Wireless Ad Hoc and Sensor Networks

40 Wireless Ad Hoc and Sensor NetworksThe MAC protocol must be power-aware and able to minimize collisionsdue to the CSMA/CA paradigm with the neighbors’ broadcast. Protocolsfor ad hoc networks cannot be ported to WSNs because, in WSN, powerefficiency is critical. In cellular networks, a mobile node is a single hopaway and QoS provisioning is given greatest importance for a MACprotocol whereas power consumption is given secondary importance.This makes MAC protocols for cellular networks to be impractical forWSNs. Besides being energy efficient, WSNs are data-centric, haveattribute-based address and location awareness, aggregate data and,therefore, MAC protocols must work under these constraints. The MAClayer protocols must allow self-organization while fairly and efficientlysharing communication resources between sensor nodes. Chapter 8 presentsthe self-organization and routing protocols for WSNs. Bluetooth is closestpeer to a WSN. Regardless of which type of medium access control schemeis used for WSNs, it certainly must support power saving modes for asensor node. The most simple form of power saving is to turn the transceiveroff when it is not required and turning it on only when desired.The data link layer is responsible for error control in communicationnetworks. Bit error rate (BER) is used as a performance parameter forassessing link reliability due to unpredictable wireless channel. A goodchoice of error correcting code such as forward error correction (FCC) orHamming codes can result in several orders of magnitude reduction in BERwithout additional energy consumption and is therefore preferred in WSNs.The physical layer addresses the need of simple but robust modulation,transmission and receiving techniques. Direct-sequence spread spectrumarchitecture, ultrawideband (UWB) and impulse radio (IR), have beenproposed for WSN applications such as indoor location determination.Low transmission power and simple transceiver circuitry make UWB anattractive candidate for WSNs. Strategies to overcome unpredictablechannels have to be dealt at this layer.UWB radio is of great interest (recently) for communications in distributedsensor networks. This is because UWB is a short-range technology that canpenetrate walls, it is suitable for multinode transmissions, and it has builtin time-of-flight properties that make it very easy to measure ranges downto 1 cm with a range of 40 m. This means that the same medium, UWB, canbe used for communications, localization, and target tracking in a distributedsurveillance network. Moreover, UWB transceivers can be made very smalland are amenable for MEMS technology; because PPM is used, no carrier isneeded, meaning that antennas are not inductive. Also, the receiver is basedon a rake detector and correlator bank so that no IF stage is needed.UWB uses signals such as (Ray 2001)∑s(t) = wt ( −jT −cT −δd )(1.18)jf j c [/ j Ns]