CONCEPT AND CHARACTERISTICS OF MOBILE AD-HOC NETWORK

Abstract

Mobile Ad-Hoc Network (MANET) has gained popularity due to its flexibility and increased application in wireless communication. Its dynamic topology, decentralization and ease of administration have enhanced its preference for inclusion in infrastructure based network. In this paper, infrastructure-based and infrastructure-less based (ad-hoc) networks are critically analyzed and the characteristics of MANET, its application and challenges, routing protocol and challenges of routing protocol are presented.

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CONCEPT AND CHARACTERISTICS OF MOBILE AD-HOC
NETWORK
Obidike, G. C., Nwabueze, C. A. and Onuzulike, V. C.
Department of Electrical/Electronic Engineering, Chukwuemeka Odumegwu Ojukwu University, Uli,
Anambra State.
ABSTRACT
Mobile Ad-Hoc Network (MANET) has gained popularity due to its flexibility and increased application
in wireless communication. Its dynamic topology, decentralization and ease of administration have
enhanced its preference for inclusion in infrastructure based network. In this paper, infrastructure-based
and infrastructure-less based (ad-hoc) networks are critically analyzed and the characteristics of
MANET, its application and challenges, routing protocol and challenges of routing protocol are
presented.
Keywords: MANET, Characteristic, Ad-Hoc Network, Routing Protocol.
1.0 INTRODUCTION
The term “Ad-hoc” means “for this purpose only”. Ad-hoc network therefore is a network
created for a special purpose which consists of autonomous nodes, each acting as a host and as a
router, connected to each other through a wireless link. The nodes here are mobile, hence the
name Mobile Ad hoc Network (MANET). As a result, MANET is said to be time varying in
nature as nodes leave and rejoin the network at will. MANET is a continuously, self-configuring,
self-healing, infrastructure-less network of mobile devices which communicate wirelessly. This
type of network has a dynamic topology [1]. Figure 1 shows a typical ad-hoc network.
Figure 1: Typical Infrastructure-less (Ad hoc) Network [1].
A wireless network can be grouped into Infrastructure-based network and Infrastructure-less
based network. In Infrastructure-based network, wireless nodes have a centralized administrator
referred to as the Base Station (BS) or the Access Point (AP). The implication is that all
messages to be communicated to a node in a region or from one region to another must be
communicated through the BS as shown in Figure 2. Example of an infrastructure based network
is seen in Cellular networks, UMTS, WLAN, etc. The base station serves as the router by mere
finding the path of transmission, while nodes are the hosts. The BS is said to provide the needed

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central control of the network which include smooth routing of packets, packet loss avoidance
and traffic decongestion. In a case where a node move out–of-range of a given BS to another
coverage area (i.e. in another region), it has to connect to another BS in that area. This process is
termed “hands off”. The major disadvantage of an infrastructure based network is that when one
BS fails (faulty) the whole nodes attached to that BS will be cut off. This is a disadvantage as the
network lacks robustness in failure.
Fig. 2: Infrastructure-based Network [2].
Observe from Fig. 2 also that each node transmits directly to the Access point as indicated by the
arrow. So, if a node wants to communicate to another, the node has to relay its message through
the gateway and vice versa. There is no provision for a node communicating directly to another
rather through AP. In contrast, ad hoc network has a distributed kind of network since there is
no provision for direct communication to a BS (Access point). Instead, nodes communicate
directly to each other (see Fig 1). Each node participates actively in the network since each of
them serve as a router and as a host. When a node goes out of range of network, the network uses
a routing protocol to find the shortest and safest path of locating the node, though at a cost of
bandwidth (BW). Failure of one node does not disrupt the entire communication of the whole
network. Example of ad hoc can be found in MANET. Wireless network can also be classified as
a single hop and a multi hop [2].
2.0 History of Ad Hoc MANET
The existence of ad hoc network is categorized into first, second and third generation.
The first generation of ad hoc network called Packet Radio Network (PRNET) as far back as
1970’s in conjunction with ALOHA (Ariel Location of Hazardous Atmosphere) and Carrier
Sense Medium Access (CSMA) approaches for medium access control and a kind of distance-
vector routing. PRNET were used in a trial basis then to provide different networking capacities
in a combat environment.
The second generation of ad hoc network emerged in 1980’s when the ad hoc network systems
were enhanced and implemented as a part of the Survivable Adaptive Radio Program (SURAN).
This provided a packet switched network to the mobile battle field in an environment without
infrastructure.
For a long time the application of ad hoc network was found only in the military until the third
generation of ad hoc, which is today’s wireless ad hoc network deployed in mid1990’s. The great
potentials and advantages of ad hoc network outside the military witnessed the creation of
mobile ad hoc networking group called IETF. Here nodes communicate within their radio range
Region A
Region B

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and outside radio transmission range. Wireless ad hoc network do not have any gateway. Instead,
every node is a gateway for another. The third generation of ad hoc network made ad hoc
network popular due to rise in its commercial application.
2.1 Types of MANET
Types of MANET include VANET, iVANET, iMANET and FANET [3]
1. Vehicular ad hoc Network (VANET): In this type of MANET, cars are used as nodes in
network to create a mobile network. It turns every participating car into a wireless router
or a node at an approximate distance of 100m to 300m to order to communicate thereby
creating a wide range of network. As mobile nodes fall out of range others join, creating a
mobile internet. VANET is aimed at providing related information and traffic
management.
2. Intelligent Vehicle Ad hoc Mobile Network (INVANET): This is an intelligent way of
using vehicular network to integrate multiple ad hoc network technologies such as Wi-Fi
IEEE 802.11, WAVE IEEE 1609, and Bluetooth. All these are deployed in the
monitoring of vehicle collision and accidents. In other words INVANET helps in
defining safety measures in vehicles. Vehicular ad hoc network can therefore be viewed
as component of the intelligent transport system.
3. Intelligent based Mobile Ad hoc Network (iMANET): This is used in linking fixed
nodes and mobile nodes via routing protocol which automatically establishes connection
in decentralized manner.
4. Flying Ad hoc Network (FANET): FANET is a special case of MANET. The topology
of this network can change frequently when compared to VANET. Here an Unmanned
Aerial Vehicle (UAV) flies autonomously without carrying any human help. It does this
by simply connecting directly to satellite or ground station to establish ad hoc network.
This type of wireless networking architecture is called Flying Ad hoc Network (FANET).
2.2 Characteristics of MANET
1. Nodes are connected through a wireless link.
2. Each of the nodes act as a router and as a host.
3. The network lacks infrastructure leading to decentralized administration (control).
4. The network is less cost effective, due to lack of infrastructure.
5. MANET can easily be setup anywhere especially where there is no internet.
6. The network topology is dynamic in nature.
7. They are more vulnerable to attacks when compared to wired network.
8. They are capable of being multi-hopped when the intended node goes out of range of
network.
9. Mobile nodes are characterized with less memory, power and light weight features.
10. The access to channel by any node is not restricted.
2.3 Advantages of MANET
1. Because the network is easy to set up, it can be setup at any place.
2. It has a distributed administration
3. Lack of infrastructure makes the network robust in network failure.
4. It is scalable. This means that network accommodates addition of more nodes.

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5. It is very cheap to set up since there is no wiring of nodes.
6. It has adaptive computing and self -configuring capability
7. It does not restrict access to channels.
2.4 Challenges of Mobile Ad hoc Network
1. Limited bandwidth: The limited radio band results in reduced data rate compared to
wireless networks. Hence, optimal usage of bandwidth is necessary by keeping low
overhead as possible.
2. Energy constraints: Most of the nodes rely on limited battery life. Some of the power of
the batteries is used for data transmission, data processing and for routing packets to their
destination. This is a critical issue in the design of an ad hoc network.
3. Dynamic network topology: The frequent movement of nodes compounds the
challenges of designing an ad hoc network due to frequent path breaks.
4. Routing overhead: Due to the mobility of nodes within the ad hoc network, stale routes
are generated in the routing table leading to routing overhead.
5. Packet loss due to transmission error: The vulnerable nature of wireless networks often
lead to frequent packet loss due to traffic collision caused by hidden terminals,
interference and frequent path breaks caused by mobility of nodes.
6. Frequent network partitions: The random movement of nodes often leads to network
partition. This affects mostly the intermediate nodes.
7. Limited physical security threats: Mobile nodes are more vulnerable to attacks within
and outside network.
2.5 APPLICATIONS OF MANET
(a)Tactical Networks
 Military communication and operations
 Coordination of military object moving at high speeds such as fleets of airplanes or ship.
 Automated battlefields such as in determining the position of troops during war fare.
(b) Sensor Networks
 Collection of embedded sensor devices used to collect real time data to automate
everyday functions.
 Deployed in the monitoring and measurement of variables such as change in pressure,
temperature etc especially in pipelining.
 In tracking of positions and movement of objects and animals.
(c) Emergency Services
 In Search, rescue, crowd control operations and in disaster control and recovery
 Replacement of a fixed infrastructure in case of earthquakes, hurricanes, fire etc.
 In policing and fire-fighting.
 Supports doctors and nurses in medical field
(d) Commercial and Civilian Environment12
 E-commerce, e.g., electronic payments from anywhere (i.e., in taxi).
 Business: dynamic database access to customer files.
(e) Home and Enterprise networking
 Home/office wireless networking such as WLAN

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 Conference meetings and lectures
 Personal area network (PAN)
(f) Educational Applications
 In universities and Campus setting
 Virtual classrooms or conference rooms
 Set up ad hoc communication during conferences, meetings, or lectures
Applications Descriptions/Services
(g) Entertainment
 Used in Multiuser games
 Robotic pets outdoor internet access
3.0 ROUTING PROTOCOL
Routing protocol has been defined in [4] as set of rules governing the routing of packets from
source to destination. This standard controls how nodes decide the way to route packets between
the computing devices. In ad hoc, nodes are not usually familiar with the topology of their
network; instead they have to discover them. The Routing protocol therefore, is very crucial in
establishing communication especially to nodes outside the communication radius. Routers are
ever ready to find the shortest and safest route for packet delivery. Since the network has been
characterized by a multi-hop network topology that can change frequently due to the mobility of
nodes, efficient routing protocols are needed between nodes without causing unnecessary traffic
overhead.
3.1Classification of Routing Protocol
Routing protocols used in MANET are classified into three, namely: Table driven, demand
driven and Hybrid routing protocol.
Figure 3: Classification of Routing Protocol [4].
I. Table Driven (Proactive) Protocol
Here each node has a routing table which contains information about the network topology. The
routing table is periodically updated since routers frequently calculate routes to various nodes.

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This protocol is therefore said to be proactive since the routers are readily available in the
network. They attempt to maintain consistent and up-to-date routing information of the whole
network, thereby minimizing delay in communication by allowing nodes to quickly determine
the node to reach out to [5]. It does this by invoking a route discovery mechanism to find the
path to the destination, which remains valid until the destination is reachable or is no longer
needed [4]. The disadvantage of this protocol is that it generates control traffic that is needed to
continually update stale route entries; especially in highly mobile environment. The
communication overhead incurred in implementing this algorithm is exorbitant. Some of the
existing proactive protocols include: Destination sequence distance vector (DSDV), Wireless
Routing Protocol (WRP), Cluster head Gateway Switch Routing (CGSR) Source Tree Adaptive
Routing (STAR) etc.
i) Destination sequence distance vector (DSDV)
Here each node in the network must be stored in the routing table. The routing table records the
nodes, the distance of nodes from other nodes (number of hops). The protocol adds a sequence of
number to the routing information protocol (RIP) routing table. This sequence number field is
used to differentiate old route from new routes. Each node maintains a routing table which
contains the next hop information for all reachable destinations. Each routing table consists of:
the sequence number (received from the destination), destination address, number of hops
required to reach the destination. The sequence number finds the new route. This happens
whenever a node receives new information about a particular route it then compares the sequence
number of the route. The one with greatest sequence number is kept while the other one is
discarded. If it receives two updates with the same sequence number, the one with lower number
of hops is used. The routing table is updated periodically or whenever information is available.
When the routing table is updated periodically it is called full dump update but when update is
sent when there is new information it is called incremental driven update. This protocol is highly
unfavourable for networks which have high mobility and large number of nodes.
ii) Clustered Head Gateway Switching Routing protocol (CGSR)
CGSR routing involves cluster routing where a node finds the best route over cluster heads from
the cluster member table. Nodes are divided into group of nodes with a head elected in each
group called the clustered head. The clustered head is the gateway to nodes in a group which
connects other nodes into a hierarchical structure. This implies that packets are not sent directly
to destination rather to clustered head which routs the packet to the intended destination. All
mobile devices maintain two tables: the clustered member table and the routing table. The
clustered member table gives information about the Clustered head in each destination while the
routing table gives routing information.
iii) Wireless Routing Protocol (WRP)
WRP makes use of routing table at each node. Each of the nodes operates four tables namely: i.
Distance table – contains information about destination, next hop distance. ii. Routing table –
contains information about routing. iii. Link cost table – cost information about each neighbour
and iv. Message retransmission list table which provides sequence number of the message, a
retransmission counter, acknowledgement and list of update sent in update message. Whenever

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there is a change in network topology, an update will be made which will be broadcasted to other
nodes [6].
II. On Demand Driven Routing (Reactive) Protocol
In Reactive protocol, each router calculates route for communication only when other nodes are
ready to communicate. The node initiates the process of route discovery when it is ready to send
packet. When it establishes connection, it maintains the route until the node is either unreachable
or is no longer needed. The route discovery occurs by mere flooding of request packets
throughout network. This protocol significantly reduces routing overhead as seen in proactive
protocol. Some of these protocols are Dynamic source routing (DSR), Ad hoc –On demand
distance vector routing (AODV), Temporary-Ordered Routing Algorithm (TORA) and
Associativity based routing (ABR) etc.
i) Ad Hoc On-Demand Distance Vector Routing (AODV)
Ad Hoc On-Demand Distance Vector Routing uses distance-vector concept. It does not maintain
a routing table, but only build a routing table when a node wants to communicate with another
node on demand. AODV uses three messages in its routing procedure: Route request (RREQ),
Route reply (RREP) and route error (RERR). When a node wants to send data to another node in
the network, the node will first of all broadcast a Route Request (RREQ) packet. When a node
receives the RREQ it will in return unicast a Route Reply (RREP) to the originator of the route
request (host). When there is a link breakage during communication time, a Route Error (RERR)
message is used to notify other neighboring nodes of the link loss. In order to enable this
reporting mechanism, each node keeps a ``precursor list'', containing the IP address for each of
its neighbors that are likely to use it as a next hop towards each destination.
ii) Dynamic Source Routing Protocol (DSR)
This is an on demand protocol in which source finds unexpired route to the destination in order
to send packets. This type of protocol is used in the network where mobile nodes move with
moderate speed thereby reducing overhead significantly. It uses three steps in routing packets (i)
Routing (ii) Route discovery and (iii) Route maintenance
Source routing is a routing technique in which the sender of a packet determines the complete
sequence of nodes through which to forward the packet. The sender explicitly lists this path in
the packet’s header, identifying each forwarding hop by the address of the next node to which to
transmit the packet on its way to the destination host [7].
Route Discovery
Route discovery is the mechanism by which a node wishing to send a packet to a destination
obtains a path to the destination. To perform this, action, node broadcasts a route request packet
with a recorded source route listing only itself. Each node that hears the route request forwards
the request (if appropriate), adding its own address to the recorded source route in the packet.
The route request packet propagates hop-by-hop outward from the source node until either the
destination node is found or until another node is found that can supply a route to the target.
Each node maintains a cache of recently received route requests and does not propagate any
copies of a route request packet after the first. All source routes learned by a node are kept
(memory permitting) in a route cache, which is used to further reduce the cost of route discovery.

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When a node wishes to send a packet, it examines its own route cache and performs route
discovery only if no suitable source route is found. Naturally, if a route request packet reaches
the destination node, the destination node returns a route reply packet to the source node with the
full source to destination path listed.
Route Maintenance
Route maintenance is a mechanism by which a node detects a break in its source route and
obtains a corrected route. Conventional routing protocols integrate route discovery with route
maintenance by continuously sending periodic routing updates. If the status of a link or node
changes, the periodic updates will eventually reflect the change to all other nodes, presumably
resulting in the computation of new routes.
However, using route discovery, there are no periodic messages of any kind from any of the
mobile nodes. Instead, while a route is in use, the route maintenance procedure monitors the
operation of the route and informs the sender of any routing errors.
If a node along the path of a packet detects an error, the node returns a route error packet to the
sender. The route error packet contains the addresses of the nodes at both ends of the hop in
error. When a route error packet is received or overheard, the hop in error is removed from any
route caches and all routes which contain this hop must be truncated at that point.
There are many methods of returning a route error packet to the sender. The easiest of these,
which is only applicable in networks which only use bidirectional links, is to simply reverse the
route contained in the packet from the original host. Route maintenance can also be performed
using end-to-end acknowledgments rather than the hop-by-hop acknowledgments described
above. As long as some route exists by which the two end hosts can communicate, route
maintenance is possible. In this case, existing transport or application level replies or
acknowledgments from the original destination, or explicitly requested network level
acknowledgments, may be used to indicate the status of the node’s route to the other node [8]..
III. Hybrid Routing Protocol
Hybrid routing protocol combines the advantages of both proactive and Reactive protocol. It uses
on demand mechanism of reactive protocol and table maintenance mechanism of proactive
protocol so as to avoid latency and overhead problem in the network. This protocol is most
appropriate for large networks where large numbers of nodes are present. Networks are divided
into zones where routing inside the zone is done by using Proactive approach and routing outside
the zone uses Reactive approach. Examples of Hybrid protocol are Zone routing protocol (ZRP)
and Zone-based hierarchical link state routing protocol (ZHLS).
Zone Routing Protocol (ZRP)
ZRP divides the topology into zones and seek to utilize different routing protocols within and
between the zones based on the weaknesses and strengths of these protocols. ZRP is totally
modular, meaning that any routing protocol can be used within and between zones. The size of
the zones is defined by a parameter r describing the radius in hops. Intra-zone routing is done by

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a proactive protocol since protocols keep an up to date view of the zone topology which result in
no initial delay when communicating with nodes within the zone. Inter-zone routing is done by a
reactive protocol. This eliminates the need for nodes to keep a proactive fresh state of the entire
network.
CONCLUSION
Ad-hoc networks are special purpose autonomous nodes that can act as hosts and routers. The
nodes are mobile, hence the name Mobile Ad hoc Network (MANET). As a result, MANET is
said to be time varying in nature as nodes leave and rejoin the network at will. MANET is a
continuously, self-configuring, self-healing, infrastructure-less network of mobile devices which
communicate wirelessly. Challenges Facing Routing in Ad Hoc networks include, but not limited
to: Movement of routers, Frequent link changes, Packet losses due to transmission error,
Frequent update of the routing table due to mobility of nodes, Inability of the routing table to
converge, Existence of routing loop. These are surmountable and make Ad-Hoc Networks very
vital in mobile communication.
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