Editing Wavelength-division multiplexing (section)

==Transceivers versus transponders==
; Transceivers: Since communication over a single wavelength is one-way ([[simplex communication]]), and most practical communication systems require two-way ([[duplex communication]]) communication, two wavelengths will be required if on the same fiber; if separate fibers are used in a so-called fiber pair, then the same wavelength is normally used and it is not WDM. As a result, at each end both a transmitter and a receiver will be required. A combination of a transmitter and a receiver is called a transceiver; it converts an electrical signal to and from an optical signal. WDM transceivers made for single-strand operation require the opposing transmitters to use different wavelengths. WDM transceivers additionally require an optical splitter/combiner to couple the transmitter and receiver paths onto the one fiber strand.{{bulleted list
| Coarse WDM (CWDM) Transceiver Wavelengths: 1271&nbsp;nm, 1291&nbsp;nm, 1311&nbsp;nm, 1331&nbsp;nm, 1351&nbsp;nm, 1371&nbsp;nm, 1391&nbsp;nm, 1411&nbsp;nm, 1431&nbsp;nm, 1451&nbsp;nm, 1471&nbsp;nm, 1491&nbsp;nm, 1511&nbsp;nm, 1531&nbsp;nm, 1551&nbsp;nm, 1571&nbsp;nm, 1591&nbsp;nm, 1611&nbsp;nm.
|  Dense WDM (DWDM) Transceivers: Channel 17 to Channel 61 according to ITU-T.
}}
; Transponder: In practice, the signal inputs and outputs will not be electrical but optical instead (typically at 1550&nbsp;nm). This means that in effect wavelength converters are needed instead, which is exactly what a transponder is. A transponder can be made up of two transceivers placed after each other: the first transceiver converting the 1550&nbsp;nm optical signal to/from an electrical signal, and the second transceiver converting the electrical signal to/from an optical signal at the required wavelength. Transponders that don't use an intermediate electrical signal (all-optical transponders) are in development.

See also [[Transponders#Optical communications|transponders (optical communications)]] for different functional views on the meaning of optical transponders.