Time division multiplexing (TDM) has been the fundamental basis for adding capacity to digital telecommunications networks for decades. However, since 1997, wavelength division multiplexing (WDM) has been emerging as an important and widely deployed complement to TDM. Sales of systems based on the new technology have risen at breathtaking speed. The driving force behind this sales explosion was the unexpected rapid exhaustion of long distance fibre network capacity. This fibre exhaust, combined with favourable economics for WDM, led to the use of this technology over other alternatives. The...