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Chapter – 3
OVERVIEW OF MOBILE AD HOC
NETWORKS
3.1 HISTORICAL DEVELOPMENTS OF MANET
In early 1970s, the Mobile Ad hoc Network (MANET) was called packet radio network, which was sponsored by Defense Advanced Research Projects Agency (DARPA). They had a project named packet radio having several wireless terminals that could communication with each other on battlefields. “It is interesting to note that these early packet radio systems predict the Internet and indeed were part of the motivation of the original Internet Protocol suite” [26].
The whole life cycle of Ad hoc networks could be categorized into the first, second, and the third generation Ad hoc networks systems. Present Ad hoc networks systems are considered the third generation [27].
The first generation goes back to 1972. At the time, they were called PRNET (Packet Radio Networks). In conjunction with ALOHA (Arial 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 (Third generation), the concept of commercial Ad hoc networks arrived with notebook computers and other viable communication equipments. At the same time, the idea of a collection of mobile nodes was proposed at several researchers gatherings. The IEEE 802.11 [3]^ 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 [28]^.
3.2 BASIC CONCEPTS OF MOBILE AD HOC NETWORKS
An Ad hoc network [29]^ is a collection of mobile nodes, which forms a temporary network without the aid of centralized administration or standard support devices regularly available as conventional networks. These nodes generally have a limited transmission range and, so, each node seeks the assistance of its neighboring nodes in forwarding packets and hence the nodes in an Ad hoc network can act as both routers and hosts. Thus a node may forward packets between other nodes as well as run user applications. By nature these types of networks are suitable for situations where either no fixed infrastructure exists or deploying network is not possible. Ad hoc mobile networks have found many applications in various fields like military, emergency, conferencing and sensor networks. Each of these application areas has their specific requirements for routing protocols.
Since the network nodes are mobile, an Ad hoc network will typically have a dynamic topology, which will have profound effects on network characteristics. Network nodes will often be battery powered, which limits the capacity of CPU, memory, and bandwidth. This will require network functions that are resource effective. Furthermore, the wireless (radio) media will also affect the behavior of the network due to fluctuating link bandwidths resulting from relatively high error rates. These unique desirable features pose several new challenges in the design of wireless Ad hoc networking protocols. Network functions such as routing, address allocation, authentication and authorization must be designed to cope with a dynamic and volatile
3.3 FEATURES OF MOBILE AD HOC NETWORKS
The mobile Ad hoc networks has the following features- Autonomous terminal Distributed operation Multihop routing Dynamic network topology Fluctuating link capacity Light-weight terminals
3.3.1 Autonomous Terminal
In MANET, each mobile terminal is an autonomous node, which may function as both a host and a router. In other words, beside 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.
3.3.2 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 like security and routing.
3.3.3 Multihop Routing
Basic types of Ad hoc routing algorithms can be single-hop and multihop, based on different link layer attributes and routing protocols. Single-hop MANET is simpler than multihop in terms of structure and implementation, with the lesser cost of 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.
3.3.4 Dynamic Network Topology
Since the nodes are mobile, the network topology may change rapidly and unpredictably and the connectivity among the terminals may vary with time. MANET should adapt to the traffic and propagation conditions as well as the mobility patterns of the mobile network nodes. The mobile nodes in the network dynamically establish routing among themselves as they move about, forming their own network on the fly. Moreover, a user in the MANET may not only operate within the Ad hoc network, but may require access to a public fixed network (e.g. Internet).
3.3.5 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 subjected to noise, fading, and interference, and has less bandwidth than a wired network. In some scenarios, the path between any pair of users can traverse multiple wireless links and the link themselves can be heterogeneous.
3.3.6 Light Weight Terminals
In most of the cases, the MANET nodes are mobile devices with less CPU processing capability, small memory size, and low power storage. Such devices need optimized algorithms and mechanisms that implement the computing and communicating functions.
3.4 CHALLENGES OF MOBILE AD HOC NETWORKS
Ad hoc networking has been a popular field of study during the last few years. Almost every aspect of the network has been explored in one way or other at different level of problem. Yet, no ultimate resolution to any of the problems is found or, at least, agreed upon. On the contrary, more questions have arisen. The topics that need to be resolved are as follows [30]^ - Scalability Routing
3.4.2 Routing
Routing in wireless Ad hoc networks is nontrivial due to highly dynamic environment. An Ad hoc network is a collection of wireless mobile nodes dynamically forming a temporary network without the use of any preexisting network infrastructure or centralized administration. In a typical Ad hoc network, mobile nodes come together for a period of time to exchange information. While exchanging information, the nodes may continue to move, and so the network must be prepared to adapt continually to establish routes among themselves without any outside support.
3.4.3 Quality of Service
The heterogeneity of existing Internet applications has challenged network designers who have built the network to provide best-effort service only. Voice, live video and file transfer are just a few applications having very diverse requirements. Qualities of Service (QoS) aware solutions are being developed to meet the emerging requirements of these applications. QoS has to be guaranteed by the network to provide certain performance for a given flow, or a collection of flows, in terms of QoS parameters such as delay, jitter, bandwidth, packet loss probability, and so on. Despite the current research efforts in the QoS area, QoS in Ad hoc networks is still an unexplored area. Issues of QoS in robustness, QoS in routing policies, algorithms and protocols with multipath, including preemptive, priorities remain to be addressed.
3.4.4 Client-Server Model Shift [33]
In the Internet, a network client is typically configured to use a server as its partner for network transactions. These servers can be found automatically or by static configuration. In Ad hoc networks, however, the network structure cannot be defined by collecting IP addresses into subnets. There may not be servers, but the demand for basic services still exists. Address allocation, name resolution, authentication and the service location itself are just examples of the very basic services which are needed but their location in the network is unknown and possibly even changing over time. Due to the infrastructureless nature of these networks and node mobility, a different
addressing approach may be required. In addition, it is still not clear who will be responsible for managing various network services. Therefore, while there have been vast research initiatives in this area, the issue of shift from the traditional client-sever model remains to be appropriately addressed.
3.4.5 Security
A vital issue that has to be addressed is the Security in Ad hoc networks. Applications like Military and Confidential Meetings require high degree of security against enemies and active/passive eavesdropping attacker. Ad hoc networks are particularly prone to malicious behavior. Lack of any centralized network management or certification authority makes these dynamically changing wireless structures very vulnerable to infiltration, eavesdropping, interference, and so on. Security is often considered to be the major "roadblock" in the commercial application.
3.4.6 Energy Conservation [34-35]
Energy conservative networks are becoming extremely popular within the Ad hoc networking research. Energy conservation is currently being addressed in every layer of the protocol stack. There are two primary research topics which are almost identical: maximization of lifetime of a single battery and maximization of the lifetime of the whole network. The former is related to commercial applications and node cooperation issues whereas the latter is more fundamental, for instance, in military environments where node cooperation is assumed. The goals can be achieved either by developing better batteries, or by making the network terminals operation more energy efficient. The first approach is likely to give a 40% increase in battery life in the near future (with Li-Polymer batteries). As to the device power consumption, the primary aspect are achieving energy savings through the low power hardware development using techniques such as variable clock speed CPUs, flash memory, and disk spin down. However, from the networking point of view, our interest naturally focuses on the device's network interface, which is often the single largest consumer of power. Energy efficiency at the network interface can be improved by developing transmission/reception technologies on the physical layer.
questions on how the Ad hoc network could be designed so that they are compatible with wireless LANs, 3 Generation (3G) and 4G cellular networks.
3.5 ISSUES TO BE CONSIDERED WHEN DEPLOYING
MANET
The following are some of the main routing issues to be considered when deploying MANETs Unpredictability of Environment Unreliability of Wireless Medium Resource-Constrained Nodes Dynamic Topology Transmission Errors Node Failures Link Failures Route Breakages Congested Nodes or Links
Unpredictability of Environment: Ad hoc networks may be deployed in unknown terrains, hazardous conditions, and even hostile environments where tampering or the actual destruction of a node may be imminent. Depending on the environment, node failures may occur frequently.
Unreliability of Wireless Medium: Communication through the wireless medium is unreliable and subject to errors. Also, due to varying environmental conditions such as high levels of electro-magnetic interference (EMI) or inclement weather, the quality of the wireless link may be unpredictable.
Resource-Constrained Nodes: Nodes in a MANET are typically battery powered as well as limited in storage and processing capabilities. Moreover, they may be situated in areas where it is not possible to re- charge and thus have limited lifetimes. Because of these limitations, they must have algorithms which are energy efficient as well as operating with limited processing and memory resources. The available bandwidth of
the wireless medium may also be limited because nodes may not be able to sacrifice the energy consumed by operating at full link speed.
Dynamic Topology: The topology in an Ad hoc network may change constantly due to the mobility of nodes. As nodes move in and out of range of each other, some links break while new links between nodes are created.
As a result of these issues, MANETs are prone to numerous types of faults including the following-
Transmission Errors: The unreliability of the wireless medium and the unpredictability of the environment may lead to transmitted packets being garbled and thus received packet errors.
Node Failures: Nodes may fail at any time due to different types of hazardous conditions in the environment. They may also drop out of the network either voluntarily or when their energy supply is depleted.
Link Failures: Node failures as well as changing environmental conditions (e.g., increased levels of EMI) may cause links between nodes to break. Link failures cause the source node to discover new routes through other links.
Route Breakages: When the network topology changes due to node/link failures and/or node/link additions to the network, routes become out-of-date and thus incorrect. Depending upon the network transport protocol, packets forwarded through stale routes may either eventually be dropped or be delayed.
Congested Nodes or Links: Due to the topology of the network and the nature of the routing protocol, certain nodes or links may become over utilized, i.e., congested. This will lead to either larger delays or packet loss.
This client-server model suffers the following major drawbacks: a user cannot play games where there is no Internet infrastructure, or when the connection is too bad, or when the server is not available (either the server is down or refuses users because the maximum number of users is reached). Another drawback is that it limits the gamers from randomly announcing, discovering and joining a networked game.
3.6.2 Military Applications [41]
Military applications have motivated early research on Ad hoc networks. The ability to quickly set up a network among military units in hostile territory without any infrastructure support can provide friendly forces with a considerable tactical advantage on the battlefield. For instance, each soldier can carry a mobile device that represents one of the mobile nodes in an Ad hoc network linking all soldiers, tanks, and other vehicles as shown in Fig 3.3. Recent advances in robotics have also motivated the idea of automated battlefields in which unmanned fighting vehicles are sent into battle. Supporting military applications requires self-organizing mechanisms that provide robust and reliable communication in dynamic battle situations.
Fig 3.3: Soldiers, Tanks and other Vehicles carrying Mobile Devices
3.6.3 Emergency Operations [42]
Another promising application area for Ad hoc networks is emergency services, including search and rescue and disaster recovery operations. As an example of search and rescue, consider an airline that attaches small wireless devices to the life jackets under each seat. Suppose that the plane has mechanical problems and has to make an emergency landing in the water. Once search and rescue teams arrive at the landing site, they are provided with detailed information about the location (the
coordinates and potentially the depth) of the victims through the transponders. As a result, the rescue teams can more effectively locate and reach the victims. The mobile devices could also monitor the vital signs of victims, such as heart rate or breathing rate, to prioritize the rescue of victims that are still alive. A similar application arises when disasters, such as earthquakes, blackouts, or bombings occur. The disaster may destroy existing communication infrastructure, preventing critical contact among emergency workers. The emergency response teams can set up Ad hoc networks quickly to replace the destroyed infrastructure, enabling the teams to better coordinate their efforts. In emergency situation the wired networks could be destroyed. There will be a need of wireless network, which could be deployed quickly for coordination of rescue. An example is the design for future public safety communications. A European project called Wireless Deployable Network System (WIDENS) [29]^ concentrated their work on this field. WIDENS have an idea that using Ad hoc network to interoperate with existing TETRA network which is used for public safety. The system structure is shown in Fig 3.4.
Fig 3.4: WIDENS System Structure
3.6.4 Home, Office, and Educational Applications [43]
Ad hoc networks also have applications in home and office environments. The simplest and most direct application of Ad hoc networks in both homes and offices is the networking of laptops, PDAs and other WLAN-enabled devices in the absence of
random, vehicles tend to move in an organized fashion. The interactions with roadside equipment can likewise be characterized fairly accurately. And finally, most vehicles are restricted in their range of motion, for example by being constrained to follow a paved high way.
Fig 3.5: A Vehicular Ad hoc Network
In addition, in the year 2006 the term MANET mostly describes an academic area of research, and the term VANET perhaps it’s most promising area of application. In VANET, or Intelligent Vehicular Ad hoc Networking, defines an intelligent way of using Vehicular Networking. In VANET integrates on multiple Ad hoc networking technologies such as WiFi IEEE 802.11 b/g, WiMAX IEEE 802.16, Bluetooth, IRA, ZigBee for easy, accurate, effective and simple communication between vehicles on dynamic mobility. Effective measures such as media communication between vehicles can be enabled as well methods to track the automotive vehicles are also preferred. In VANET helps in defining safety measures in vehicles, streaming communication between vehicles, infotainment and telematics. Vehicular Ad hoc Networks are expected to implement variety of wireless technologies such as Dedicated Short Range Communications (DSRC) which is a type of WiFi. Other candidate wireless technologies are Cellular, Satellite, and WiMAX. Vehicular Ad hoc Networks can be viewed as component of the Intelligent Transportation Systems (ITS).
3.6.6 Wireless Sensor Networks [48-51]
Advances in processor, memory and radio technology will enable small and cheap nodes capable of sensing, communication and computation. Networks of such nodes called wireless sensor networks can coordinate to perform distributed sensing of environmental phenomena.
Fig 3.6: Applications of Sensor Networks
Sensor networks have emerged as a promising tool for monitoring (and possibly actuating) the physical world, utilizing self-organizing networks of battery- powered wireless sensors that can sense, process and communicate. A sensor network] is a network of many tiny disposable low power devices, called nodes, which are spatially distributed in order to perform an application-oriented global task. These nodes form a network by communicating with each other either directly or through other nodes. One or more nodes among them will serve as sink(s) that are capable of communicating with the user either directly or
