 band SO 1550 nanometres is arguably operationally better. Additionally, longer wavelengths have lower photon energies. A 1550-nanometre photon has half the energy of an 800-nanometre photon, which means that for the same amount of energy (watts of power) a beam of 1550 nm has twice the number of photons. 2.6.4 Infrared Optical Systems Most of us of course use at least one optical wireless system every day when we turn a television or audio system on or off. Infrared remotes have been around now for 25 years. They require line of site and have limited range of at most thirty feet but seem to work well enough to keep us sitting on the sofa which is their main function. Infrared controllers get easily confused by sunlight and strong fluorescent lighting. To achieve selectivity infrared receivers usually work at around 980 nanometres with filters on the receive path that block out light at other wavelengths. Table 2.4 sets these frequencies and wavelengths in the context the range of light waves (colours) visible to the human eye. Cable Performance Benchmarks Cable data rates are lower than fibre but in common with fibre are increasing over time. The increase in data rate has primarily been delivered through the adoption of higher-order Making Telecoms Work Table 2.4 Visible wavelengths Colour Frequency Wavelength violet 668-789 THz 380-450 nm 631-668 THz 450-475 nm 606-630 THz 476-495 nm green 526-606 THz 495-570 nm yellow 508-526 THz 570-590 nm orange 484-508 THz 590-620 nm 400-484 THz 620-750 nm modulation schemes. The downstream frequencies used in cable are typically from 42 to 850 MHz, with an upstream between 5 and 42 MHz in the US and between 5 and 65 MHz in Europe. Table 2.5 shows how data rates have increased over the past ten years over three generations of DOCSIS (data over cable service interface specification). DOCSIS 3.0/Eurodocsis is needed to support a credible offering of high-definition TV and designed IPV6 compatible. 18 The higher data rates are achieved partly by higher-order modulation schemes