A Comparative Study OF Routing Protocols IN Mobile
Ad-hoc Network
Neha Pharswan1, Prof. Dr. (Mrs.) Rama Sushil2
1
Department of Computer Science and Engineering, DIT University, Dehradun, Uttarakhand
nehapharswan.86@gmail.com
2
Professor, Department of Computer Science & Application, DIT University, Dehradun, Uttarakhand
ramasushil@yahoo.co.in

Abstract: Mobile Ad hoc Networks (MANET) are a kind of temporary
networks in which nodes are moving without any fixed infrastructure
or centralized administration. Due to the diverse applications it is an
interesting current research area. There are different current research
area in MANET are routing, synchronization, power consumption etc.
This paper concentrates on routing protocols which is the most
challenging issue due to the dynamic topology of ad hoc networks.
There are different strategies proposed for efficient routing which
claimed to provide improved performance. It is a kind of challenge and
quite difficult to determine which protocol is best suitable protocol for
different network conditions. This paper provides an overview of
different routing protocols proposed in literature.
I.




Multi-hop radio relaying- When a source node and
destination node for a message is out of the radio range,
the MANETs are capable of multi-hop routing.
Distributed nature of operation for security, routing and
host configuration. A centralized firewall is absent here.
The nodes can join or leave the network anytime, making
the network topology dynamic in nature.
Mobile nodes are characterized with less memory, power
and light weight features.
The reliability, efficiency, stability and capacity of
wireless links are often inferior when compared with wired
links. This shows the fluctuating link bandwidth of
wireless links.
Introduction
A mobile ad hoc network (MANET) is a self-configuring
infrastructure less network of mobile devices connected by wireless.
Each device in a MANET is free to move independently in any
direction, and will therefore change its links to other devices
frequently. Each must forward traffic unrelated to its own use, and
therefore be a router. The primary challenge in building a MANET
is equipping each device to continuously maintain the information
required to properly route traffic. Such networks may operate by
themselves or may be connected to the larger Internet. MANETs are
a kind of Wireless ad hoc network that usually has a routable
networking environment on top of a Link Layer ad hoc network. In
recent years MANET has gained popularity and lots of research is
being done on different aspects of MANET. It is an infrastructure
less network having no fixed base stations MANET is characterized
by dynamic topology low bandwidth and low power consumption.
All the nodes in the network are moving i.e. topology of the
network is dynamic so the nodes can act both as host as well as
router to route information unnecessary for its use.
II.
Characteristics of MANET

In MANET, each node acts as both host and router. That is
it is autonomous in behavior.
III.
Routing Protocols
Routing protocols define a set of rules which governs the journey of
message packets from source to destination in a network. In
MANET, there are different types of routing protocols each of them
is applied according to the network circumstances. Basic
classification of routing protocols is:
1.
Reactive Routing Protocol: Reactive routing protocol is
also known as on demand routing protocol. In this protocol
route is discovered whenever it is needed Nodes initiate
0route discovery on demand basis. Source node sees its
route cache for the available route from source to
destination if the route is not available then it initiates
route discovery process. The on- demand routing protocols
have two major components.
Route discovery: In this phase source node initiates route
discovery on demand basis. Source nodes consults its route
cache for the available route from source to destination
otherwise if the route is not present it initiates route
discovery. The source node, in the packet, includes the
destination address of the node as well address of the
intermediate nodes to the destination. Route
maintenance: Due to dynamic topology of the network
cases of the route failure between the nodes arises due to
link breakage etc, so route maintenance is done. Reactive
protocols have acknowledgement mechanism due to which
route maintenance is possible Reactive protocols add
latency to the network due to the route discovery
mechanism. Each intermediate node involved in the route
discovery process adds latency. These protocols decrease
the routing overhead but at the cost of increased latency in
the network. Hence these protocols are suitable in the
situations where low routing overhead is required. There
are various well known reactive routing protocols present
in MANET for example DSR, AODV and DYMO.
Dynamic source routing (DSR) is based on source
routing where the source specifies the complete path to the
destination in the packet header. All intermediary nodes
along the path simply forwards the packet to the next node
as specified in the packet header. This means that
intermediate nodes only need to keep track of their
neighboring nodes to forward data packets. The source on
the other hand, needs to know the complete hop sequence
to the destination. This eliminates the need for maintaining
latest routing information by the intermediate nodes as in
DSDV. In DSR, all nodes in a network cache the latest
routing information. When more than one route to the
destination is found, the nodes cache all the route
information so that in case of a route failure, the source
node can look up their cache for other possible routes to
the destination. If an alternative route is found, the source
node uses that route; else the source node will initiate route
discovery operations to determine possible routes to the
destination. During route discovery operation, the source
node floods the network with query packets. Only the
destination or a node which already knows the route to
destination can reply to it, hence avoiding the further
propagation of query packets from it. If a broken link is
detected by a node, it sends route error messages to the
source node. The source node on receiving error messages
will initiate route discovery operations. Unlike DSDV,
there are no periodically triggered route updates.
Ad Hoc On-Demand Distance Vector (AODV) routing protocol
shares DSR’s on demand characteristics in that it also discovers
routes on an “as needed’’ basis via a similar route discovery
process. However, AODV adopts different mechanism to maintain
routing information. It uses traditional DSDV routing tables, with
one entry per destination. AODV uses sequence numbers
maintained at each destination to determine whether the routing
information is latest and up to date and to prevent looping of
packets across routes. Just like DSDV, during route discovery
AODV floods broadcast route request packets to its neighboring
nodes. Each route request contains the source node id, the
destination node id, the sequence numbers of the source and the
destination, the hop count and the broadcast id. Every time a route
discovery operation is initiated, the source sequence number is
incremented. AODV depends on sequence numbers to avoid count
to infinity problem. Like DSR, a response message is generated
once the request packet reaches the destination or a node which
knows the route to the destination. This response message contains
the latest route information from the source to the destination.
AODV also has a feature called "precursor list" maintained by
every node. It contains the list of neighbor nodes that are most
likely to use the current node for routing packets. Routing table
entry of every node contains a list of predecessor nodes that most
likely to use that entry to route data packets. This becomes essential
when a broken link is encountered. On encountering such broken
links, all the predecessor nodes will be notified with route error
messages.
DYMO(Dynamic MANET On Demand) routing protocol has
been proposed by Perkins & Chakeres as advancement to the
existing AODV protocol. It is also defined to as successor of
AODV and keeps on updating till date. DYMO operates similar to
its predecessor i.e. AODV and does not add any extra modifications
to the existing functionality but operation is moreover quite simpler.
DYMO is a purely reactive protocol in which routes are computed
on demand i.e. as and when required.
Unlike AODV, DYMO does not support unnecessary HELLO
messages and operations purely based on sequence numbers
assigned to all the packets. It is a reactive routing protocol that
computes unicast routes on demand or when required. It employs
sequence numbers to ensure loop freedom. It enables on demand,
multi-hop unicast routing among the nodes in a mobile ad hoc
network. The basic operations are route discovery and maintenance.
Route discovery is performed at source node to a destination for
which it does not have a valid path. And route maintenance is
performed to avoid the existing obliterated routes from the routing
table and also to reduce the packet dropping in case of any route
break or node failure. Basic DYMO protocol shows in a figure.
2.
Proactive Routing Protocol: Proactive routing protocols
are also called as table driven routing protocols. In this
every node maintain routing table which contains
information about the network topology even without
requiring it. This feature although useful for datagram
traffic, incurs substantial signaling traffic and power
consumption. The routing tables are updated periodically
whenever the network topology changes. Proactive
protocols are not suitable for large networks as they need
to maintain node entries for each and every node in the
routing table of every node. These protocols maintain
different number of routing tables varying from protocol to
protocol. There are various well known reactive routing
protocols present in MANET for example DSDV and
OLSR.
Destination sequenced distance vector (DSDV) routing protocol
is table driven protocol based on the Distributed Bellman Ford
Algorithm. In DSDV, each node maintains a routing table which
contains an entry for destination node in the network. The routing
table contains entries such as the next hop address, metric or the
number of hop counts, and the sequence number. Sequence
numbers are assigned by destination node for identification of the
routes. DSDV tags each route with a sequence number and
considers a route X more favorable than Y if X has a greater
sequence number, or if the two routes have equal sequence numbers
but X has a lower metric. This was done so that the routing tables
have the latest updated path. The sequence number for a route is
updated every time a new route discovery is initiated. When a
broken link is encountered, the sequence number is set to infinity
and it is broadcasted to all the nodes so that the routing tables of the
node containing the broken link can be updated to infinity and the
link is discarded. The sequence number of every route is assigned
by the destination and it is incremented for every route discovery
operation. Thus in case of mobile ad-hoc networks, the sequence
numbers enable DSDV to maintain up to date routing information at
the nodes ensuring the consistency of routing data across all routing
tables. Both periodic and triggered route updates are initiated by
DSDV to maintain consistency of routing information. In case of
periodic updates, fresh route discovery operations are initiated after
the elapse of fixed interval of time. Triggered route updates are
initiated whenever a node encounters a broken link which can be a
result of sudden network topology change or communication link
failure.
Optimized Link State Routing Protocol (OLSR) is an IP routing
protocol optimized for mobile ad hoc networks, which can also be
used on other wireless ad hoc networks. OLSR is a proactive link-
state routing protocol, which uses hello and topology control (TC)
messages to discover and then disseminate link state information
throughout the mobile ad hoc network. Individual nodes use this
topology information to compute next hop destinations for all nodes
in the network using shortest hop forwarding paths. Link-state
routing protocols such as Open Shortest Path First (OSPF) and ISIS elect a designated router on every link to perform flooding of
topology information. In wireless ad hoc networks, there is different
notion of a link, packets can and do go out the same interface;
hence, a different approach is needed in order to optimize the
flooding process. Using Hello messages the OLSR protocol at each
node discovers 2-hop neighbor information and performs a
distributed election of a set of multipoint relays (MPRs). Nodes
select MPRs such that there exists a path to each of its 2-hop
neighbors via a node selected as an MPR. These MPR nodes then
source and forward TC messages that contain the MPR selectors.
This functioning of MPRs makes OLSR unique from other link
state routing protocols in a few different ways: The forwarding path
for TC messages is not shared among all nodes but varies
depending on the source, only a subset of nodes source link state
information, not all links of a node are advertised but only those
that represent MPR selections. Since link-state routing requires the
topology database to be synchronized across the network, OSPF
and IS-IS perform topology flooding using a reliable algorithm.
Such an algorithm is very difficult to design for ad hoc wireless
networks, so OLSR doesn't bother with reliability; it simply floods
topology data often enough to make sure that the database does not
remain unsynchronized for extended periods of time.
Proactive protocols also has various advantages and disadvantages,
being table driven protocols they increase the control messages in
the network due which message overhead in the network increases
.But at the same time due to routing information already present
latency is reduced in the network. Proactive approaches also suffer
from either out of date states or flooding of periodic updates.
IV.
Parametric comparison
Parameters
Routing philosophy
Routing scheme
Routing overhead
Latency
Scalability level
Availability
of
routing
information
Periodic updates
Storage capacity
Mobility support
Reactive
Protocol
Flat
On demand
Low
High
due
to
flooding
Not suitable for
large network
Available
when
required
Proactive
Protocol
Flat/Hierarchical
Table driven
High
Low due to routing
tables
Low
Not needed as route
available
On demand
Low generally
Depends upon the
number of routes
Route maintenance
Yes. Whenever the
topology of the
network changes
High ,due to the
routing tables
Always
available
stored in tables
Periodical updates
V.
Conclusion
Wireless mobile ad-hoc network has very enterprising applications
in today’s world. With fast growing technology mobile laptop
computers and wireless hardware costs are becoming very
affordable. There is increasing use of wireless devises. The field of
mobile ad-hoc networks is very vast and there are various
challenges that need to be met, so these networks are going to have
widespread use in the future. Reactive and Proactive protocols are
active research area in the field of ad-hoc mobile network. There
are still lots of simulations to be done in this promising field.
VI.
Future Work:
In future, we intend to design an optimized protocol, for a specific
area of application e.g. for battlefield a protocol could be designed
with basic MANET needs, and Maximum life time data gathering
approach, customized for AODV routing or OLSR routing.
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