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16.1.5 Distributed Control Systems using none toincoporate none for web,windows applicationpdf-417 printing Ad hoc wireless ne none none tworks also enable distributed control applications, with remote plants, sensors and actuators linked together via wireless communication channels. Such networks allow coordination of unmanned mobile units, and greatly reduce maintenance and recon guration costs over distributed control systems with wired communication links. Ad hoc wireless networks can be used to support coordinated control of multiple vehicles in an automated highway system, remote control of manufacturing and other industrial processes, and coordination of.

iReport Application unmanned airborne vehicles for military applications. Current distributed control designs provide excellent performance as well as robustness to uncertainty in model parameters. However, these designs are based on closed-loop performance that assumes a centralized architecture, synchronous clocked systems, and xed topology.

Consequently, these systems require that the sensor and actuator signals be delivered to the controller with a small, xed delay. Ad hoc wireless networks cannot provide any performance guarantee in terms of data rate, delay or loss characteristics: delays are typically random and packets may be lost. Unfortunately, most distributed controllers are not robust to these types of communication errors, and effects of small random delays can be catastrophic [28, 29].

Thus, distributed controllers must be redesigned for robustness to the random delays and packet losses inherent to wireless networks [30]. Ideally, the ad hoc wireless network can be jointly designed with the controller to deliver the best possible end-to-end performance..

16.2 Design Principles and Challenges The most fundament al aspect of an ad hoc wireless network is its lack of infrastructure, and most design principles and challenges stem from this characteristic. The lack of infrastructure inherent to ad hoc wireless networks is best illustrated by contrast with the most prevalent wireless networks: cellular systems and wireless LANs. Cellular systems divide the geographic area of interest into cells, and mobiles within a cell communicate with a base station in the cell center that is connected to a backbone wired network.

Thus, there is no peer-to-peer communication between mobiles. All communication is via the base station through single hop routing. The base stations and backbone network perform all networking functions, including authentication, call routing, and handoff.

Most wireless LANs have a similar, centralized, single hop architecture: mobile nodes communicate directly with a centralized access point that is connected to the backbone Internet, and the access point performs all networking and control functions for the mobile nodes 1 . In contrast, an ad hoc wireless network has peer-to-peer communication, networking and control functions that are distributed among all nodes, and routing that can exploit intermediate nodes as relays. Ad hoc wireless networks can form an infrastructure or node hierarchy, either permanently or dynamically.

For example, many ad hoc wireless networks form a backbone infrastructure from a subset of nodes in the network to improve network reliability, scalability, and capacity [7]. If a node in this backbone subset leaves the network, the backbone can be recon gured. Similarly, some nodes may be chosen to perform as base stations for neighboring nodes [8].

Thus, ad hoc wireless networks may create structure to improve network performance, however such structure is not a fundamental design requirement of the network. A lack of canonical structure is quite common in wired networks. Indeed, most metropolitan area networks (MANs) and wide area networks (WANs), including the Internet, have an ad hoc structure.

However, the broadcast nature of the radio channel introduces characteristics in ad hoc wireless networks that are not present in their wired counterparts. In particular, with suf cient transmit power any node can transmit a signal directly to any other node. For a xed transmit power, the link SINR between two communicating nodes will typically decrease as the distance between the nodes increases, and will also depend on the signal propagation and interference environment.

Moreover, this link SINR varies randomly over time due to fading of the signal and interference. Link SINR determines the communication performance of the link: the data rate and associated probability of packet error or BER that can be supported on the link. Links with very low SINRs are not typically used due to their extremely poor performance, leading to partial connectivity among all nodes in the network, as shown in Figure 16.

1. However, link connectivity is not a binary decision, as nodes can adapt to the SINR using adaptive modulation or change it using power control. The different SINR values for different links are illustrated by the different line widths in Figure 16.

1. Thus, in theory, every node in the network can transmit data directly to any. The 802.11 wireles none none s LAN standard does include ad hoc network capabilities, but this component of the standard is rarely used..

other node. Howeve r, this may not be feasible if the nodes are separated by a large distance, and direct transmission even over a relatively short link may have poor performance or cause much interference to other links. Network connectivity also changes as nodes enter and leave the network, and this connectivity can be controlled by adapting the transmit power of existing network nodes to the presence of a new node [9].

The exibility in link connectivity that results from varying link parameters such as power and data rate has major implications for routing. Nodes can send packets directly to their nal destination via single hop routing as long as the link SINR is above some minimal threshold. However, the SINR is typically quite poor under single hop routing, and this method also causes excessive interference to surrounding nodes.

In most ad hoc wireless networks, packets are forwarded from source to destination through intermediate relay nodes. Since path loss causes an exponential decrease in received power as a function of distance, using intermediate relays can greatly reduce the total transmit power (the sum of transmit power at the source and all relays) needed for endto-end packet transmission. Multihop routing using intermediate relay nodes is a key feature of ad hoc wireless networks: it allows for communication between geographically-dispersed nodes and facilitates the scalability and decentralized control of the network.

However, it is much more challenging to support high data rates and low delays over a multihop wireless channel than over the single-hop wireless channels inherent to cellular systems and wireless LANs. This is one of the main dif culties in supporting applications with high data rate and low delay requirements, such as video, over an ad hoc wireless network. Scalability is required for ad hoc wireless networks with a large number of nodes.

The key to scalability lies in the use of distributed network control algorithms: algorithms that adjust local performance to account for local conditions. By forgoing the use of centralized information and control resources, protocols can scale as the network grows since they only rely on local information. Work on protocol scalability in ad hoc wireless networks has mainly focused on self-organization [10, 11], distributed routing [12], mobility management [7], and security [13].

Note that distributed protocols often consume a fair amount of energy in local processing and message exchange: this is analyzed in detail for security protocols in [14]. Thus, interesting tradeoffs arise as to how much local processing should be done versus transmitting information to a centralized location for processing. This tradeoff is particularly apparent in sensor networks, where nodes close together have correlated data, and also coordinate in routing that data through the network.

Most experimental work on scalability in ad hoc wireless networks has focused on relatively small networks, less than 100 nodes. Many ad hoc network applications, especially sensor networks, could have hundreds to thousands of nodes or even more. The ability of existing wireless network protocols to scale to such large network sizes remains unclear.

Energy constraints are another big challenge in ad hoc wireless network design [15]. These constraints arise in wireless network nodes powered by batteries that cannot be recharged, such as sensor networks. Hard energy constraints signi cantly impact network design considerations.

First, there is no longer a notion of data rate, since only a nite number of bits can be transmitted at each node before the battery dies. There is also a tradeoff between the duration of a bit and energy consumption, so that sending bits more slowly conserves transmit energy. Standby operation can consume signi cant energy, so sleep modes must be employed for energy conservation, but having nodes go to sleep can complicate network control and routing.

In fact, energy constraints impact almost all of the network protocols in some manner, and therefore energy consumption must be optimized over all aspects of the network design..
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