The goal of the PicoRadio project is to build a wireless sensor network that is versatile, self-organizing, dynamically reconfigurable, and multi-functional. The primary constraint in building the nodes is the extremely low energy budget. This imposes tight constraints on the entire design, and instead of the traditional layered protocol design, we are tightly integrating the protocol stack vertically for maximum energy optimization. The network layer of the PicoNode protocol stack has two primary functions: routing and addressing of nodes.
Addressing of nodes is based on the geographical positions of nodes. Queries in sensor networks are typically not concerned with a particular node, rather they are directed to any nodes which satisfy particular criteria of position, type, etc. This leads to our concept of class-based addressing, which is a triplet consisting of location, node type, and node sub-type. This eliminates the overhead of assigning and maintaining fixed addresses across the network as well as the problem of distributing them dynamically.
For the routing of packets across the network, we have developed an energy efficient protocol we call "energy aware routing." This scheme takes the view that trying to optimize every route to consume the least amount of energy is not in the best interest of the network. Instead, we try to optimize the power of the entire network and in the process maximize the lifetime. Thus, we find a set of routes on demand and choose between them in a probabilistic fashion. The choice is based on a metric which combines both transmit/receive power and residual energy at a node. This means that all nodes across the network participate more actively in routing, and a few nodes do not die out just because they are in a good position to forward packets. Simulations verify this behavior and the network lifetime also increases substantially.
In addition, we are working on defining the capacity of sensor networks at the network level. This capacity is basically determined by a set of nodes that are the most energy constrained, forming a cutset. The challenge is to find a routing scheme that can route packets in such a way as to minimize the energy consumption of nodes in this set. The probabilistic scheme discussed earlier performs this task to some extent, but more work is needed to actually find the optimal scheme.