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amount of noise to the channel for a neighbouring node/receiver that may not be relatedto our communication process. A CDMA-Based MAC Protocol for general purposeMobile Ad-Hoc networks has been proposed [17]. It uses channel-gain informationobtained from overheard RTS and CTS packets over an out-of-band control channel. Theimplementation of this MAC protocol for WSNs is very complex as it requires specialtransceiver capabilities and also needs a second control channel. Furthermore, it is basedon the overhearing and as we have seen above the former is a primary reason for energywaste.IEEE 802.11 [15]Whenever IEEE 802.11 is used for WSNs, it operates in the Ad-Hoc mode withDistributed Coordination Function (DCF). Many control packets (RTS-CTS) are used inorder to avoid the hidden/exposed terminal problems (see section 2.2.2). However controlpackets and possible collisions are not the main energy waste reasons. Recent work [16]has shown that the energy consumption using the 802.11 MAC is very high when nodesare in idle mode. Idle listening consumes about 50-100% of the energy that is required toreceive data. Several measurements have shown that idle:receive:send ratios are1:1.05:1.4 . To sum up the above observations an IEEE 802.11 MAC protocol is notsuitable for a WSN in terms of energy consumption but due to its big popularity it isusually a choice for several experiments and simulations with small modifications.2.2.5 Proposed MAC schemes for WSNsIn the literature of sensor networks, several MAC protocols/schemes have beenproposed.Most of them use techniques like:• turning off the radio when channel is idle,• having two separate channels (one for data and one for control),• supporting several operating states with different energy consumption levels and• performing power control (use a variation in the transmission power).
It is not within the scopes of this paper to describe them one by one. The reader isreferenced to read [2] for a nice survey on the current MAC Protocols for WirelessSensor Networks. To get an idea of how does a typical MAC protocol for WSNs operatewe describe in short the S-MAC protocol.S-MAC [1]The designers of S-MAC set energy conservation and self-configuration as primarygoals, while per-node fairness and latency were less important for them. It is known thatlengthy packets lead to higher energy consumption because in the case of a collision thewasted energy is higher than that of a smaller one (more bytes are transmittedineffectually). Additionally, overhearing and control overhead due to large packets, spendserious amounts of energy. Here comes the first technique of S-MAC, message passing,to achieve efficient transmission of a very long message. It divides it into smallerfragments and then transmits them in bursts. Unfortunately, this is not a fair modification,as the nodes that have more data to send acquire more times the medium in a per-hopMAC perspective. This drawback though, is not so crucial for WSNs comparing to theenergy savings. Another technique that is used is the scheme of periodic listen and sleepof a node. In sleep mode, only a few circuit elements of the transceiver have to bepowered on (radio is off), leading to minimal power needs during this operating mode. Itmainly saves the expended energy due to idle listening but the latency is increased whilethe sender has to wait for the receiver to “wake up”. Periodic listen-sleep is implementedusing synchronization to form virtual clusters of nodes on the same sleep schedule inorder to minimize additional latency through their coordinated operation. After extensiveexperiments the authors of [1] found that S-MAC can reduce the energy consumption upto 2-6 times (when messages sent every 1-10sec) compared to IEEE 802.11. What ismore, S-MAC supports parameterized tradeoffs between energy and latency.2.3 Network Layer2.3.1 Routing challenges in WSNsIn the first look, wireless sensor networks seem to have a lot of similarities with acommon Mobile Ad-Hoc Network (MANET). Nodes are mobile and they can change
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Page 53 and 54: REFERENCES[1] W. Ye, J. Heidemann,

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