What is an ad hoc network



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Introduction
Nowadays, there is a huge increase of the use of handled devices. Indeed, laptops, mobile phones and PDAs take an important part in the everyday life. Hence, the challenge is now that all these devices can communicate together in order to act as a network. Obviously, this kind of networks has to be wireless. In this context, the idea of ad hoc networks was developped.

Historical part
In 1968, engineers tried to develop a network in Hawaii which was responsible of the interconnection between educational facilities. This project has been named ALOHA and created by the University of Hawaii under the leadership of Norman Abramson. The main drawback is that ALOHA is based on a single-hop protocol. Hence, the pair source-destination has to be always within the same transmission range. [1] [3] [4]
In 1973, the Defense Advanced Research Projects Agency (DARPA) developed the Packet Radio Network (PRnet) which was inspirated by ALOHA. It was expected that a node could be able to communicate with all the other nodes. Nevertheless, those nodes had a thin radio range. Hence, two distant nodes could not exchange data. The engineers took into consideration this problem and implemented the multihopping technique. Thus, a node can deliver data on behalf of another one to a determined destination.[1]
It is considered that those two projects lead to the known definition of the ad hoc network. Indeed, the IEEE decided to replece the term Packet Radio Network by the term of Mobile Ad hoc NETwork (MANET).


What is an ad hoc network?
Before the definition of the ad hoc network, we have to talk about the node. A node can be a laptop, a PDA or another communication device. A node is characterized by its limited CPU capacity, storage capacity, bandwidth and battery power. Hence, the chosen protocol has to take into consideration that those ressources are scarce. [1] [2]
An ad hoc network is a wireless network without any infrastructure and composed by nodes. In such a network, the nodes are mobile and can communicate dynamically in an arbitrary manner. The network is characterised by the absence of central administration devices such as base stations or access points. Furthermore, nodes should be able to enter or to leave the network easily.

In these networks, the nodes act as routers. They play an important role in the discovery and the maintenance of the routes from the source to the destination or from a node to another one. This is the principal challenge to such a network. If link breakages occur, the network has to stay operational by building new routes.

The main technique used is the multi-hopping which increase the overall network capacity and performances. By using multihopping, one node can deliver data on behalf of another one to a determined destination. Thus, the problem of range radio is solved.

Multihopping
Multihopping is a technique which allows to extend virtualy the limited transmission range of each node. Indeed, a node can send a packet toanother one even if this destination node is not in the radio range of the source. A sended packet traverses several wireless links to reach the destination. That allows to economize energy since a node needn’t to reach the destination through a single hop. But, to be efficient, it is necessary that the different nodes which participate to the transmitting process are well distributed in space. According to [1], the multihopping technique:

Conserves transmit energy ressources.

Reduces interference.

Increases overall network throughput.



Challanges of ad hoc networks
Ad hoc engineering has to solve three main problems:

Security: trust relationships must be set up ( use of criptography). Indeed the employment of multiple hops could be a problem because it facilitates the interception of the data by an unauthorized person and the intentional interference (jamming) or unintentional interference because of the fact that several nodes share the same air interface domain.

Routing: the mobility of the nodes leads to the fact that routing is not a simple task. Indeed, depending of the chosen scenario, nodes can move frequently. Hence, because of the topology changes, a dyanamic routing protocol is required to maintain routes between the source and the destination.


Mobility Management: linked with the problem of routing. Indeed, the routing algorithm has to manage the mobility of the nodes which move randomly and unpredictably in the network. In order to achieve this task, it will be helped by the nodes which have to store informations concerning the topology of the network in their routing table. Hence, the correct working of the ad hoc network depends of both the quality of the informations collected by the node and the efficience of the routing protocol.
Two categories of protocols have been proposed to implement ad hoc networks: the proactive (table driven) and the reactive (on demand). This last category seems to take the advantage with protocols such as AODV and DSR. Experimentations made on the both have shown that they increase the overall performances and give a partial answer to the bandwidth limitation, the energy consumption, the limited radio range of the nodes,... [2]

Different scenarios:
An ad hoc network would be useful for military troops which want to settle a network in an hostile area where wired devices are not adequate.

In a conference, the audience would be able to share informations. We can also imagine the case of an airport where associates want to exchange data and, in the same time, are informed on the weather, the schedule of the departures and so on. They can also choose to establish a

connection with a backbone network like Internet.

Ad Hoc Network will also take an important role in domains such as domotic where several robots will be able to communicate in order to achieve the different tasks they are implemented for. [1] [2] [3]


Conclusion:
The future for this kind of networks seems to be very clear, due to the fact that the market of wireless communications grows up every years. Ad hoc will be needed everywhere people will want to communicate by deploying a cheap or temporary network. Efforts are also made to implement such a network in Personal Area Network and in order to interconnect, for instance, domestic appliances. This leads to lower prices which is one of the main reason while mobile computing growth rapidly. [1]

Bibliography

[1]Magnus Frodigh, Per Johansson, Peter Larsson : Wireless ad hoc networking-the art of networking without a network. This document deals with the definition of an ad hoc network and its challenges.

Retrieve on 20th September 2004 from the World Wide Web:

http://itpapers.news.com/abstract.aspx?kw=laptops&tag=wpr_more&promo=300111&docid=23006


[2] Tony Larsson, Nicklas Hedman : Routing protocol in wireless ad hoc networks- a simulation study. This document is a master thesis published in 1998.

Retrieve on 25th September 2004 from the World Wide Web:

http://epubl.luth.se/1402-1617/1998/362/
[3] Liang Qin, Thomas Kunz : Survey on mobile ad hoc network routing protocols and cross-layer design. This document is a Technical report published in August 2004.

Retrieve on 20th September 2004 from the World Wide Web:

http://kunz-pc.sce.carleton.ca/Thesis/RoutingSurvey.pdf
[4] Sushant Jain, Ratul Mahajan : Wireless LAN MAC protocols

Description of the main protocols used in wireless LAN published in May 2000.

Retrieve on 1st October 2004

from the World Wide Web:



http.//www.cs.washington.edu/homes/ratul/papers/wmac.pdf


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