Many people believe that if a good wireless sensor node is built with a good radio, a good wireless sensor network can be built. Although a good sensor node/radio is an important part of the whole wireless sensor network, this is not whole story. The characteristics of a “good” sensor network include: scalability, reliability, responsiveness, mobility, and power efficiency.
A “sensor node” is just a very small part of the wireless sensor network. There are more important design challenges that go into making a network “good.” To build a good wireless sensor network, all of the above factors must work in harmony. The challenge is that this harmony can be difficult to achieve. The complex inter-relationships between these factors is a balance; if these factors are not managed well, the network can suffer from overhead that negates its applicability in the real world.
Scalability refers to the ability of the network to grow, in terms of the number of nodes, without excessive overhead. This is an important real-world requirement where networks must support more than the small handful of nodes typical in a pilot implementation.
This is due to the network overhead that comes with the increased size of the network. In ad hoc networks, the network is formed without any predetermined topology or shape. Therefore, any node wishing to communicate with other nodes should generate more packets than its data packets – i.e. “control packets” or “network overhead.” As the size of the network grows, more control packets will be needed to find and keep the routing paths. Moreover, as the network size increases, there is higher chance that communication links get broken in communication paths, which will end up with creating more control packets. In a small network, the amount of control packets is almost negligible. But when the network size starts increasing, the overhead increases rapidly. Since the available overall bandwidth is limited, the increase of overhead results in the decrease of usable bandwidth for data transmission. As the network size grows further, there will be very small amount of bandwidth left for application data transmission.
Reliability is the ability of the network to ensure reliable data transmission in a state of continuous change of network structure. Typically there is an inverse relationship between scalability and reliability in ad hoc wireless networks; as the number of nodes in the network increases, the more difficult it becomes to ensure reliability.
This scalability characteristics of ad hoc networks described above imposes an interesting question on the reliability of the network. Since an ad hoc network is designed to automatically adapt itself to a changing environment or interference, it will issue more control packets when it faces dynamics. More dynamics in the environment will increase the number of control packets and, at some point, the network cannot sustain the amount of overhead caused by the dynamics, which will result in less reliability of data transmission. This breaking point will show up earlier in a large-sized network. So, network scalability and reliability are closely coupled and typically they act against each other.
C. Responsiveness (and Latency)
Responsiveness is the ability of the network to quickly adapt itself to changes in topology. To achieve high responsiveness, an ad hoc network should issue and exchange more control packets, which will naturally result in less scalability and less reliability. In general, the latency of packet delivery in dynamic environment decreases in the network with high responsiveness.
Mobility refers to the ability of the network to handle mobile nodes and changeable data paths. Generally, a wireless sensor network that includes a number of mobile nodes should have high responsiveness to deal with the mobility. So, it is not easy to design a large scale and highly mobile wireless sensor network.
E. Power Efficiency
Power efficiency – the ability of the network to operate at extremely low power levels – also plays an important role in this complex equation. A typical method for designing a low-power wireless sensor network is to reduce the duty cycle of each node. The drawback is that as the wireless sensor node stays longer in sleep mode to save power, there is less chance that the node can communicate with its neighbors. This will decrease the network responsiveness and may also lower reliability due to the lack of the exchange of control packets and delays in packet delivery. In addition, a more complicated synchronization technique will be necessary to keep more nodes in low duty cycle, which may also affect scalability.
F. Managing the Design Tradeoffs
The complex issue of managing these tradeoffs comes down to how the communication overhead can be minimized while maintaining the network reliability and responsiveness. As explained above, there are many conflicting factors involved in the design of wireless sensor networks, and there are always tradeoffs. When choosing a wireless sensor network for an application, careful consideration of the balance of these factors within the context of the needs of the application is critical.