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					Dept. of Electronics & Communication,Govt. Engg. College,Thrissur.                1




                            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.




Seminar Report 2004                 MANET:-The Art of Networking without a
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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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Dept. of Electronics & Communication,Govt. Engg. College,Thrissur.           3




             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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Dept. of Electronics & Communication,Govt. Engg. College,Thrissur.                6
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
Seminar Report 2004                  MANET:-The Art of Networking without a
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Dept. of Electronics & Communication,Govt. Engg. College,Thrissur.               11
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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Dept. of Electronics & Communication,Govt. Engg. College,Thrissur.               13
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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Dept. of Electronics & Communication,Govt. Engg. College,Thrissur.               28
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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Dept. of Electronics & Communication,Govt. Engg. College,Thrissur.              30
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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Dept. of Electronics & Communication,Govt. Engg. College,Thrissur.                  31
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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Dept. of Electronics & Communication,Govt. Engg. College,Thrissur.               32
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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Dept. of Electronics & Communication,Govt. Engg. College,Thrissur.         35
[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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