Dynamic Spectrum Access systems exploit temporarily available spectrum
(`white spaces') and can spread transmissions over a number of non-contiguous
sub-channels. Such methods are highly beneficial in terms of spectrum
utilization. However, excessive fragmentation degrades performance and hence
off-sets the benefits. Thus, there is a need to study these processes so as to
determine how to ensure acceptable levels of fragmentation. Hence, we present
experimental and analytical results derived from a mathematical model.

As passive optical networks (PON) are increasingly deployed to provide high
speed Internet access, it is important to understand their fundamental traffic
capacity limits. The paper discusses performance models applicable to
wavelength division multiplexing (WDM) EPONs and GPONs under the assumption
that users access the fibre via optical network units equipped with tunable
transmitters. The considered stochastic models are based on multiserver polling
systems for which explicit analytical results are not known.