channel wavelength spacing. In contrast, DWDM, which is widely used in long-haul networks and some metro core Figure 6-2 Wavelengths used by Area used Area used by CMDM by BWDM Wavelength (nm) DWDM refers to the ability to carry a large number of channels, where large may be a hundred or more. DWDM equipment usually uses the 1530- to 1565-nm wavelength range with very high laser accuracy requiring temperature-controlled components. The standards today define a 0.8-nm wavelength gap between any two channels, with some manufacturers demonstrating equipment with 0.4-nm and even 0.2-nm accuracy. CWDM uses a much wider wavelength range (1200 to 1600 nm) with a minimum 20-nm wavelength gap between any two channels. Optical components used for CWDM are less accurate and thus less expensive because of the 20-nm wavelength gap. Bidirectional WDM (BWDM) refers to the ability to carry two channels in different directions on a single fiber. Two distinct wavelengths may be used in BWDM technology: 1310 and 1550 nm. Coarse Wavelength Division Multiplexing (CWDM) Technology networks (particularly those with large diameters), uses lasers with much narrower wavelength spacing, typically 0.8 or 0.4 nm. The wide channel spacing of CWDM means that a lower system cost can be achieved. This lower equipment cost is the result of a lower transmitter cost because no temperature and wavelength control is needed and a lower optical mux/demux cost is due to wider tolerance on the wavelength stability and bandwidth. Transmission-side technological advancements are currently taking place. For example, Lucent recently demonstrated the highest-capacity (20- Gbps) transmission on an access fiber (AllWaveTM zero water peak) using O and E bands (1,300- to 1,440-nm band), and 8-channel, 20-nm-spaced CWDM technology; each channel supports 2.5 Gbps. This kind of full- spectrum-capable CWDM solution provides the lowest cost per bit and is upgradeable to DWDM in the future. This CWDM technology is based on a 20-nm channel spacing that allows lower-cost