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Mobile ad-hoc networks are autonomously self-organized and self-configured multi-hop wireless networks without the aid of any stand-alone infrastructure or centralized administration. Basically mobile ad-hoc network is a collection of wireless mobile nodes that communicate with each other and the nodes in the network move arbitrarily and acts as routers that route data to/from other nodes in network. Nodes in these networks utilize the same random access wireless channel. The wireless links in this network are highly error prone and can go down frequently due to mobility of nodes and interference .Therefore, routing in ad-hoc network is a challenging task and the reason for this is the constant change in network topology. This paper attempts to provide a comprehensive overview of mobile ad-hoc networks and also includes design issues and applications of MANET.
Keywords: - MANET
The wireless network can be classified into two types: Infrastructured or Infrastructure less. In Infrastructured or access point wireless networks, the mobile node can move while communicating, the base stations are fixed and when the node goes out of the range of a base station, it gets into the range of another base station. Examples of this kind of wireless networks are GSM, UMTS, WLL, WLAN, etc. In Infrastructure less or ad-hoc wireless network, the mobile node can move while communicating, there are no fixed base stations and all the nodes in the network act as routers. Mobile ad-hoc network is a collection of independent mobile hosts 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 other nodes need the help of intermediate nodes to route their packets. Ad-hoc networking allows the devices to maintain connections to the network as well as easily adding and removing devices to and from the network. Due to nodal mobility, the network topology may change rapidly and unpredictably over time. These networks introduced a new art of network establishment and can be well suited for an environment where either the infrastructure is lost or where deploy an infrastructure is not very cost effective. The popular IEEE 802.11 "WI-FI" protocol is capable of providing ad-hoc network facilities at low level, when no access point is available. Mobile ad-hoc networks can operate in a standalone fashion or could possibly be connected to a larger network such as the Internet. Ad-hoc networks can be classified, depending on their coverage area as: Body (BAN), Personal (PAN), Local (LAN), Metropolitan (MAN) and Wide (WAN) area networks
Fig.-1 Infrastructured and Infrastructure less ad-hoc wireless networks
II. MANET History
The concept of mobile ad hoc networking is not a new one. As a technology for dynamic wireless networks, MANET has been deployed in military since 1970s. Commercial interest in such networks has recently grown due to the advances in wireless communications. A new working group for MANET has been formed within the Internet Engineering Task Force (IETF), aiming to investigate and develop candidate standard Internet routing support for mobile, wireless IP Autonomous segments and develop a framework for running IP based protocols in ad-hoc networks. The recent IEEE standard 802.11 has increased the research interest in the field. For a long time, ad-hoc network research stayed in the realm of the military, and only in the middle of 1990, with the advent of commercial radio technologies, did the wireless research community become aware of the great potential and advantages of mobile ad hoc networks outside the military domain, witnessed by the creation of the Mobile ad-hoc Networking working group within the IETF. Currently, mobile ad hoc network research is a very vibrant and active field and the efforts of the research community, together with current and future MANET enabling radio technologies.
III MANET Features
MANET has the following features:
1) Autonomous terminal-In MANET, each mobile terminal is an autonomous node, which may function as both a host and a router. In other words, besides the basic processing ability as a host, the mobile nodes can also perform switching functions as a router. So usually endpoints and switches are indistinguishable in MANET.
2) Distributed operation- Since there is no background network for the central control of the network operations, the control and management of the network is distributed among the terminals. The nodes involved in a MANET should collaborate amongst themselves and each node acts as a relay as needed, to implement functions e.g. security and routing.
3) Multi-hop routing- Single-hop MANET is simpler than multi-hop in terms of structure and implementation, with the cost of lesser functionality and applicability. When delivering data packets from a source to its destination out of the direct wireless transmission range, the packets should be forwarded via one or more intermediate nodes. In Fig.-2 Nodes A and C are not in direct transmission range of each other therefore nodes A and C must discover the route through B in order to communicate.
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Fig.-2 Multi-hop routing
4) Fluctuating link capacity- The nature of high bit-error rates of wireless connection might be more profound in a MANET. One end-to-end path can be shared by several sessions. The channel over which the terminals communicate is subject to noise, fading, and interference, and has less bandwidth than a wired network.
5) Light-weight terminals- In most cases, the MANET nodes are mobile devices with less CPU processing capability, small memory size, and low power storage.
IV. Issues in Designing MANET
Mobile Ad-hoc Networks are highly dynamic in nature and no fixed infrastructure in these type of networks. Due to this, issues in designing mobile ad-hoc networks using a routing protocol are explain as:
Error-prone channel state- The wireless links in this network are highly error prone and can go down frequently due to mobility of nodes and interference .Therefore, routing in ad-hoc network is a challenging task.
Hidden problem- Node A and node C are in range for communicating with node B, but not with each other. In the event that both try to communicate with node B simultaneously, A and C might not detect any interference on the wireless medium. Thus, the signals collide at node B, which in turn will be unable to receive the transmissions from either node. The typical solution for this is that the nodes coordinate transmissions themselves by asking and granting permission to send and receive packets. This scheme is often called RTS/CTS (Request to Send/Clear to send).
Fig.-3 Hidden Problem using three Nodes
Bandwidth-constrained, variable capacity links- Wireless links will continue to have significantly lower capacity than their hardwired counterparts. In addition, the realized throughput of wireless communications--after accounting for the effects of multiple access, fading, noise, and interference conditions etc. is often much less than a radio's maximum transmission rate.
Energy-constrained operation- Some or all of the nodes in a MANET may relay on batteries or other exhaustible means for their energy. For these nodes, the most important system design criteria for optimization may be energy conservation. It should be noted that the energy consumed during sending a packet is the largest source of energy consumption of all modes. Despite the fact that while in idle mode, it has been found that the wireless interface consumes a considerable amount of energy nevertheless. Idle energy is a wasted energy that should be eliminated or reduced through energy-efficient schemes.
Security issues- Mobile ad-hoc networks are generally more prone to security threats than are fixed cable nets. The increased possibility of eavesdropping, spoofing, and denial-of-service attacks should be carefully considered. The feature of distributed operation requires different schemes of authentication and key management. Further, wireless link characteristics introduce also reliability problems, because of the limited wireless transmission range, hidden terminal problem, and data transmission errors.
IV. Broadcasting Approaches in MANET
In MANET, a number of broadcasting approaches on the basis of cardinality of destination set:
Unicasting: Sending a message from a source to a single destination.
Multicasting: Sending a message from a source to a set of destinations.
Broadcasting: Flooding of messages from a source to all other nodes in the specified network.
Geocasting: Sending a message from a source to all nodes inside a geographical region.
V. Routing Protocols
A routing protocol is needed whenever a packet needs to be transmitted to a destination via number of nodes and numerous routing protocols have been proposed for such kind of ad-hoc networks. These protocols find a route for packet delivery and deliver the packet to the correct destination. Basically, routing protocols can be broadly classified into two types as (a) Table Driven or Proactive Protocols and (b) On-Demand or Reactive Protocols (c) Hybrid Protocols
Table Driven or Proactive Protocols: In proactive protocols each node maintains one or more tables containing routing information to every other node in the network. All nodes keep on updating these tables to maintain latest view of the network. Proactive routing protocols maintain the routing information of all the participating nodes and update their routing information frequently irrespective of the routing requests. The major drawback of proactive protocols is the heavy load created from the need to flood the network with control messages. Examples of proactive protocols are: DSDV (Destination Sequenced Distance-Vector), WRP (Wireless Routing Protocol), CGSR (Cluster head Gateway Switch Routing) , OLSR (Optimized Link State Routing).
On Demand or Reactive Protocols: In these protocols, routes are created as and when required. When a transmission occurs from source to destination, it invokes the route discovery procedure. The route remains valid till destination is achieved or until the route is no longer needed. This leads to higher latency than with proactive protocols, but lower overhead. Examples of reactive protocols are: DSR (Dynamic Source Routing), ABR (Associativity Based Routing), SSR (Signal Stability Routing), LAR (Location Aided Routing, 1998), AODV (ad hoc On-Demand Distance Vector Routing).
Hybrid Routing Protocols: Hybrid routing protocols are a new generation of protocols that combine the characteristics of both reactive and proactive routing protocols under different scenarios. These protocols are designed to increase scalability by allowing nodes with close proximity to work together to form some sort of a backbone in order to reduce the route discovery overheads. Hybrid routing protocols are predominantly zone or cluster based. Examples of hybrid protocols are: ZRP (Zone Routing Protocol) ,TORA (Temporally-Ordered Routing Algorithm).
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Fig.-4 MANET routing protocols
VI. MANET Applications
With the increase of portable devices as well as progress in wireless communication, ad-hoc networking is gaining importance with the increasing number of widespread applications. The set of applications for MANETs is diverse, ranging from large-scale, mobile, highly dynamic networks, to small, static networks that are constrained by power sources. Typical applications include:
1) Military battlefield- Military equipment now routinely contains some sort of computer equipment. Ad-hoc networking would allow the military to take advantage of commonplace network technology to maintain an information network between the soldiers, vehicles, and military information head quarters.
Fig.- 5 Applications of MANET
2) Commercial sector- Ad-hoc can be used in emergency/rescue operations for disaster relief efforts, e.g. in fire, flood, or earthquake. Emergency rescue operations must take place where non-existing or damaged communications infrastructure and rapid deployment of a communication network is needed. Information is relayed from one rescue team member to another over a small handheld. Other commercial scenarios include e.g. ship-to-ship ad hoc mobile communication, law enforcement, etc.
3) Local level- Ad hoc networks can autonomously link an instant and temporary multimedia network using notebook computers or palmtop computers to spread and share information among participants at a e.g. conference or classroom. Another appropriate local level application might be in home networks where devices can communicate directly to exchange information.
4) Personal Area Network (PAN)- Short-range MANET can simplify the intercommunication between various mobile devices (such as a PDA, a laptop, and a cellular phone). Tedious wired cables are replaced with wireless connections. Such an ad hoc network can also extend the access to the Internet or other networks by mechanisms e.g. Wireless LAN (WLAN), GPRS, and UMTS.
VIII Conclusion and Future Scope
This paper describes the fundamental designing issues and history of MANET. Firstly, the background information of MANET is introduced. Then the main design issues and applications of MANET are discussed. The focus of the study is on these issues in our future research work and effort will be made to propose a solution for routing in ad-hoc networks by tackling these core issues of secure and power aware/energy efficient routing. The future of ad-hoc networks is really appealing, giving the vision of anytime, anywhere and cheap communications. In coming years, mobile computing will keep flourishing, and an eventual seamless integration of MANET with other wireless networks, and the fixed Internet infrastructure, appears inevitable. Ad hoc networking is at the center of the evolution towards the 4th generation wireless technology. Its intrinsic flexibility, ease of maintenance, lack of required infrastructure, auto-configuration, self-administration capabilities, and significant costs advantages make it a prime candidate for becoming the reliable technology for personal pervasive communication.