• A mobile ad-hoc network is a collection of mobile nodes forming an ad-hoc network without the assistance of any centralized structures. 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. • There are quite a number of uses for mobile ad-hoc networks which can be discussed later .

The whole life-cycle of ad-hoc networks could be categorized into the first, second, and the third generation ad-hoc networks systems.

FIRST GENERATION:- The first generation goes back to 1972. At the time, they were called PRNET (Packet Radio Networks). In conjunction with ALOHA (Areal Locations of Hazardous Atmospheres) and CSMA (Carrier Sense Medium Access), approaches for medium access control and a kind of distance-vector routing PRNET were used on a trial basis to provide different networking capabilities in a combat environment.

The second generation of ad-hoc networks emerged in 1980s,when the ad-hoc network systems were further enhanced and implemented as a part of the SURAN (Survivable Adaptive Radio Networks) program. This provided a packet-switched network to the mobile battlefield in an environment without infrastructure. This program proved to be beneficial in improving the radios' performance by making them smaller, cheaper, and resilient to electronic attacks.

In the 1990s, the concept of commercial ad-hoc networks arrived with notebook computers and other viable communications equipment. At the same time, the idea of a collection of mobile nodes was proposed at several research conferences. The IEEE 802.11 subcommittee had adopted the term "ad-hoc networks" and the research community had started to look into the possibility of deploying ad-hoc networks in other areas of application.

Meanwhile, work was going on to advance the previously built ad-hoc networks. GloMo (Global Mobile Information Systems) and the NTDR (Near-term Digital Radio) are some of the results of these efforts.

Present ad-hoc networks systems are considered as the third generation.

Mobile ad hoc network’s routing protocols are characteristically subdivided into three main categories. These are proactive routing protocols, reactive routing protocols and hybrid routing protocols.

The Efficient routing protocols can provide significant benefits to mobile ad hoc networks, in terms of both performance and reliability.Reactive Routing Protocols : Reactive MANET protocols only find a route to the destination node when there is a need to send data. The source node will start by transmitting route requests throughout the network. The sender will then wait for the destination node or an intermediate node (that has a route to the destination) to respond with a list of intermediate nodes between the source and destination. This is known as the global flood search, which in turn brings about a significant delay before the packet can be transmitted. It also requires the transmission of a significant amount of control traffic. Thus, reactive MANET protocols are most suited for networks with high node mobility or where the nodes transmit data infrequently.

AD HOC ON DEMAND DISTANCE VESTOR(AODV)

DYNAMIC SOURCE ROUTING(DSR)

ADMISSION CONTROL ENABLED ON DEMAND ROUTING(ACOR)

ASSOCIATIVITY BASED ROUTING(ABR)

•Proactive MANET protocols are table-driven and will actively determine the layout of the network. Through a regular exchange of network topology packets between the nodes of the network, a complete picture of the network is maintained at every single node. There is hence minimal delay in determining the route to be taken. This is especially important for time-critical traffic . • However, a drawback to a proactive MANET of protocol is that the life span of a link is significantly short. This phenomenon is brought about by the increased mobility of the nodes, which will render the routing information in the table invalid quickly. Thus, proactive MANET protocols work best in networks that have low node mobility or where the nodes transmit data frequently.

DESTINATION SEQUENCED DISTANCE VECTOR(DSDV)

OPTIMISED LINK STATE ROUTING(OLSR)

WIRELESS ROUTING PROTOCOL(WRP)

CLUSTER HEAD GATE WAY SWITCH ROUTING(CGSR)

Since proactive and reactive routing protocols each work best in oppositely different scenarios, there is good reason to develop hybrid routing protocols, which use a mix of both proactive and reactive routing protocols. These hybrid protocols can be used to find a balance between the proactive and reactive protocols. The basic idea behind hybrid routing protocols is to use proactive routing mechanisms in some areas of the network at certain times and reactive routing for the rest of the network. The proactive operations are restricted to a small domain in order to reduce the control overheads and delays. The reactive routing protocols are used for locating nodes outside this domain, as this is more bandwidth-efficient in a constantly changing network.

oTEMPORARY ORDERED ROUTING ALOGORITHM(TORA)

oZONE ROUTING PROTOCOL(ZRP)

oORDERONE ROUTING PROTOCOL(OORP)

The first, and the most important scenario, is the ability to establish a network in places where it's not possible otherwise: i.e., in a disaster relief setting, or in a situation where the entire communication structure has been destroyed. One thing we should be clear about is, there's no doubt two machines in an ad-hoc network could be networked, but with a completely different art of network formation. No router, no administrator, no access server, etc. What does it mean? It means we can't use the existing strategies for network control as efficiently as we do in a fixed wireless network. New technology requires new techniques. This is one of the major issues that appears to be an obstacle in deploying an ad-hoc network as a leading technology in the future wireless generation.

Routing, security, limited bandwidth, and low power are some of the important challenges to the technology.

As far as low power is concerned, it's more of a hardware issue. However, it does effect the other operations of the ad-hoc network.

Consider that in a network there are three nodes named as node1,node2,node3. Assume that the node1 wants to transmits data to the node3 through node2(router) .If node2 refuses to transmit the data or unable to do so then this leads to a problem.

In mobile ad-hoc networks, a mobile node meets two types of security challenges: import and export authorization.

Import authorization requires a node acting as a router to decide weather or not it should modify its routing information when it receives information from somewhere outside.

Export authorization requires the router to make a decision whenever it receives a request for routing information. Source authentication: we need to be able to verify that the node is the one it claims to be.

Integrity: we need to be able to verify that the routing information that it is being sent to us has arrived safely.

From the security perspective, in mobile ad-hoc networks there are two kinds of messages--the routing, and the data message--with different natures and different security needs. Data messages are point-to-point and can therefore be saved by using an existing point-to-point security mechanism, such as IPSec (Secure Internet Protocol).However, for routing messages, there will always be some parts of those messages that will change during their propagation. This is perhaps the main challenge posed by routing messages to the ad-hoc environment. Normally, routing messages carry two types of information, mutable and non-mutable. It's desirable that the mutable information in a routing messages is secured in such a way that no trust in intermediate nodes is required. Otherwise, securing the mutable information will be much more computationally intensive. Plus, the overall security of the system will greatly decrease.

•One possible solution is to use a trusted certificate server C, whose public key is known to all participating nodes.

•Keys are priority generated and are exchanged through a mutual relationship between C and each node. Each node obtains a certificate with exactly a single key from the trusted certificate server upon joining the network.

•The certificate details different aspects of the connecting node. These details include node addresses, a public key, and a time stamp t1 and t2, where t1 and t2 represent the certificate issue and certificate expiration time. These certificates are authenticated and signed by the server C.

The goal of communication between the source and the destination is to make sure that the data safely reaches the destination. Therefore, whenever a node wants to transmit data to a destination for which it does not have any routing entry in its routing tables, it can adopt one of several mechanisms, such as invoking a route discovery mechanism (on-demand protocols), or invoking route discovery and data delivery processes (mobile ad-hoc on-demand data delivery protocol). As for the security perspective, besides an IP address of the destination, a broadcast ID, and a source ID, the packet also contains a certificate A and the expiration time t2. All of these are assigned to the public key, which was allocated to the source node at the time it joined.

 

o Each intermediate or receiving node of the package extracts the public key from the certificate C attached to the packet to validate the signature and make sure the certificate is still valid before forwarding it to other nodes.o To explain further, consider a scenario in which node A wants to transmit data to node D. Node B on the receiving route request verifies the public key and the certificate validation time by extracting this information from the certificate attached to the packet. Once this has been done, B then removes the A certificate signature, records B as predecessor, signs the contents of the message originally broadcast by A, appends its own certificate, and forwards the broadcast message to the neighboring node until it reaches D.

One of many possible uses of mobile ad-hoc networks is in some business environments, where the need for collaborative computing might be more important outside the office environment than inside, such as in a business meeting outside the office to brief clients on a given assignment.A mobile ad-hoc network can also be used to provide crisis management services applications, such as in disaster recovery, where the entire communication infrastructure is destroyed and resorting communication quickly is crucial. By using a mobile ad-hoc network, an infrastructure could be set up in hours instead of weeks, as is required in the case of wired line communication.

Another application example of a mobile ad-hoc network is Bluetooth, which is designed to support a personal area network by eliminating the need of wires between various devices, such as printers and personal digital assistants.

We have presented a detailed performance comparison of routing protocols for mobile ad hoc wireless networks. Reactive protocols performed well in high mobility scenarios than proactive protocol. High mobility result in highly dynamic topology i.e. frequent route failures and changes. Both proactive protocols fail to respond fast enough to changing topology.Security is one of the fundamental issues in mobile ad-hoc networks. I've highlighted different security requirements, along with a possible solution to secure transmission in these types of networks. The proposed security solution is one of several ways to protect data communication in mobile ad-hoc networks. However, there's a solid need to deploy more efficient strategies to resolve various issues besides security in mobile ad-hoc networks.