A wireless ad hoc network is a decentralized type of wireless network. The network is ad hoc because it does not rely on a pre-existing infrastructure, such as routers in wired networks or access points in managed wireless networks.
Mobile Ad Hoc Network
In ad hoc network, all the nodes themselves are responsible for routing and forwarding of packets. If the wireless nodes are within range of each other, no routing is necessary. But, on the other hand, if the nodes have moved out of range from each other, and therefore are not able to communicate directly, intermediate nodes are needed to make up the network in which the packets are to be transmitted. Much wireless technology is based upon the principle of direct point-topoint communication. Popular solutions like Group Standard for Mobile communications (GSM) and Wireless Local Area Network (WLAN) both uses an approach where mobile nodes communicate directly with some centralized access point. These types of networks demand centralization for configuration and operation.
Ad hoc network
Ad hoc Network is Centralized network, such as GSM, cannot be used in all situations. Significant examples of such scenarios include establishing survivable, efficient, dynamic communication for rescue operations, disaster relief efforts and military networks. Such network scenarios cannot rely on centralized and organized connectivity; they can be conceived as applications of MANETs. The set of applications for MANETs is diverse, ranging from small, static networks that are constrained by power sources, to large-scale, mobile, highly dynamic networks.
To enable multi-hop communication in a distributed manner, all nodes should be able to act as routers for each other. Routes are set up and maintained by a routing protocol. MANET routing protocol design is a complex issue considering the possible rapidly changing topology of such networks.
For route maintenance one has two main approaches in MANETs, reactive and proactive. Reactive routing protocols set up traffic routes on-demand, whilst proactive protocols attempts to dynamically maintain a full understanding of the topology
The wireless network consists of at least one access point and a set of wireless nodes. This configuration is called a Basic Service Set (BSS). An Extended Service Set is a set of two or more BSSs (multiple cells).
Ad hoc mode
This is a peer-to-peer mode. This configuration called Independent Basic Service Set (IBSS), and is useful for establishing a network where nodes must be able to communicate directly and without any centralized access point.
The Ad-hoc mode is obviously the mode to use when setting up a MANET, but it lacks one basic requirement: multi-hop. Traffic is only transmitted to neighbors within radio range when using the ad-hoc mode. Therefore there is a need for MANET routing protocols to set up and maintain traffic paths. Nodes in an ad hoc network are responsible for dynamically discover which other nodes they can directly communicate with.