhead as each packet needs a

RTS CTS packet. SMAC works around this by sending "bursts" of data using only one RTS

CTS pair. There is a possibility of a new node entering the range of the transmitting node and

this scenario might disrupt the current transmission. This way the node knows that atransmission is in progress and will remain silent. Nodes on waking up will expect a clear




channel and this will detect this condition of time extension by detecting the

acknowledgements or data fragments (these both contain duration information) [7].

4.2.4 Overhearing Avoidance

In 802.11 each node goes to listen to all the transmissions from its neighbors in order

to perform effective virtual carrier sensing. As a result, each node overhears a lot of packets

that are not directed to it. This is a significant waste of energy, especially when node density

is high and traffic load is heavy.

S-MAC protocol tries to avoid overhearing by letting interfering nodes go to sleep

after they hear an RTS or CTS packet. Since DATA packets are normally much longer than

control packets, the approach prevents neighboring nodes from overhearing long DATA

packets and the following ACKs (acknowledgements).

Figure 9 Overhearing Avoidance is shown using the above figure. In the above figure 9, A,B, C, D, E, and F forms a multi-hop network where each node can only hear the

transmissions from its immediate neighbors.

Node A is currently transmitting a data packet to B. Node D should go to sleep since its

transmission interferes with B’s reception. Node E and F do not produce interference, so

they do not need to go to sleep. C is two-hop away from B, and its transmission does not

interfere with B’s reception, so it is free to transmit to its other neighbors like E. However, C

is unable to get any reply from E, e.g. , CTS or data, because E’s transmission collides with

A’s transmission at node C. So C’s transmissio n is simply a waste of energy [4].

4.3 Causes of Latency

Various kinds of delays contributing to latency

Carrier sense delay

Determined by the contention window size




Back off delay

Because the node detects another transmission or the collision occurs

Transmission delay

Determined by channel bandwidth, packet length and the coding scheme adopted

Propagation delay

Determined by the distance between the sending and receiving nodes.

Processing delay

Depends on the computing power of the node and the efficiency of in-network data

processing algorithms.

Queuing delay

Depends on the traffic load.

The above kinds of delays are common to S-MAC as well as IEEE802.11.

Sleep delay

This is an extra kind of delay in S-MAC caused by the nodes periodic sleeping.

A complete cycle of the listen and sleep is called a frame.

The average sleep delay on the sender is

D s = T frame /2 (1)

Where,

T frame = T listen + T sleep (2)

and, T listen = T RTS + T CTS (3)

The relative energy saving in SMAC is:-

Es =T sleep /T frame

=1- (T listen /T frame ) (4)

So from equation 4 we can see, lesser the duty cycle more is the energy saving.




CHAPTER 5 ADAPTIVE SMAC PROTOCOL

5.1 INTRODUCTION

In the earlier chapter we have studied SMAC protocol in detail and also have shown

the advantages SMAC protocol gives over the other MAC protocols. Even then we have seen

that the S-MAC protocol can only be implemented provided a trade-off is made between the

energy used for throughput and the latency. The throughput is reduced because only the

active part of the frame is used for communication. The latency is increased because a

message generating event may occur during the sleeping part. In this case, the message will

be queued until the start of the next active part.To solve this problem, the ASMAC (ADAPTIVE SMAC) protocol was proposed.

ASMAC, which adds flexibility to S-MAC's duty cycle, can be adapted to the network traffic

state. A node will keep listening, and potentially transmitting, as long as it is in an active

period. Therefore, ASMAC is more energy efficient than S-MAC [2].

Figure 10 Example of SMAC protocol’s unnecessary active intervals are shown in the

above figure. Looking at the above figure we can see that S-MAC has unnecessary activeintervals. When burst messages are generated in part A and there are no messages in parts B

and C, unnecessary active intervals occur which decrease the energy efficiency.




5.2 WORKING

5.2.1 Adaptive SMAC Algorithm

Adaptive S-MAC is a protocol that dynamically changes the entire frame size by

adjusting the network frame states. The fundamental idea is to decide whether the interval is

to be spent in the active or sleep state by means of a two bit flag that records the data

generation, transmission and reception. In adaptive S-MAC, if the flag value is 0, in the next

active interval the node will sleep and when the flag value changes to 1 in this sleeping active

interval, in the next active interval node wakes up and can sense the messages. [3].

Figure 11 Basic working of AS-MAC protocol is shown. The figure 11 signifies that theactive interval is determined by the flag value. If the flag value is 0 in the previous activeinterval, during the next active interval, node follows its sleeping schedule. On the other

hand, if the flag value is 1, then the next active interval has an active schedule [3].

. In the case where no message is generated, as in the case of Frames A, B and D, the

entire frame length is prolonged, whereas in the case where a message is generated, as in the

case of Frame C, the frame interval is reduced. In this manner, adaptive S-MAC candynamically control the entire frame length according to the network traffic state.

5.2.1.1 Improvement to the AS-MAC

The above proposed adaptive S-MAC protocol is useful when the data rate is moderate or

low. The adaptive S-MAC gives better power conservation with increased latency due to the

lower duty cycle used when traffic load is low. In the lifetime of a WSN, during certain timeperiod, when the activity of the environment increases, the rate of the fundamental S-MAC




In state C, D the WSN node has the frame length of an S-MAC protocol. The WSN

fundamentally has the duty cycle of S-MAC protocol. The states are chosen based on the

traffic load.

In state A and B the two bit flag ranges from 00 to 11 i.e. from decimal 0 to 3.The

flag value increments with every pulse .When it is 11 that means the next state is going to be

active with flag value 00,rest all are sleeping states.

In states C and D the flag ranges from 00 to 01.The frame size here is half that of in

states A and B. Here if flag is 01 it means next state will be active.

In state E the flag remains in 00 values, with 00 means the WSN node will be active

during next pulse.

Figure 13 Scenario 1 of AS-MAC protocol is shown. We can see that at state A when there’sincoming data the node moves into state B and at this particular state if no data is received ,

then node moves back to state A.

Figure 14 Scenario 2 of AS-MAC protocol is shown. Here we see that the node moves all theway into state C from state A, following the Moore state machine diagram. At state C it

doesn’t receive any data. So it moves back into state A.




Figure 15 Scenario 3 of AS-MAC protocol is shown. According to the state diagram if thenode enters state C, it can move into state D and on not receiving any data it changes to stateC. On the other hand if it receives data in state D it moves into state E and will stay there as

long as data keeps on coming in. and when the steady stream of data stops coming in thenode moves back into state C from E.

5.2.2 Synchronization

When the node synchronizes with its neighbors, AS-MAC operates just like SMAC

protocol. Especially, when the SYNC packets are sent to the neighbors or the cluster head,

they contain the schedule .A node fundamentally stays in state A or has largest frame length.

But that does not matter because as soon as it gets SYNC packet from its neighbor it starts

following the schedule contained in the SYNC packet [3].

Figure 16 Synchronization of AS-MAC protocol is shown here. In the above figure it canbe seen that B has a different duty cycle and also has a clock drift with respect to A. So when

A is looking to communicate with B it sends a SYNC packet containing its schedule. Bupdates its schedule upon receiving it.




5.2.3 Operation with neighbors

Figure 17 Operation of a node with its neighbors following AS-MAC protocol is seen here.The figure shows the schedules of the neighbor nodes in the case of transmission, after

synchronizing with the neighbors.

In State 1, node 3 sends a RTS packet to node 2 requesting permission to transmit. If node

2 is ready to receive the messages, it sends a CTS packet to node 3. As in State 2, nodes 2 and

3 send and receive DATA and ACK packets to each other. At this time, nodes 2 and 3 follow

the same schedule with their corresponding flag values and while staying in the same states,

so nodes 2 and 3 function using half or double of their usual frame size. In this situation,

nodes 1 and 4 enter sleep mode, because of the duration in the header of nodes 2 and 3's

DATA and ACK packets. As a result, nodes 0 and 5 don't send or receive messages and

maintain an established frame length.

5.3 Protocol Analysis

In front of experimenting S-MAC and adaptive S-MAC, we analyze the performance of the

AS-MAC protocol, and compare it with that of the S-MAC protocol. Following delays have

been considered.

Carrier sense delayThe latency incurred due to the carrier sensing procedure when a sensor contends for

the channel. We denote the average value of carrier sense delay by T ics , which is

determined by the contention window size.

Transmission delay

The transmission time is related to the channel bandwidth, packet length. Therefore,

we assume a fixed packet length as mentioned in the table (GIVE TABLE NUMBER

FOR SIM PARAMETERS) and denote the transmission delay by T tx.




Propagation delay

It is determined by the distance between the sending and receiving nodes. It has been

ignored in the simulation as it is negligible.

Processing delay

It depends on the computing power of the node and the efficiency of in-network data

processing algorithms.

Back off delay

It is the delay incurred when a collision occurs while trying to send the data. It is

denoted by T b .

Sleep delay

The latency incurred due to the periodic sleeping of each sensor. The latency incurred

by the periodic sleeping algorithm is denoted as T s in this context. This contributes

most to the delay and hence the latency. It depends upon the frame length mentioned

below.

Frame length : We denote the entire frame length by T f.

We look at the latency for a packet transmission between two neighboring nodes that are one

hop away from each other. We have ignored propagation delay and the processing delay.

5.3.1 Latency of S-MAC

When a node sends to its neighboring nodes using the S-MAC protocol, it takes

carrier sensing time and latency by the end of the sleep interval. Therefore, we can obtain the

latency of a single-hop communication experienced in the network as follows:

D= T cs + T tx + T s+T b

The sleep delay is a random variable from ( 0, T f ).Therefore the expression for average

delay of S-MAC is

E(D)= E( T cs + T tx + T s)

= Tcs + T tx + T f /2 +T b




5.3.2 Latency of AS-MAC

For Adaptive S-MAC, as described above, we assume three duty cycle levels. Let pl,

p2 and p3 denote probabilities that T f /2, T f and 2 T f are the adjusted duty cycles inalgorithm, respectively. We have

E(D)= E( T cs + T tx + T s)

= T cs + T tx + T b+ T f ( p1+2p1+4p3 )/4

Depending on the values of p1, p2 and p3, the latency of AS-MAC can be less or more than

S-MAC. These probabilities depend on the activity of the WSN.




Chapter 6. SIMULATION

6.1 Simulation Configuration

Simulation parameters have been taken from wireless sensor nodes, Motes , developed by

University of California, Berkeley [13]. The mote has a 8-bit Atmel AT90LS8535

microcontroller running at 4 MHz. It has a low power radio transceiver module TR1000 from

RF Monolithics, Inc [14], which operates at 916.5 MHz frequency and provides a

transmission rate of 19.2 Kbps. The mote runs on a very small event-driven operating system

called TinyOS [15].The WSN has been implemented in single cluster two hop network using

OMNeT++ software. We have simulated a hierarchical topology WSN with a single cluster.

The cluster head has all the nodes in its range and is aware of the schedules of other nodes.

Simulation Parameters

Table 2

The simulation parameters can vary with hardware aspects of the WSN nodes.

Depending on requirement and resources we can have different battery power, bandwidth etc

parameters.The purpose of the simulation is to compare between S-MAC and A-SMAC with

same parameters for both. The functionality of the protocol is more important than the

numerical values of parameters and results. If the protocol simulation gives desired results it

should definitely work for other simulation parameters also.

Parameter Value

Channel Bandwidth 20kbps

Average data packet size 50 Byte

RTS,CTS,ACK size 30 Byte

Reception Power 13mw

Transmission Power 24.75mw

Idle Power 13mw

Sleep Power 15µw

Transmission Range 250m

Battery power 1.6 Joules




6.2 Simulation Results for S-MAC protocol

6.2.1 Variation of Residual energy with time

0 20 40 60 80 100 120 140 1600

200

400

600

800

1000

1200

1400

1600

Time of Simulation

A v e r e a g e

E n e r g y

l e f t i n t h e n o

d e s

( m i l l i j o u

l e s

)

Energy VS Time curve for SMAC and 802.11 MAC protocol

SMAC

Without SMAC

Figure 18

Figure 18 compares the energy efficiencies of SMAC with IEEE 802.11 protocol.

By 100 second the battery power of IEEE 802.11 is exhausted but S-MAC nodes still

have a lot of residual energy left. From the graph above it can be seen that the energyefficiency of S-MAC is more than 10 times that of IEEE 802.11.The time of exhaustion

for 802.11 can vary with the simulation parameters but the network employing 802.11

will always die before a WSN using S-MAC protocol.




6.2.2 Transmission rate variation with Duty Cycle

Figure 19

Figure 19 is a plot of number of packets sent against the duty cycle. We can get an

idea about the latency of the WSN from this curve. This curve also shows the tradeoff

between duty cycle and the latency. As we increase the duty cycle more packets are sent

hence less the latency whereas as we move towards the right less packets are sent

indicating more latency. So lesser the duty cycle or time of listening, more is the powerconservation but latency performance depletes.

6.2.3 Variation of Residual Node percentage with time

0 100 200 300 400 500 600 700 800 900 1000 11000

20

40

60

80

100

120

Time of simulation in millis econds

P e r c e n

t a g e o

f N o

d e s

A l i v e

Residual Nodes Vs Time

Figure 20

0 10 20 30 40 50 60 70 80 0

0.5

1

1.5

2

2.5

Duty Cycle Percentage

No. of Packets per unit time




Figure 20 is a plot of the lifetime of the network. A thirty node WSN has been simulated

with simulation parameters mentioned above. This curve is subjective to various conditions

like the geographical distribution of the nodes and the activity in the area employing WSN.

The important observation we can make is that almost all the nodes have survived till 80% of

the WSN lifetime. It goes to show good synchronization among the neighboring nodes. The

shape of the above curve heavily depends on the state of the WSN. So we cannot standardize

this as the way the WSN dies. This is just one of the ways.

6.3 Simulation of ASMAC protocol

6.3.1 Power Consumption Comparison

Figure 21

The curve is a plot of average power consumption of a node against packets per unit

time. The x axis shows the activity of the network. When the packets are less frequent the

AS-MAC adapts to lower frame length and hence the power consumption is much lesser than

the S-MAC. As packets per unit time increases, the frame length also increases and so is the

power consumption. Since a WSN is mostly inactive during its life time so we can say that

power conservation is more as compared to S-MAC. The power consumption is significantly

higher when activity is higher than about 15 packets per unit time. Though this should be

looked at qualitatively, as the values will differ with hardware and environment .When the




WSN is less active the performance of A-SMAC is significantly better than S-MAC with a

fixed duty cycle.

6.3.2 Packet loss analysis

Figure 22

One of the primary objectives for implementing AS-MAC was to prevent loss of data.

The AS-MAC performs remarkably better than S-MAC here. With packet loss of AS-MAC

and S-MAC being almost same when packets are less frequent the packet loss of S-MAC

increases at a greater rate than that of AS-MAC. S-MAC finds it difficult to keep up with the

messages generated. It may be possible that at very high rate even AS-MAC will show high

packet loss. This is particularly useful when WSN is deployed in a very dynamic situationwhere different data is generated very frequently. When the same kind of data is generated S-

MAC will do the job as there will be redundancy in data.




Chapter 7 Conclusion

The simulation of S-MAC protocol gave the expected results with improved energy

conservation, which is the primary issue in a WSN. Also in a WSN following S-MAC

protocol, there is a trade-off between energy efficiency and latency which varies according to

traffic conditions. The AS-MAC protocol when implemented takes care of both unnecessary

energy consumption as well as latency. It adds flexibility to the WSN in a manner not fully

provided by the conventional S-MAC protocol. Besides, ASMAC protocol provides a

solution to the secondary issue of packet loss. For the given simulation parameters the

ASMAC protocol was successfully implemented. Graphs were plotted using the various data

taken from the results of the simulation. Graphs were plotted for variation of power

consumption ,latency performance and packet loss with packets per unit time .From the

above results we can clearly say that ASMAC protocol provides better energy conserving

properties when compared to SMAC protocol.




REFERENCES

[1] John A. Stankovic, “Wireless Sensor Networks”, Department of Computer Science,

University of Virginia, Charlottesville, Virginia, June 19, 2006.

[2] Wei Ye, Member, IEEE, John Heidemann, Member, IEEE, and Deborah Estrin, Fellow,

IEEE, “Medium Access Control With Coordinated Adaptive Sleeping for Wireless Sensor

Networks” , IEEE/ACM TRANSACTIONS ON NETWORKING, VOL. 12, NO. 3, JUNE 2004.

[3] Dae-Suk Yoo, Su- Sung Park, Seung Sik Choi and Se Hwa Park ,“Dynamic S -MAC

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APCC, October 2008,Tokyo ,Department of Computer Science, University of Incheon.

Dohwa-dong Nam-gu Incheon, South Korea.

[4] Subah Ramakrishnan, Hong Huang, Manikande n Balakrishnan and John Mullen, “ Impact

of Sleep in a Wireless Sensor MAC Protocol”, 60th Vehicular Technology ConferenceIEEE

VTS Fall 60th Vehicular Technology Conference IEEE Vehicular Technology Conference

N o60 , Los Angeles CA, Klipsch School of Electrical Engineering, New Mexico State

University, Las Cruces, USA.

[5]Suresh Singh and C.S. Raghavendra, “PAMAS protocol with signaling for Adhoc

netwo rks”, Oregon State University,Corvallis .

[6] Tamer Nadeem and Ashok Agrawala, “ Performance of IEEE 802.11 based Wireless

Sensor Networks in Noisy Environments”, Department of Computer Science, University of

Maryland.

[7] Anton Bilos and David Hardy, “ Survey wireless sensor network MAC protocols”,

Technical University of Eindhoven, Eindhoven, The Netherlands.

[8] Ilker Demirkol, Cem Ersoy, and Fatih Alagöz, “MAC Protocols for Wireless Sensor

Networks: a Survey”.

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