Transponder = receiver + transmitter?

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Hello. Reading the literature on the design of DWDM systems, a question arose. In one book they write that Transponder = transmitter + receiver, a kind of 2 in 1 box. The functions of which are to convert electrical signals to optical on different light waves and back at reception. However, another source writes that the DWDM system should include separate receivers, transmitters, transponders and repeaters. Question: Transponder = transmitter or receiver? If not, what is their difference?