DWDM puts dozens of separate signals onto a single pair of fibers by giving each one its own colour of light. The channels do not interfere with each other and can carry different services at different rates, so one fiber pair does the work that would otherwise need dozens.
Trenching fiber is expensive and slow; adding a wavelength to fiber already in the ground is neither. DWDM is what turns installed plant into a scalable asset — capacity grows by adding channels and upgrading optics rather than by building new routes, which is the economic foundation of modern optical networks.
Wavelengths sit on a standardised frequency grid, commonly spaced at 100 GHz or 50 GHz, with flexible-grid systems allowing wider channels for very high rates. Each channel is independent: one can carry 10G Ethernet, another 100G, another a customer's wavelength service, all on the same fiber pair.
Coarse WDM uses much wider channel spacing, which makes the optics cheaper and the channel count far lower. It is common for short metro and enterprise links where a handful of channels is plenty. DWDM is what you use when channel count or distance makes the extra cost worthwhile.
DWDM is why a wavelength can be bought as a product and why bandwidth upgrades on a lit circuit are usually a configuration change rather than a construction project. For a buyer it mostly matters as an explanation of why capacity is elastic on an existing route and not elastic at all on a route that does not exist.
DWDM is one of the specifications these comparisons turn on.
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