Wireless Ad Hoc and Sensor Networks

28 Wireless Ad Hoc and Sensor Networksinfrastructure. Each node in the network must act both as a host and arouter in addition to a transmitter. Every node in the network shouldforward packets that are intended for other nodes that are not in directtransmission range.1.4.4.1 Ad Hoc NetworkAccording to the IEEE 802.11 standard, an ad hoc network is defined as anetwork composed solely of stations within mutual communicationrange of each other via the WM. An ad hoc network is typically createdin a spontaneous manner. The principal distinguishing characteristic ofan ad hoc network is its limited temporal and spatial extent. Theselimitations allow the act of creating and dissolving the ad hoc networkto be sufficiently straightforward and convenient so as to be achievableby nontechnical users of the network facilities; i.e., no specialized technicalskills are required and little or no investment of time or additionalresources is required beyond the stations that are to participate in the adhoc network. The term ad hoc is often used as a slang for an independentbasic service set (IBSS).The extended set network on the other hand relies on existing infrastructurefor the nodes to communicate with other nodes including thewired network. The network infrastructure consists of one or more accesspoints (AP), which provide the routing for the nodes in the network.Similar to the case of cellular networks, transmitter power of a node inan ad hoc network affects the channel capacity. Hence, power controlschemes are necessary. Efficient power control in ad hoc wireless networks,which can increase the channel reuse and the number of nodes inthe network, is considered in this book. To understand the operation ofpower control, knowledge of how the nodes communicate in the networkis required. In the next section, network simulator (NS) (Fall and Varadhan2002) implementation and MAC protocol for ad hoc networks are given.1.4.5 IEEE 802.11 MAC ProtocolIEEE 802.11 specifications provide information about two MAC protocols,PCF (point coordination function) and DCF (distributed coordinationfunction). PCF is a centralized scheme and DCF is a distributed scheme.The specifications of DCF are considered in this thesis. Important definitionsrequired to understand the specifications are given next.Transmission range: This is the distance range around the transmitterwithin which any user in the network can receive, and correctlydecode, packets sent by the transmitter. When highest transmitterpower is used, the transmission range is 250 m.