Ad Hoc Networks
by Jie Wu, Florida Atlantic University; Ivan Stojmenovic, University
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
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
HOC NETWORK 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.
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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