Multiplexing in OSI model?

The textbook "Data Communications and Computer Networks", by Curt M. White, (Pub. Thomson Learning, Mass.) claims,...

on p 144, "Multiplexing is a technique performed at the physical layer of the OSI model..." Frankly, I find this not only difficult to understand, but difficult to believe. I would have thought that it should go into the Transport, or perhaps the Network layer of OSI. I've just done a multiple-choice test; one question asked about the placement of multiplexing in OSI, and I said, Transport. What do you say?

Multiplexing means adding signals together for transmission and breaking them apart upon reception. Because we're talking only about signals (not frames, packets, or other protocol data units) this activity indeed occurs at the Physical Layer in the OSI model. I agree with your textbook (and in fact, wrote more or less the same kind of thing in my own Course Technology textbook for Thomson Learning as well).