 In vacuum, this is the velocity of light, usually denoted by the lower case letter, c. In glass fiber, it is substantially slower, usually about 0.7 times c. The data rate, which ideally might be at the carrier frequency, in practical systems is always a fraction of the carrier frequency.A WDM system uses a multiplexer at the transmitter to join the several signals together and a demultiplexer at the receiver to split them apart.With the right type of fiber, it is possible to have a device that does both simultaneously and can function as an optical add-drop multiplexer.