Arguments for cross-layer optimizations in Bluetooth scatternets

The Bluetooth standard defines a multi-hop routing structure, called a scatternet, to address the limitations caused by short-range and small fanout of the underlying link technology. We identify several characteristics, the combination of which makes scatternets different from previously considered networks.  Importantly, Bluetooth links are connection-oriented with low-power link modes. We show that the unique aspects of the technology require a redesign of the protocol structure for link formation, IP routing, and service discovery. When existing approaches to these protocols are applied to scatternets, the multiple protocol layers would operate without knowledge of each other, resulting in inefficient use of power in many cases. We suggest an alternative approach where there is a single protocol layer providing a level of indirection within the scope of a scatternet. That is, we argue for extensive cross-layer optimizations. Specifically, this allows (a) links to be kept active only when absolutely required, and (b) scatternet-wide floods to be minimized by caching service discovery results at all intermediate nodes. Our experiments with an implementation of an emulated Bluetooth scatternet show that this could be more efficient than the traditional approach of layered protocol design.