Potentially spread across any terrain, wireless ad hoc sensor networks use communication capability and intelligence to wirelessly transmit data and process signals. Defined as two or more mobile devices connected to each other without the help of intervening infrastructure, the possibilities for use are endless. If two nodes are not within mutual transmission range, they locate intermediary nodes to relay the message. Upon deployment, pre-programmed devices discover each other and then self organize into a wireless network. Sensing the environment and sharing information with other nodes to carry out their mission.

Ad hoc networks are classified into two main types, mobile ad hoc networks (MANET) and ad hoc sensor networks. Ad hoc sensors are "wireless computer networks that consist of spatially distributed autonomous devices using sensors to cooperatively monitor physical or environmental conditions such as temperature, sound, vibration, pressure, motion or pollutants at different locations." [1]

MANET's are "self-configuring networks of mobile routers (and associated hosts) connected by wireless links - the union of which forms an arbitrary topology. The routers are free to move randomly and organize themselves arbitrarily; thus, the network's wireless topology may change rapidly and unpredictably. Alternatively, a MANET may be considered as a mobile version of an ad hoc network." [2]

The organization of a network is dependent upon the detection of a locally occurring event by most if not all of the nodes. Processing power, broadcast authority and distance all play a role in establishing a node hierarchy where one node serves as master of the cluster. Common deployment of these networks is anticipated as short range communication costs decrease and advancements in wireless networking continue to occur.

There are few requirements of ad hoc sensor networks, including the use of a large number of sensors, minimal energy use, network self organization, collaborative signal processing and the ability to query individual nodes. Future consideration given in support of the advancement of ad hoc wireless sensor networks include the topics of medium access control, routing, topology control, deployment strategies, energy efficient system design and development tools. 

The application of sensor networks are numerous and varied, a few tasks common to multiple networks include determining temperature, atmospheric pressure, sunlight and relative humidity. Also, sensor networks are used to track and classify objects while determining their speed and direction of travel. The military has used sensor networks in enemy surveillance. There are also strict timeliness requirements of data delivered within the network.

References:

[1] K.Romer and F. Mattern. The Design Space of Wireless Sensor Networks. IEEE Wireless Communications, 11(6):54-61, 2004.

[2] Charles E. Perkins. Ad Hoc Networking. Addison-Wesley, Boston, MA, First Edition, 2001.