Design Issues of Routing Protocols for WSNs
The following are factors that must be considered to achieve efficient communications between nodes in a wireless sensor networks.
- Node deployment may be manual/deterministic or random. In the former, nodes are manually placed and data is routed via pre-determined paths. In the latter case, nodes are scattered randomly, forming an ad hoc infrastructure. If the nodes are not uniformly distributed, it may be necessary to cluster the nodes to support connectivity and enable energy-efficient operations.
- Sensor nodes are very dependent on battery lifetime because they are often left unattended for long periods of time, thus, it is imperative that communications and computations take an energy-conserving approach.
- Data reporting method is either time-driven, even-driven, query-driven or a hybrid of these methods, depending on the criticality of the data. The time-driven method allows sensor nodes to periodically turn off their transmitters to conserve power. The event- and query-driven methods require sensors to react immediately to sudden and drastic changes in the environment, and are therefore, suitable for time-critical applications. The routing protocol is very influenced by the data reporting method in terms of energy consumption and route calculations.
- Sensor nodes are often assumed to be homogeneous, but depending on the application, sensor nodes may have different roles or capabilities. For example, an application may require different types of sensors to monitor different aspects of the environment, such as temperature, vibration and humidity level. In this case, special sensors can be deployed independently or a sensor may incorporate different functionalities. Data reading and reporting by the sensors may be done at different rates, depending on the QoS requirement. If the nodes are clustered, the burden of transmission to the base station is placed on the cluster heads.
- The failure of a sensor node should not affect the rest of the network. If several nodes fail, the MAC layer and the routing protocol should accommodate the formation of new links and routes to the data collection base station.
- The routing protocol must be able to scale and work in a large network consisting of hundreds or thousands of sensor nodes. The routing protocol must also be scalable enough to respond to events in the environment.
- In cases where sensor nodes are mobile, routing messages from/to moving nodes is more challenging because routes and topology stability become important issues. In addition to node mobility, the event monitored may also be mobile, e.g. tracking application. Sensing fixed events allows the network to be in a reactive mode whereas dynamic events usually require periodic reporting.
- The required bandwidth of WSN is usually in the order of 1 – 100 kbps. The MAC design that uses TDMA conserves more energy that contention-based protocols like CSMA.
- In a high density network, it is unlikely that sensors will be isolated from each other. Even though nodes would be highly connected, it does not mean the network topology will be fixed or that the network size is static. The network size may vary and shrink due to node failures.
- A node has a certain view of the environment that is limited in range and accuracy. A node covers a limited physical area, hence, area coverage is an important parameter.
- Sensor nodes may generate a significant amount of redundant data, making data aggregation (e.g duplicate suppression, minima, maxima and average) an important function of sensor networks. Aggregation can be used to support energy efficiency and data transfer optimisation for routing protocols. Signal processing methods can be used for data aggregation (referred to as data fusion), where a node produces a more accurate output signal by using techniques such as beamforming to combine incoming signals and reduce noise in the signals.
- Certain applications require that data is delivered within a certain time frame after it is sensed, or it will become useless. Bounded latency for data delivery is a condition for time-constrained applications. On the other hand, there are applications that consider energy conservation, which is directly related to network lifetime, to be more important than the quality of data sent. Therefore, as energy is depleted, the network may be required to reduce the quality of data in order to reduce energy dissipation at nodes to lenghten the network lifetime. This requirement must be considered by energy-aware routing protocols.
Source:
Routing techniques in wireless sensor networks: a survey
