DWDM basics

First of all i must say gr8 post on DSLAM by Ranger .I'v seen many tutorials on ATM and SDH but Ranger explained it very beautifully.

I wanto know whts the concept of DWDM ,how it works ?and how is it different from other technologies?.I've read some stuff but havnt been able to get into it deeply.Ranger as u r into Broadband Network support , i think you'll be the best person for explaining this technology.And one more thing...Wht r the basic differences between PDH and SDH?

Rgds
Taqqi14

DWDM : Short for Dense Wavelength Division Multiplexing, an optical technology used to increase bandwidth over existing fiber optic.

DWDM works by combining and transmitting multiple signals simultaneously at different wavelengths on the same fiber. In effect, one fiber is transformed into multiple virtual fibers.

I've been having unanswered questions in this field for ages,

Papers in DWDM research area talk about 40 to 80 wavelengths incorporated into one fibre!! I wonder what kind of fibre they are using here. Can Multimode do that :!: :

DWDM uses wavelengths in the 1550-nm band and, as far as I know, multimode fibers have a too high attenuation in this band... So singlemode fibers are best suited for DWDM.

DWDM uses wavelengths in the 1550-nm band and, as far as I know, multimode fibers have a too high attenuation in this band... So singlemode fibers are best suited for DWDM.

Great, But isn't single mode designed to only carry one wavelength at a time? Or at most only a few like 4 or 8 wavelengths ? A CCNP once told me that he hasn't seen a fibre carrying more than 8 wavelngths.

But I'm sure they are using 40 to 80 and more in metro networks. I wonder how is this possible?

I don't know the details but to carry 80 wavelengths the fiber must have a very very low loss for the 80 wavelengths. I've read they use non-zero dispersion-shifted fiber... But I dont know much about this...