In Network Processing and Data Aggregation in

made a discussion over the issues that come up when communicating through thewireless medium. The next step in designing an effective MAC scheme for WSNs is thedefinition of the properties that it should have.A general purpose MAC-layer protocol takes into serious consideration the assuranceof:Fairness: Ensure that all nodes have equal opportunities to access the medium. It isalso important for the node that acquired the medium to use it prudentially.Small Latency: It is a vital requirement for a network to respond quickly to the needsof an application. If a MAC scheme is fair enough but has very strict and complex rulesthen it is possible that it will suffer from lengthy delays (the time elapsed between themoment a station requested to send a frame and the moment it finally managed to startsending the first bytes).High throughput: Wasted network resources (like bandwidth) due to stiff architectureof the lower level affect the performance of the upper layers. The MAC layer sets theupper bound of the network throughput and thus it has to achieve high channelutilization.WSNs have very tight constraints in power consumption, computational capabilities,transceiver strength and storage size. Very few of their characteristics are common withthese of a traditional wireless system like GSM for example. It sounds a bit weird that afair, with small latency and high throughput MAC may not be ideal for a WSN. However,this is true. A MAC protocol designed for a sensor network is supposed to be “welldefined” if:Is energy efficient: Sensor nodes use batteries for power supply. These batteries areusually small sized and it is difficult (if not impossible) to be recharged. The totalnetwork lifetime then, is a major design factor. In section 2.2.4 we discuss why existingMAC schemes for wireless systems are not suitable for WSNs. Additionally, the majorreasons for energy waste are defined.Has good scalability: Scalability to the change in network size, node density andtopology is an additional property of great importance. WSNs are frequently deployed inareas where the topology changes rapidly. Nodes may fail at any time (due to hardwarefailures or empty battery), change location unexpectedly and finally new nodes may joinlater. A good MAC protocol should easily adapt to such network changes.