Robin Manet

Dept. of Electronics & Communication,Govt. Engg. College,Thrissur.

Seminar Report 2004 MANET:-The Art of Networking without a Network

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ABSTRACT In the recent years communication technology and services have

advanced. Mobility has become very important, as people want to communicate

anytime from and to anywhere. In the areas where there is little or no

infrastructure is available or the existing wireless infrastructure is expensive and

inconvenient to use, Mobile Ad hoc NETworks, called MANETs, are becoming

useful. They are going to become integral part of next generation mobile services.

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. The special features of MANET bring this technology great

opportunities together with severe challenges. The military tactical and other

security-sensitive operations are still the main applications of ad hoc networks,

although there is a trend to adopt ad hoc networks for commercial uses due to their

unique properties. However, they face a number of problems. In this paper, we

describes the fundamental problems of ad hoc networking by giving its related

research background including the concept, features, status, and applications of

MANET. Some of the technical challenges MANET poses are also presented

based on which the paper points out the related kernel barrier. Some of the key

research issues for ad hoc networking technology are discussed in detail that are

expected to promote the development and accelerate the commercial applications

of the MANET technology.



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1. INTRODUCTION

During the last decade, advances in both hardware and software

techniques have resulted in mobile hosts and wireless networking common and

miscellaneous. Generally there are two distinct approaches for enabling wireless

mobile units to communicate with each other:

1) Infrastructured:-

Wireless mobile networks have traditionally been based on the

cellular concept and relied on good infrastructure support, in which mobile devices

communicate with access points like base stations connected to the fixed network

infrastructure. Typical examples of this kind of wireless networks are GSM,

UMTS, WLL, WLAN, etc.

2) Infrastructureless:- As to infrastructureless approach, the mobile wireless network is

commonly known as a mobile ad hoc network (MANET) [1, 2]. 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. It has

many important applications, because in many contexts information exchange

between mobile units cannot rely on any fixed network infrastructure, but on rapid

configuration of a wireless connections on-the-fly. Wireless ad hoc networks

themselves are an independent, wide area of research and applications, instead of

being only just a complement of the cellular system.



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In this paper, we describes the fundamental problems of ad hoc

networking by giving its related research background including the concept,

features, status, and applications of MANET. Some of the technical challenges

MANET poses are also presented based on which the paper points out the related

kernel barrier. Some of the key research issues for adhoc networking technology

are discussed in detail that are expected to promote the development and

accelerate the commercial applications of the MANET technology.

The paper is structured as follows. In Section II, the background

information related to ad hoc wireless networks is introduced; including the

MANET concept, difference between wireless LAN and MANET, features,

current research status, and some of its applications. The working and various

important protocols related to MANET are presented in section III. The various

challenges related to the implementation of MANET are given in section IV.

Section V mainly discusses the key research issues of MANET with the emphasis

on network layer routing strategies. Finally, we summarize the paper by

conclusions in Section VI. Main references are included as section VII.



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2. RELATED BACKGROUND

2.1 MANET Concept A mobile ad hoc network is a collection of wireless nodes that can

dynamically be set up anywhere and anytime without using any pre-existing

network infrastructure. It is an autonomous system in which mobile hosts

connected by wireless links are free to move randomly and often act as routers at

the same time.

The topology of such networks is likely highly dynamic because each

network node can freely move and no pre-installed base stations exist. Due to the

limited wireless transmission range of each node, data packets then may be

forwarded along multi-hops.



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2.2Difference between MANET and WLAN MANETs are dynamically created and maintained by the individual

nodes comprising the network. They do not require a pre-existing architecture for

communication purposes and do not rely on any type of wired infrastructure; in an

ad hoc network all communication occurs through a wireless median.

MANET comprises a special subset of wireless networks since they

do not require the existence of a centralized message-passing device. Simple

wireless networks require the existence of access points or static base stations (BS),

which are responsible for routing messages to and from mobile nodes (MNs)

within the specified transmission area. Ad hoc networks, on the other hand, do not

require the existence of any device other than two or more MNs willing to

cooperatively form a network. Instead of relying on a wired BS to coordinate the

flow of messages to each MN, the individual MNs form their own network and

forward packets to and from each other. This adaptive behavior allows a network

to be quickly formed even under the most adverse conditions. Other characteristics

of ad hoc networks include “team collaboration of a large number of MN units,

limited bandwidth, the need for supporting multimedia real time traffic and low

latency access to distributed resources (e.g. distributed database access for

situation awareness in the battlefield)” (Hong et al., 1999).

Two different architectures exist for an ad hoc network: flat and

hierarchical (Haas, 1997). Flat networks are the simplest because all MNs are

“equal”. Flat networks require each MN to participate in the forwarding and

receiving of packets depending on the implemented routing scheme. Hierarchical

networks use a tiered approach and consist of two or more tiers. The bottom layer



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consists of MNs grouped into smaller networks. A single member from each of

these groups acts as a gateway to the next higher level. Together, the gateway

MNs create the next higher tier. When an MN belonging to group A wants to

interact with another MN located in the same group,routing is the same as in a flat

ad hoc network. However, if an MN in group A wants to communicate with

another MN in group B, more advanced routing techniques incorporating the

higher tiers must be implemented. For the purposes of this thesis, further reference

to ad hoc networks assumes a flat architecture.

Figure shows the examples of both infrastructured and infrastructureless ad hoc

wireless networks.

(a) Infrastructure-based wireless network (b) Ad hoc wireless network

Infrastructured and infrastructureless wireless networks



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

infrastructured wireless network [3], including:

1) Peer-to-Peer:- Communication between two nodes which are within one hop.

Network traffic (Bps) is usually consistent.

2) Remote-to-Remote:- 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 communication range of each other in a single area or possibly

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

3) Dynamic Traffic:- This occurs when nodes are dynamic and moving around. Routes must

be reconstructed. This results in a poor connectivity and network activity in short

bursts.

2.3 MANET Features MANET has the following features:

1) Autonomous terminal:- In MANET, each mobile terminal is an autonomous node, which may

function as both a host and a router. In other words, besides the basic processing



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ability as a host, the mobile nodes can also perform switching functions as a router.

So usually endpoints and switches are indistinguishable in MANET.

2) Distributed operation:- Since there is no background network for the central control of the

network operations, the control and management of the network is distributed

among the terminals. The nodes involved in a MANET should collaborate

amongst themselves and each node acts as a relay as needed, to implement

functions e.g. security and routing.

3) Multihop routing:- Basic types of ad hoc routing algorithms can be single-hop and

multihop, based on different link layer attributes and routing protocols. Single-hop

MANET is simpler than multihop in terms of structure and implementation, with

the cost of lesser functionality and applicability. When delivering data packets

from a source to its destination out of the direct wireless transmission range, the

packets should be forwarded via one or more intermediate nodes.

4) Dynamic network topology:- Since the nodes are mobile, the network topology may change

rapidly and unpredictably and the connectivity among the terminals may vary with

time. MANET should adapt to the traffic and propagation conditions as well as the

mobility patterns of the mobile network nodes. The mobile nodes in the network

dynamically establish routing among themselves as they move about, forming

their own network on the fly. Moreover, a user in the MANET may not only

operate within the ad hoc network, but may require access to a public fixed

network (e.g. Internet).



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Topology change in ad hoc networks: nodes A, B, C, D, E, and F constitute an ad hoc network. The circle represents the radio range of node A. The network initially has the topology in (a). When node D moves out of the radio range of A, the network topology changes to the one in (b).

5) Fluctuating link capacity:- The nature of high bit-error rates of wireless connection might be

more profound in a MANET. One end-to-end path can be shared by several

sessions. The channel over which the terminals communicate is subject to noise,

fading, and interference, and has less bandwidth than a wired network. In some

scenarios, the path between any pair of users can traverse multiple wireless links

and the link themselves can be heterogeneous.

6) Light-weight terminals:- In most cases, the MANET nodes are mobile devices with less CPU

processing capability, small memory size, and low power storage. Such devices



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need optimized algorithms and mechanisms that implement the computing and

communicating functions.

2.4 MANET Status Ad hoc networking is not a new concept. As a technology for

dynamic wireless networks, it has been deployed in military since 1970s.

Commercial interest in such networks has recently grown due to the advances in

wireless communications. A new working group for MANET has been formed

within the Internet Engineering Task Force (IETF) [2], aiming to investigate and

develop candidate standard Internet routing support for mobile, wireless IP

autonomous segments and develop a framework for running IP based protocols in

ad hoc networks. The recent IEEE standard 802.11 [4] has increased the research

interest in the field.

Many international conferences and workshops have been held by

e.g. IEEE and ACM. For instance, MobiHoc (The ACM Symposium on Mobile

Ad Hoc Networking & Computing) has been one of the most important

conferences of ACM SIGMOBILE (Special Interest Group on Mobility of

Systems, Users, Data and Computing). Research in the area of ad hoc networking

is receiving more attention from academia, industry, and government. Since these

networks pose many complex issues, there are many open problems for research

and significant contributions.

2.5 MANET Applications With the increase of portable devices as well as progress in wireless

communication, ad hoc networking is gaining importance with the increasing

number of widespread applications. Ad hoc networking can be applied anywhere

where there is little or no communication infrastructure or the existing



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infrastructure is expensive or inconvenient to use. Ad hoc networking allows the

devices to maintain connections to the network as well as easily adding and

removing devices to and from the network. The set of applications for MANETs is

diverse, ranging from large-scale, mobile, highly dynamic networks, to small,

static networks that are constrained by power sources. Besides the legacy

applications that move from traditional infrastructured environment into the ad hoc

context, a great deal of new services can and will be generated for the new

environment. Typical applications include:

1) Military battlefield:- Military equipment now routinely contains some sort of computer

equipment. Ad hoc networking 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. The basic techniques

of ad hoc network came from this field.

2) Commercial sector:- Ad hoc can be used in emergency/rescue operations for disaster relief

efforts, e.g. in fire, flood, or earthquake. Emergency rescue operations must take

place where non-existing or damaged communications infrastructure and rapid

deployment of a communication network is needed. Information is relayed from

one rescue team member to another over a small handheld. Other commercial

scenarios include e.g. ship-to-ship ad hoc mobile communication, law enforcement,

etc.



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3) Local level:-

Ad hoc networks can autonomously link an instant and temporary

multimedia network using notebook computers or palmtop computers to spread

and share information among participants at a e.g. conference or classroom.

Another appropriate local level application might be in home

networks where devices can communicate directly to exchange information.

Similarly in other civilian environments like taxicab, sports stadium, boat and

small aircraft, mobile ad hoc communications will have many applications.

4) Personal Area Network (PAN):- Short-range MANET can simplify the intercommunication

between various mobile devices (such as a PDA, a laptop, and a cellular phone).

Tedious wired cables are replaced with wireless connections. Such an ad hoc

network can also extend the access to the Internet or other networks by

mechanisms e.g. Wireless LAN (WLAN), GPRS, and UMTS. The PAN is

potentially a promising application field of MANET in the future pervasive

computing context.

3. How MANETs Work

A Mobile Ad hoc NETwork (MANET) [1], [2] is a network

architecture that can be rapidly deployed without relying on existing fixed wireless

network infrastructure. This means that the network nodes should be able to

communicate to each other even if no static infrastructure, such as backbone



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network, base stations, and centralized network management function are available.

Under these situations a node itself provides these functions.

The transmission range of a node is limited to a circular region around

the node, whose radius depends on the transmitted power, receiver sensitivity and

propagation loss model. If the destination node is not in the transmission range of

the source node, then the mobile ad hoc network works like a multi hop network

with one or more node acting as routing node.

All the active nodes of the MANET need to transmit a hello message

at regular intervals, to indicate their presence. Other nodes, in the transmission

range of a node, can use that node as next hop to forward their packets toward the

destination. The selection of hello interval is an important parameter for mobile ad

hoc network. For on-demand routing protocols it is taken as 1second and it varies

from 1sec to 5sec for table driven-routing protocols [3]. In the Mobile Ad hoc

Network, nodes are communicating on ad hoc basis. Till now it has not been

standardized.

Figure shows a MANET connected to the Internet. A wired/wireless gateway

provides the Internet connection to a MANET. Since the gateway is fixed, the

mobility of the network is restricted around the gateway. A series of gateways may

provide a free movement to the MANETs.



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MANET has its own limitations in terms of coverage area, bandwidth,

scarce battery power, scalability and security. The major problem a MANET faces

is its requirement of large number of nodes in a given area for a scalable network.

It requires 50 nodes in 1000m*1000m terrain and 100 nodes in 1500m*1500m

area for the proper operation of the network.

Since the MANET is highly dynamic and there is no centralized

control, the existing shortest path routing algorithms [4], and adaptive shortest

path algorithms [5] are not suitable for it. Some routing algorithms [6], which have

been developed for such environment are Destination-Sequence Distance Vector

(DSDV) routing protocol [7], Wireless Routing Protocol (WRP) [8], Dynamic

Source Routing (DSR) [9], Associativity based routing protocol, Clustered based

routing protocol, Signal Stability Routing and Ad hoc On-demand Distance Vector

(AODV)[10] Routing protocol. The applicable MAC protocols are Multiple

Access Collision Avoidance (MACA) [11]. Carrier Sense Multiple Access-



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Collision Avoidance (CSMA/CA)[1] and IEEE 802.11 Distributed Coordination

Function (DCF)[12].

3.1 ROLE OF MAC AND ROUTING PROTOCOLS IN

MANET:- In the MANETs, since the medium is common, simultaneous

communication will collide. A suitable MAC layer protocol avoids the collision.

The transmission of unicast packet is preceded by a Request-to-Send/Clear-to-

Send (RTS/CTS) exchange that reserves the channel for transmission of the data

packets. Routing protocols are used to set up and maintain the route between the

source and destination by means of Route-Request/Route-reply (RREQ/RREP)

packet exchange. Route-Error (RERR) packet is used to detect link/route failure.

3.1.1 MAC Protocols The traditional carriers sense multiple Accesses/Collision detection

(CSMA/CD) has the probability of collision during vulnerable period, which is

equal to the one-way propagation delay. The CSMA/CA is designed to avoid

collision of data packets. It uses small control packets before data packets to avoid

collision. In CSMA/CA when a station wants to send data to another node, it first

sends a short Request To Send (RTS) packet to the destination. The receiver

responds with a Clear To Send (CTS) packet. After receiving the CTS packet the

source can send its queued data packet. In the paper we have considered following

two MAC protocols.

a) Multi Access Collision Avoidance (MACA):-

MACA is well suited for MANET. MACA uses packet sensing

multiple access (PSMA), along with RTS and CTS. In MACA carrier sensing is

provided only for data packet transmission, not for control packet. So RTS packet



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duration is the vulnerable period for MACA protocol and collision can occurs in

this period. These RTS packets are small in compare to data packet so loss is also

small, and collision can further reduced by introducing randomized exponential

back off.

Back Off Interval:- When a node wishes to transmit a packet, it first waits until the channel is

idle. Once channel becomes idle, the node waits for a randomly chosen duration

before attempting to transmit.

When transmitting a packet, nodes choose a backoff interval in the range

[0,cw] where cw is contention window.

Nodes which wants to send a data starts count down the backoff interval

when medium is idle and the count-down is suspended if medium becomes busy.

When backoff interval reaches 0, transmit RTS. The time spent counting

down backoff intervals is a part of MAC overhead.

Choosing a large cw leads to large backoff intervals and can result in larger

overhead. Choosing a small cw leads to a larger number of collisions (when two

nodes count down to 0 simultaneously).

Since the number of nodes attempting to transmit simultaneously may

change with time, some mechanism to manage contention is needed.

b) IEEE 802.11 DCF:-

The IEEE 802.11 DCF uses the collision avoidance before RTS

transmission and ACK transmitted by the receiver ensures successful reception of

the data packet. The addition of collision avoidance, to control packet exchange,

aids in the prevention of control packet collision. The protocol is suited for highly

loaded network. IEEE 802.11 DCF also provides feedback when next hope is



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unreachable by means of negative ACK, thus reduces MAC layer and routing

loads considerably.

Binary Exponential Backoff in IEEE 802.11 DCF

Here the contention window cw is chosen dynamically depending on

collision occurrence. When a node fails to receive CTS in response to its RTS, it

increases the contention window. For example it is doubled (up to an upper

bound).When a node successfully completes a data transfer, it restores cw to

Cwmin. Or in other words cw follows a sawtooth curve

3.1.2 Routing Protocols

One of the most important aspects of classifying ad hoc routing

protocols is whether or not the nodes of the network should keep track of route to

all possible destinations. Protocols, that keep track of routes for all destinations

before the communication to start, are called proactive or table- driven routing

protocols. On the other hand, the protocols that acquire routing information only

when it is actually needed is called reactive or on-demand routing protocols.

The various proactive routing protocols are Dynamic Destination-

Sequenced Distance Vector (DSDV) routing, Wireless Routing Protocols (WRP),

Global State Routing (GSR), Fisheye State Routing (FSR), and Hierarchical State

Routing (HSR) etc. The various on-demand routing protocols are Cluster-Based

Routing (CBR), Ad hoc On-demand Distance Vector (AODV) routing, Dynamic

Source Routing (DSR), Temporally Ordered Routing Algorithm (TORA),

Associativity-Based Routing (ABR), and Signal Stability Routing (SSR) protocol

etc. In this paper following two on-demand and two table-driven protocols have

been studied.



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1) Table-driven Routing protocols

In table-driven routing protocols all possible routes for each

destinations are stored and maintained at every node, even before communication

to start. The DSDV and the WRP are two table-driven routing protocols

considered in this paper.

a) Destination-Sequence Distance Vector (DSDV) Protocol :- In DSDV all possible routes for each destination are stored in routing

table. The routing table is transmitted periodically and also if significant change

has occurred in routing information. A sequence number is used to distinguish

stale route from new one. The route with higher sequence number is newer.

Destination-Sequence Distance Vector protocol works as follows i)Each node maintains a routing table which stores

-next hop towards each destination

-a cost metric for the path to each destination

-a destination sequence number that is created by the destination itself

-Sequence numbers used to avoid formation of loops.

ii)Each node periodically forwards the routing table to its neighbors

-Each node increments and appends its sequence number when sending its

local routing table.

-This sequence number will be attached to route entries created for this node.

Assume that node X receives routing information from Y about

a route to node Z.



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Let S(X) and S(Y) denote the destination sequence number for node Z as stored at

node X, and as sent by node Y with its routing table to node X, respectively.

Node X takes the following steps:

-If S(X) > S(Y), then X ignores the routing information received from Y

-If S(X) = S(Y), and cost of going through Y is smaller than the route known to

X, then X sets Y as the next hop to Z

-If S(X) < S(Y), then X sets Y as the next hop to Z, and S(X) is updated to

equal S(Y)

b) Wireless Routing Protocol (WRP) :- In WRP, distance of each destination, possible routes and next hops

are stored in the routing table at each node. The routing information is broadcast

only if there is a significant change in the routing information. If there is no

change in routing table, an idle Hello message is used to ensure connectivity. This

reduces the routing load of the network.

2) On demand routing protocols In the on-demand routing protocols, a route is established only if it is

required. The DSR and the AODV are two on-demand routing protocols

considered in this paper.

a) Dynamic Source Routing (DSR) protocol:-

DSR is an improved version of DSDV. It minimizes the

number of broadcasts by creating route on demand. To find out a route, source;S



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broadcasts RREQ(Route REQuest) to all of its neighbours, which then rebroadcast

it to their neighbours. Before rebroadcasting the packet each node will append its

own identifier to the packet header. Packet numbers are used such that a node will

retransmit the packet only once. The process continues till the destination;D is

reached. The reverse path is used by the destination to send RREP(Route REPly)

to the source. RREP includes the route from source to destination through which

RREQ was received by destination node. Node S on receiving RREP, caches the

route included in the RREP. When node S sends a data packet to D, the entire

route is included in the packet header so this routing is called source routing.

Intermediate nodes use the source route included in a packet to determine to whom

the packet should be forwarded.

RERR(Route ERRor )is used to indicate any link failure during

data packet transfer. On receiving the RERR the source starts another route request

by transmitting RREQ.

Dynamic Source Routing can be optimized by using route

caches. The route caches work as given below. Assume that node S wants to

communicate with node D and A,B,C,E,F,G,H,I,J,K,L,M, etc are the other nodes

present in the network.

i) Each node caches a new route it learns by any means

ii) When node S finds route [S,E,F,J,D] to node D, node S also learns route [S,E,F]

to node F

iii) When node K receives Route Request [S,C,G] destined for node, node K

learns route [K,G,C,S] to node S

iv) When node F forwards Route Reply RREP [S,E,F,J,D], node F learns route

[F,J,D] to node D

v) When node E forwards Data [S,E,F,J,D] it learns route [E,F,J,D] to node D



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vi) A node may also learn a route when it overhears Data packets

The main uses of route caching are, i) When node S learns that a route to node D is broken, it uses another route from

its local cache, if such a route to D exists in its cache. Otherwise, node S

initiates route discovery by sending a route request

ii)Node X on receiving a Route Request for some node D can send a Route Reply

if node X knows a route to node D

iii)Thus use of route cache

-can speed up route discovery

-can reduce propagation of route requests

While using route caches we should be careful because

i) Stale caches can adversely affect performance

ii) With passage of time and host mobility, cached routes may become invalid

iii) A sender host may try several stale routes (obtained from local cache, or

replied from cache by other nodes), before finding a good route

The main advantage of dynamic source routing is its simplicity.

Routes are maintained only between nodes who need to communicate, thus it

reduces the overhead of route maintenance.

The disadvantages of dynamic source routing are

1) Packet header size grows with route length due to source routing 2) Flood of route requests may potentially reach all nodes in the network



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3) Care must be taken to avoid collisions between route requests propagated by

neighboring nodes

-insertion of random delays before forwarding RREQ

4) Increased contention if too many route replies come back due to nodes replying

using their local cache

-Route Reply Storm problem

-Reply storm may be eased by preventing a node from sending RREP if it

hears another RREP with a shorter route

5) An intermediate node may send Route Reply using a stale cached route, thus

polluting other caches

6) This problem can be eased if some mechanism to purge (potentially) invalid

cached routes is incorporated.

7) For some proposals for cache invalidation,

-Static timeouts

-Adaptive timeouts based on link stability

b) Ad Hoc On Demand Distance Vector (AODV) routing

protocol:-

AODV routing protocol is designed specially for Ad-Hoc network

and provides quick and efficient route establishment, with minimal control

overhead and minimal route acquisition latency. The route recovery of AODV is

similar to that of DSR. In addition to that it stores the previous routing information



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for time out duration. If stored route is not used within the time-out period, it is

eliminated from the table. The protocol is suitable for a highly mobile network.

Comparison of AODV and DSR

DSR includes source routes in packet headers. It results in large packet headers

when the route is very long. Resulting large headers can sometimes degrade

performance particularly when the data contents of a packet are small. AODV

attempts to improve on DSR by maintaining routing tables at the nodes, so that

data packets do not have to contain routes. AODV retains the desirable feature of

DSR that routes are maintained only between nodes which need to communicate.

The various steps in AODV are

i) Route Requests (RREQ) are forwarded in a manner similar to DSR.

ii)When a node re-broadcasts a Route Request, it sets up a reverse path pointing

towards the source that is AODV assumes symmetric (bi-directional) links.

iii)When the intended destination receives a Route Request, it replies by sending a

Route Reply.

iv)Route Reply travels along the reverse path set-up when Route Request is

forwarded.

v)On receiving the RREP the sourge will send the data packet to the destination

similar to that in DSR.



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vi) An intermediate node (not the destination) may also send a Route Reply

(RREP) provided that it knows a more recent path than the one previously

known to sender S.

vii)To determine whether the path known to an intermediate node is more recent,

destination sequence numbers are used.

viii)The likelihood that an intermediate node will send a Route Reply when using

AODV not as high as DSR.A new Route Request by node S for a destination is

assigned a higher destination sequence number. An intermediate node which

knows a route, but with a smaller sequence number, cannot send Route Reply.

Timeouts in AODV

A routing table entry maintaining a reverse path is purged after a

timeout interval and the timeout should be long enough to allow RREP to come

back

A routing table entry maintaining a forward path is purged if not used

for a active_route_timeout interval so that if no is data being sent using a

particular routing table entry, that entry will be deleted from the routing table

(even if the route may actually still be valid)

Link Failure Reporting in AODV

The various steps in reporting link failure in AODV are,

i) A neighbour of node X is considered active for a routing table entry if the

neighbour sent a packet within active_route_timeout interval which was

forwarded using that entry.



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ii)When the next hop link in a routing table entry breaks, all active neighbors are

informed.

iii)Link failures are propagated by means of Route Error messages, which also

update destination sequence numbers.

iv)When node X is unable to forward packet P (from node S to node D) on link

(X,Y), it generates a RERR message.

v)Node X increments the destination sequence number for D cached at node X.

vi)The incremented sequence number N is included in the RERR.

vii)When node S receives the RERR, it initiates a new route discovery for D using

destination sequence number at least as large as N.

vii)When node D receives the route request with destination sequence number N,

node D will set its sequence number to N, unless it is already larger than N.

Link Failure detection in AODV

Neighboring nodes periodically exchange hello message. Absence of

hello message is used as an indication of link failure. Alternatively, failure to

receive several MAC-level acknowledgement may be used as an indication of link

failure.



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Why Sequence Numbers in AODV

i)To avoid using old/broken routes and to determine which route is newer.

ii)To prevent formation of loops as explained below.

-Assume that A does not know about failure of link C-D because RERR sent by C is lost

-Now C performs a route discovery for D. Node A receives the RREQ (say, via -path C-E-A) -Node A will reply since A knows a route to D via node B -Results in a loop (for instance, C-E-A-B-C )



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Summary: AODV

i)Routes need not be included in packet headers

ii)Nodes maintain routing tables containing entries only for routes that are in

active use

iii)At most one next-hop per destination maintained at each node in AODV where

as in DSR may maintain several routes for a single destination

iv)Unused routes expire even if topology does not change

4. CHALLENGES AND KERNEL BARRIER

4.1 MANET Challenges:- 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:

1) 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. Multicast

routing is another challenge because the multicast tree is no longer static due to the

random movement of nodes within the network. Routes between nodes may



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potentially contain multiple hops, which is more complex than the single hop

communication.

2) Security and Reliability:- In addition to the common vulnerabilities of wireless connection,

an ad hoc network has its particular security problems due to e.g. nasty neighbour

relaying packets. The feature of distributed operation requires different schemes of

authentication and key management. Further, wireless link characteristics

introduce also 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.

3) Quality of Service (QoS):- Providing different quality of service levels in a constantly changing

environment will be a challenge. The inherent stochastic feature of

communications quality in a MANET makes it difficult to offer fixed guarantees

on the services offered to a device. An adaptive QoS must be implemented over

the traditional resource reservation to support the multimedia services.

4) Internetworking In addition to the communication within an ad hoc network,

internetworking between MANET and fixed networks (mainly IP based) is often

expected in many cases. The coexistence of routing protocols in such a mobile

device is a challenge for the harmonious mobility management.

5) Power Consumption For most of the light-weight mobile terminals, the

communication-related functions should be optimized for lean power consumption.

Conservation of power and power-aware routing must be taken into consideration.



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4.2 Kernel Barrier It has been widely recognized that routing strategy is the most

important research problem among others. To determine viable routing paths and

deliver messages in a decentralized environment where network topology

fluctuates is far less than a well-defined problem. New models are needed to

describe the mobile ad hoc feature of the target wireless networks, while new

algorithms are required to safely and efficiently route information to mobile

destination in order to support different types of multimedia applications. Factors

such as variable wireless link quality, propagation path loss, fading, multi-user

interference, power expended, and topological changes become relevant issues that

add more difficulties and complexities to the routing protocol design.

Many routing protocols have been proposed with the form of IETF

working documents of both Internet Drafts and Request For Comments (RFC) [2].

Numerous projects related to different aspects of MANET are employed by

academics and institutes all over the world, with individual standards being

presented occasionally in literatures [5-9]. They serve the purpose of

demonstrating the functionality and performance of ad hoc routing with

comparatively simple protocols, whereas very few of them can be regarded to

really fulfill the requirements of a real application scenario. There are still many

relative aspects to be deeply researched before the wide deployment of the

commercial ad hoc systems.

5. KEY RESEARCH ISSUES

This section analyses key Research issues concerning MANET

network layer routing strategies, including four selected key problems in MANET:

X-cast routing, security & reliability, QoS, and interworking with outside IP



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networks. These issues are currently main challenges of ad hoc wireless networks.

The lack of robust solutions to these problems prevents MANET from wide

commercial deployment.

5.1 X-cast Routing Algorithms:- As in the infrastructured wireless networks, all kinds of X-cast

communication schemes should be supported in an ad hoc mobile environment.

These include unicast, anycast, multicast, and broadcast. MANET also brings new

X-cast modes into communications, e.g. geocast [10] and content-based. In

particular, multicast is desirable to support multiparty wireless communications

[11]. Since the multicast tree is no longer static (i.e. its topology is subject to

change over time), the multicast routing protocol must be able to cope with

mobility, including multicast membership dynamics (e.g., leave and join).

In a multihop ad hoc context, the routing problem becomes more

complex because of the mobility of both hosts and routers. The random movement

of the nodes and the uncertainty of path quality render the traditional routing

protocols impractical. Trade-off between reactive and proactive schemes in terms

of latency and overhead of route discovery and maintenance are to be considered

depending on different traffic and mobility patterns. Issues to be taken into

account include routing discovery and flooding, caching, data delivery, location-

aided and power-aware, broadcast storm issue, route request and reverse path.

5.2 QoS Supporting Model:- Just like in wired networks, QoS protocols can be used to prioritize

data within ad hoc networks in order to reserve better connections for high data

rate applications while still maintaining enough bandwidth for lower bit rate

communication. The support of multimedia services will most likely be required



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within and throughout the MANET, for which different QoS classes (e.g. voice,

video, audio, web, and data stream) are needed to facilitate the use of multimedia

applications. In such a stochastic changing environment involving dynamic nodes,

hidden terminals, and fluctuating link characteristics, supporting end-to-end QoS

at different levels will be a great challenge that requires in-depth investigation [12].

An adaptive QoS must be implemented over the traditional plain resource

reservation to support the multimedia services. Special emphasis should be put on

achieving a new QoS model for MANETs by taking into account the ad hoc

features of the target networks: dynamic node roles, data flow granularity, traffic

profile, etc.

5.3 Security, Reliability, and Availability Schemes:- Security, reliability, and availability are three crucial aspect of

MANET, especially in security-sensitive applications. Since ad hoc relies on

wireless communication medium, it is important to deploy a security protocol to

protect the privacy of transmissions. The requirements regarding confidentiality,

integrity, and availability are the same as for any other public communication

networks. However, the implementation schemes of key management,

authentication, and authorization are quite different because there is no aid of a

trusted third-party certification authority to create trusted relationships by

exchanging private/public keys [13]. Different types of threats and attacks against

routing in MANET should be analyzed leading to the requirement of ad hoc

routing security, and advanced solutions are needed for the secure routing of

MANET.

Wireless communication is subject to many types of problems due

to interference and poor signals. As for reliability and availability issues, besides

low level error masking and recovery mechanisms (i.e. link layer error detection

and correction coding), special attention should be paid to studying fault-tolerant



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routing algorithm. In multihop ad hoc wireless networks, there exists an inherent

attribute of redundant routing paths between nodes. Exploiting this property, it’s

possible to provide a fault-tolerant routing scheme [14], for increasing the

reliability and security of the target routing algorithm. Since overhead occurs in

this reliable-increasing algorithm, research should also study the tradeoff between

performance and reliability in order to calculate the most efficient solution.

5.4 Internetworking Mechanisms:- To integrate the two mobility management schemes in the domains

of both traditional infrastructured wireless networks and the new mobile ad hoc

networks is an important issue. The mobility mode of an ad hoc network is quite

different from that of infrastructured networks. In infrastructured networks only

the nodes (terminals) at the very edges (the last hop) of fixed networks are moving,

whereas an ad hoc network can be completely mobile, since a device can serve

both as router and host at the same time. Consequently, in an ad hoc network

mobility is handled directly by the routing algorithm.

In many cases, device accesses both within the ad hoc network and to

public networks (e.g. the Internet) can be expected to form a universal

communication scenario. In other words, a terminal in an ad hoc wireless network

is able to connect to nodes outside the MANET while being itself also accessible

by external nodes. The interworking between ad hoc and fixed networks is

necessary. In particular, the coexistence and cooperation with the public IP based

wireless networks is necessary to many contexts. The Mobile IP protocol for

MANET should be deeply studied in order to give nodes in ad hoc networks the

ability of accessing the Internet and other IP based networks to take advantage of

the services of Mobile IP.



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6. CONCLUSIONS

This seminar describes the fundamental issues and analyses key

research problems of MANET. Firstly, the background information of MANET

are introduced, including the MANET concept, features, current status, and

application areas. Important MAC and routing protocols are also described.

Then the main challenges of MANET are discussed that lead to the analysis of

relevant kernel barrier. Finally, four key network layer research issues of MANET

routing strategies are described in detail. The novel and advanced solutions to

these issues are necessary to fulfil the requirements of wide commercial

deployment of MANET. Mobile ad hoc networking is one of the most important

and essential technologies that support future pervasive computing scenario. The

special characters of MANET bring this technology great opportunities together

with severe challenges. Currently MANET is becoming more and more interesting

research topic and there are many research projects employed by academic and

companies all over the world. Various interesting issues are investigated that cover

all aspects of ad hoc wireless networks. Meanwhile, many routing protocols

designed for ad hoc networks have been proposed as Internet Draft and RFC of

IETF. MANETs can be exploited in a wide area of applications, from military,

emergency rescue, law enforcement, commercial, to local and personal contexts.



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7. REFERENCES [1] M. Frodigh, P. Johansson, and P. Larsson. “Wireless ad hoc networking: the art of networking without a network,” Ericsson Review, No.4, 2000, pp. 248-263. [2] IETF Working Group: Mobile Adhoc Networks (manet). http://www.ietf.org/html.charters/manet-charter.html. [3] Ad Hoc Networking Extended Research Project. Online Project. http://triton.cc.gatech.edu/ubicomp/505. [4] IEEE 802.11 Working Group. http://www.manta.ieee.org/groups/802/11/. [5] E.M. Royer and C.K. Toh, “A review of current routing protocols for ad hoc mobile wireless networks,” IEEE Personal Communications, 1999, 6(2), pp. 46-55. [6] S.R. Das, R. Castaneda, and J. Yan, “Simulation-based performance evaluation of routing protocols for mobile ad hoc networks,” Mobile Networks and Applications, 2000, 5, pp. 179-189. [7] S.-J. Lee, M. Gerla, and C.-K. Toh, “A simulation study of table-driven and on-demand routing protocols for mobile ad-hoc networks,” IEEE Network, 1999, 13(4), pp. 48-54. [8] M. Joa-Ng and I.-T. Lu, “A peer-to-peer zone-based two-level link state routing for mobile ad hoc networks,”. IEEE Journal on Selected Areas in Communications, 1999, 17(8), pp. 1415-1425. [9] L. Ji, M. Ishibashi, and M.S. Corson, “An approach to mobile ad hoc network protocol kernel design,” In Proceedings of IEEE WCNC’99, New Orleans, LA, Sep. 1999, pp. 1303-1307.



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[10] Y.-B. Ko and N. H. Vaidya, “Geocasting in mobile ad hoc networks: Location-based multicast algorithms,”. Technical Report TR-98-018, Texas A&M University, Sep. 1998. [11] M. Gerla, C.-C. Chiang, and L. Zhang, “Tree multicast strategies in mobile, multihop wireless networks,” ACM/Baltzer Mobile Networks and Applications, speical issue on Mobile Ad Hoc Networking, 1999, 4(3), pp. 193-207. [12] S. Chakrabarti and A. Mishra, “QoS issues in ad hoc wireless networks,” IEEE Communications Magazine, 2001, 39(2), pp. 142–148. [13] L. Zhou and Z. J. Haas, “Securing ad hoc networks,” IEEE Network Journal, 1999, 13(6), pp. 24-30. [14] E. Pagnani and G. P. Rossi, “Providing reliable and fault tolerant broadcast delivery in mobile ad-hoc networks,” Mobile Networks and Applications, 1999, 5(4), pp. 175-192.

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