A virtual switching supervisor (VSS) is a solution that binds two Catalyst 6500 chassis in a single switching system, allowing you to bundle links terminated on both chassis in a single EtherChannel (without VSS, all links in an EtherChannel have to be terminated on the same box). This solution reduces the impact that spanning tree has on your network, particularly on bisectional bandwidth. (You can find more information in this introduction to data center technologies for network engineers.)
Dense wavelength division multiplexing (DWDM) is a layer-1 transport technology that multiplexes several optical signals into the same fiber by using different wavelengths (colors). It allows you to transport more data across existing dark fiber infrastructure.
Comparing the two is like comparing a car to a highway; they perform completely different functions.
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