The range of frequencies or wavelengths emitted by a transmitter and surrounding the center frequency or wavelength at a power level equal to half the maximum power level.The ITU-T has defined a number of wavelength bands, or windows, for use in standards-based fiber optics transmission systems (FOTS). As fairly crude light sources, light-emitting diodes (LEDs) emit signals of the greatest spectral width. More sophisticated diode lasers emit very narrowly defined signals that may be only 1 nm wide, or less. The more narrowly defined the spectral width, the tighter the windows can be packed and the greater the spectral efficiency. Some radio systems are similarly concerned with spectral efficiency, as they must pack multiple channels into a relatively narrow radio frequency (RF) band. Bluetooth and DECT both employ Gaussian frequency-shift keying (GFSK) as a means of both signal modulation and pulse shaping. See also Bluetooth, channel, DECT, DFB laser, Fabry-Perot laser, FOTS, GFSK, laser diode, LED, modulation, pulse shaping, RF, and window.