CWDM vs DWDM vs SWDM4: Choosing a Wavelength Plan

Once a link is long enough or busy enough that a single wavelength is no longer enough, the next question is how to pack several signals onto the same fiber. Coarse WDM, dense WDM and short-wavelength WDM all multiplex wavelengths, but they trade capacity, reach and cost in very different ways. This article compares the three plans by channel count, spacing, fiber type, reach and amplification.

What Each Letter Actually Means

WDM stands for wavelength-division multiplexing: several optical signals share one fiber because each rides its own colour of light. The descriptors tell you how tight the spacing is. Coarse WDM (CWDM) spaces channels widely, dense WDM (DWDM) packs them tightly, and short-wavelength WDM (SWDM) uses a handful of wavelengths near the 850nm band over multimode fiber.

Spacing drives everything else. Wider spacing tolerates cheaper, uncooled lasers that drift a few nanometres with temperature without bleeding into the next channel. Tighter spacing demands temperature-stabilised lasers and, in return, unlocks far higher channel counts and optical amplification. That single design choice cascades into cost, power and maximum distance.

Channel Count, Spacing and Fiber Type

CWDM is standardised by ITU-T G.694.2 on a 20nm grid from 1271nm to 1611nm, giving up to 18 channels, though many deployments use only eight to avoid the water-peak loss region of older fiber. DWDM follows ITU-T G.694.1, a frequency grid anchored at 193.1 THz with spacings of 100GHz (about 0.8nm) or 50GHz (about 0.4nm), yielding 40 to 96 or more channels in the C-band. SWDM4 is a de facto agreement built around four wavelengths near 850nm, designed to run over the multimode fiber already installed.

The plans also differ in the rate each channel can carry, and that often decides the case before distance does. A CWDM channel today tops out around 10G to 25G, because the direct-detect optics that suit wide spacing do not scale to higher PAM4 rates without complex equalisation. A DWDM channel can carry 100G, 400G or more through coherent modulation, which is why DWDM is the backbone of high-capacity transport. SWDM4 is purpose-built for 40G and 100G links and is not intended to scale beyond the short-reach multimode case.

Reach and Amplification

The reach difference comes from amplification rather than from wavelength count. Erbium-doped fiber amplifiers work in the C-band around 1530 to 1565nm, which is exactly where DWDM channels sit, so a DWDM system can be boosted every 60 to 100km and span hundreds or thousands of kilometres. CWDM channels are spread far outside the amplifier window, so a CWDM link is effectively unamplified and is limited to roughly 40 to 80km. SWDM4 is a short-reach plan by design, reaching about 100m on OM4 and 150m on OM5 multimode fiber.

Cost and Deployment Tradeoffs

CWDM wins on simplicity and cost per channel: uncooled optics, low power and passive multiplexers make it a natural fit for campus and metro access where a handful of 10G or 25G channels is enough. DWDM costs more per channel and per watt but scales to 400G and beyond per wavelength and reaches carrier distances, which is why it underpins long-haul transport and large data center interconnect.

SWDM4 is the odd one out in a useful way. It reuses installed multimode fiber instead of demanding new single-mode cabling, which makes it attractive for lifting an existing 40G or 100G multimode plant to 100G without a re-cable. The classic hybrid is to run CWDM across a campus and nest a DWDM system inside one of its slots later, since DWDM channels fit within the width of a single coarse channel.

How Apex Helps

Send us the route lengths, the per-channel rate you expect to grow into, and whether the fiber is single-mode or multimode, and we will propose a wavelength plan with the matching optics and passive components. Send your requirement list and we will quote within hours.