Introduction

In Chapter 4, we basically studied how to assign time slots to communication links in quasi-static networks (e.g., WMNs and WSNs) such that the scheduled transmissions are interference-free and the scheduling period is minimized (thus maximizing the throughput). We assumed that there is only one spectrum used by all links in the network. In this chapter, we mainly focus on multichannel networks when there are multiple spectrums available for wireless terminals in the network. The wireless devices may have multiple wireless NICs or just a single wireless NIC. Because close by mesh routers compete for certain wireless channels, the capacity of a WMN will be increased tremendously when the single channel is extended to multiradio, multichannel, and multihop. For example, if two nodes vi and vj could communicate with each other by channel f1, and both nodes have at least one more available NIC that could operate on another channel f2, if f2 is also available for both nodes, vi and vj could use both f1 and f2 to communicate simultaneously. When such cases are applied to more wireless nodes, the throughput of the wireless network will be increased tremendously.

On the other hand, with recent fast-growing spectrum-based services and devices, the remaining spectrum available for future wireless services is being exhausted, known as the spectrum-scarcity problem.

Introduction

When designing routing protocols, it is often implicitly assumed that each participant (users or routers) will faithfully follow the prescribed protocols without any deviation–except, perhaps, for a few faulty or malicious ones. For example, in wireless ad hoc networks, it is commonly assumed that each terminal contributes its own resources to forward the data for other terminals to serve the common good and benefits from resources contributed by other terminals to route its packets in return. However, the critical observation that individual users who own these wireless devices are generally selfish, aiming to maximize their own benefit instead of contributing to the system, may severely undermine the expected performances of the wireless networks. The limitations of energy supply, memory, and computing resources of these wireless devices raise concerns about the traditional assumption about terminals' conforming to protocols. Sometimes, wireless devices owned by individual users may prefer not to participate in the routing in order to save its energy and resources. Therefore, if all users are selfish, providing incentives is a natural and common way to encourage contribution and thus maintain the robustness and availability of networking systems. The question turns to how to design the proper incentives.

Introduction

Network-wide broadcasting in MANETs provides important control and route establishment functionality for a number of unicast and multicast protocols. In this chapter, an overview is presented of the recent progress of energy-efficient broadcast and multicast in wireless ad hoc networks.

Notice that, in general, there are four basic techniques for energy-efficient communication (Jones et al., 2001).

1. The first technique is to turn off nonused transceivers to conserve energy. Then, we need to schedule, for every node, when it should sleep, when it should be idle, when it should receive, and when it should transmit such that a networking task is finished in a certain time period while simultaneously saving the energy cost.

2. The second technique is scheduling the competing nodes to avoid wasting energy because of contention. This can reduce the number of retransmissions and increase the nodes' lifetime by turning off the nonused transceivers for a period of time when they are not scheduled to transmit or receive. (This was studied in Chapter 4.)

3. The third technique is to reduce communication overhead, such as to defer transmission when the channel conditions are poor.

4. The fourth technique is to use power control to conserve energy. Each node will dynamically adjust its transmission power based on the downstream neighboring nodes to a level that is sufficient to reach the downstream neighboring node(s). This has the added advantage of reducing interference with other ongoing transmissions.

Introduction

Most of the methods developed in the literature for backbone construction try to minimize the number of cluster-heads; i.e., the number of nodes in the backbone. However, in many applications of wireless ad hoc networks, minimizing the size of the backbone is not sufficient. For example, different wireless nodes may have different costs for serving as a cluster-head because of device differences, power capacities, and information loads to be processed. Therefore, in the rest of this chapter, for succinctness of presentation, we assume that each wireless node has a generic cost (or weight). The cost may also represent the fitness or priority of each node to be a cluster-head. Lower cost means higher priority. In practice, cost could represent the power-consumption rate of this node if a backbone with small power consumption is needed; the robustness of this node if a fault-tolerant backbone is needed; or a function of its security level if a secure backbone is needed. Y. Wang et al. (2005a) studied how to construct a sparse backbone efficiently for a set of weighted wireless nodes such that the total cost of the backbone is approximately minimized and there is a cost (or hops) efficient route connecting every pair of wireless nodes via the constructed network backbone.

Introduction

Wireless multihop radio networks such as ad hoc, mesh, or sensor networks are formed of autonomous nodes communicating via radio. Wireless networks have drawn a great deal of attention in recent years because of their potential applications in various areas. For example, WMNs are being used as the last mile for extending the Internet connectivity for mobile nodes. These wireless mesh or sensor networks behave almost like wired networks because they have infrequent topology changes, limited node failures, and so on. For WMNs or WSNs, the aggregate traffic load of each routing node changes infrequently also. A unique characteristic of wireless networks is that the radio sent out by a wireless terminal will be received by all the terminals within its transmission range and also possibly cause signal interference to some terminals that are not intended receivers. In other words, the communication channels are shared by the wireless terminals. Thus, one of the major problems facing wireless networks is the reduction of capacity because of interference caused by simultaneous transmissions. Using multiple channels and multiple radios can alleviate, but not eliminate, the interference. To achieve robust and collision-free communication, there are two alternatives. One is to utilize a random-access MAC layer scheme; this was discussed in detail in Chapter 3. The other is to carefully construct a transmission schedule. One variant, link scheduling in the context of time-division multiplexing (TDM), is the subject of this chapter.

Introduction

Hundreds of protocols (Bose et al., 2001; Chlamtac and Farago, 1999; Das et al., 2000; Johnson and Maltz, 1996; Ko and Vaidya, 1997; X.-Y. Li et al., 2002a; Maltz et al., 1999; Perkins, 1997a; Ramanathan and Steenstrup, 1996; Royer and Toh, 1999; Stojmenovic and Lin, 2001; Y. Wang and Li, 2002b; Zaruba et al., 2001) that take into account the unique characteristics of wireless ad hoc networks have been developed. Among them, energy efficiency, routing, and MAC layer protocols have attracted the most attention. One of the remaining fundamental and critical issues is to have fault-tolerant network deployment without sacrificing the spectrum-reusing property. In other words, the network should support multiple disjoint paths connecting every pair of nodes. Obviously, we can increase the transmission range of all nodes to increase the fault tolerance of the network. However, increasing the transmission range will cause more signal interference (thus reducing the throughput) and increase the power consumption of every node. Because power is a scarce resource in wireless networks, it is important to save the power consumption without losing the network connectivity. The universal minimum power used by all wireless nodes such that the induced network topology is connected is called the critical power.

Introduction

Wireless multihop radio networks such as ad hoc, mesh, or sensor networks are formed of autonomous nodes communicating via radio. Wireless networks have drawn lots of attention in recent years because of their potential applications in various areas. For example, wireless mesh networks (WMNs) are being used as the last mile for extending Internet connectivity for mobile nodes. Many U.S. cities (e.g., Medford, Oregon; Chaska, Minnesota; and Gilbert, Arizona) have already deployed mesh networks. AWA, the Spanish operator of WLANs, will roll out commercial WLANs and mesh networks for voice and data services. Several companies, such as MeshDynamics, have recently announced the availability of multihop, multiradio mesh-network technology. These networks behave almost like wired networks because they have infrequent topology changes, limited node failures, and so forth. For WMNs or WSNs, the aggregate traffic load of each routing node also changes infrequently. A unique characteristic of wireless networks is that the radio sent out by a wireless terminal will be received by all the terminals within its transmission range and also possibly cause signal interference to some terminals that are not intended receivers. In other words, the communication channels are shared by the wireless terminals. Thus, one of the major problems facing wireless networks is the reduction of capacity that is due to interference caused by simultaneous transmissions. Using multiple channels and multiple radios can alleviate but not eliminate the interference. This raises the scalability issue of WMNs.

Introduction

Unlike in traditional fixed infrastructure networks, there is no centralized control over ad hoc networks, which consist of an arbitrary distribution of radios in a certain geographical area. An important requirement of wireless ad hoc networks is that they should be self-organizing; i.e., transmission ranges and data paths are dynamically restructured with changing topology. Energy conservation and network performance are probably the most critical issues in wireless ad hoc networks because wireless devices are usually powered by batteries only and have limited computing capability and memory. Recently, significant research (Grünewald et al., 2002; L. Li et al., 2001; X.-Y. Li et al., 2001, 2002b; Rajaraman, 2002; Ramanathan and Rosales-Hain, 2000; Wang et al., 2003; Wattenhofer et al., 2001) has been conducted on designing a power-efficient network topology for wireless networks. Many research results applied a computational geometry technique (specifically, geometrical spanner) to achieve power efficiency. In this chapter, we review these approximation algorithms of a power spanner for ad hoc networks.

Ad Hoc Networks: Graph Model

A WAN consists of a set V of n wireless nodes distributed in a two-dimensional plane. Each node has the same maximum transmission range of R meters; e.g., a typical IEEE 802.11g WLAN adapter has a transmission range of around 100–500 m. By a proper scaling, we assume that all nodes have the maximum transmission range equal to one unit.

Introduction

Having location information can be very useful and it has so many applications. It can answer questions such as: Are we almost to the campsite? What lab bench was I standing by when I prepared these tissue samples? How should our search-and-rescue team move to quickly locate all the avalanche victims? Can I automatically display this stock devaluation chart on the large screen I am standing next to? Where is the nearest cardiac defibrillation unit?, and so on. Service providers can also use location information to provide some novel location-aware services. The navigation system based on a GPS is an example. A user can tell the system his destination and the system will guide him there. Phone systems in an enterprise can exploit locations of people to provide follow-me services.

Researchers are working to meet these and similar needs by developing systems and technologies that automatically locate people, equipment, and other tangibles. Indeed, many systems over the years have addressed the problem of automatic location sensing. Because each approach solves a slightly different problem or supports different applications, they vary in many parameters, such as the physical phenomena used for location determination, the form factor of the sensing apparatus, power requirements, infrastructure versus portable elements, and resolution in time and space.

For outdoor environments, the most well-known positioning system is the global positioning system (GPS) (Peng and Mirsa, 1999), which uses 24 satellites set up by the U.S. Department of Defense to enable global 3D positioning services.

Introduction

A MAC protocol is used to address resolving potential contention and collision when the communication medium is used. Many MAC protocols have been proposed for wireless networks (e.g., Bharghavan et al., 1994; Fullmer, 1998; Fullmer and Garcia-Luna-Aceves, 1995; Garcia-Luna-Aceves and Tzamaloukas, 1999; Garcias and Garcia-Luna-Aceves, 1996; Lin and Gerla, 1997b; Lu et al., 1999; Vaidya et al., 2000), which often assume a common channel shared by mobile hosts.

Contention-Based MAC

The MAC protocol is essential for stations that share a common broadcast channel. CSMA protocols (Kleinrock and Tobagi, 1975) have been used in a number of packetradio networks in the past (Leiner et al., 1987). These protocols attempt to prevent a station from transmitting simultaneously with other stations within its transmitting range by requiring each station to listen to the channel before transmitting. Unfortunately, the performance of the CSMA protocol suffers from hidden-terminal problems and exposed-terminal problems substantially. To remedy these problems, several approaches (Bambos and Kandukuri, 2000; Bharghavan et al., 1994; Colvin, 1983; Fullmer and Garcia-Luna-Aceves, 1995; Karn, 1990; Monks et al., 2001) were proposed in the literature. Karn (1990) proposed a protocol called MACA that attempts to detect collision at the receiver by establishing a request-response dialog between senders and intended receivers. When a sending station wants to transmit, it sends an RTS to the receiver, which responds with a CTS if it receives the RTS correctly.

Introduction

The (localized) topology-control technique lets each wireless device (locally) adjust its transmission range and select certain neighbors for communication while maintaining a decent global structure to support energy-efficient routing and to improve the overall network performance. A distributed method is localized if it runs in a constant number of rounds (Naor and Stockmeyer, 1993). By enabling each wireless node to shrink its transmission power (which is usually much smaller than its maximum transmission power) enough to cover its farthest selected neighbor or selecting only a portion of nodes to forward data for others, topology-control schemes can not only save energy and prolong network life but can also improve the network throughput through mitigating the MAC-level medium contention. Unlike traditional wired and cellular networks, the movement of wireless devices during communication could change the network topology to some extent. Hence, it is more challenging to design a topology-control algorithm for ad hoc wireless networks: The topology should be locally and self-adaptively maintained at a low communication cost, without affecting the whole network's performance.

Wireless networks have drawn a good deal of attention in recent years because of the potential applications in various areas. Many routing protocols have recently been proposed for wireless ad hoc networks. The simplest routing method is to flood the message, which not only wastes the rare resources of wireless nodes but also diminishes the throughput of the network.

Introduction

In Chapter 4, we basically studied how to assign time slots to links such that the simultaneous transmissions will be interference-free. In Chapter 5, we studied how to assign frequency channels to wireless terminals such that (1) they are interference-free; i.e., the spectrums assigned to nearby terminals will be disjoint; or (2) the links formed by the assigned channels will form a network with certain networking properties such as being connected. In this chapter, we study the channel assignment to wireless networks when the channels are defined by CDMA codes.

Code-division multiple access (CDMA) provides a higher capacity, flexibility, scalability, reliability, and security than conventional frequency-division multiple access (FDMA) and time-division multiple access (TDMA). It has already been widely deployed in 2G cellular communication systems and was proposed for the emerging and future wireless systems, including WLANs and wireless ad hoc networks. In a CDMA system, the communication channels are defined by pseudo-random codewords, which are carefully designed to cancel each other out as far as possible. Each communication utilizes the entire available spectrum, and every bit of data is multiplied by the codeword used by the communication channel. Thus, many duplicates of the same information are transmitted to ensure that at least one gets through. The number of duplicates, which is equal to the length of the codeword, is known as the spreading factor. The inverse of the length of the codeword is known as the rate of the codeword.