Editing Wavelength-division multiplexing (section)

===Wavelength-converting transponders===
{{tone|section|date=December 2018}}
Wavelength-converting transponders originally translated the transmit wavelength of a client-layer signal into one of the DWDM system's internal wavelengths in the 1,550&nbsp;nm band. External wavelengths in the 1,550&nbsp;nm most likely need to be translated, as they almost certainly do not have the required frequency stability tolerances nor the optical power necessary for the system's EDFA.

In the mid-1990s, however, wavelength-converting transponders rapidly took on the additional function of [[signal regeneration]]. Signal regeneration in transponders quickly evolved through 1R to 2R to 3R and into overhead-monitoring multi-bitrate 3R regenerators. These differences are outlined below:

; 1R: Retransmission. Basically, early transponders were [[garbage in, garbage out]] in that their output was nearly an analog ''copy'' of the received optical signal, with little signal cleanup occurring. This limited the reach of early DWDM systems because the signal had to be handed off to a client-layer receiver (likely from a different vendor) before the signal deteriorated too far. Signal monitoring was basically confined to optical domain parameters such as received power.

; 2R: Re-time and re-transmit. Transponders of this type were not very common and utilized a quasi-digital [[Schmitt trigger|Schmitt-triggering]] method for signal clean-up. Some rudimentary signal-quality monitoring was done by such transmitters that basically looked at analogue parameters.

; 3R: Re-time, re-transmit, re-shape. 3R Transponders were fully digital and normally able to view SONET/SDH section layer overhead bytes such as A1 and A2 to determine signal quality health. Many systems will offer {{nowrap|2.5 Gbit/s}} transponders, which will normally mean the transponder is able to perform 3R regeneration on OC-3/12/48 signals, and possibly gigabit Ethernet, and reporting on signal health by monitoring SONET/SDH section layer overhead bytes. Many transponders will be able to perform full multi-rate 3R in both directions. Some vendors offer {{nowrap|10 Gbit/s}} transponders, which will perform Section layer overhead monitoring to all rates up to and including OC-192.

; Muxponder: The [[muxponder]] (from multiplexed transponder) has different names depending on vendor. It essentially performs some relatively simple time-division multiplexing of lower-rate signals into a higher-rate carrier within the system (a common example is the ability to accept 4 OC-48s and then output a single OC-192 in the 1,550&nbsp;nm band). More recent muxponder designs have absorbed more and more TDM functionality, in some cases obviating the need for traditional [[SONET/SDH]] transport equipment.