Wireless Sensor Networks : Technology, Protocols, and Applications

290 PERFORMANCE AND TRAFFIC MANAGEMENT
query messages to sensor nodes to get the up-to-date value for the information.
Query messages may also carry a command from the sink to the sensors about
the information, reporting frequency and other parameters of interest to the sink.
In this delivery model, the sink broadcasts the query message, a path is constructed
automatically when the query arrives at the sensor nodes, and the sensor nodes
report their findings according to the request in the query message.
Hybrid Delivery In some WSNs, the types of sensors and the data they sense may
be very diverse. For example, data may be reported continuously by some nodes,
and the sink may need to query information from other sensor nodes.
Energy Models The radio communication function of sensor nodes is the most
energy-intensive function in the node. Compared with that, the actual sensing
operation consumes the least energy (see Figure 11.3 from [11.7]). There are two
approaches to reducing consumption for sensor communications. The first approach
is to design a communication scheme that conserves energy inherently: for example,
turning off the transceiver for a period of time. The second approach is to
reduce the volume of communications through in-network processing. These
would entail functions such as data aggregation and data compression since computation
tasks usually require less energy than do communication tasks.
Model for Sensing Usually, the least amount of energy is consumed for sensing.
Let the sensing range be r s . It can be assumed that the power consumed to perform
sensing over a circle with radius r s is proportional to rs 2 or r4 s [11.10].
Model for Communication An energy model for communications is as follows
[11.4,11.5]. The energy for transmitting l-bit data over a distance d is E tx ðl; dÞ
Figure 11.3 Energy consumption for each subsystem in sensor nodes. (From [11.7].)