 divided by wavelength, with the speed of light defined as 299 792 458 metres per second. Physical Layer Connectivity The 18 billion dollar FioS Optical network being deployed by Verizon in the US is an example of a passive optical network based on a combination of multimode and single-mode fibre transport. Even allowing for multiplexing on the multimode drop, these networks offer super fast downloads of recorded content including high-definition video, multiple (200 or more) stan- dard TV channels and high-definition TV. Peak rates are capped per user but can be 45 Mbit/s and could theoretically be much higher. Both multimode and single-mode fibre have substantial untapped bandwidth potential that will be realised as the cost performance ratio of optical components improves over time. So, for example, Fabry-Perot lasers remain cost effective but have a wide spectral line that makes them unsuitable for long-distance transmission (the effects of chromatic dis- persion) and dense WDM (requiring close frequency spacing). The devices are, however, temperature tolerant, which makes them suitable for CPE (customer premises equipment) applications. Distributed feedback lasers have a wide spectral range and narrow line width, of the order of a few hundred MHz. When temperature stabilised they are good for both long-haul and DWDM systems but are much more expensive. They can also be wavelength tuned over several nanometres. Vertical cavity resonators are cheaper and now perform very similarly to DFB lasers. Other enabling technologies already mentioned include modulators that control optical phase as well as amplitude, optical receive filters and photonic integrated circuits that combine previously discrete components such as transmitters modulators and receivers into a single chip, though some standards work needs to be done to ensure sufficient economies of scale are realised to cover R and D and manufacturing investment. The standardisation of 100 Gbit/s transceivers by the Internetworking Forum (OIF) is one example. A