A PON takes advantage of (WDM), using one wavelength for downstream traffic and another for upstream traffic on a (ITU-T, typically OS2). BPON, EPON, GEPON, and have the same basic wavelength plan and use the 1490 nanometer (nm) wavelength for downstream traffic and 1310 nm wavelength for upstream traffic. 1550 nm is reserved for optional overlay services, typically RF (analog) video.