Image Processing Reference
In-Depth Information
Bandwidth is also a constraint when dealing with sensor networks. he low-power communication
devices used (most of the time radio transceivers) can only work in simplex mode. hey offer low data
rates due also to the fact that they are functioning in the free unlicensed bands where traffic is strictly
regulated.
3.4.2 Diversity and Dynamics
As we already suggested, there may be several kinds of sensor nodes present inside a single sensor
network. We could talk of heterogeneous sensor nodes from the point of view of hardware and soft-
ware.Fromthepointofviewofhardware,itseemsreasonabletoassumethatthenumberofacertain
kind of devices will be in an inversely proportional relationship to the capabilities offered. We can
assist to a tiered architecture design, where the resource-poor nodes will ask more powerful or spe-
cialized nodes to make more accurate measurements of a certain detected phenomenon, to perform
resource-intensive operations or even to help in transmitting data at a higher distance.
Diversity can also refer to sensing several parameters and then combine them in a single decision,
or in other words to perform data-fusion. We are talking about assembling together information from
different kinds of sensors like: light, temperature, sound, smoke, etc. to detect, for example, that a fire
has started.
Sensor nodes are to be deployed in the real world, most probably in harsh environments. his puts
them in contact with an environment that is dynamic in many senses and has a big influence on the
algorithms that the sensor nodes should execute. First, the nodes are deployed in a random fashion
intheenvironmentandinsomecases,someofthemcanbemobile.Second,thenodesaresubjectto
failures at random times and they are also allowed to change their transmission range to better suit
their energy budget. his leads to the full picture of a network topology in a continuous change. he
algorithms for the wireless sensor networks have as one of their characteristic the fact that they do
not require a predefined well-known topology.
One more consequence of the real world deployment is that there are many factors influencing
the sensors in contact with the phenomenon. Individual calibration of each sensor node will not be
feasible and probably will not help much as the external conditions will be in a continuous change.
The sensor network should calibrate itself as a whole, as a reply to the changes in the environment
conditions. More than this, the network will be capable of self-configuration and -maintenance.
Another issue we need to talk about is the dynamic nature of the wireless communication medium.
Wireless links between nodes can periodically appear or disappear due to the particular position
of each node. Bidirectional links will coexist with unidirectional ones and this is a fact that the
algorithms for wireless sensor networks need to consider.
3.4.3 Needed Algorithms
For a sensor network to work as a whole, some building blocks need to be developed and deployed
in the vast majority of applications. Examples can include a localization mechanism, a time synchro-
nization mechanism, and some sort of distributed signal processing. A simple justification can be
that data hardly has any meaning if some position and time values are not available with it. Full,
complexsignalprocessingdoneseparatelyateachnodewillnotbefeasibleduetotheresource
constraints.
The self-localization of sensor nodes gained a lot of attention lately [-]. It came as a response
tothefactthatglobalpositioningsystemsarenotasolutionduetohighcost(intermsofmoneyand
resources) and it is not available or provides imprecise positioning information in special environ-
ments as indoors, etc. Information such as connectivity, distance estimation based on radio signal
strength, sound intensity, time of flight, angle of arrival, etc. were used with success in determining
the position of each node within degrees of accuracy using only localized computation.
