l spectrum map, multiple radios could also collaborate in a vertical sharing mode, aggregating channel measurements via a CSCC protocol, in order to identify and exploit "white space" opportunities. If multiple radios compete for transmit opportunities, the CSCC channel would enable the contention to be resolved via a priority system, or alternatively dynamic auctions could be implemented, allowing devices to bid for media access. A further issue in spectrum sharing is identifying and preserving redundant "stand-by" channels, in case a primary user resumes broadcasting and the spectrum agile radio has to shift channels in order to avoid interfering. Solving these MAC and PHY layer challenges, starting with the efforts of the 802.22 Working Group in defining the specifications for wireless regional area networks, will enable cognitive and spectrum agile radios to open up the potential of a duty cycle approaching 100% in every hertz of the RF spectrum, unblocking spectrum availability as a constraint for a long time to come. Summary of Part VI The four developing technologies briefly described in this chapter promise a not-too-distant future in which wireless networks become truly ubiquitous - with virtually unlimited bandwidth available over a variety of different network types and ranges, enabling always on, mobile connectivity at any desired service level. The technology challenges of bandwidth, media access, QoS and mobility, have been recurring themes as each scale of wireless networking has been explored in earlier chapters. These new technologies may look set to consign these particular challenges to the history books, but there can be little doubt that new usage models Leading Edge Wireless Networking Technologies and services will also emerge that will put greater demands on wireless networks and pose new challenges for the standards, software and hardware developers. It is likely still to be some time before the final chapter can be written on wireless networking technology. This page intentionally left b