DEFINITION OF WIRELESS AD HOC NETWORKS
In the last few years we have seen the proliferation of wireless communications technologies. Wireless technologies are being widely used today across the globe to support the communications needs of very large numbers of end users. There are over 1 billion wireless subscribers of cellular services today utilizing wireless devices for voice communications (e.g. phone calls) and data services. Data services include activities such as sending e-mail and instant messages, and accessing the Web. In fact, in some areas of the world wireless technologies are more prevalent than traditional wireline communications technologies. There are several reasons for the current popularity of wireless technologies. The cost of wireless equipment has dropped significantly, allowing service providers to significantly reduce the price of wireless services and making them much more affordable to end users. The cost of installing wireless networks in emerging markets has dropped well below the cost of installing wireline networks. The wireless technologies themselves have improved tremendously, making it possible to offer both voice and data services over such networks. The resulting allure of anytime, anywhere services makes such services very attractive for the end users. In wireless networks, nodes transmit information through electromagnetic propagation over the air. The signal transmitted by a node can only be received by nodes that are located within a specific distance from the transmitting node. This distance is typically called the transmission range. The transmission range depends not only on the power level used for the transmission, but also on the terrain, obstacles, and the specific scheme used for transmitting the information. Typically, for simplicity, the transmission range of nodes is assumed to be a circle around the transmitting node, as shown in Figure 1.1. Typically multiple nodes exist within an area and these nodes might need to make use of the wireless medium for communication. If many such transmissions happen at the same time within the transmission range of a node, then this will result in the transmissions colliding with each other. Such collisions make it impossible for receivers to interpret the data being transmitted by individual nodes. The effect here is similar to many people talking simultaneously to a person, in which case the person involved will not be able to understand any of them. Therefore, it is vital to prevent or minimize such collisions. This can be done by controlling access to the wireless medium. This is the approach typically followed by the collision avoidance or minimization schemes. Many collision avoidance or minimization schemes have been developed for sharing the available wireless spectrum among wireless nodes transmitting concurrently. Typical schemes include: (1) time division multiple access (TDMA), which divides time into small time slots and requires nodes to take turns transmitting data during separate time slots; (2) frequency division multiple access (FDMA), which provides for different frequencies such that each node transmits on a different frequency; (3) carrier sense multiple access (CSMA), which requires for every node to listen for transmissions on the wireless channel (on a given frequency) and transmit its own data when the node perceives the channel to be free of any other wireless transmissions; and (4) code division multiple access (CDMA), which allows nodes to transmit at the same time but requires them to use different spreading codes so that the signals from different nodes can be distinguished by the receivers. Nodes might need to communicate with other nodes that are outside their transmission range. This is typically accomplished by having other nodes that are within the transmission range of the transmitting node receive and then retransmit the signal. As a result of this retransmission, nodes within transmission range of the node repeating the original signal receive the data. Depending on the location of the destination, multiple nodes may need to retransmit/repeat the data, as shown in Figure 1.2.
Various network architectures have been introduced based on the high-level concepts discussed so far. Such architectures allow wireless services and provide for end-to-end communication among users often located far away from each other. Figure 1.3 shows a typical architecture that is used for cellular networks. In a typical cellular architecture, radio transmission towers are placed across the area that the service provider desires to offer cellular service in. These towers are often built on top of buildings, on big towers, on high ground, and so on, and are hence stationary. These radio transmission towers are responsible for receiving the data transmitted by other nodes and then retransmitting the data as needed in order to reach the destination. The devices used by end users for accessing the service are typically small and mobile (e.g. mobile phones). End devices typically only communicate directly with the radio transmission tower that is closest to them. The radio transmission tower is then responsible for transmitting that information towards the node that needs to receive that information. The radio transmission tower might also enlist the help of other radio transmission towers in order to do this. In a cellular network, towers are typically interconnected through a static wireline network (e.g. SONET network) with each other. An end device transmits information to the local tower. If the destination end device is unreachable from the local tower, then the local tower locates the tower closest to the destination. Following this, the local tower transmits the information to the tower closest to the destination end device through the wireline network. The tower closest to the destination is then responsible for transmitting the information to the destination end device. Cellular technology is not the only wireless technology in existence. Another widely used wireless technology is IEEE 802.11-based wireless local area network (WLAN), also popularly referred to as Wi-Fi. Wi-Fi has mostly been used for providing wireless data connectivity inside buildings for personal computers and laptops. This technology allows such devices to communicate potentially at very high speeds (but over relatively smaller distances) as compared with cellular networks. In fact, these networks are called WLAN networks since they typically provide the equivalent of LAN connectivity Figure 1.3. Architecture of cellular networks. inside buildings. Figure 1.4 shows the typical network architecture used today for 802.11. This architecture utilizes fixed access points (APs) that play a similar role to that played by radio towers in the cellular environment. APs are responsible for receiving the signal from end devices and then retransmitting them to the destination. The APs also have the responsibility for interconnecting the wireless LAN to external networks such as the internet or other WLANs (through other access points to which they could be connected over wireline links). The wireless networks that we have discussed so far are dependent on fixed nodes (the radio towers and access points) for connecting the mobile nodes. In addition, these networks require some fixed infrastructure to interconnect the fixed nodes with each other. This type of architecture has been very successful and widely deployed throughout the world for offering a variety of voice and data services, despite being inflexible (by requiring fixed nodes). This is because the architecture has been sufficient for services typically offered by service providers. Having a communications network that relies on a fixed infrastructure, however, is not always acceptable for some applications (see Section 1.2). For example, when emergency responders move into an area (say to deal with a disaster), it is possible that the fixed infrastructure may have been destroyed or may be unavailable (e.g. in some remote areas). Emergency responders might not have enough time to establish a fixed infrastructure in such cases. A similar situation might also arise in a battlefield environment. In the past few years, a new wireless architecture has been introduced that does not rely on any fixed infrastructure. In this architecture, all nodes may be mobile and no nodes play any special role. One example of this architecture is the “ad hoc” mode architecture of 802.11, as shown in Figure 1.5. In this architecture, 802.11 nodes do not rely on access points to communicate with each other. In fact, nodes reach other nodes they need to communicate with using their neighbors. Nodes that are close to each other discover their neighbors. When a node needs to communicate with another node, it sends the traffic to its neighbors and these neighbors pass it along towards their neighbors and so on. This repeats until the destination of the traffic is reached. Such an architecture requires that every node in the network play the role of a router by being able to determine the paths that packets need to take in order to reach their destinations. Networks that support the ad hoc architecture are typically called wireless ad hoc networks or mobile ad hoc networks (MANET). We will use these two terms interchangeably throughout the book. Such networks are typically assumed to be self-forming and selfhealing. This is because the typical applications of such networks require nodes to form networks quickly without any human intervention. Given the wireless links and mobility of nodes, it is possible that nodes may lose connectivity to some other nodes. This can happen if the nodes move out of each other’s transmission range. As a result, it is possible for portions of the network to split from other portions of the network. In some applications it is also possible that some nodes may get completely disconnected from the other nodes, run out of battery, or be destroyed. For these reasons, nodes in a MANET cannot be configured to play any special role either in the way nodes communicate or in the way of providing communication services (e.g. naming services). This leads to a symmetric architecture where each node shares all the responsibilities. The network needs to be able to reconfigure itself quickly to deal with the disappearance (or reappearance) of any node and continue operating efficiently without any human intervention. Routing in such networks is particularly challenging because typical routing protocols do not operate efficiently in the presence of frequent movements, intermittent connectivity, network splits and joins. In typical routing protocols such events generate a large amount of overhead and require a significant amount of time to reach stability after some of those events. The Internet Engineering Task Force (IETF), which is the main standardization body for the internet, has recognized that existing routing protocols cannot meet the unique requirements of MANET and has played a key role in the creation of novel MANET routing protocols. This is done through the IETF MANET Working Group, which has been a focal point for a lot of the related research. This group was established in 1997 and since then has created some of the most widely cited MANET routing protocols such as the ad hoc on demand distance vector (AODV) and optimized link state routing (OLSR) routing protocols (see www.ietf.org/html.charters/manet-charter.html). Its efforts are continuing with a focus on additional routing protocols and multicast.
APPLICATIONS OF WIRELESS AD HOC NETWORKS So far we have discussed the unique concept of MANET. We next discuss the applications that have motivated much of the research on MANETs and are well suited for their use. Perhaps the most widely considered application of a MANET is battlefield communications. The Department of Defense (DoD) future transformation is based on a key initiative called Network Centric Warfare (NCW). It is expected that there will typically be a large number of nodes in the battlefield environment that need to be interconnected, including radios carried by soldiers, and radios mounted on vehicles, missiles, unattended air vehicles (UAV), and sensors. In such an environment the network plays a critical role in the success of the military mission. The vast majority of these nodes move around at varying speeds and nodes may lose connectivity to other nodes as they move around in the battlefield because of the terrain (e.g. obstacles may prevent line of sight), distance among the nodes, and so on. Because of the rapid pace and the large degree of unpredictability it is not possible to assume a fixed infrastructure in the battlefield environment. Network administrators have little time to react and reconfigure the networks. Existing networking technologies cannot support such an environment efficiently. MANETs are viewed as a potential solution for providing a much more flexible network in support of NCW. The DoD has been funding a large number of research efforts exploring the use of MANETs for battlefield communication. As a result, a large number of research papers are motivated by such applications. The other widely considered application for MANETs is interconnection of sensors in an industrial, commercial, or military setting. Sensors are typically small devices measuring environmental inputs (such as temperature, motion, light, etc.) and often alerting users and/or taking specific reactions (e.g. starting an air-conditioner) when those inputs reach specific ranges. Sensors have been used extensively in industrial applications and even for applications inside the home (such as in security systems, heating systems, etc.). Most recently, advanced sensors are being considered for the detection of harmful agents (such as anthrax) or nuclear material. The availability of very inexpensive network interfaces has made it possible to provide network connectivity to sensors. Certain uses of sensors seem to be well suited for MANETs. For example, the military has considered scenarios where large numbers of sensors are dropped in an area of interest and those sensors then establish connectivity to each other and to the soldiers for providing advanced reconnaissance. In some cases, applications are considered where a very large number of sensors (hundreds or even thousands) is dropped in areas that need to be monitored closely. Sensors in such areas then establish a network. For example, “Smart Dust” which is a project at the University of California, Berkeley, (see http://robotics.eecs.berkeley.edu/_pister/ SmartDust/) has focused on the development of small devices that have both sensor and communication capabilities and are smaller than 1 cubic millimeter. Typically in such applications it is not possible to have a fixed infrastructure and therefore these applications seem to be well suited for MANETS. Another relevant application is that of emergency response. During major emergencies and disasters such as hurricanes or large explosions, the communications infrastructure in the immediate area of the disaster or emergency may be unusable, unavailable, or completely destroyed. When emergency responders first arrive in the disaster-struck area, it is critical for them to be able to communicate with each other. The communications make it possible for the team to coordinate the relief operations with each other. Since the communication infrastructure is often unavailable, first responders need to be able to establish connectivity immediately. MANETs are well suited for such an application because of their ability to create connectivity rapidly with limited human effort. Several other applications of MANETs are also being considered. For example, municipalities are considering deployment of wireless ad hoc networks (in the form of so called mesh networks) for offering broadband access to end users including employees of the municipality, first responders, and even residents of the municipality. Such networks have already been deployed in a small (but increasing) number of municipalities. More recently researchers have considered the use of MANET in the vehicular environment. Making MANET networking capabilities available in such environments can enable a variety of new applications such as sharing of up-to-date traffic information between vehicles.
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