In this chapter important issues for the conservation of power for wireless sensor networks (WSNs) are identified and related to the design of communication protocols. The ability to do real- time data collection from both hostile and friendly environments without communication lines makes WSNs an interesting research area. Resource constraint and application specific conditions have to be catered for during the communication protocol development. Different set of requirements are required by different applications. Chapter 2 points out two main categories of WSNs applications including the periodic and the event based. Sensors periodically send their readings in the periodic based whilst heavy traffic are generated during event detection in the event based. Three main requirements in communication protocol development for WSNs are addressed as follows:
Lifetime or throughput as a main goal: In the periodic-based scenario, sensors are
sometimes deployed in a remote area and have to operate on their own throughout the targeted lifetime and data reporting rate is fairly constant. Power conservation is therefore important as the sensors may be expected to run for several months. Instead of achieving a longer lifetime, some applications focus on how to make the base station receive the desired number of packet receptions. Throughput is thus a major concern in the event- based application.
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Duty cycle: The required duty cycle is mainly concerned about the amount of generated
traffic at a specific time. In a surveillance system, many readings may be collected from a site and sent within a short duration. Hence the sensors have to be often in an active mode for communications. However, the sensors switch their radios off more often in an environmental monitoring system. Current temperature and humidity may be read every minute [MPS+02, TPS+05]. A lower duty cycle is desired in such cases and the sensors can be in sleep mode most of the time.
Amendment to the locations of sensors: The sensors are either fixed at specific
locations or moved. Mobility of sensors is crucial for network routing. In a fixed topology, the same communication route can be assumed if all of the nodes are still performing. A new path has to be discovered if the nodes are moved. Localisation is another key issue if the sensors are attached to objects, such as in the local-aware or habitat monitoring system.
This dissertation aims at building a communication protocol for WSNs. The targeted scenario is the periodic-based where a low duty cycle is required. The network consists of a fixed set of sources and a base station. Furthermore, direct data communications between the base station and its sources are feasible. The communication protocol to be developed will effectively support the single-hop WSNs. Such a structure forms a network cluster which can be used in some environmental or habitat monitoring system such as [MPS+02] and [TPS+05]. As the number of sources is fixed throughout the communications, the data reporting rate is fairly constant. The communication of the sources can be therefore scheduled and controlled by the base station. A time slot is allocated to each source and will be used for data communication. Only one source can use the shared medium whilst the others switch to sleep mode by turning their radios off and consuming the least amount of energy. Data collision can be avoided and idle listening can be minimised.
In order to develop a protocol to support the described scenario, several questions arise as follows:
Of the total energy consumed by a sensor node, how much of it is attributed to
communication? If communication accounts for a high proportion of a sensor's total
energy budget, then optimising energy consumption for communication is itself important.
How does the energy used for communication change at different transmission
levels would motivate adapting the power used for transmission to the minimum required for effective communication.
What factors affect signal reception and provides the basis for determining whether
the models discussed are good predictors for sensor network communication. If the
important factors affecting the receiving signal strength are not included in the existing models, the indices reflecting the current link quality should be measured.
What metrics should be used to measure the signal strength? The relationship
between the metrics can justify whether the transmission power adaptation is desired to support power conservation.
Are measurements required to determine the power required for effective
communication? Experimental results indicate that transmission power, location,
heterogeneity in sensor manufacture and time of day are four key factors introducing variations in link quality.
Can scheduling be done with sufficient accuracy to save energy when compared to a
contention based approach? There are two main approaches which resolve contention in
the shared medium system. In this dissertation, the single-hop topology is assumed to remain unchanged. The schedule-based protocol is adopted to schedule the communications. The slot length can be accurately estimated if the relationship between delays and packet sizes are addressed. The two-way propagation delay should be significantly small as data travels at the speed of light.