SONET time division multiplexing (TDM) multiplexes synchronous and asynchronous signals to a single higher bit rate for single-wavelength transmission over fiber. Before being multiplexed, source signals might be converted from electrical to optical, or from optical to electrical and back to optical. Wave division multiplexing (WDM) maps multiple optical signals to individual wavelengths and multiplexes the wavelengths over a single fiber. Another difference between TDM and WDM is that WDM can carry multiple protocols without a common signal format, whereas SONET cannot. Some of the key differences between TDM and WDM interfaces are illustrated