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synchronize perfectly all the nodes it is undoubted that two or more signals will reach thesame receiver at different power levels. That is called interference.Multipath propagation: The same signal is possible to follow two or more differentpaths on its way to the receiver because of the possible reflections (on objects or even onthe ground). The multiple paths have various lengths; different path lengths lead todifferent reception times and this finally results in the blurring of the signal at thereceiver.Another function that the physical network layer may support is the physical carriersense. This is done by listening to the channel for possible transmissions in order to avoidsending signals that may interfere with others. Carrier sense is performed at random timesexcept for the case that the MAC sublayer demands it.To get an idea about the specifications of the wireless channel of a typical node of aWSN we present the following table (Table 1) that demonstrates the transceivercharacteristics of the most popular commercial sensor nodes (for more information thereader is encouraged to read [14]).PrototypeUCBerkeleyCrossbowWeCUCBerkeleyCrossbowReneUCBerkeleyCrossbowMICAUCBerkeleyCrossbowMICA2InteliMote(2003)MicrostrainGalbreath et al.(2003)(1999) (2000) (2002) (2003)Radio TR1000 TR1000 TR1000RFChipcon WirelessMonolithicsCC1000 BT ZeevoDR-3000-1315, 433Frequency 868 / 916 868 / 916 868 / 916 orBand MHz MHz MHz 868 / 9162.4 GHz 916.5 MHzMHzStandard - - - -IEEE802.15.1-SpreadSpectrumNo No No Yes (s/w) Yes -Data Rate 10 kbps 10 kbps 40 kbps 38.4 kbps 600 kbps 75 kbpsTable 2. Transceiver characteristics of the most popular commercial sensor nodes
2.1.3 Proposed schemesSeveral systems suggested physical layer solutions that provide APIs for upper layerslike setting the radio into different states (sleep, idle, reception, transmission). It isnoticeable the lack of standardization at lower and the (physical) sensor hardware. In thefollowing paragraphs we mention the most significant and popular schemes that wereproposed.Bluetooth [13]Operates in the 2.4 GHz ISM band. It employs binary Gaussian Frequency ShiftKeying (GFSK), 1 MBaud - symbol rate of. Performs frequency hopping spreadspectrum, randomly hopping across 79 channels (1 MHz each one), performing 1600hops per second. Although it is frequently suggested for sensor applications, theBluetooth physical layer is not very suitable for WSNs [12]. This is mainly because itconsumes a lot of power searching the band for the network on a packet rate time scale(frequency hopping). What is more, the narrow channel separation makes the phase noiserequirements of signal sources more difficult.IEEE 802.11b WLAN [15]Operates in the 2.4 GHz ISM band. This worldwide IEEE standard, specifies threedifferent layer-1 options at 1 Mb/s and optionally 2 Mb/s:• Infrared,• Frequency hopping spread spectrum 2.4 GHz and• Direct Sequence spread spectrum 2.4 GHz [DBPSK at 1 Mb/s and DQPSK at 2Mb/s. Using complementary code keying CCK it achieves till 11Mb/s].The 1- and 2-MHz direct sequence 802.11 physical layer is a possible option for aWSN because it has minimal hardware requirements, the provided data rate is more thanenough and it has not the problems of the frequency hopping systems (like bluetooth).Using the extended version (CCK) of the direct sequence option has a big cost in powerconsumption and transceiver hardware complexity (e.g. have to support complexidentification and decoding functions for the CCK sequences).μAMPS [12]
Page 1: Athens University of Economics and
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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
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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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