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of it and as a consequence B already receives packets from C (even though B is not thedestination). The result of this scenario is that B decides to defer its own transmission toA. This is not a right decision because there is no possibility of a collision at A (A is notwithin the transmission range of C).ABCDFigure 6. Exposed Terminal ProblemSolution:Protocol 802.11 uses a nice technique to overcome the above problems. It uses twokinds of control frames the RTS (Request to Send) and CTS (Clear to Send). RTS is usedby the transmitter to reserve the channel when senses the channel to be idle. Afterwards,it waits until it receives the CTS frame by the receiver indicating that the latter is ready tostart “listening”. That’s it, the sender is indirectly informed that someone else is keepingbusy the receiver by not receiving CTS within a given timeout t.2.2.3 MAC Properties for WSNsWhen designing a protocol for a system, the focus should be tuned on the designconsiderations and the operational requirements of that system. These specialcharacteristics may include the physical ones (hardware, node size, supportedtechnologies etc.), the practical ones (e.g. what kind of applications will run on top of theprotocol stack) and finally the performance related ones (scalability, delays etc.). Insection 2.2.1 we reviewed the functions that a MAC layer must support and then we
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.
Page 1: Athens University of Economics and
Page 4 and 5: 1 IntroductionRecently, the technol
Page 7 and 8: Thus, in-network processing is one
Page 11 and 12: The following figure (Figure 4) vis
Page 13 and 14: 2.1.3 Proposed schemesSeveral syste
Page 15 and 16: Medium access functionality is so i
Page 17 and 18: encodes the data before transmissio
Page 19: o CSMA/CA - Is also an extension of
Page 24 and 25: amount of noise to the channel for
Page 27 and 28: wild animals are monitored inside a
Page 29 and 30: than the whole network (GAF, GEAR).
Page 31 and 32: In the data-centric approach we ass
Page 33 and 34: 3 Data Aggregation in WSNs3.1 Gener
Page 35 and 36: 2. Less packet collisions.Packet co
Page 37 and 38: Exemplary aggregates, on the other
Page 39 and 40: consumes more energy than the initi
Page 41 and 42: Tree. To be more specific, as the q
Page 43 and 44: a) Sensors that heard aggregate req
Page 45 and 46: of communication failures because e
Page 47 and 48: function SE() to translate its loca
Page 49 and 50: Benefits1. It is a completely distr
Page 51 and 52: application can tolerate big estima
Page 53 and 54: REFERENCES[1] W. Ye, J. Heidemann,

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