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CHAPTER 1

MOBILE AD HOC NETWORK

1.1 INTRODUCTION

Mobile Ad hoc Network is a collection of wireless mobile devices

such as laptops, handheld digital devices, personal digital assistants and

wearable computers forming a temporary network without the aid of any

infrastructure or centralized administration. The vision of MANET (Corson

and Macker 1999) is to form a wireless Internet, where users can move

anywhere anytime with the rest of the world and by incorporating routing

functionality into mobile nodes. The Internet Engineering Task Force (IETF)

has created a MANET working group to standardize the Internet Protocol (IP)

routing protocol functionality suitable for wireless routing application within

both static and dynamic network topologies.

MANETs are self-organized and self-configured multi-hop wireless

networks where the structure of the network changes dynamically due to the

mobility of mobile nodes. MANETs are being increasingly used due to their

decentralized and dynamic nature as well as the fact that they do not require

any fixed, preexisting infrastructure. In general, there are two distinct

approaches for enabling wireless mobile units to communicate with each

other. They are Infrastructure-based Wireless Local Area Networks (Wireless

LANs) (Murthy and Manoj 2004) and Infrastructure-less Ad hoc Wireless

LANs (Jochen Schiller 2003).

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Infrastructure-based Wireless Network: Wireless networks have

traditionally been based on the cellular concept and relied on good

infrastructure support such as Access Point (AP) and backbone. Here mobile

devices communicate with access points as base stations connected to the

fixed network infrastructure. It is depicted in Figure 1.1(a).

Infrastructure-less Wireless Network: Figure 1.1(b) shows infrastructure-

less approach namely ad hoc wireless network, wherein the wireless devices

can communicate with each other, which is commonly known as a MANET.

A MANET is a collection of wireless nodes that can dynamically form a

network to exchange information without using any pre-existing fixed

network infrastructure.

Figure 1.1 MANET Approaches

Like wired Local Area Networks, a Wireless LAN has a

communication range typical of a single building or a cluster of buildings, i.e.,

100–500 meters. A Wireless LAN should satisfy the same requirements

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typical of any LAN, including high capacity, full connectivity among attached

stations and broadcast capability. However, to meet these objectives, Wireless

LANs should be designed to face some issues specific to the wireless

environment, such as security on the air, power consumption, mobility and

bandwidth limitation of the air interface. Infrastructure-based Wireless LANs

require a special node called Access Point, which hosts or terminal connects

via existing wired LANs. APs can act as a router and arbiter between

mobile devices and the rest of the networks. This approach is used in

Wireless-Fidelity (Wi-Fi) hotspots to provide wireless Internet access at

coffee shops, airports, conferences and other public places.

The set of mobile nodes that are associated with a particular

AP is called the Basic Service Set (BSS). A number of BSSs can be

connected together by means of a backbone network to form an Extended

Service Set (ESS), in order to extend the Wi-Fi coverage area. In ESS, every

AP is given with the same service set identifier, which serves as a

network “name” for the network users. In many dynamic environments such

as disaster sites, battlefields and temporary conference meetings where

people and/or vehicles need to be temporarily interconnected, it may be

difficult and/or expensive to deploy infrastructure-based Wireless LANs.

For these environments, infrastructure-less or ad hoc Wireless LANs

provide a viable alternative solution.

Ad hoc Wireless LANs do not need any fixed infrastructure

and require only the mobile nodes to cooperate in a peer-to-peer fashion to

form a temporary network in order to exchange data. However, this

configuration of the Institute of Electrical and Electronics Engineering (IEEE)

802.11 standard is limited to single-hop communication which is only

applicable to mobile nodes within a mutual transmission range.

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Due to increase in the processing power and transceiver capability

of the mobile nodes, it has become feasible to increase the communication

range of the temporary network using the mobile nodes itself as forwarding

agents and relying on the upper layers of the protocol stack for multi-hop

path formation. Therefore, mobile nodes acting as routers and it may form

the backbone of a spontaneous network that extends the range of the ad

hoc wireless LAN beyond the transmission radius of the source. This

latter category of ad hoc wireless LANs is popularly referred as MANET.

The MANET is characterized by energy constrained nodes,

bandwidth constrained links and dynamic topology. In real-time applications,

such as audio, video, and real-time data, the ad hoc networks need Quality of

Service (QoS) in terms of delay, bandwidth, and packet loss is important.

Providing QoS in ad-hoc networks is a challenging task because of the

dynamic nature of network topology and imprecise state information. Hence it

is important to have a dynamic routing protocol with fast re-routing

capability, which also provides stable route during the life-time of the flows.

The infrastructure-less approach is increasingly becoming a very important

part of communication technology, because in many contexts, information

exchange between mobile units cannot rely on any fixed network

infrastructure, but on rapid configuration of wireless connections on-the-fly.

In Infrastructure-less network, a MANET consists of a set of

mobile nodes that may communicate with one another from time to time

where no base stations are present. In MANET, each host is equipped with a

Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA) (Crow et al

1997) transceiver. In such cases, a mobile node may communicate with each

other directly or indirectly. If it is an indirect communication, a multi-hop

scenario occurs, where the packets originated from the source host are relayed

by several intermediate mobile nodes before reaching the destination. The

traffic types in ad hoc networks are quite different from those in an

infrastructure-based wireless network, including:

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Peer-to-Peer: Communication between two nodes that are within

one hop. The peer-to-peer is shown in Figure 1.2.

Figure 1.2 Peer-to-Peer Communication

Remote-to-Remote: Figure 1.3 depicts the communication between

two nodes beyond a single hop but which maintain a stable route

between them. This may be the result of several nodes staying

within the communication range of each other in a single area or

possibly moving as a group. The traffic is similar to standard

network traffic.

Figure 1.3 Remote-to-Remote Communication

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There are various types of communications based on the type of

recipients namely unicasting, anycasting, multicasting and broadcasting.

Unicasting: One to one communication is called unicasting.

In unicasting, the packet is transmitted to a specific recipient.

Anycasting: From a single node to any one node, out of a set of

nodes.

Multicasting: Multicasting is a limited case of broadcasting in

which a subset of nodes from the entire nodes participates and

communicates among them. It is the transmission of datagrams

to a group of nodes identified by a single destination address.

Broadcasting: One to all communication is referred to as

broadcasting. The packet is transmitted to each and every node

of the network. Broadcasting is often used as a building block

for route discovery in on-demand ad hoc routing protocols. For

designing the ad hoc networks, one of the primary goal is to

reduce the broadcast overhead (redundant retransmission,

channel contention and packet collision) while reaching all the

nodes in the network.

The nodes in the MANET are not only acting as hosts but also as

routers that route data to or from other nodes in a network. In MANET, where

there is no infrastructure support as is the case with wired network since a

destination node might be out of range of a source node transmitting packets,

a routing procedure (Ramanathan and Steenstrup 1996) is always needed to

find a path as it forwards the packets appropriately between the source and the

destination. Network-wide broadcasting (Jetcheva et al 2001) provides route

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establishment and control functionality for a number of unicast and multicast

protocols.

1.1.1 MANET Characteristics

A MANET is an autonomous system consisting of mobile nodes

which are free to move in a random manner. In MANET, mobile nodes are

equipped with wireless transceivers to communicate with each other. MANET

has the following salient characteristics (Corson and Macker 1999):

1. Dynamic Network Topologies: In MANET, mobile nodes

location changes as they move around causing the network

topology. Typically the multi-hop may change arbitrarily and

rapidly at un predictable times.

2. Bandwidth constrained variable constraints links: The

bandwidth of the wireless links in MANET are significantly

lower than the wired links. MANET has relatively low

bandwidth links, high bit error rates, unstable and asymmetric

links.

3. Energy constrained operations: Mobile devices such as

laptops and personal digital assistants rely on battery power

for their energy sources. Mobile devices act as both an end

system and a router for forwarding operation at the same time.

The important tasks of mobile devices in MANET are

wireless signal transmission, retransmission and broadcasting

operations which require battery power.

4. Physical Security: Wireless networks are more prone to the

physical security threats of eavesdropping, denial-of-service

and routing attacks when compared to wired networks.

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1.1.2 MANET Features

Regardless of the application, there are certain features that can

determine the effectiveness and efficiency of a MANET. MANETs share

many of the properties of wired-infrastructure LANs and it posses the

following unique features (Corson and Macker 1999):

1. Autonomous nodes: In MANET, each mobile node is an

autonomous node, which may function as a host and also as a

router that relays packets along network paths.

2. Distributed operation: The nodes in a MANET should

associate among itself and each node act as a relay to

implement routing and security functions. Since there is no

background network for the central control of the network

operations, the control and management of the network must

be distributed among nodes.

3. Multi-hop routing: When delivering packets from a source

to its destination (when the sender node are not directly

connected with destination node), the packets should be

forwarded via one or more intermediate nodes.

4. Infrastructure-less network: The MANET is an autonomous

system of mobile nodes that are connected without any

infrastructure or centralized administration. Each node also

acts as a router for forwarding message to other nodes that

may not be within the same transmission range.

5. Heterogeneity: The scope of MANET applications show that

the number of mobile nodes can range from small number of

nodes to tens of thousands of nodes. The size, memory,

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computational power and battery power of theses mobile

nodes are different from one another.

1.1.3 Applications of MANET

With the increase of portable devices as well as progress in wireless

communication, MANET is gaining more importance with the increasing

number of widespread applications. MANET can be applied anywhere where

there is little or no communication infrastructure or the existing infrastructure

is expensive or inconvenient to use.

Significant examples of MANETs include establishing survivable,

efficient and dynamic communication for emergency/rescue operations,

disaster relief efforts, and military networks. Such network scenarios cannot

rely on centralized or organized connectivity, but it can be conceived as

applications of MANET. However, MANETs are not solely intended for

disconnected autonomous operations or scaled scenarios (i.e. hundreds or

even thousands of cooperation wireless nodes in a region). They may be used

as hybrid infrastructure extensions in fixed infrastructure operations. A hybrid

infrastructure extension is a dynamic enhancement to a home or campus

wireless networking environment. It provides an extended service and allows

low-cost, low complexity dynamic adjustments to provide coverage regions

and range extensions away from the more fixed infrastructures networks.

The majority of applications for the MANET technology are in

areas where rapid deployment and dynamic reconfiguration are necessary and

the wired network is not available. There are numerous scenarios that do not

have an available network infrastructure and could benefit from the creation

of an ad hoc network. Typical applications include (Corson and Macker

1999):

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Military battlefield: MANET would allow the military to take

advantage of commonplace network technology to maintain an

information network between the soldiers, vehicles, and military

information head quarters.

Rescue/Emergency operations: Rapid installation of a

communication infrastructure during a natural/environmental

disaster that demolished the existing communication

infrastructure like telephone lines, backbones and access points.

Commercial sector: Emergency rescue operations (like fire,

flood, earthquake, etc.,) must take place where non-existing or

damaged communications infrastructure need rapid deployment

of a communication network.

Local level: MANET can autonomously link an instant and

temporary multimedia network using notebook computers or

palmtop computers to spread and share information among

participants at a conference or classroom.

Personal Area Network (PAN): Short-range MANET can

simplify the intercommunication between various mobile

devices such as Personal Digital Assistants (PDAs), a laptop,

and a cellular phone.

Educational classrooms: Simple installation of a

communication infrastructure to create an interactive classroom

on demand.

Commercial projects: Simple installation of a communication

infrastructure for commercial gatherings such as conferences,

exhibitions, workshops and meetings.

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1.1.4 Challenges Faced in MANET

Regardless of the attractive applications, the features of MANET

introduce several challenges that must be studied carefully before a wide

commercial deployment can be expected. These include (Corson and Macker

1999):

Routing: Since the topology of the network is constantly

changing, the issue of routing packets between any pair of nodes

becomes a challenging task. Most protocols should be based on

reactive routing instead of proactive.

Security and Reliability: MANET has security problems

particularly due to nasty neighbour relaying packets. Further,

wireless link characteristics introduce reliability problems,

because of the limited wireless transmission range, the

broadcast nature of the wireless medium (e.g. hidden terminal

problem), mobility-induced packet losses, and data transmission

errors.

Quality of Service (QoS): Providing different quality of service

levels in a constantly changing environment will be a challenge.

Internetworking: The coexistence of routing protocols, for the

sake of internetworking a MANET with a fixed network, in a

mobile device is a challenge for the mobility management.

Power Consumption: For most of the lightweight mobile

terminals, the communication-related functions should be

optimized for less power consumption (Toh 2001).

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1.1.5 MANET Mobility Models

To thoroughly and systematically study a new MANET protocol, it

is important to evaluate its performance. Protocol simulation (Kurkowski et al

2005) has several key parameters, including mobility model and

communicating traffic pattern, among others. The Mobility model (Lin et al

2004) is designed to describe the movement pattern of mobile users, and how

their location, velocity and acceleration change over time. Since mobility

patterns may play a significant role in determining the protocol performance,

it is desirable for mobility models to emulate the movement pattern of

targeted real life applications in a reasonable way. Camp et al (2002)

described seven different synthetic entity mobility models for ad hoc

networks which are listed below:

Random Walk Mobility Model (including its many

derivatives): A simple mobility model based on random

directions and speeds.

Random Waypoint Mobility Model: A model that includes

pause times between changes in destination and speed.

Random Direction Mobility Model: A model that forces

mobile nodes to travel to the edge of the simulation area before

changing direction and speed.

A Boundless Simulation Area Mobility Model: A model that

converts a two dimensional rectangular simulation area into a

torus-shaped simulation area.

Gauss-Markov Mobility Model: A model that uses one tuning

parameter to vary the degree of randomness in the mobility

pattern.

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A Probabilistic Version of the Random Walk Mobility

Model: A model that utilizes a set of probabilities to determine

the next position of an MN.

City Section Mobility Model: A simulation area that represents

streets within a city.

1.2 MANET AND OSI REFERENCE MODEL

The International Organization for Standardization (ISO)

proposed the Open System Interconnection (OSI) reference model (Day and

Zimmerman 1983). In OSI reference model, there are seven layers: physical,

datalink, network, transport, session, presentation and application layer. It is

primarily designed to enable multi-vendor computers to interact and

communicate. OSI defines seven layers in a hierarchy that goes from physical

to application layers as shown in Figure 1.4. OSI is still a reference model,

often used to describe and outline the different levels of networking

protocols and their relationships with each other. The communication

mechanisms of a MANET are mainly associated with the protocols operating

at layers 1 to 3 of the OSI reference model.

1.2.1 Physical Layer

The IEEE 802.11 standards (Crow et al 1997) specify a Physical

Layer (PHY) and Medium Access Control (MAC) layer for Wireless LAN.

The physical layer serves as an interface between the MAC sub layer and

wireless medium where frames are transmitted and received. The physical

layer variants include the provision of Clear Channel Assessment (CCA),

which is needed for sensing the wireless medium and indicating to the MAC

layer when a signal is detected or when the channel is busy. The Physical

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Layer is subdivided into the Physical Layer Convergence Protocol (PLCP)

and Physical Medium Dependent (PMD) sub layer.

Application layer

Network layer

Presentation layer

Session layer

Transport layer

PHYSICAL LAYER

802.11

FHSS

PHY

802.11

DSSS

PHY

802.11 a

OFDM

PHY

802.11b

HR/DSSS

PHY

Data link

Layer

LOGICAL LINK CONTROL LAYER

(LLC)

MEDIUM ACCESS CONTROL SUB

LAYER (MAC)

Figure 1.4 The OSI Reference Model and its Relationship with MANET

The PLCP sub layer provides a carrier channel assessment and

provides a common PHY Service Access Points (SAP) with 1 / 2 Mbps

transfer rate. Finally the Physical Medium Dependent (PMD) sub layer

handles modulation and encoding and decoding of signals.

The IEEE 802.11 wireless standard support three different physical

layers: one layer based on Infra Red (IR) (Kahn and Barry 1997) and other

two layers based on radio transmission. Radio based standards operate within

the 2.4 Giga Hertz (GHz) ISM band. The 802.11 standard defines data rates of

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1 Mbps and 2 Mbps via radio waves using Frequency Hopping Spread

Spectrum (FHSS) and Direct Sequence Spread Spectrum (DSSS).

Using FHSS techniques, the 2.4 GHz band is divided into 75

channels of 1 MHz each. The sender and receiver agree on a hopping pattern,

and data is sent over a sequence of the sub channels. Each conversation within

the 802.11 network occurs over a different hopping pattern, and the patterns

are designed to minimize the chance of two senders using the same sub

channel simultaneously. The standard specifies Gaussian Shaped Frequency

Shift Keying (GSFSK) modulation scheme for the FHSS PHY. For 1 Mbps a

2 level Digital GSFSK (Chang and Shiu 2006) is used, (1 bit is mapped to one

frequency) a 4 level GFSK for 2 M bit/s (i.e., 2 bit are mapped to one

frequency).

Direct Sequence Spread Spectrum (DSSS) is the alternative spread

spectrum techniques separated by code and not by frequency. In 802.11,

DSSS spreading is achieved using a chipping sequence. IEEE 802.11 DSSS

PHY also uses a 2.4 GHz ISM band and offers both 1 and 2 Mbps data rates.

The system uses Differential Binary Phase Shift Keying (DBPSK) (Lo et al

1993) for 1 Mbps transmission and Differential Quadrature Phase Shift

Keying (DQPSK) for 2 Mbps as modulation scheme.

1.2.2 Datalink Layer

The IEEE 802.11 standard specifies a MAC layer and a Physical

Layer for Wireless LANs. The MAC layer provides to its users both

contention-based and contention-free access control on a variety of physical

layers. The basic access method in the IEEE 802.11 MAC protocol is the

Distributed Coordination Function (DCF), which is a CSMA/CA MAC

protocol. Besides the DCF, the IEEE 802.11 also incorporates an alternative

access method known as the Point Coordination Function (PCF). As the PCF

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access method cannot be adopted in ad hoc networks. In wireless network, the

sender may apply carrier sense and detect an idle medium. The sender starts

sending but a collision happens at the receiver due to a second sender. The

sender detects no collision, assumes that the data has been transmitted without

errors, but actually a collision might have destroyed the data at the receiver.

Thus, this common MAC Scheme from wired network fails in a wireless

network. The major challenge of the MAC sublayer is the hidden node

problem (Crow et al 1997).

Figure 1.5 Hidden Node Problem

The Hidden Node Problem is depicted in Figure 1.5, when node A

wants to transmit a frame to node B, node C is not aware of this transmission

due to its distance from node A (node C is a hidden terminal for node A). If

node C simultaneously transmits a frame to node B, a collision occurs at node

B. In wireless network, many MAC protocols (Xu and Saadawl 2002) have

been designed to solve this hidden node problem through CSMA/CA scheme.

This scheme is sender initiated including an exchange of channel reservation

control frames between the communicating nodes prior to data transmission.

In this case, all the neighbouring nodes of a given communicating node need

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to be informed that the channel will be occupied for a time period. As shown

in Figure 1.5, node A wishing to transmit a data packet to node B, node A

first broadcasts a Request To Send (RTS) control frame (Ju et al 2003)

containing the length of the data and address of node B. Upon receiving the

RTS, node B responds by broadcasting a Clear To Send (CTS) control frame

containing the length of data and address of node A. Any node overhearing

either of these two control frames remains silent for the entire transmission

period.

Considering the situation in wireless networks, where the exposed

terminal problem occurs when a node is prevented from sending packets to

other nodes due to a neighbouring transmitter. As shown in Figure 1.6, four

nodes labeled R1, S1, S2, and R2, where the two receivers (R1 and R2) are

out of range of each other, yet the two transmitters (S1 and S2) in the middle

are in range of each other. Here, if a transmission between S1 and R1 is

taking place, node S2 is prevented from transmitting to R2 as it concludes

after carrier sense that it will interfere with the transmission by its neighbour

S1. However, it is to be noted that R2 could still receive the transmission of

S2 without interference because it is out of range from S1. This is known as

Exposed Terminal Problem, depicted in Figure 1.6.

Figure 1.6 Exposed Terminal Problem

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1.2.3 Network Layer

Residing above the data link layer, the network layer (Basagani

et al 2004) is responsible for routing packets from source to destination. In

wired network, routing paths are usually fixed since nodes are static.

However, in wireless networks, mobile nodes are free to move, resulting in

frequently changed routing paths. The wireless network layer hence needs to

take care of mobility management, such as location tracking or update

information, which is not considered in static wired networks. Network layer

provides end-to-end transmission service. Due to the dynamic nature of

MANET, the destination node may not be located within the transmission

range of the source node. Therefore, the need for a routing protocol to set up

route from source to destination and a mechanism to maintain the route, since

some links may be broken due to node mobility. The important task of

network layer includes the exchange of routing information, finding a route to

a destination and providing efficient utilization of the available

communication bandwidth of the network. As a result, a good routing

protocol should be able to solve the above issues with a low communication

overhead.

1.3 BROADCASTING IN MANET

Network-wide broadcasting (Jetcheva et al 2001) means one node

sends a packet to all other nodes in a network. Broadcasting is a building

block for unicast routing protocols such as Dynamic Source Routing(DSR)

(Johnson et al 2007), Ad hoc On-Demand Distance Vector(AODV) (Perkins

2003), Zone Routing Protocol(ZRP)(Haas et al 2002), and Location Aided

Routing(LAR) (Ko and Vaidya 1998) use broadcasting to establish routes for

delivery of packets. A MANET consists of a set of wireless mobile hosts that

may communicate with one another from time to time. In MANET, no base

stations are supported for communication among the mobile hosts. Each host

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is equipped with a CSMA/CA transceiver. In such an environment, a host

may communicate with another directly or indirectly. In the latter case, a

multi-hop scenario occurs, where the packets originated from the source host

are relayed by several intermediate hosts before reaching the destination.

Hence it is important to carefully design an efficient broadcasting method so

as to avoid redundancy in the dissemination process for discovering the route

in the multi-hop network. Broadcasting is a major communication primitive

required by many applications and protocols in MANETs. Traditional

broadcast method often causes unproductive and harmful bandwidth

congestion and wastes node resources.

Further, broadcasting is frequently used to discover and advertise

resources. A simple example of resource discovery is the route discovery in

many reactive routing protocols. Broadcasting is also frequently used to

distribute content to all network participants, such as alarm signals or

announcements. In highly dynamic scenarios, broadcasting serves as a robust

way of realizing other communication primitives, such as multicast.

1.3.1 Design Issues

MANET consists of a set of mobile hosts that may communicate

with each other without any infrastructure and centralized administration.

Each mobile node is equipped with wireless transceiver which can access the

air medium. The nature of broadcasting is spontaneous; any mobile node can

issue a broadcast operation at any time. Collision avoidance is difficult in

overcoming the hidden node problem, where a node is able to determine

whether its neighbours are busy receiving transmissions from an uncommon

neighbour.

The 802.11 MAC (I. C. S. L. M. S. Committee 1997) utilizes a

Request To Send (RTS) / Clear To Send(CTS) / Data / Acknowledgement

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(ACK) procedure used for hidden node problem when unicasting packets.

However, the RTS/CTS/Data/ACK procedure is too difficult to implement for

broadcast packets as it would be difficult to coordinate. Therefore, the only

requirement made for broadcasting nodes is that they assess a clear channel

before broadcasting. Unfortunately, clear channel assessment does not prevent

collisions from hidden nodes. Additionally, no recource is provided for

collision when two neighbours assess a clear channel and transmit

simultaneously.

Suppose a source node originates a broadcast packet, the radio

waves propagate at the speed of light, all neighbours will receive the

transmission simultaneously and also rebroadcast the packet at the same time.

To overcome this problem, broadcast protocols jitter the scheduling of

broadcast packets from the network layer to the MAC layer by some uniform

random amount of time. This (small) offset allows one neighbour to obtain

the channel first, while other neighbours detect that the channel is busy (clear

channel assessment fails). The jitter time can be relatively small, just enough

to allow one node access to the channel before the others.

Broadcasting protocols require a node to keep track of the

redundant packets received over a short time interval in order to determine

whether to rebroadcast or not. That time interval is termed as Random Delay

Time (RDT), it is randomly chosen from a uniform distribution between 0 and

Tmax seconds, where Tmax is the highest possible delay interval. This delay in

transmission accomplishes two things. First, it allows nodes sufficient time to

receive redundant packets and assess whether to rebroadcast. Second, the

randomized scheduling prevents the collisions.

An important design consideration is the implementation of the

random delay time. One approach is to send broadcast packets to the MAC

layer after a short random time similar to the jitter. In this case, packets

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remain in the InterFace Queue (IFQ) until the channel becomes clear for

broadcast. While the packet is in the IFQ, redundant packets may be received,

allowing the network layer to determine if rebroadcasting is still required. If

the network layer protocol decides the packet should not be rebroadcast, it

informs the MAC layer to discard the packet.

A second approach is to implement the random delay time as a

longer time period and keep the packet at the network layer until the RDT

expires. Retransmission assessment is done considering all redundant packets

during the RDT. After RDT expiration, the packet is either sent to the MAC

layer or dropped. No attempts are made by the network layer to remove the

packet after sending it to the MAC layer. Each broadcast protocol requires

node to rebroadcast a given packet not more than one time in order to prevent

infinite transmission loops. Thus, each broadcast protocol requires that node

cache the original source node ID of the packet and the packet ID. It allows

the protocol to uniquely identify each broadcast packet and assign appropriate

behavior upon reception of a packet. This characteristic strictly avoids the

infinite loops behavior of broadcasting.

1.3.2 Applications of Broadcasting

Content Distribution Applications

A typical application for broadcasting in MANETs is news

spreading. Examples are the broadcasting of aid information in a disaster area

to coordinate relief actions (e.g. fire fighting (Karumanchi et al 1999)), the

dissemination of parking availability information in a city scenario,

dissemination of accident information in Vehicle Ad hoc NETwork (VANET)

and the dissemination of alarms and announcements. Further content

distribution applications are publish-subscribe applications, where some

nodes are subscribers to content providers. These applications typically run in

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the background for a few hours or even a few days. Examples are Usenet-on-

fly (Becker et al 2002), latency insensitive data (Su et al 2004) and file

sharing in a peer-to-peer (P2P) manner (Lindemann and Waldhorst 2005).

Resource Discovery and Advertisement

Further typical broadcast scenarios are resource (or service)

discovery and advertisement. MANET nodes may have little or no knowledge

at all about the capabilities and services offered by each other. Therefore,

mechanisms for resource discovery or advertisement are important for these

self-configurable networks. Due to the decentralized and highly dynamic

nature of MANETs, service discovery and service advertisement frequently

use broadcasting strategies. An example of resources is a multi-hop routing

path to a given destination. For highly dynamic topologies the route is

continuously changing and the resource is so highly dynamic that maintaining

a route to all nodes at every time is very costly. However, most of the time, it

is not necessary to have an up-to-date route to all other nodes. Hence, a novel

class of reactive routing protocols, such as DSR and AODV has been

developed. Reactive routing protocols only set up routes to nodes they

communicate with and these routes are kept alive as long as they are needed.

This is realized by a route discovery mechanism, which uses broadcasting

strategies to distribute control messages for route discovery.

Sensor Data Dissemination

Another important application of broadcasting is the sensor data

dissemination. Real-time sensor data may be disseminated to other nodes in

order to realize a fully-replicated database, where every database node has a

consistent view of real time events. Data consistency algorithms act on

disseminated observation data and chronologically order observed events in

MANETs.

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1.4 PROBLEM DEFINITION

Broadcasting is a communication primitive and important network

service for routing protocols in MANET. On-demand routing protocols such

as DSR and AODV use flooding as the basic mechanism to propagate

control message namely Route REQuest (RREQ) for determining the route

in the MANET. This simplest mechanism is known as simple flooding.

Despite its simplicity, simple flooding generates excessive amount of

redundant retransmission, channel contention and packet collision. Such a

phenomenon is referred to as broadcast storm problem and it can lead to a

collapse in the operation of the network, since it requires every node

involved to retransmit the message. Broadcasting a large number of RREQ

packets may guarantee a high chance of discovering routes to the

destinations. But, this will consume a lot of energy resources of mobile node

and cause congestion of the network. Therefore, a route discovery method

that can guarantee an efficient utilization of these limited system resources

while achieving acceptable levels of other performance metrics such as

overhead and power consumption is required.

In most existing probabilistic approaches of network-wide

broadcasting, the rebroadcast probability at a given node is fixed. Here, every

node in the network has the same probability to rebroadcast a packet

regardless of the number of its neighbouring nodes. This could lead to poor

reachability or more number of redundant retransmissions. In counter-based

scheme, packets received at a node are less than that of a threshold value.

However, most of the existing solutions are inadequate in reducing the

number of redundant broadcast while still guarantee that most nodes receive

the packet. In some cases, the schemes require near topological information or

used additional hardware devices for distance measurement or location

identification in order to reduce the redundant transmissions. The aim of this

24

thesis is to propose an efficient probabilistic scheme for MANET in order to

reduce the broadcast overhead and alleviate the broadcast storm problem.

1.5 ABOUT THE PROPOSED WORK

Simple flooding and Probability flooding are the broadcast methods

to provide a solution for network-wide broadcasting but consume more

network resources. Further k-means clustering based flooding is also

evaluated and its performance is compared with the simple and probability

flooding methods. Ideally, the rebroadcast probability p should be high in a

node located in a sparse region and relatively low in a node located in a dense

region. If the rebroadcast probability p is too low then the reachability might

be poor while if the rebroadcast probability p is set too high then many

redundant rebroadcasts might be generated.

In this work, an optimum density based model for probability

flooding is proposed and its performance is compared with the existing

broadcasting methods. This research work suggests and investigates the

performance of a new method namely optimum density based model for

probability flooding where the rebroadcast probability is determined at a

transmitting node densities and as well as its previous neighbour node’s

densities instead of assigning fixed probability for each node. The primary

goal of this research is to study the existing protocols for flooding through

network simulator-2(ns-2) simulations (Hogie et al 2006) and device a new

and efficient broadcasting method which will cause minimum broadcast

overhead in the MANET.

Secondly, the another new method namely Flooding Reduced-

Destination Sequenced Distance Vector routing protocol is proposed for

reducing the overhead due to periodic and triggered update messages using

optimum density based probability flooding in Destination Sequenced

25

Distance Vector routing protocol. By doing an extensive literature survey, the

resources for doing the proposed research on ns-2 and the required knowledge

will be acquired from various resources. First a MANET with required

number of nodes with suitable mobility and transmission power is proposed to

be constructed on ns-2. This ns-2 is widely used by academia and the research

community for simulation of MANETs and ad hoc routing protocols and can

effectively simulate an 802.11 network. The CMU/Rice University Monarch

group has made extensions to ns-2 to support mobility including ability to

simulate multi-hop wireless ad-hoc networks. The key reason for selecting ns-

2 is the fact that it supports the use of co-ordinate system by default and

without major modification should serve our purposes. The other advantage is

the simulator code for most protocols. The researchers are interested in

comparing exist for ns-2 which are freely available. A suitable version of

ns-2 is proposed to be installed on Red Hat Linux 9 for implementing the

idea. While evaluation, the various standard metrics are proposed to be used

for selecting suitable protocols. Suitable trace files will be used for measuring

various performances. The trace files will be filtered and analyzed by using

suitable perl scripts or grep utility to find the performance. Finally, based on

the information gathered from the trace files, various graphs will be prepared

for visualizing the performance of the flooding algorithms.

1.6 CONTRIBUTIONS OF THIS RESEARCH

A detailed study and analysis of the impact of broadcast mechanism

in routing protocols on network performance is done. This analysis obviously

proved the problems due to the overhead of broadcast or flooding algorithms

in their design.

An improved version of broadcast algorithm based on K-means

clustering algorithm is proposed and evaluated with suitable network

26

scenario. The performance of cluster based broadcasting is compared with the

simple flooding and probability flooding method.

This research proposes a density based broadcast approach namely

optimum density based model for probability flooding for reducing the

broadcast overhead in MANET. In several previous works, particularly in

probability based flooding schemes, the rebroadcast of a message will be done

only based on the density of a particular transmitting node. But in this

algorithm, each node will forward or rebroadcast a message based on its

density as well as its previous neighbour node’s density. This characteristic

makes this algorithm most unique and produced excellent improvements in its

performance. The simulation results reveal that the new approach improves

the route discovery process by reducing the Medium Access Control (MAC)

load, routing load, collision and power consumption when compared to

simple and probability flooding.

This research work also proposes a new method namely Flooding

Reduced-Destination Sequenced Distance Vector routing protocol for

reducing the broadcast overhead due to periodic and triggered update

messages in Destination Sequenced Distance Vector routing protocol. The

simulation results reveal that the new approach FR-DSDV improves the

network performance by reducing the Medium Access Control (MAC) load,

routing load, collision and power consumption when compared to DSDV.

1.7 LITERATURE SURVEY

1.7.1 Basic Routing Protocol Families

1.7.1.1 Distance Vector Routing Protocols

Elizabeth Royer and Chai-Keong Toh (1999) have described about

routing protocols of MANET. In distance vector routing protocols, every host

maintains a routing table containing the distance from itself to possible

27

destinations. Each routing table entry contains the next hop to the destination

and the distance to the destination. Nodes only feed the estimated link costs

for each destination (e.g. the number of hops to destination) to their

neighbours, instead of flooding the whole network. All nodes calculate the

shortest paths to the destinations using that broadcasted information.

1.7.1.2 Link State Routing Protocols

Elizabeth Royer and Chai-Keong Toh (1999) have briefed about

Link state routing protocols. These protocols keep a routing table for

complete topology, which is built up by finding the shortest path of link costs.

Link cost information is periodically transmitted and received by all nodes

using a flooding technique; these periodic floods are called Link State

Advertisements (LSA). Flooding means that a node sends out its information

to all other neighbour nodes and they forward all received information to their

neighbours and so on. Each node updates its table using the new link cost

information gathered from these floods.

1.7.1.3 Source Routing Protocols

Elizabeth Royer and Chai-Keong Toh (1999) presented about

source routing protocols. In source routing, all data packets carry their

routing information as their header. The originating node could learn this

routing information by means of a source routing protocol: When a node

receives a (broadcast) route request packet for a destination, it adds its own

address to the header and then forwards the packet. The destination uses the

recorded route in reverse order to send a route reply to the requesting node.

Thus, the originating node is provided with the complete route to the

destination. The routing decision is made at departure. Loops are avoided,

since nodes can determine if they are already in the packet header.

28

1.7.2 Types of Routing Protocols

Routing protocols are needed whenever delivered data packets need

to be handed over several nodes to arrive at their destinations. Routing

protocols have to find routes for packet delivery and make sure that the

packets are delivered to the correct destinations. The existing routing

protocols such as distance vector routing and link-state routing were

originally designed for static, wired networks and dynamic topology was not

considered. Mehran et al (2004) have classified the Routing protocols for ad

hoc networks into different categories.

Pro-active, re-active and hybrid

Centralized or distributed

Dynamic or static

As for the second criterion, when a routing protocol is centralized,

all decisions are made at a center node, whereas in a distributed routing

protocol, all nodes cooperate, usually in a symmetric way, in order to reach a

routing decision. The third criterion is concerned with the nature of the

information used for the routing process.

A dynamic protocol may change behavior according to the network

status, which can cause congestion on a link or many other possible factors. A

link may fail unexpectedly, or a new link may be added. A dynamic routing

protocol must discover these changes, automatically adjust its routing tables,

and inform other routers of the changes. The process of rebuilding the routing

tables based on new information is called convergence. Static protocols on the

other hand do not change when the network status changes and the changes

must be added manually. An example of a static protocol is flooding, in which

a node always retransmits an incoming packet, unless it has already sent the

same packet earlier.

29

Broadcasting may be used to disseminate data to all other nodes in

the network or may be used by MANET unicast or multicast routing protocols

to disseminate control information. For example, unicast protocols such as

DSR, AODV, ZRP and LAR use broadcasting to establish routes. Currently

these protocols all rely on a simple form of plain or blind flooding technique,

in which each node retransmits each received unique packet exactly one time.

The main role of broadcasting methods in routing protocols are route

discovery process, when advising an error message to erase invalid routes

from the routing table, or as an efficient mechanism for reliable multicast in a

fast-moving MANET. Generally speaking, currently known routing protocols

for ad hoc networks can be classified in three different classes: pro-active

protocols, re-active protocols and the hybrid protocols. These three classes

differ in a number of ways.

1.7.2.1 Proactive Routing Protocols

These routing protocols are similar to and come as a natural

extension of those for the wired networks. In proactive routing (Das et al

1999), each node has one or more tables that contain the latest information of

the routes to any node in the network. The two kinds of table updating in

proactive protocols are the periodic update and the triggered update. In

periodic update, each node periodically broadcasts its table in the network.

Each node just arriving in the network receives that table. In triggered update,

as soon as a node detects a change in its neighbourhood, it broadcasts entries

in its routing table that have changed as a result.

Proactive routing tends to waste bandwidth and power in the

network because of the need to broadcast the routing tables/updates.

Furthermore, as the number of nodes in the MANET increases, the size of the

table will increase; this can become a problem in and of itself. DSDV, which

is not to be suitable for large dense network. A route table at each node

30

enumerates all available destinations and the corresponding hop-count from

the node. Each route table entry is tagged with a sequence number, which is

created by a destination node. To maintain consistency of the route tables in a

dynamically changing network topology, each node transmits table updates

either periodically (periodic update) or when new significant information is

available (triggered update).

Routing information is advertised by broadcasting or multicasting.

The packets are transmitted periodically and incrementally as topological

changes are detected. Topological changes include movement of a node from

place to place or the disappearance of the node from the network. Information

about the time interval between arrival of the very first routing solution and

the arrival of the best routing solution for each particular destination is also

maintained.

On the basis of this information, a decision may be made to delay

advertising routes that are about to change, thus, reducing fluctuations in the

route tables. The advertisement of possible unstable routes is delayed to

reduce the number of rebroadcasts of possible route entries that normally

arrive with the same sequence number. Some of the Proactive routing

protocols are:

Destination-Sequenced Distance Vector (DSDV)

Wireless Routing Protocol (WRP)

Global State Routing (GSR)

Fisheye State Routing (FSR)

Source-Tree Adaptive Routing (STAR)

Distance Routing Effect Algorithm for Mobility (DREAM)

Multimedia Support in Mobile Wireless Network (MMWN)

31

Cluster-Head Gateway Switch Routing (CGSR)

Hierarchical State Routing (HSR)

Optimized Link State Routing (OLSR)

Topology Broadcast Reverse Path Forwarding (TBRPF)

Open Shortest Path First (OSPF)

1.7.2.2 Reactive Routing Protocols

Maltz et al (1999) presented on-demand routing protocols were

designed to reduce the overheads in proactive protocols by maintaining

information for active routes only. This means routes are determined and

maintained for nodes that require sending data to a particular destination.

Route discovery usually occurs by flooding a route request packets through

the network.

Reactive protocols can be classified into two categories: source

routing and hop-by-hop routing. In source routed on-demand protocols each

data packets carry the complete source to destination address. Therefore, each

intermediate node forwards these packets according to the information kept in

the header of each packet. This means that the intermediate nodes do not need

to maintain up-to-date routing information for each active route in order to

forward the packet towards the destination. Furthermore, nodes do not need to

maintain neighbour connectivity through periodic beacon messages.

The major drawback with source routing protocols is that in large

networks they do not perform well. This is due to two main reasons; firstly as

the number of intermediate nodes in each route grows, then so does the

probability of route failure. Secondly, as the number of intermediate nodes in

each route grows, then the amount of overhead carried in each header of each

packet will grow as well. Therefore, in large networks with significant levels

32

of multi-hoping and high levels of mobility, these protocols may not scale

well.

In hop-by-hop routing (also called as point-to-point routing) each

data packet only carries the destination address and the next hop address.

Therefore, each intermediate node in the path to the destination uses its

routing table to forward each data packet towards the destination. The

advantage of this strategy is that routes are adaptable to the dynamically

changing environment of MANETs, since each node can update its routing

table when receive fresher topology information and hence forward the data

packets over fresher and better routes. Using fresher route recalculations are

required during data transmission.

The disadvantage of this strategy is that each intermediate node

must store and maintain routing information for each active route and each

node may require to be aware of their surrounding neighbours through the use

of beaconing messages. Some of the reactive routing protocols are:

Ad hoc On-Demand Distance Vector (AODV)

Dynamic Source Routing (DSR)

Routing On-demand Acyclic Multi-Path (ROAM)

Light-Weight Mobile Routing (LMR)

Temporally Ordered Routing Algorithm (TORA)

Associated-Based Routing (ABR)

Signal Stability Adaptive (SSA)

Relative Distance Micro-discovery Ad hoc Routing (RDMAR)

Location-Aided Routing (LAR)

Ant-colony-based Routing Algorithm (ARA)

Flow Oriented Routing Protocol (FORP)

Cluster-Based Routing Protocol (CBRP)

33

1.7.2.3 Hybrid Routing Protocols

Hybrid protocols combine the advantages of both pro-active and re-

active routing, by locally using pro-active routing and inter-locally using re-

active routing. This is partly based on the assumption that most

communication in MANETs takes place between nodes that are close to each

other, and the assumption that changes in topology are only important if they

happen in the vicinity of a node. When a link fails or a node disappears on the

other side of the network, it has only effect on local neighbourhoods; nodes

on the other side of the network are not affected. The ZRP is an example of a

hybrid routing protocol.

In MANET, mobile nodes communicate with each other using

multi-hop wireless transceivers. There is no infrastructure such as base

stations. A big challenge in the design of ad hoc network is the development

of dynamic routing protocols that can efficiently find or establish the routes

between two communicating nodes. The routing protocol must be able to keep

up with the high degree of node mobility that often changes the network

topology frequently and unpredictably.

1.7.3 Broadcasting Methods

Ni et al (1999) classified several proposed broadcast algorithms in

two categories: deterministic and probabilistic. In a deterministic broadcast

protocol, the set of relay nodes is chosen deterministically to cover the entire

network. When a node receives a broadcast message, it will decide

deterministically whether to forward the message or not. The probabilistic

approaches require each node to rebroadcast the packet to its neighbours with

a given forwarding probability. The advantages of deterministic broadcast

schemes are low broadcast overhead and low potential packet collisions. In

34

contrast to deterministic broadcast, a probabilistic broadcast protocol usually

causes redundant retransmissions, which incurs relatively more overhead and

potential packet collisions. To solve the impact of the broadcast storm

problem several broadcast schemes have been proposed. Each of these

methods operates differently according to their inherent characteristics.

These methods can be broadly categorized into four groups as follows:

1. Simple Flooding

2. Probability Based Methods

a. Probabilistic Scheme

b. Counter-Based Scheme

3. Area Based Methods

a. Distance-Based Scheme

b. Location-Based Scheme

4. Neighbour Knowledge Methods

a. Flooding with Self Pruning

b. Scalable Broadcast Algorithm (SBA)

c. Dominant Pruning

d. Multipoint Relaying

e. Ad Hoc Broadcast Protocol (AHBP)

f. Connected Dominating Set (CDS)-Based Broadcast

Algorithm

g. Lightweight and Efficient Network-Wide Broadcast

(LENWB)

35

1.7.4 Simple Flooding Method

Ni et al (1999) described about the simple flooding with broadcast

storm problem. In simple flooding, source node broadcasts a packet to all its

neighbours. Each of those neighbours in turn rebroadcasts the packet it

receives the packet for the first time. This behaviour continues until all

reachable network nodes have received and rebroadcast the packet once. The

dissemination of packets in this way often consumes valuable network

resources such as bandwidth and node power due to redundant

retransmissions of broadcast packets. These redundant retransmissions of

packets cause high contention and collision in the network. This is known as

broadcast storm problem which can lead to a total collapse in the operation of

the network.

Jetcheva et al (2001) proposed in IETF internet draft, the use of

flooding as s simple protocol for broadcasting and multicasting in ad hoc

networks which are characterized by low node densities and/or high mobility.

Williams and Camp (2002) compared the performance of several proposed

broadcast approaches including the probabilistic, counter-based, area based

and neighbour knowledge methods.

1.7.5 Probability Based Methods

1.7.5.1 Probabilistic Scheme

Probability based method is one of the simplest and most efficient

broadcast method that have been suggested by Ni et al (1999). This method is

similar to simple flooding, except that nodes only rebroadcast with a pre

determined forwarding probability p so that every node has the same

probability to rebroadcast the message, regardless of its number of

neighbours.

36

The study of Ni et al (1999) and Tseng et al (2003) shown that the

probability scheme has poor reachability. The problem comes from the

uniformity of the algorithm; every node has the same probability to

rebroadcast the packet regardless of network topology.

Zhang and Agrawal (2005) suggested dynamic probabilistic

algorithm that combines the properties of probabilistic and counter-based

methods. In this method the forwarding probability at a node is set based on

the number of duplicate packets received at a node. But the values of a packet

counter at a node does not necessarily correspondent to the exact number of

neighbours of the node, since some of its neighbours may have suppressed

their rebroadcasts according to their local rebroadcast probability.

Cartigny and Simplot (2003) described a probabilistic scheme

where the forwarding probability p is computed from the local density n. The

authors have also introduced a fixed value parameter k to achieve high

reachability. This broadcast scheme has a drawback of being locally uniform. This

is because each node in the network determines its forwarding probability

based on the fixed efficiency parameter k which is not globally optimal.

1.7.5.2 Counter Based Scheme

In the counter-based techniques, when a node receives a broadcast

packet, it initiates a Random Assessment Delay (RAD) timer and a counter

which counts the number of received duplicate packets. When the timer

expires, if the counter exceeds the threshold value, the node assumes that all

its neighbours might have received the same packet and will not rebroadcast

the packet. Otherwise, the node will broadcast the packet. The predefined

counter threshold is the key parameter of this technique and it has been shown

in Tseng et al (2003) that transmission redundancy could be reduced by

37

choosing a threshold value between 2 and 4. The algorithm for counter-based

scheme is described as follows:

Algorithm: Counter-Based Scheme

Upon reception of a broadcast packet ‘m’ at a node X for the first time

- Initialize the packet counter C=1

- Set the RAD timer

- Wait for RAD timer to expire

- While waiting

For every duplicate packet m received

increment C by 1

- if (c < C) (here c is the counter threshold value)

Forward the packet m

else

Drop the packet m

End Algorithm.

1.7.6 Area Based Methods

Suppose a node receives a packet from a sender that is located only

one meter away. If the receiving node rebroadcasts, the additional area

covered by the retransmission is quite low. On the other side, if a node is

located at the boundary of the sender node’s transmission distance, then a

rebroadcast would reach significant additional area. Area based methods only

consider the coverage area of a transmission; they don’t consider whether

nodes exist within that area.

1.7.6.1 Distance-Based Scheme

Ni et al (1999) described the distance based scheme for reducing

the redundant rebroadcast, contention and collision. In distance-based

38

scheme, a node compares the distance between itself and each neighbouring

node that has previously forwarded a given packet. In this scheme, a node

upon reception of a broadcast packet for the first time initiates a RAD timer.

Before the expiration of the RAD timer, the node checks the location of the

senders of each received packet. If any sender is closer than a threshold

distance value (D), the node will not rebroadcast the packet. Otherwise, the

node rebroadcast it when the RAD expires. The algorithm for distance-based

scheme is described as follows:

Algorithm: Distance-Based Scheme

Upon reception of a broadcast packet m at a node X for the first time

- Initialize a RAD timer

- Before the timer expires:

Check the location of the sender of packet m.

- If the sender is closer than the threshold distance D

The packet m is dropped

else

Forward the packet m after the RAD expires

End Algorithm

Here, a node using the distance-based scheme requires the

knowledge of the geographical location of its neighbours in order to make a

rebroadcast decision. This can be achieved by Global Positioning System

(GPS) receiver, where nodes could include their location information in each

packet transmitted. Although distance-based scheme achieve high reachability

they suffer from high number of redundant broadcast packets because a node

that has received a broadcast many a time may still rebroadcast the packet if

all the neighbouring nodes transmission distances are greater than the

threshold value.

39

1.7.6.2 Location-Based Scheme

Tseng et al (2002) have described the location-based scheme,

where each node is expected to know its own position relative to the sender’s

position using geo-location technique such as GPS (Latiff et al 2005).

Whenever a node originates or forwards a broadcast packet it adds its own

location to the header of the packet. Upon the reception of a previously

unseen packet, the node initiates a waiting timer and accumulates the

coverage area that has been covered by the arrived packet. When the waiting

timer expires, if the accumulated coverage area is larger than a threshold

value, the node will not rebroadcast the packet. Otherwise, the node broadcast

the packet. The operation of the location-based scheme is summarized as

follows:

Algorithm: Location-Based Scheme

Upon reception of a broadcast packet m at a node X for the first time

- Initiate a waiting timer(RAD)

- Before the timer expires

Calculate the coverage area covered by the received packet m

- When the waiting timer expires:

- If the coverage area is larger than the threshold location L

The packet m is dropped

else

Forward the packet m

End Algorithm

Deterministic Schemes

Deterministic scheme requires some sort of topological knowledge

of the network to build a fixed backbone that guarantees full coverage of the

network for a broadcast operation. The topological knowledge of the network

is collected by maintaining information about nodes neighborhood via

40

periodic exchange of hello packets. This deterministic scheme uses only a

subset of nodes in the network to forward the broadcast packet. Williams and

Camp (2002) referred to this category as neighbour knowledge-based

algorithm. The descriptions of the various neighbour knowledge-based

schemes are presented below:

1.7.7 Neighbour Knowledge Method

1.7.7.1 Flooding with Self-Pruning

Lim and Kim (2000) and Wu and Dai (2003) presented the simple

neighbour knowledge-based method namely flooding with self-pruning. This

protocol requires that each node have knowledge of its one-hop neighbours,

which is obtained via periodic hello packets. A node includes its list of known

neighbours in the header of each rebroadcast packet. A node receiving a

broadcast packet compares its neighbour list to the sender’s neighbour list. If

the receiving node would not reach any additional nodes, it refrains from

rebroadcasting; otherwise the node rebroadcasts the packet.

4

56

1 2

7

3

Figure 1.7 Self Pruning Approach

In Figure 1.7, after receiving a message from node 2, node 1 will

rebroadcast the message to node 4 and node 3 as its only additional nodes. It

is noted that node 5 also will rebroadcast the same message to node 4

41

as it’s only additional node. In this situation still the message redundancy

takes place in the network.

1.7.8 Forwarding Neighbour Schemes

In forwarding neighbours schemes, the forwarding status of each

node is determined by its neighbours. Specifically, the sender proactively

selects a subset of its 1-hop neighbours as forwarding nodes. The forwarding

nodes are selected using a Connected Dominating Set (CDS) (Ivan et al 2002)

algorithm and the identifiers (IDs) of the selected forwarding nodes are

piggybacked on the broadcast packet as the forwarder list. Each designated

forward node in turn designates its own list of forward nodes before

forwarding the broadcast packet.

The Dominant Pruning algorithm is a typical example of the

forwarding neighbour’s schemes. Ideally, the number of forwarding nodes

should be minimized to decrease the number of redundant transmissions.

However, the optimal solution is NP-complete and requires that nodes know

the entire topology of the network.

1.7.8.1 Scalable Broadcast Algorithm

Peng and Lu (2000) presented the Scalable Broadcast Algorithm

(SBA). It requires that all nodes have knowledge of their neighbours within a

two hop radius. This neighbour knowledge coupled with the identity of the

node from which a packet is received allows a receiving node to determine if

it would reach additional nodes by rebroadcasting. 2-hop neighbour

knowledge is achievable via periodic "Hello" packets; each "Hello" packet

contains the node's identifier (IP address) and the list of known neighbours.

After a node receives a "Hello" packet from all its neighbours, it has two hop

topology information centered at itself.

42

1.7.8.2 Dominant Pruning

Dominant Pruning also uses 2-hop neighbour knowledge, obtained

via "Hello" packets, for routing decisions described in Lim and Kim (2000).

Unlike SBA, however, Dominant Pruning requires rebroadcasting nodes to

proactively choose some or all of its 1-hop neighbours as rebroadcasting

nodes. Only those chosen nodes are allowed to rebroadcast. Nodes inform

neighbours to rebroadcast by including their address as part of a list in each

broadcast packet header.

When a node receives a broadcast packet it checks the header to see

if its address is part of the list. If so, it uses a Greedy Set Cover algorithm to

determine which subset of neighbours should rebroadcast the packet, given

knowledge of which neighbours have already been covered by the sender’s

broadcast. The Greedy Set Cover algorithm, as adapted in Lim and Kim

(2000) recursively chooses 1-hop neighbours which cover the most 2-hop

neighbours and recalculates the cover set until all 2-hop neighbours are

covered.

1.7.8.3 Multipoint Relaying

Multipoint Relaying is presented in Qayyum et al (2000). It is

similar to Dominant Pruning in that rebroadcasting nodes are explicitly

chosen by upstream senders. For example, say Node A is originating a

broadcast packet. It has previously selected some, or in certain cases all, of it

one hop neighbours to rebroadcast all packets they receive from Node A. The

chosen nodes are called Multipoint Relays (MPRs) and they are the only

nodes allowed to rebroadcast a packet received from Node A. Each MPR is

required to choose a subset of its one hop neighbours to act as MPRs as well.

Since a node knows the network topology within a 2-hop radius, it can select

1-hop neighbours as MPRs that most efficiently reach all nodes within the

43

two hop neighbourhood. Qayyum et al (2000) propose the following

algorithm for a node to choose its MPRs:

They propose the following algorithm for a node to choose its

MPRs:

Find all 2-hop neighbours that can only be reached by one 1hop

neighbour. Assign those 1-hop neighbours as MPRs.

Determine the resultant cover set (i.e., the set of 2-hop

neighbours that will receive the packet from the current MPR

set).

From the remaining 1-hop neighbours not yet in the MPR set,

find the one that would cover the most 2-hop neighbours not in

the cover set.

Repeat from step 2 until all 2-hop neighbours are covered.

Multipoint Relaying is described in detail as part of the Optimized

Link State Routing (OLSR) (Clausen and Jacquet 2003) protocol defined by

an Internet draft. In this implementation, “Hello” Packets include fields for a

node to list the MPRs it has chosen. Anytime a node receives a “Hello”

packet, it checks if it is an MPR for the source of the packet. If so, it must

rebroadcast all data packets received from that source. Clearly, the update

interval for “Hello” packets must be carefully chosen and, if possible,

optimized for network conditions.

1.7.8.4 Ad Hoc Broadcast Protocol

Peng and Lu (2002) described Ad Hoc Broadcast Protocol (AHBP)

an approach similar to Multipoint Relaying. In AHBP, only nodes who are

designated as a Broadcast Relay Gateway (BRG) within a broadcast packet

44

header are allowed to rebroadcast the packet. BRGs are proactively chosen

from each upstream sender which is a BRG itself. The algorithm for a BRG to

choose its BRG set is identical to that used in Multipoint Relaying (see steps

1-4 for choosing MPRs).

AHBP differs from Multipoint Relaying in three ways:

A node using AHBP informs 1-hop neighbours of the BRG

designation within the header of each broadcast packet. This

allows a node to calculate the most effective BRG set at the

time a broadcast packet is transmitted. In contrast, Multipoint

Relaying informs 1-hop neighbours of the MPR designation via

“Hello” packets.

In AHBP, when a node receives a broadcast packet and is listed

as a BRG, the node uses 2-hop neighbour knowledge to

determine which neighbours also have received the broadcast

packet in the same transmission. These neighbours are

considered already “covered” and are removed from the

neighbour graph used to choose next hop BRGs. In contrast,

MPRs are not chosen considering the source route of the

broadcast packet.

AHBP is extended to account for high mobility networks.

Suppose Node A receives a broadcast packet from Node B, and

Node A does not list Node B as a neighbour (i.e., Node A and

Node B have not yet exchanged “Hello” packets). In AHBP-EX

(extended AHBP); Node A will assume BRG status and

rebroadcast the node. Multipoint relaying could be similarly

extended.

45

4

5

1 2

6

3

Figure 1.8 Ad hoc Broadcasting Approach

Figure 1.8 shows ad hoc broadcasting approach, node 2 has 1, 5 and

6 nodes as one hop neighbours, 3 and 4 nodes has two hop neighbours. Node

3 can be reached through node 1 as a one hop neighbour of node 2. Node 4

can be reached through node 1 or node 5 as one hop neighbours of node 2.

Node 2 selects node 1 as a gateway to rebroadcast the message to nodes 3 and

4. Upon receiving the message node 5 will not rebroadcast the message as it is

not a gateway.

1.7.8.5 Connected Dominated Set - Based Broadcast Protocol

Peng and Lu (1999) described the Connected Dominating Set

(CDS)-Based Broadcast Algorithm, a more calculation intensive algorithm for

selecting Broadcast Relay Gateways (BRGs). Where AHBP only considers

the source of the broadcast packet to determine a receiving node’s initial

cover set, CDS-based broadcast algorithm also considers the set of higher

priority BRGs selected by the previous sender.

For example, suppose Node A has selected Nodes B, C and D (in

this order) to be BRGs. When Node C receives a broadcast packet from Node

A, AHBP requires Node C to add neighbours common to Node A to the initial

cover set.

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CDS-Based Broadcast Algorithm also requires that Node C adds

neighbours common to Node B, because Node B is a higher priority BRG.

Likewise, Node D is required to consider common neighbours with nodes A,

B and C respectively. Once the initial cover set is determined, a node then

chooses which neighbours should function as BRGs. The algorithm for

determining this is the same as that for AHBP and Multipoint Relaying.

1.7.8.6 Lightweight and Efficient Network-Wide Broadcast

Sucec and Marsic (2000) described the Lightweight and Efficient

Network-Wide Broadcast (LENWB) protocol. LENWB relies on

2-hop neighbour knowledge obtained from “Hello” packets. However, instead

of a node explicitly choosing nodes to rebroadcast, the decision is implicit. In

LENWB, each node decides to rebroadcast based on knowledge of which of

its other one and two hop neighbours are expected to rebroadcast. The

information required for that decision is knowledge of which neighbours have

received a packet from the common source node and which neighbours have a

higher priority for rebroadcasting. The priority is proportional to a node’s

number of neighbours; the higher the node’s degree the higher is the priority.

Since a node relies on its higher priority neighbours to rebroadcast, it can

proactively compute if all of its lower priority neighbours will receive those

rebroadcasts; if not, the node rebroadcasts.

Fei Dai and Jie Wu (2005) proposed and implemented an efficient

broadcast protocol for ad hoc networks using directional antennas. This

protocol, called Directional Self-Pruning (DSP), is a non-trivial generalization

of an existing localized deterministic broadcast protocol using omni

directional antennas. Compared with its omni directional predecessor, DSP

achieves much lower broadcast redundancy and conserves bandwidth and

energy consumption. DSP is based on 2-hop topology information and does

not rely on any location or Angle-of-Arrival (AoA) information.

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A special case of DSP can be used for preserving shortest paths in

on-demand route discovery processes. Another special case of DSP is

proposed to use the directional reception mode in broadcasting. It is proved

that the average number of forward nodes in DSP is within a constant factor

of the minimal value in an optimal solution. Extensive simulation results

show that DSP outperforms many existing directional and omni directional

broadcast protocols in terms of efficiency and/or reliability.

Jie Wu and Fei Dai (2004) proposed and evaluated a mobility

management method based on the use of two transmission ranges. Using this

mechanism, it can also be extended Wu and Dai’s coverage condition to a

dynamic environment where network topology is allowed to change, even

during the broadcast process. In addition, connectivity, link availability, and

consistency issues related to neighbourhood information of different nodes

have also been addressed. This scheme can also be extended to provide

mobility management for other activities such as topology control in

MANETs.

Ivan et al (2002) proposed the broadcasting method significantly to

reduce or eliminate the communication overhead of a broadcasting task by

applying the concept of neighbouring nodes. Retransmissions by only internal

nodes in a dominating set are sufficient for reliable broadcasting. It is also

proposed to eliminate neighbours that already received the message and

rebroadcast only if the list of neighbours that might need if the message is

nonempty. The important features of this work is reliability (reaching all

nodes in the absence of message collisions), significant rebroadcast savings,

and their localized and parameter less behavior. The reduction communication

overhead for broadcasting take is measured experimentally.

Liu et al (2007) presented a distributed and efficient flooding

scheme. The authors first study the sufficient and necessary condition of

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100% deliverability for flooding schemes that are based on only one-hop

neighbor information. The proposed flooding algorithm achieves the local

optimality in two senses: 1) the number of forwarding nodes in each step is

the minimal; 2) the time complexity for computing forwarding nodes is the

lowest, which is O (n log n), where n is the number of neighbors of a node.

1.7.9 Summary

In this chapter, the description of routing protocols and its

classification are described. On-demand routing protocols are designed to

reduce the overheads in proactive protocols by maintaining information for

active routes only. This means routes are determined and maintained for

nodes that require sending data to a particular destination. In proactive

routing, each node has one or more tables that contain the latest information

of the routes to any node in the network. The two kinds of table updating in

proactive protocols are the periodic update and the triggered update. In

periodic update, each node periodically broadcasts its table in the network.

Each node just arriving in the network receives that table. In triggered update,

as soon as a node detects a change in its neighbourhood, it broadcasts entries

in its routing table that have changed as a result. This chapter has also

provided a general overview of the existing various broadcasting methods

proposed in MANETs including simple flooding, probabilistic schemes,

neighbour knowledge-based, distance-based, location-based and counter-

based schemes.

From the literature survey, it concludes that simple flooding

requires each node to rebroadcast all packets. In simple flooding, where each

mobile node forwards every received packet exactly once. Simple flooding

guarantees that packets reach every node in the network, but it produces high

redundant retransmissions in MANET. To reduce the broadcast storm

problem in flooding, a number of broadcasting methods have been suggested

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in the literature. The broadcasting methods can be categorized into

deterministic and probabilistic. The deterministic scheme requires global

network topological information to build a virtual backbone that covers all the

nodes in the network. It is not scalable because of the excessive overhead

associated with building and maintaining network topological information

especially in the presence of mobility.

In probabilistic flooding method, each node decides to rebroadcast

based on same fixed probability p. The problem of probability flooding is

fixed probability or uniformity of the algorithm; every node has the same

probability to rebroadcast the packet, regardless of its number of neighbours.

The poor reachability exhibited by the fixed probability is due to assigning the

same forwarding probability at every node in the network. In an attempt to

address this issue, this thesis proposes a new broadcasting method namely

optimum density based model for probability flooding to reduce the redundant

rebroadcasts by assigning forwarding or rebroadcast probability for a node

based on the type of network namely dense and sparse network. In this

method, the rebroadcast probability for a node is based on its densities and as

well as densities of its previous neighbor node instead of assigning fixed

rebroadcast probability for each node in MANET.

1.8 OUTLINE OF THE THESIS

This thesis has been arranged in seven chapters. A brief description

of each chapter is given below.

Chapter 1 provides an introduction to the MANET, characteristics

of MANET, features and applications of MANET, OSI reference model,

broadcasting in MANET, problem definition, proposed work, contributions of

the research, literature review and thesis organization.

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Chapter 2 presents the analysis of the impact of broadcast

mechanism in proactive routing protocol (DSDV) and reactive routing

protocols (DSR and AODV) on network performance. This chapter also

analysis the performance of cluster based broadcasting in MANETs and its

performance is compared with the basic flooding and probability based

flooding method.

Chapter 3 presents a proposed broadcast method namely optimum

density based model for probability flooding algorithm. The performance of

the optimum density based model for probability flooding is compared with

the existing broadcasting methods such as simple flooding and probability

flooding method.

Chapter 4 presents a second proposed method namely flooding

reduced-destination sequenced distance vector routing protocol for reducing

the broadcasting overhead in destination sequenced distance vector routing

protocol using density based flooding method. The performance of the

proposed method is compared with the existing destination sequenced

distance vector routing protocol.

Chapter 5 summarizes the results presented in this thesis and

discusses some possible directions for future research work.