Recent Press

Ad Hoc Networks
by Jie Wu, Florida Atlantic University; Ivan Stojmenovic, University of Ottawa

Ad hoc networks are a key factor in the evolution of wireless communications. Self-organized ad hoc networks of PDAs or laptops are used in disaster relief, conference, and battlefield environments. These networks inherit the traditional problems of wireless and mobile communications, such as bandwidth optimization, power control, and transmission-quality enhancement. In addition, their multihop nature and the possible lack of a fixed infrastructure introduce new research problems such as network configuration, device discovery, and topology maintenance, as well as ad hoc addressing and self-routing. Various approaches and protocols have been proposed to address ad hoc networking problems, and multiple standardization efforts are under way within the Internet Engineering Task Force, as well as academic and industrial research projects.

NETWORK TOPOLOGY

In ad hoc networks, wireless hosts can communicate with each other in the absence of a fixed infrastructure.1 These networks typically consist of equal nodes that communicate over wireless links without central control. Sensor networks,2 also called hybrid ad hoc networks, are linked to monitoring centers that collect data such as temperature, chemical detection, or movement. In recent years, government agencies in several countries have supported research on sensor networks. For example, the US National Science Foundation launched a multidisciplinary program on sensors and sensor network research in 2003. Some ad hoc networks are linked to a fixed infrastructure via access points. For example, mesh or rooftop networks consist of antennas placed on top of buildings to provide wireless Internet access. Vehicles on a highway can create an ad hoc network for use in disseminating traffic information. They can operate as a pure ad hoc network in which an individual vehicle detects traffic events and initiates a broadcast to other vehicles. Alternatively, cellular or Internet access points placed near the road can transmit the information. Multihop cellular networks3 have recently emerged as a communication alternative at events where huge numbers of users are concentrated in a small area such as a stadium. Peer-to-peer networks are ad hoc networks in which an overlay network is built on the Internet. In a P2P network, two or more peers can use appropriate information and communication systems to collaborate spontaneously without requiring central coordination.

AD HOC NETWORK COMMUNICATION

Communication between two hosts in an ad hoc network is not always direct—it can proceed in a multihop fashion so that every host is also a router. Ad hoc network hosts can use protocols such as the IEEE 802.11 media-access control standard to communicate via the same frequency, or they can apply Bluetooth or other frequency-hopping technology. Because power consumption is directly proportional to the distance between hosts, direct single-hop transmissions between two hosts can require significant power, causing interference with other such transmissions. To avoid this routing problem, two hosts can use multihop transmission to communicate via other hosts in the network. With IEEE 802.11 technology, avoiding collisions—transmission interferences—is difficult because of the hidden station problem: Two hosts that do not communicate directly can transmit messages simultaneously to a common neighbor on the same frequency. In addition to maintaining an ongoing routing task or facilitating route establishment, mobile networks also must support location management by keeping track of the host's location.

NETWORK LAYER PROBLEMS

The problems encountered in the network layer of ad hoc networks include topology control, data communication, and service access. Topology control problems include discovering neighbors, identifying position, determining transmission radius, establishing links to neighbors, scheduling node sleep and active periods, clustering, constructing the dominating set (each node either belongs to or has a neighbor from the dominating set), and maintaining the selected structure. Data communication problems include routing—sending a message from a source to a destination node, broadcasting—flooding a message from a source to all other nodes in the network, multicasting—sending a message from a source to a set of desirable destinations, geocasting—sending a message from a source to all nodes inside a geographic region, and location updating—maintaining reasonably accurate information about the location of other nodes. Service access problems include Internet access, cellular network access, data or service replication upon detection or expectation of network partition, and unique IP addressing in merge or split-network scenarios.

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