26 Introduction to Mobile Ad hoc Networks (MANETs)

Suchit Purohit

Overview of Adhoc Network

Communication between various devices makes it possible to provide unique and innovative services. Although this inter-device communication is a very powerful mechanism, it is also a complex and clumsy mechanism, leading to a lot of complexity in the present-day systems. This not only makes networking difficult but limits its flexibility as well. Many standards exist today for connecting various devices. At the same time, every device has to support more than one standard to make it interoperable between different devices.

Wireless networks can be classified in two types:

• Infrastructure dependent network
• Ad hoc wireless networks (Infrastructure less network)

Ad hoc network is a kind of Wireless Network which can be designed for fulfilling particular purposes that is served by establishment of the whole set up on the fly. Example types of an Ad hoc network are –

• Mobile Ad hoc Network (MANET)
• Vehicular Ad hoc Network (VANET)
• Wireless Sensor Network (WSN)

Mobile Ad hoc Networks (MANETs)

Mobile ad hoc network is a collection of independent mobile nodes that can communicate to each other via radio waves. The mobile nodes can directly communicate to those nodes that are in radio range of each other, whereas others nodes need the help of intermediate nodes to route their packets. These networks are fully distributed, and can work at any place without the aid of any infrastructure. This property makes these networks highly robust.

Mobile Ad hoc Networks are autonomous systems which comprise a collection of mobile nodes that use wireless transmission for communication. They are self-organized, self-configured, and self-controlled infrastructure-less networks. This type of network can be set up or deployed anywhere and anytime because it poses very simple infrastructure setup and no or minimal central administration. These networks are mainly used by community users such as military, researchers, business, students, and emergency services. Nodes are using Internet Protocol and IP addresses are assigned to each of the nodes. Individual nodes discover dynamically which other nodes they can communicate with.

Features / Characteristics of MANETs

• Rapidly deployable, self-organized, self-configured, and self-controlled infrastructure- less networks
• Wireless links
• Every computer or device (node) is a router as well as end host
• Nodes are mobile, topology can be very dynamic
• Nodes must be able to relay traffic since communicating nodes might be out of range
• Can be a standalone network or it can be connected to external networks(Internet)
• Radio communication – shared medium

Applications of Wireless Ad hoc Network

Military Purpose & Rescue Mission

Military networks are designed to maintain a low probability of intercept and/or a low probability of detection. Hence, nodes prefer to radiate as little power as necessary and transmit as infrequently as possible, thus decreasing the probability of detection or interception. A lapse in any of these requirements may degrade the performance and dependability of the network.

Home Networking

Home networking is the collection of electronic products and systems, enabling remote access and control of those products and systems, and any available contents such as music, video or data. In home networks, a user wants to accomplish data communication in ways that are affordable, reliable, easy to learn, and easy to use.

Vehicular Ad hoc network (VANET)

Vehicular Ad hoc Network (VANET) is a subset of mobile ad hoc network, which supports data communications among nearby vehicles and between vehicles and nearby fixed infrastructure, and generally represented as roadside entities. Depending on the range of data communications, nodes in VANET communicate among themselves in type of short-range (vehicle-to-vehicle) or medium-range (vehicle-to-roadside) communications.

In addition, the major application view of VANETs includes real-time and safety applications. Non-safety applications include real-time traffic congestion and routing information, high-speed tolling, mobile infotainment, traffic condition monitoring, and many others. Vehicular safety applications include emergency, collision, car accident, and other safety warnings.

Sensor network

Sensor networks are composed of a large number of small nodes with sensing, computation, and wireless communication capabilities. In sensor networks, sensor nodes are usually scattered and the position of sensor nodes needs not be predetermined. It means that sensor network protocols and algorithms must provide self-organizing capabilities. The features of sensor networks provide a wide range of applications such as health, military, and home. The realization of these and other sensor network applications require wireless ad hoc networking techniques.

Conferencing

Ad hoc network is widely used to create a network in place like Conferences, where for a short time of period the network is formed. One main pc is wired to which the internet facility is given and the rest of the computers or laptops are connectionless. One main computer forms the Ad hoc network and tries to connect with all the rest of the computers to make communication and data transfer possible.

Campus use

Ad hoc Network is also used for Campus use. It’s a small area in which network is created or formed whenever it is required. Professor from one building can communicate with the professor of the other building. No need to set up any wired connection.

Limitations & Current Challenges of MANETs

Each node must have full performance. Throughput is affected by system loading. Reliability requires a sufficient number of available nodes. Sparse networks can have problems. Large networks can have excessive latency (time delay), which affects some applications.

The other considerable challenges are pointed out as follows:

• Multihop operation requires a routing mechanism for mobile nodes
• Internet access mechanisms
• Self-configuring networks requires an address allocation mechanism
• Mechanism to detect and act on, merging of existing networks
• Dynamic topology maintenance
• Scaling to large networks
• Limited energy and computing resources
• Security mechanisms

Advantages & Disadvantages of Mobile Ad hoc Networks

It is independent from central network administration. It is having ability of Self-configuring in which nodes are also routers. It is having ability of Self-healing through continuous re configuration. It is having ability of Scalability in which it accommodates the addition of more nodes. It is Flexible which is similar to being able to access the Internet from many different locations. If a Computer is connected with internet and is shut down due to power failure so all the pc which is part of Ad hoc network lost their internet connection. At least one PC should have wired internet connection to provide internet to all other wireless pc.

Routing Protocols for MANETs

An ad hoc routing protocol is a convention, or standard, that controls how nodes decide which way to route packets between computing devices in a mobile ad hoc network. In ad hoc networks, nodes do not start out familiar with the topology of their networks; instead, they have to discover it. The basic idea is that a new node may announce its presence and should listen for announcements broadcast by its neighbors. Each node learns about nodes nearby and how to reach them, and may announce that it, too, can reach them.

Routing Architecture

The routing architecture of a self-organized network can be either hierarchical or flat. In most self-organized networks, the hosts will be acting as independent routers, which imply  that routing architecture should conceptually be flat, that is, each address serves only as an identifier and does not convey any information about one host that is topologically located with respect to any other node. In a flat self-organized network, the mobility management is not necessary because all of the nodes are visible to each other via routing protocols. In flat routing algorithms, the routing tables have entries to all hosts in the self-organized network. However, a flat routing algorithm does not have good scalability. The routing overhead increases rapidly when the network becomes larger. Hence, to control channel reuse spatially (in terms of frequency, time, or spreading code) and reduce routing information overhead, some form of hierarchical scheme should be employed. Clustering is the most common technique employed in hierarchical routing architectures. The idea behind hierarchical routing is to divide the hosts of a self-organized network into a number of overlapping or disjoint clusters. One node is elected as cluster-head for each cluster. cluster. This cluster-head maintains the membership information for the Nodes that are not cluster-heads will, henceforth, be referred to as “ordinary nodes”. When an ordinary node wants to send a packet, the node can send the packet to the cluster-head that routes the packet toward the destination. Cluster-head Gateway Switch Routing (CGSR) and Cluster- Based Routing Protocol (CBRP) belong to this type of routing scheme. Hierarchical routing involves cluster, address, and mobility management.

Classification of Routing Protocols

One of the most popular methods to distinguish mobile ad hoc network routing protocols is based on how routing information is acquired and maintained by mobile nodes. Using this method, mobile ad hoc network routing protocols can be divided, as discussed above, into proactive routing, reactive routing, and hybrid routing.

Proactive or Table-driven Routing Protocol

A proactive routing protocol is also called a “table-driven” routing protocol. Using a proactive routing protocol, nodes in a mobile ad hoc network continuously evaluate routes to all reachable nodes and attempt to maintain consistent, up-to-date routing information. Therefore, a source node can get a routing path immediately if it needs one. In proactive routing protocols, all nodes need to maintain a consistent view of the network topology. When a network topology change occurs, respective updates must be propagated throughout the network to notify the change. Most proactive routing protocols proposed for mobile ad hoc networks have inherited properties from algorithms used in wired networks.

To adapt to the dynamic features of mobile ad hoc networks, necessary modifications have been made on traditional wired network routing protocols. Using proactive routing algorithms, mobile nodes proactively update the network state and maintain a route regardless of whether data traffic exists or not, and the overhead to maintain up-to-date network topology information is high.

Examples are –

• Wireless Routing Protocol (WRP)
• Destination-Sequenced Distance Vector (DSDV)
• Optimized Link State Routing (OLSR) Protocol
• Fisheye State Routing (FSR)
• Topology Broadcast Reverse Forwarding (TBRF)

Reactive or On-demand Routing Protocol

Reactive routing protocols for mobile ad hoc networks are also called “on-demand” routing protocols. In a reactive routing protocol, routing paths are searched only when needed. A route discovery operation invokes a route-determination procedure. The discovery procedure terminates when either a route has been found or no route is available after examination for all route permutations. In a mobile ad hoc network, active routes may be disconnected due to node mobility. Therefore, route maintenance is an important operation of reactive routing protocols.

Compared to the proactive routing protocols for mobile ad hoc networks, less control overhead is a distinct advantage of the reactive routing protocols. Thus, reactive routing protocols have better scalability than proactive routing protocols in mobile ad hoc networks. However, when using reactive routing protocols, source nodes may suffer from long delays for route searching before they can forward data packets.

Examples are –

• Ad hoc On-Demand Distance Vector (AODV)
• Dynamic Source Routing (DSR) Protocol
• Temporally Ordered Routing Algorithm (TORA)
• Cluster-Based Routing Protocol (CBRP)

Various  parameter  comparison  of  on  demand  and  table  driven  routing protocol

Parameters	On Demand	Table Driven
Availability of Routing Information	Available when needed	Always available regardless of need
Routing Philosophy	Flat	Mostly Flat except for CGSR
Periodic route  updates	Not Required	Yes
Coping with Mobility	Using Localized route discovery	Inform other nodes to achieve consistent routing tables
Signaling Traffic Generated	Grows with increasing mobility of active nodes	Greater than that of On Demand Routing
QoS Support	Few Can Support QoS	Mainly Shortest Path as QoS Metric

Hybrid Routing Protocol

Hybrid routing protocols are proposed to combine the merits of both proactive and reactive routing protocols and overcome their shortcomings. Normally, hybrid routing protocols for mobile ad hoc networks exploit hierarchical network architectures. The proper proactive routing approach and reactive routing approach are exploited in different hierarchical  levels, respectively.

Examples are –

• Zone Routing Protocol (ZRP)
• Zone-Based Hierarchical Link State (ZHLS)
• Distributed Spanning Trees Based Routing Protocol (DST)
• Distributed Dynamic Routing (DDR) Protocol

References

1. Ad-hoc-mobile-wireless-networks-principles-protocols-and-applications By Subir Kumar Sarkar and Basavraju
2. Ad Hoc Networkingby Charles E. Perkins, Addison-Wesley.
3. http://www.windowsnetworking.com/articles_tutorials/introduction-wireless-networking-part1.html