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Wireless LAN networks can be classified into two categories. The first and most common is infrastructure networks with fixed and wired gateways (wireless network built on-top of a wired network). In this kind of networks mobile nodes connect to a network via an AP (Access Point) which is within its coverage range in a single hop communication technique. The second type of wireless network is the infrastructure-less mobile network, this commonly known as mobile ad hoc network (MANET) as in figure... shows example of the two kinds, In MANET nodes can communicate directly, nodes operate both as router and host, sending and receiving packets of data to and from other nodes in the network. Also MANETs are called multi-hop networks.
One advantage of wireless is the ability to transmit data among users in a common area while remaining mobile. However, the distance between participants is limited by the range of transmitters or their proximity to wireless access points. On the other hand Mobile Ad hoc wireless networks (MANET) solve this problem by allowing out of range nodes to route data through intermediate nodes.
Diagram showing a traditional wireless networkadhocnet.gif
In recent years various wireless network technologies have been developed separately to offer different services, coverage area and data rates. In this introduction we will describe in overview:
Wi-Fi (short for Wireless Fidelity) is a class of wireless (LAN) devices; the technology is based on the IEEE 802.11 standards. Today, Wi-Fi devices can be found in many desktop computers, smart phones, printers, and indeed all modern laptops and (PDAs) are equipped with Wi-Fi technology .
Wi-Fiâ€™s original purpose was mobile computing devices (eg.Laptops in LANs), but is now progressively more used for more purposes, including VoIP phones, games, and televisions and DVD players.
Wi-Fi is more commonly used to provide an Internet LAN connection to Wi-Fi enabled devices like a computers, smart phones or PDAs. The above functions require the device to be within range of an access point
Wi-Fi however is short range (10's of meters) can be encrypted with WEP or WPA encryption but it has problems including interference as in crowded areas where there are many users.
The most common Wi-Fi standard IEEE 802.11G has a data transfer rate of around 54 Mbits/sec, the range indoors is a maximum 150 feet (approximately 45 meters) and double outdoors. This depends on the conditions, like obstacles, power and weather.
In Wi-Fi MAC (Media Access Controller) users are competing when they are connected to Wi-Fi access point, and users therefore have different levels of bandwidth.
In Wi-Fi except for 802.11a, which operates at 5 GHz, Wi-Fi uses the spectrum near 2.4 GHz, which is standardized and unlicensed by international agreement, although the exact frequency allocations vary slightly in different parts of the world, as does maximum permitted power. However, channel numbers are standardized by frequency throughout the world, so authorized frequencies can be identified by channel numbers.
Wi-Fi Hotspots are typically backhauled over ADSL in most coffee shops therefore Wi-Fi access is typically highly contended and has poor upload speeds between the router and the internet.
WiMAX is a short name for Worldwide Interoperability of Microwave Access. based on the IEEE 802.16 standard (also called Broadband Wireless Access). WiMAX was formed in 2001 by the WiMAX Forum, in order to endorse WiMAX as a standard.
WiMAX was described as a standard based technology for use as â€œlast mileâ€ broadband delivery rather than using wires.
WiMAX is planned to be used to link Wi-Fi hotspots together
WiMax technology is not affected by obstacles like buildings. This advantage makes Wimax especially useful and cost-effective for country side homes where setting a traditional wire would be more difficult and very expensive.
WiMAX is equipped with stronger encryption than Wi-Fi, and typically suffrers less interference .
How ever wimax is still in the early stages of development, and it suffers from such limitations like limited bandwidth and sheer cost in creation.
WiMAX will deliver 70 Mbit/s, 112 kilometres in theory. But these numbers will
WiMAX technology (unlike Wi-Fi) allows each user a constant amount of bandwidth disadvantage of WiMAX is the spectral limitation, in other words limitation of wireless bandwidth. For use in high density areas, it is possible that the bandwidth may not be sufficient to satisfy the needs of large customers.
One huge disadvantage of WiMax over Wi-Fi is with Wi-Fi anyone can set up a wireless network for free, in comparison not everyone is permitted to set up a WiMax wireless network; and one must pay incredibly huge amounts of money to be given the right to set up a WiMax wireless network in a region.
But WiMax is still in the early stages of development, and it will need a significant boost in support and infrastructure before it gains any traction in the wireless market. WiFi, on the other side, has already filled a significant percentage of the wireless market, and it has shown both easy to use and cheap. While businesses with large physical space might want to move to WiMax to avoid buying the many repeater access points required with WiFi, it will be several years before WiMax becomes cheap enough to enter the residential and small commercial market.
In short, WiMax technology promises a future of more powerful and more accessible wireless Internet access. For the meantime, however, WiFi will be the mainstay.
Wireless sensor network
Sensor network: is a group of distributed autonomous sensors working together to monitor physical or environmental conditions, such as temperature, sound, vibration, pressure, motion or pollutants. These sensors are small with limited processing and computing resources up to the smart sensors that are equipped with one or more sensors, memory, processor, a power supply, a radio and an actuator.
Each sensor has wireless communication capability or radio transceiver, small microcontroller, energy source (usually battery, solar panel might be added) and some intelligence for signal processing and networking of the data.
Sensor network is used in military applications such as battlefield surveillance; equipment ammunition; targeting; and nuclear, biological and chemical attack detection and reconnaissance. And it used in many industrial and civilian application areas, including monitoring product quality and control, machine health monitoring, environment monitoring, healthcare applications, home automation(smart home), and traffic control.
There are two types of wireless sensor networks: structured and unstructured. An unstructured WSN is one that includes an intense collection of sensor nodes. Sensor nodes may be deployed in an ad hoc mode, sensor nodes may be randomly placed into the field. In this way of WSN, network maintenance such as managing connectivity and detecting failures is difficult because there are so many nodes.
While the structured WSN, most of the sensor nodes are deployed in a pre-planned manner (sensor nodes are pre-determined to be placed at fixed locations). The advantage of a structured network is that fewer nodes can be deployed with lower network maintenance and management cost .
1.1Wireless ad hoc networks
Ad hoc networks are autonomous systems which are comprised of a number of mobile nodes that communicate between themselves using wireless transmission. They are self-organized, self-configured and self-controlled infrastructure-less networks. This kind of network has the advantage of being able to be set up and deployed anywhere and anytime because it has a simple infrastructure setup and no central administration .
Mobile ad hoc networks (MANET) are kind of wireless ad hoc networks, increasingly popular and successful in the marketplace of wireless technology. Examples include Bluetooth and Wireless Local Area Networks (WLANs).
These networks are therefore particularly useful to those mobile users who need to communicate in situations where no fixed wired infrastructures are available. Obvious examples are the military or the emergency services: one clear situation might be a fire fighter who needs to connect to an ambulance en route to an emergency. In such situations a collection of mobile hosts with wireless network interface can form a temporary network because many modern laptops are powerfully equipped with this technology .
Recently, ad hoc network received extensive attention in both commercial and military applications, due to the attractive properties of creating a network on the fly and not requiring any pre planned infrastructure such as a base station .
Applications of mobile ad hoc networks
These have been listed in :
Home network and enterprise network: unlike a fixed wireless network, wireless ad hoc nodes are free to move and organise themselves in an arbitrary fashion. Any user is free to roam about while communicating with others. The path between each pair of nodes may have multiple links. This allows an association of various links to be part of the same network. Mobile ad hoc networks can work either in stand-alone fashion or be connected to a larger network such as the Internet one of many possible users of mobile ad hoc networks is in some business environments, ad hoc networks are suited for use in cases where an infrastructure is unavailable or to deploy one is not cost effective.
Military applications: ad hoc network can be very useful to soldiers in establishing communication for tactical operations, setting up a fixed infrastructure in enemy territories or in inhospitable terrains where it may not otherwise be possible, in such environments. Whereas, ad hoc wireless networks provide the required communication mechanism quickly. The coordination of military objects moving at high speeds such as fleets of airplanes or warships is another application in this area.
Emergency response network: a mobile ad hoc network can be used to provide crisis management services applications, such as in disaster recovery, where the entire communication infrastructure is destroyed and restoring communication quickly is crucial. By using a mobile ad hoc network, an infrastructure could be set up in hours instead of in weeks, as with wired communication.
Sensor Network: wireless sensor networks can be deployed in ad hoc manner to facilitate monitoring and controlling of physical environments from remote locations with better accuracy. These sensors are might be equipped with a variety of devices in order to measure various physical attribute such as temperature, humidity, barometric pressure and velocity.
The Characteristics of MANET
A mobile ad hoc network (MANET) is an autonomous system of mobile routers (and associated hosts) connected by wireless links. The routers are thus free to move randomly; therefore, the topology of wireless network may change swiftly and unpredictably.
The only possible direct communication in MANETs is between neighbouring nodes. Therefore, communication between remote nodes is based on multiple-hop. These locations of these nodes may change dynamically, therefore the interconnections between the adjacent nodes may change continually. Each mobile node acts as a host and a router, relaying information from one neighbour to others. MANETs have therefore various defining characteristics that differentiate them from other wired and wireless networks [-,-,-]: which in details are:
Infrastructure less: MANETs are formed based on the collaboration between independent, peer-to-peer nodes that need to communicate with each other for some purpose. No prior organisation or base is defined and all devices have the same rule in the network. In addition, there are no pre-set roles such as servers, routers or gateways for the nodes participating in the network.
Dynamic Topology: MANET nodes are free to move around; thus they could be in and out of the network, constantly changing its links and topology, and the routing information will be changing all the time because of the movement of the nodes. In addition, the links between nodes could be bi-directional or unidirectional.
Low and Variable Bandwidth: Wireless links which connect the MANET nodes have much smaller bandwidth than wired links, while the effects of interference, noise and congestion may be more significant, causing the available bandwidth to vary with the surrounding conditions and to be reduced.
Constrained Resources: In general, most of the MANET devices are small handheld devices ranging from laptops and personal digital assistants (PDAs) down to cell phones. These devices have limited power (battery-operated), processing capabilities and storage capacity, since on ad hoc network consists of multiple nodes, and depletion of batteries in these nodes will have a significant influence on overall network performance. Therefore, the energy consumption is important criterion when designing the system.
Multi-hop Communication: The communication in MANET between two end nodes is carried out through a number of intermediate nodes whose function is to relay information from one point to another. Thus, ad hoc networks require the support of multi-hop communications. For example, in figure 1.1, nodes A and D must enlist the aid of nodes B and C to relay packets between them in order to communicate.
Figure 1.1: Mobile ad hoc network of four nodes, node A communicates with node D
Limited Device Security: MANETs devices are usually small and portable, and are therefore not restricted by location. Unfortunately, therefore these devices can be easily lost, damaged or stolen.
Limited Physical Security: MANET are generally more susceptible to physical layer's attacks than wired network, the increased possibility of eavesdropping, spoofing, jamming and denial of service (DoS) attacks should be carefully considered , the decentralised nature of MANETs however makes them better protected against single failure points.
Short Range Connectivity: MANETs depend on radio frequency (RF) or infrared (IR) technology for connectivity, both of which are generally short range. Therefore, the nodes that wish to communicate directly need to be in close proximity to each other. To overcome this limitation, multi-hop routing techniques have therefore to be used to connect distant nodes through intermediate ones that act as routers.
The Challenges of MANET
The main challenges in the design and operation of MANETs come from the lack of a centralised entity such as a base station, access point or server (infrastructure less), the possibility of rapid node movement and the fact that all communication is conducted through the wireless medium. These unique vulnerabilities present appreciable challenges for MANETs such as:
Media Access: As opposed to cellular networks, there is a lack of centralised control and global synchronisation. Also many Media Access Control (MAC) protocols do not deal with host mobility, a deficiency which should be considered when it comes to designing MAC protocols for wireless ad hoc networks [28, 29].
Routing: The presence of mobility implies that links make and break randomly and often. Existing distance vector and link state-based routing protocols are unable to catch up with such frequent link changes, especially with the absence of fixed routers and stable links between them. Hence, new routing protocols are needed [30, 31].
TCP Performance: Unfortunately, Transmission Control Protocol (TCP) is able to distinguish neither the presence of mobility nor network congestion. Hence, some changes are required such the transport protocol is able to perform properly without affecting the end-to-end communication throughput [34, 35].
Service Location, Provision, and Access: there are continued assumptions that the traditional client/Server RPC (Remote Procedure Call) paradigm could be appropriate for MANETs, because of the limitations in bandwidth and the heterogeneity of the devices in ad hoc networks this may not be attractive. Also, how can a mobile device access a remote service in an ad hoc network or how it can advertise its ability to provide services to the rest of the devices in the network, all these issues require research .
Security: The unique characteristics of MANETs present a new set of serious challenges to security design such as open peer-to-peer network architecture, shared wireless medium, stringent resource constraints, and highly dynamic network topology. These challenges clearly make a case for building security solutions that achieve both broad protection and desirable network performance [36- 66].
Lack of Secure Boundaries: There is no clear secure boundary in
the mobile ad hoc network compared with the clear line of defence in the
traditional wired network where the node must get physical access to the network medium, and pass through through several lines of defence such as firewall and gateway. This vulnerability be created from the nature of the mobile ad hoc network; freedom to join, leave and move inside the network .once the node is in the radio range of any other nodes in the mobile ad hoc network, it can communicate with those nodes in its radio range and thus join the network automatically, As a result, the mobile ad
hoc network does not provide secure boundary to protect the network from some potentially dangerous network accesses.
Unreliability of wireless links between nodes: Because of the limited battery supply for the wireless nodes and the mobility of the nodes, the wireless links between mobile nodes in the ad hoc network are not consistent for the communication participants, some nodes in MANET may behave in a selfish manner when it finds that there is only limited power supply or intends to be non-cooperation.
Threats from Compromised nodes Inside the Network: Because of the mobility of the ad hoc network, a compromised node can frequently change its attack target and perform malicious behaviour to different node in the network, thus it is very difficult to track the malicious behaviour performed by a compromised node especially in a large scale ad hoc network. Therefore, threats from compromised nodes inside the network are far more dangerous than the attacks from outside the network, and these attacks are much harder to detect because they come from the compromised nodes, which behave well before they are compromised.
Lack of Centralized Management Facility: The absence of centralized management entity makes the detection of attacks a very difficult problem since highly dynamic large networks cannot be easily monitored, benign failures in the mobile ad hoc network is fairly common such as path breakages, transmission impairments and packet dropping. And therefore, malicious failures will be more difficult to distinguish.
Restricted Power Supply: The nodes in the MANET are generally operated by small batteries with limited lifetimes, which will cause several problems. Such a node cannot be expected to be able to carry out intensive computations. Here attackers have opportunity to launch a denial-of-service attack by creating additional transmissions or intensive computations (like to send additional routing packets to a targeted node) in order to be carried out by the node in an attempt to exhaust its battery.
Scalability: In MANET nodes entering and leaving the network causes frequent changes of the network topology, the network may consist of hundreds or even thousands of nodes, therefore the protocols and services that are applied to the ad hoc network such as routing protocol and key management service should be compatible to the continuously changing scale of the ad hoc network.