, which boosts the signal before being received. In the S, C, and L bands, it is necessary to use a separate absorption and multi- plication layer structure, wherein the light is absorbed in a layer of InGaAs and the resulting signal is amplified in an InP multiplication layer. Unfor- tunately, avalanche photodiodes have been plagued historically by low-yield processes and a reputation of not being as reliable as standard PIN diodes. Additionally, the need to provide a high-voltage supply (40 to 60 volts) and track the APD supply voltage over temperature complicated their use in many systems. Improvements in epitaxial growth and processing tech- niques have enabled the vendors to achieve high yields (90 percent), with reliabilities equivalent to PIN diodes. 6.2.6 Standards Underway CWDM systems development and standards efforts come at a critical time for metro access service providers. Work is underway for the transport plane, but additional work is needed in the management plane, as noted earlier. One organization working to define standards for CWDM systems in the 1,400-nm region is the Commercial Interest Group (CIG). CIG par- ticipants include component suppliers, system vendors, and service providers. The group's focus to date has been primarily on defining the CWDM wavelength grid and investigating the cost/performance compar- isons of DWDM versus CWDM architectures. The CWDM wavelength grid under consideration for proposal is divided into three bands. The O band consists of 1,290, 1,310, 1,330, and 1,350 nm. The E band consists of 1,380, 1,400, 1,420, and 1,440 nm. The S, C, and L bands consist of 8 wavelengths from 1,470 to 1,610 nm in 20-nm increments. These wavelengths take advantage of the full optical fiber spectrum, including the legacy optical sources at 1,310, 1,510, and 1,550 nm, while maximizing the number of channels. The 20-nm channel spacing supports lower component costs with the use of uncooled lasers and a wideband filter. It also avoids the high-loss 1,270-nm wavelength and maintains a 30