In no other field these developments have been more evident than in field of wireless technology. Though wireless systems have existed since the 1980’s, it is only in recent times that wireless systems have started to make in-roads into all aspects of human life. Mobile Ad hoc Network (MANET) is advanced wireless communication networks. MANET appeared in the 1970s with the Packet Radio Network (PRNET) program of the Defense Advanced Research Projects Agency (DARPA). Initially designed for tactical networks, MANET has benefited from a growing interest in the research community since the 1980s. Indeed, since no fixed infrastructure manages the network, the MANET experiences several problems, such as frequent disconnections of links, hidden nodes, varying topology due to mobility, more interference and limited bandwidth due to the shared medium, and quick power consumption on the mobile nodes. Every networking functions, such as formative the network topology, multiple accesses, and routing of data over the most suitable paths are to be performed in a spread way. These tasks are particularly difficult due to the restricted communication bandwidth offered in the wireless channel. These networks operate without the support of any fixed infrastructure or centralized administration (i.e. Infrastructure less Networks) and are completely self-organizing and self-configuring as in figure 1 Nodes are connected dynamically and in an arbitrary manner to form a network, depending on their transmission ranges and positions. Nodes can communicate directly with all nodes within transmission range. As transmission ranges are limited, two nodes may not be able to communicate directly and they must rely on other nodes to forward their packets. Different types of ad hoc networks like Mobile Ad hoc Network (MANET), Wireless Mesh Network (WMN), Wireless Sensor Networks (WSN) and Vehicular Ad hoc Networks (VANET) etc. In recent years, the interest in ad hoc networks has grown due to the availability of wireless communication devices that its capability to work in the ISM bands. While designing an ad hoc network in particular we are concerned with the capabilities and limitations that the physical layer imposes on the network performance. Each of the nodes has a wireless interface and communicates with each other over either radio or infrared. Laptop computers and Personal Digital Assistants (PDA) that communicate directly with each other are some of the examples of nodes in an ad hoc network. Nodes in the ad hoc network are often mobile, but can also consist of stationary nodes, such as access points to the internet. Figure 1 shows a simple MANET with three nodes. The outermost nodes are not within transmitter range of each other. However the middle node can be used to forward packets between the outermost nodes. The middle node is acting as a router and these three nodes have formed an ad hoc network.
MANET does not use any centralized administration. This is to ensure that the network won’t collapse just because one of the mobile nodes moves out of transmitter range of the others. Nodes should be able to enter/leave the network as they wish. Because of the transmission range limit of the nodes, multiple hops may be needed to reach other nodes. Every node wishing to participate in an ad hoc network must be willing to forward packets to other nodes. Thus each node in a MANET can act as both a host and a router to receive and forward packets, and it can randomly move around, leave the network or switch off. Moreover, new nodes may join the network unexpectedly. MANET takes advantages of the nature of the wireless communication medium. In other words, in a wired network the physical cabling is done a priori restricting the connection topology of the nodes, this restriction is not present in the wireless domain and, provided that two nodes are within transmitter range of each other, an instantaneous link between them may form. These characteristics make a MANET an unstable network where links between nodes frequently break. In such dynamic topologies, each node is responsible for establishing connections with neighbour nodes and for relaying packets on behalf of other nodes, and therefore, temporary networks can be set up with no or limited infrastructure support. Due to these properties, MANET has great application potential in various scenarios, such as battle field communication and disaster recovery. Application scenarios of MANET include disaster relief, personal communication in battlefields, vehicle-to-vehicle communication in automated highway systems, emergency services, personal entertainment and mobile conferencing.
Characteristics of MANET: MANET has several distinct characteristics that make them quite different from conventional cellular networks.
- Dynamic topologies: Nodes are free to move arbitrarily at different speeds. It is difficult to predict a node’s movement and its trajectory. Therefore, the network topology may change randomly and unpredictably
- Limited bandwidth: Wireless links have lower capacity than wired networks. Nowadays however, some standards offer better data rates comparable to those of Ethernet (e.g., WiMAX). Additionally, the throughput in wireless communication is affected by the conditions of the environment, fading, noise or interference.
- Energy-constrained: In a MANET, nodes may rely on limited batteries as source of energy. Nodes do not have enough transmitting power to reach all nodes in the network. This restricts the nodes communication to only the neighbouring nodes.
- Limitations of the medium: In a wireless network, it is not possible to transmit and to listen at the same time. Message loss is mainly due to collisions and interference.
- Limited physical security: In MANETs, data packets travel across the air. Hence, it is relatively easy for an intruder to every data packets by operating in promiscuous mode and by using a packet sniffer. Nodes may also be affected by intentional jamming or denial of service (DoS) attacks.
Challenges of MANET Network: A number of challenges are given below which affect the MANET. Figure 2 depicts assorted MANET challenge where security related challenge is at application layer, quality of service at transport layer, routing at network layer, power control at link/application layer.
- Application Layer: The application layer with which a user interacts is application-specific. All the other layers are used to create a seamless interface for the networking needs of the application layer. The functionalities of the other layers change on the basis of requirement of application. There are Numbers of identifications like communication partners, privacy quality of service, user authentication, and a number of constraints on data syntax are identified. The application services provided by application layer are file transfers, e-mail and network software services.
- Transport Layer: The transport layer is accountable for end-to-end connection establishment, end-to-end data packet delivery, congestion control, and flow control. Transport protocols help to create communication sessions between computers and that reliable data movement between computers is guaranteed.
- Network Layer: In MANET the source and the destination are not in the direct transmission/reception range of each other. Multi-hop forwarding is used for the nodes that are not in direct communication range. Such indirect communication through multi-hop forwarding is called routing. Set up and maintain of routes from a source to any desired destination is the responsibility of the network layer. The network layer needs sufficient information about the topology of the communication network to provide efficient routes. There are two main function of the network layer: route discovery and route maintenance. Route discovery is used to find a route from a source to a destination, although route maintenance is used to maintain an existing route as the topology changes suitable to node mobility.
- Physical Layer: The physical layer is the modem hardware in simple terms. Electronics parts of the physical layer are the antenna and the transmitter/receiver as an e.g. in a wireless node. Main functions are Modulation and coding of the physical layer. IEEE protocols at physical layer are:
- To transmit data at 10 Mbps 802.3 (Ethernet) is used which is a logical bus network. To every computer on the network, data is transmitted.
- To transmit data at 4Mbps or 16bps 802.5 (Token Ring) is used which is a logical ring network. Each computer is branching of hub so it resembles more like star.
- 802.4 (Token Passing) uses a token-passing scheme which is a bus layout. Computers that are addressed respond all the received data. A token that travels the network determine which computer is capable to broadcast.
- Link Layer: The limited bandwidth of the wireless channel combined with radio propagation loss and the broadcast nature of radio transmission make communication over a wireless channel inherently unreliable. Link-layer protocols are used to add information bits to the data bits to protect them against channel errors. Forward error correction protocols add controlled redundancy to the data, in order to enable reliable transmission of data over unreliable channels. Typical channel coding systems contain a source code, to reduce redundancy from the data, followed by a channel coder that adds controlled redundancy to the compressed data.
Basically, routing protocols are often generally classified into three varieties as Table–Driven routing protocol, On-Demand routing protocol and Hybrid routing protocol.
- Table Driven / Proactive: Proactive routing protocols acquire routing information periodically and store them in one or more routing tables. The differences among the protocols in this class are routing structure, number of tables, frequency of updates, use of hello messages and the existence of a central node.
- On-Demand / Reactive: Reactive routing protocols discover or maintain a route as needed. This reduces overhead that is created by proactive protocols. Flooding strategy is used to discover a route. Reactive routing protocols can be classified into two groups: source routing and hop by hop routing.
- Hybrid: This protocols exhibit both reactive and proactive features. Proactive strategy is used to discover and maintain routes to nearby nodes, while routes to far away nodes are discovered reactively. Consequently, overheads and delay that are introduced by proactive protocols and reactive protocols, respectively, are minimized.
Problem with Routing in MANET: Routing is the most fundamental research issue in MANET and must deal with limitations of power, bandwidth, and mobility of nodes. Some of the problems with routing in MANET are as follows: Asymmetric links, Routing Overhead, Interference and Dynamic Topology.