Power
awareness is prominent in all aspects of WSNs ranging from the hardware
design to algorithms and protocols designed at all layers of the network architecture.
90 Ali Abu-el Humos et al.
Chapter 4 Medium Access Control Protocols for Wireless Sensor Networks
The main sources of energy consumption are sensing, data processing and
communication. Among these, communication is the major power consumer.
For example, the communication to computation power usage ratio is between
1500 and 2700 for Rockwell??™s WIN sensor nodes [9].
In general, radios can operate in four distinct modes of operation: transmit,
receive, idle and sleep. When a node is not sending or receiving packets,
it is in the idle mode, with power consumption nearly as high as in the receiving
mode. The highest power consumption is in the transmit mode and
the lowest in the sleep mode, when a node turns o?® its transceiver. For example,
the power consumption for Rockwell WINS seismic sensors [9] for transmit:
receive:idle:sleep modes is (0.38??“0.7W):(0.36W):(0.34W):(0.03W), while
the sensing power is 0.02 W.
Based on these results, an important method to save power resources is
to have sensor nodes enter the sleep mode whenever they are not actively
participating in data transmission or reception. This way they avoid large
energy consumption in idle states. This energy saving method is actually
used extensively by the current MAC protocols for sensor nodes, and such
mechanisms are further detailed in the following sections.
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