be a function of time, the spatial position of the sensor, and its relationship to the environment.
By following where sensor nodes are positioned, spatial data will less likely be in error.
In comparison to traditional computer networks, it is also important that networks composed of such sensors be more fault tolerant and adaptive to environmental changes.
There are some commercial sensor-network nodes we can use. One of them is NeuronChip, the network node of LonWorks. It communicates using LonTalk Protocol, a protocol stack ensuring that nodes can interoperate using an efficient and reliable communication standard ( Hoske 1996). Applications can be developed for NeuronChip in a GUI-based programming environment using Neuron-C which provides implicit message passing through network variables.
It is essential for sensor systems to be fault tolerant and have real-time capabilities. Takegaki proposed a task mapping algorithm based on a diffusion model for distributed real-time systems ( Takegaki 1991). His approach included the analysis of task scheduling on the basis of a Rate Monotonic algorithm. Nadig proposed a network architecture composed of a hierarchical de Bruijn network to prevent node isolation due to route failures ( Nadig 1993). The above works were generally intended for plants or large scale factories, but not for more human-centric environments such as offices or homes.
Exemplary works having such a goal include Ubiquitous Computing ( Weiser 1991) and TRON ( Koshizuka 1996). In their networks, components are mostly electrical equipment, while we aim at connecting not only electrical equipment but also non-electrical objects by embedding networked sensors in them. In addition, since they did not concern dynamic changes of a sensor network, any mechanism for position dependency was never implemented.
Some work considered the location of a network node. Active Badge Location System ( Want 1992) aimed at constructing an active office with providing various services using computers on which some sensors are attached. Every user of the system wears an active badge that broadcasts a message by infra-red waves in a certain period. The system handles update of users' location information, but it cannot estimate the location of sensors dynamically because the location of each sensor is assumed to be fixed. In addition, the system cannot obtain a precise location, while our sensor network system can obtain the almost precise locations of several sensors in a short period.