[attachment=11901]
Abstract
Wireless ad hoc networks (also referred to as packet radio networks and multi-hop radio networks) consist of mobile nodes communicating over a shared wireless channel. Contrary to cellular networks, where the nodes are restricted to communicate with a set of carefully placed base stations, in wireless ad hoc networks there are no base stations; any two nodes are allowed to communicate directly if they are close enough, and nodes must use multi-hop routing to deliver their packets to distant destinations. The lack of wired infrastructure, the nature of the wireless channel, and the mobility of the nodes create many challenging problems in the link, network, and higher layers of the OSI hierarchy. On the other hand, the lack of wired infrastructure and their topology make these networks ideal for many applications, from personal area networks, to search and rescue operations, to massive networks of millions of sensors. It is therefore expected that, once all the technological issues are solved, wireless ad hoc networks will become an integral part of our society's communication network infrastructure.
Adhoc Networks Introduction
An ad hoc network is a collection of wireless mobile nodes dynamically forming a temporary network without the use of existing network infra-structure or centralized administration. Due to the limited transmission range of wireless network interfaces, multiple network hops may be needed for one node to exchange data with another across the network. In such a network, each mobile node operate not only as a host but also as a router, forwarding packets for other mobile nodes in the network, that may not be within the direct reach wireless transmission range of each other. Each node participates in an ad hoc routing protocol that allows it to discover multi hop paths through the network to any other node. The idea of an ad hoc network is sometimes also called an infrastructure-less networking, since the mobile nodes in the network dynamically establish routing among themselves to form their own network on the fly.
Some examples of the possible use of ad hoc networks include students using laptop computers to participate in an interactive lecture, business associates sharing information during a meeting, soldiers relaying information for situational awareness on the battlefield, and emergency disaster relief personnel coordinating efforts after a hurricane or earthquake.
Routing
A lot of work has already been done in the area of unicast routing in ad hoc networks. These routing protocols can be broadly classified into two categories:
• Table driven
• Source initiated (demand driven)
Table driven Routing Protocols
Table driven routing protocols attempt to maintain consistent, up to date routing information from each node to every other node in the network. These protocols require each node to maintain one or more tables to store routing information, and they respond to changes in network topology by propagating updates throughout the network in order to maintain a consistent network view. The areas in which they differ are the number of necessary routing related tables and the methods by which changes in network structure are broadcast.
Some of such routing protocols are:
• Destination Sequence Distance Vector Routing
• Cluster Head Gateway Switch Routing
• Wireless Routing Protocol
Source Initiated Routing Protocols
A different approach from table driven routing is source initiated on demand routing. This type of routing creates routes only when desired by the source node. When a node requires a route to a destination, it initiates a route discovery process within the network. This process is completed once a route is found or all possible route permutations have been examined. Once a route has been established, it is maintained by a route maintenance procedure until either the destination becomes inaccessible along every path from the source or until the route is no longer desired.
The following protocols fall in this category:
• Ad Hoc On Demand Routing Protocol
• Dynamic Source Routing
• Temporally Ordered Routing Algorithm
• Associativity Based Routing
• Signal Stability Routing
Wireless Network Architectures
In planning the wireless network, we will have to determine which wireless network architecture to adopt in the network environment. There are two architectures available, namely standalone and centrally coordinated wireless network.
Standalone architecture (Ad hoc mode)
By using ad hoc mode, all devices in the wireless network are directly communicating with each other in peer to peer communication mode. No access point (routers/switches) is required for communication between devices.
For setting up ad hoc mode, we need to manually configure the wireless adaptors of all devices to be at ad hoc mode instead of infrastructure mode, and all adaptors must use the same channel name and same SSID for making the connection active.
Ad hoc mode is most suitable for small group of devices and all of these devices must be physically present in close proximity with each other. The performance of network suffers while the number of devices grows. Disconnections of random device may occur frequently and also, ad hoc mode can be a tough job for network administrator to manage the network. Ad hoc mode has another limitation is that, ad hoc mode networks cannot bridge to wired local area network and also cannot access internet if without the installation of special gateways.
However, Ad hoc mode works fine in small environment. Because ad hoc mode does not need any extra access point (routers/switches), therefore it reduces the cost. Ad hoc can be very useful as a backup option for time being if network based on centrally coordinated wireless network (infrastructure mode) and access points are malfunctioning.
An ad hoc mode uses the integrated functionality of each adaptor to enable wireless services and security authentication. The characteristics of an Ad hoc wireless network are listed as below:
• All access points in the network operate independently and has own configuration file.
• Access point is responsible for the encryption and decryption.
• The network configuration is static and does not respond to changing network conditions.