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In this paper, we discuss about what is an Adhoc network and its operation. In addition to, we also discuss about MANET mobile adhoc network and outline idea of MANET Routing protocols. Finally we compare cellular networks with Adhoc networks.
Today, People carry many portable devices such as Laptop, Mobile Phone, MP3 player so on. For most of the time these devices are used separately in our day to day life i.e., their application do not interact with each other. Think what happens when they interact each other directly at any place and any time : a person in a seminar hall wants to share presentation to others in a short time, an incoming email can directly diverted to their PDA's and finally when a student enter a University campus all communication could be routed through wireless campus network.
These are the examples of Adhoc wireless network that is devices establish the communication any time and any place without the help of any central infrastructure. In past , Adhoc network communication used in combat fields and at disaster areas. Now with "Bluetooth" technology adhoc networking changes as its importance.
What is an Adhoc Network:
The Term "Ad Hoc" is actually derived from a Latin phrase which means "for this purpose". An Adhoc network is a wireless network used to establish a connection between nodes in a decentralized manner. It does not depends on any pre-existing infrastructure such as router(wired network) or access point( wireless network) instead each node acts as router or access point and forward the data to other nodes in the network. In addition to, Adhoc network uses "Flooding" algorithm for forwarding the data.
Example Of Adhoc Network:
Fig 1.0: Example of Adhoc Network
From the fig 1.0, shows the real time example of adhoc network where people carry devices works on basis of adhoc network. A person carry devices such as laptop, PDA and Mobile phone which are interconnected to each other and also connected to local information points (HiperLan/2 or WCDMA indoor base station). This local information points provide details about the flight departure, Gate no and so on . These information are transmitted through access point and person get those information on their devices which are interconnected.
Fig 1.1: Adhoc network
Fig 1.1 operation of adhoc network. First node A (mobile node) established a connection directly to node B and node D by sending "beaconing message". The beaconing message are control messages which includes node address information and location information. When the channel is free node A is connected to node B directly otherwise multi-hop communication take place i.e node A -> D -> B. For multi-hop communication intermediate nodes should act as router and forward the data. In the above scenario, if communication between node A and B or C , then node D & node E act as router.
Fig 1.3: Routing table updated
In the Fig 1.3, shows a link is down between node B & node C. When a link is down routes are updated for instance, consider the Fig 1.3 in which Link between B & C is down then Node A updates its table from A -> B -> C to A -> D-> E -> C.
Mobile Adhoc network (MANET) is a type of wireless adhoc network which can change locations and configure itself. In general MANET are mobile which uses wireless and also satellite and cellular communication to connect to various network.
There are two types of MANET,
Vehicular Adhoc Networks(VANET) - used to communicate between vehicles.
Internet based MANET - used to link mobile nodes.
MANETs are generally used the areas which do not have any communication infrastructure or areas where infrastructure is damaged such as military operation, disaster relief efforts .
MANET Routing Protocol:
It is Generally classified into
Table Driven Protocols
On demand protocols
The Fig 1.1(below) shows the classification of MANET routing Protocol
Fig 1.1: MANETs Routing Protocol 
Table Driven Protocols (Proactive):
These type protocol maintains routes to destination even if they are not needed. They sends periodic updates of the routes to each node. It maintains a table with all active links in network and also calculate the shortest path from the table. The main drawback in these type of protocols are lower latency as the routes should be maintained all the time, requires very high overheads for routing.
Examples for Table Driven Protocols are Destination sequence distance vector (DSDV), Wireless Routing Protocol (WRP).
On Demand Protocols (Reactive):
These type of algorithm finds routes to destination only when they are needed by flooding the network with Route Request packets. The main drawback are high transmission delay, network clogging due to flooding.
Examples for On demand protocols are Adhoc On-demand distance vector (AODV),Dynamic Source Routing (DSR).
These type of protocols are the combination of Table Driven and On demand Protocols . That is, it maintains routes to closer nodes even they are not needed and it maintains route to far away nodes only when they needed.
Examples for Hybrid protocols are Zone routing protocols(ZRP).
Benefits of MANETs:
It does not require any infrastructure to be installed.
It uses the unlicensed frequency spectrum.
It distributes the information very quick around sender.
It is kind of ad-hoc networks which increases mobility and flexibility.
It support multi-hop, therefore communication beyond the Line of Sight (LOS) is possible at high frequencies.
Reduce Power Consumption due to multi-hop adhoc network.
Advantage of Adhoc Network:
Ease of Deployment
Disadvantage of Adhoc Network:
Wireless Range is limited
low quality communication
Hidden node problem
Cellular Networks Vs Adhoc Networks:
Base stations are fixed
No fixed base stations
Static network topologies
Dynamic Network topologies
Requires large setup time
Less setup time
In this paper, we have tried to briefly explain about adhoc network and its operation and benefits. Adhoc network mainly used in Military sectors. In commercial point of view, successful example of Adhoc network is "Bluetooth". Furthermore, we discuss about MANET and its routing protocols. However, Adhoc network have some serious problems in terms security and Quality of service. On the other hand it is less expensive and easy to deploy.