White Space Networking with Wi-Fi like Connectivity

This paper describes a MAC protocol for Wi-Fi-like whitespace networking ("WhiteFi"). The primary challenge is to choose and maintain choices of channels that avoid interference with existing users of the channel. To achieve this goal the hardware structure of a WhiteFi node includes both a scanner that drives a software-defined radio (used for access point detection and to detect ‘incumbents’ of channels) as well as a Wi-Fi-like radio.

To assign channels, the authors measure the utilization of each 6MHz frequency range and the number of active access points on each. Based on this, they estimate the share of bandwidth each node can expect receive and send these measurements to the access point. The AP choses a channel by aggregating these measurements (after taking into account which channels are used by incumbents seen by any end host or the AP).

To allow devices to quickly switch channels after incumbents are detected, a backup channel to which the AP listens (using its scanner radio) and clients fall back to in quiet periods is used.

The authors evaluated their active channel detection, AP discovery, and channel switching speed on an (apparently two-node) prototype deployment. They evaluated their channel selection algorithm in simulation and showed that it appeared to perform close the optimum in the presence of simulated background traffic, spatial variation.

Disappointingly, the authors don't appear to evaluate the effects their channel switching will have on end users. On their only experiment on the real testbed with this issue (Fig. 14) shows that there appears to be some serious throughput penalty when channel switching must occur, and from their evaluation it is not clear what effect that would likely have on transport protocols.