Although Moore's law predicts that hardware for sensor networks will inexorably become smaller, cheaper, and more powerful, technological advances will never prevent the need to make tradeoffs. Even as our notions of metrics such as ``fast'' and ``small'' evolve, there will always be compromises: nodes will need to be faster or more energy-efficient, smaller or more capable, cheaper or more durable.
Instead of choosing a single hardware platform that makes a particular set of compromises, we believe an effective design is one that uses a tiered platform consisting of a heterogeneous collection of hardware. Larger, faster, and more expensive hardware (``sensors'') can be used more effectively by also using smaller, cheaper, and more limited nodes (``tags''). An analogy can be made to the memory hierarchy commonly found in desktop computer systems. CPUs typically have extremely expensive, fast on-chip cache, backed by slower but larger L2 cache, main memory, and ultimately on-disk swap space. This organization, combined with a tendency in computation for locality of reference, results in a memory system that appears to be as large and as cheap (per-byte) as the swap space, but as fast as the on-chip cache memory. In sensor networks, where localized algorithms are a primary design goal, similar benefits can be realized by creating the network from a spectrum of hardware ranging from small, cheap, and numerous, to large, expensive, and powerful.
The smaller ``tag'' devices will trade functionality and flexibility for smaller form factor and power. Alone, they would not be adequate to support our sensor network application. However, in conjunction with more endowed nodes, they significantly enhance the network's capabilities. There are many possible advantages to augmenting sensor nodes with small-form-factor tags, including: (1) Density: Tags, by definition, can be significantly lower cost and therefore can be deployed in larger numbers, more densely, than larger, higher capacity sensor nodes, (2) Longevity: Tags can be significantly lower power and therefore can be deployed for longer periods of time, or at higher duty cycles, than larger, higher capacity sensor nodes, particularly if we are able to exploit higher density, and (3) Form factor: Tags are smaller and therefore can be (a) more easily and unobtrusively attached to a wider variety of targets (e.g., for tracking, condition based maintenance, and other logging applications), and (b) deployed with high density.