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Introduction
Everyone knows wavelength-division multiplexing (WDM) lets you push more traffic over the same strand of glass. What trips people up is deciding which flavor belongs in a given design. In real deployments, that decision pivots on four levers: distance, capacity, budget, and growth path.
This guide demystifies Coarse WDM (CWDM) and Dense WDM (DWDM) using standards-anchored numbers, practical tables, and a simple selection workflow. You’ll see how many channels you can realistically get, why DWDM reaches farther, when CWDM is the better buy, and how hybrid overlays (DWDM over CWDM) extend life without ripping out your fiber plant.
Fast definitions (anchored to the standards)
- CWDM uses a fixed wavelength grid defined by ITU-T G.694.2, with 20 nm channel spacing over the 1271–1611 nm range, yielding 18 nominal channels. This wide spacing enables cheaper uncooled lasers and passive filters, which is why CWDM is popular in access/metro builds that need capacity increases at low cost.
- DWDM uses a frequency grid defined by ITU-T G.694.1, anchored at 193.1 THz. Fixed grids commonly use 100 GHz (~0.8 nm) or 50 GHz (~0.4 nm) spacing; the latest edition also defines a flex-grid (variable slot widths) for modern coherent systems. DWDM’s tighter spacing, combined with amplification, unlocks far more channels and long-haul reach.
CWDM vs DWDM Overview
| Dimension | CWDM | DWDM |
| Standards & grid | ITU-T G.694.2; 20 nm spacing; 1271–1611 nm | ITU-T G.694.1; 100/50 GHz fixed grids; flex-grid option |
| Typical channel count (per fiber) | Up to 18 | ~80 in C-band at 50 GHz is very common; ~96 also seen; C+L band designs push >160 |
| Lasers & optics | Uncooled (lower cost, wider tolerances) | Cooled lasers & tighter filters (higher cost/power) |
| Amplification | Limited across whole band; (C-band slice can be boosted) | Native with EDFA in C-band; Raman options for extra reach |
| Typical reach & role | Short/medium (metro/access, campus, enterprise DCI light) | Metro core & long-haul (amplified spans, regional/backbone) |
| Cost profile | Lower CapEx/Watts up front | Higher CapEx, but best $ per bit at high scale/long reach |
Notes on the numbers: ITU-T G.694.2 lists the 18 CWDM center wavelengths (1271…1611 nm). For DWDM, ITU-T G.694.1 sets the 193.1 THz-anchored grids; vendors commonly ship 80–96 channels in C-band at 50 GHz spacing, with additional channels available when using L-band or denser spacing.
Channels & spacing - what you actually get
CWDM channel plan (the quick list)
CWDM gives you 18 nominal channels at 20 nm spacing: 1271, 1291, 1311, …, 1611 nm. Historically, using the mid-band (around 1383 nm) depended on low-water-peak fiber (e.g., G.652.D) to avoid OH⁻ absorption; most modern plants qualify, but older cables may restrict you from using some of those wavelengths. Verify your fiber spec before you plan all 18.
DWDM channel plan (why the grid is in frequency)
DWDM grids are defined in frequency (THz) and then expressed as wavelengths. The 193.1 THz anchor translates to the familiar C-band ITU tables. With 100 GHz spacing you’ll often see ~40/80 channels; with 50 GHz spacing ~80/96 channels in C-band is standard; C+L designs can push >160 channels depending on filters and amplification.
Distance & why DWDM goes farther
DWDM isn’t “long-haul” simply because channels are closer together. It goes farther because the ecosystem supports optical amplification and tighter optical control:
- EDFA (Erbium-Doped Fiber Amplifier) boosts signals in the C-band (and L-band), simultaneously amplifying many DWDM channels with good noise figures—perfect for WDM backbones.
- Raman amplification pushes gain inside the transmission fiber itself and is widely deployed in long-haul/ultra-long-haul systems to extend span lengths and improve OSNR.
Tie that amplification to better-stabilized (cooled) lasers and, in modern designs, coherent transponders, and you get reliable multi-hundred-kilometer spans and continent-scale networks.
Costs, power, and operating model
- CWDM keeps CapEx low: uncooled lasers and passive components are simpler and cheaper, ideal for short/medium spans where you can’t justify amplification. The trade-off is fewer channels and limited reach.
- DWDM costs more upfront (cooled optics, mux/demux, EDFAs/Raman, tighter filters), but it delivers far higher capacity over long distances and excellent scale economics as you fill channels—especially across regional and long-haul routes.
Hybrid overlays: DWDM within a CWDM plant
A practical upgrade path is to add DWDM channels inside a CWDM window (typically in the C-band portion of the CWDM spectrum) using suitable filters or “upgrade ports.”
That lets you keep existing CWDM equipment and fiber while adding high-density DWDM payloads where you’re capacity-constrained. It’s a proven way to extend metro/access builds without wholesale replacement.
A simple decision framework
Step 1 — Distance
- ≤ 40–80 km, unamplified and modest capacity? CWDM usually wins on simplicity and cost.
- > 80 km or you need optical amplification anyway? Move to DWDM. (If you’re already deploying EDFAs, you’re in DWDM territory.)
Step 2 — Capacity (channels and growth)
- Need ≤ 8–16 waves and unlikely to grow past ~18? CWDM can be perfect.
- Need ≥ 40–96 waves in C-band (with room to grow to C+L)? That’s DWDM.
Step 3 — Budget & operating model
- If you can’t justify active amplification, stick with CWDM.
- If you can amortize DWDM optics and amplifiers across many services and years, DWDM’s $ per bit becomes compelling.
Two mini-scenarios
- Access/metro add: 8 business services across a few rings, <50 km → CWDM.
- Regional backbone: 80-wave C-band, amplified spans across 300+ km → DWDM.
Common pitfalls
| Pitfall | Why it happens | Fix |
| Planning all 18 CWDM channels on old fiber | Water-peak attenuation near 1383 nm on legacy fiber limits usable channels | Verify fiber spec; prefer G.652.D low-water-peak; channel-plan accordingly |
| Expecting an EDFA to boost the entire CWDM band | EDFAs work in C-band (and L-band)—not across the whole CWDM grid | If you need amplification in C-band only, consider a hybrid plan or go DWDM |
| Underestimating temperature drift with uncooled lasers | CWDM lasers have wider tolerances | Add margin and design for worst-case temperature/load |
| Channel math that ignores OSNR | Long spans need EDFA/Raman planning and noise budgets | Do a proper link budget; add pre-/line/booster amps as required |
| Assuming “DWDM over CWDM” is plug-and-play everywhere | Not all CWDM nodes support upgrade filters/windows | Use compatible add/drop/upgrade components; lab-test the overlay first |
Realistic channel counts
- 80 channels in C-band @ 50 GHz is a classic DWDM configuration (numerous Cisco ONS releases/stateful docs show 80-channel C-band implementations), and ~96 channels C-band is also common using 50 GHz grids and matching mux/demux/OCMs.
- C+L band systems can exceed 160 channels, depending on spacing, filters, and the platform. Even older Cisco documentation mentions growth paths beyond 160 channels; modern vendor collateral cites similar ranges.
FAQs
Q1: How many channels does CWDM provide?
A: Up to 18 channels at 20 nm spacing across 1271–1611 nm (per ITU-T G.694.2). If your plant isn’t low-water-peak, you may not be able to use all mid-band wavelengths.
Q2: How many channels does DWDM provide?
A: In the C-band, ~80–96 channels at 50 GHz spacing are common. With C+L bands and appropriate spacing/filters, designs can exceed 160 channels. (Exact counts depend on platform and spacing.)
Q3: Why does DWDM reach farther?
A: Because you can use EDFA (and Raman) amplification and better-stabilized lasers, preserving OSNR and enabling multi-hundred-kilometer spans.
Q4: Can I mix DWDM with CWDM on the same fiber?
A: Yes, often by inserting DWDM within the CWDM C-band window using upgrade filters. It’s a common way to scale capacity without a full replacement. Validate components and budgets first.
Q5: Do CWDM and DWDM require different fiber types?
A: Both run over standard single-mode fiber. To use all CWDM wavelengths including the mid-band, you typically want low-water-peak fiber (e.g., ITU-T G.652.D).
Q6: Why is DWDM gear more expensive?
A: Cooled lasers, tighter optical filters, and amplification hardware drive cost—but they deliver higher capacity and longer reach, often with better $/bit when fully utilized.
A quick chooser
- Pick CWDM when…Distances are short/medium (unamplified, typically ≤40–80 km). You need ≤18 total wavelengths. Budget and simplicity outrank long-haul scale.
- Pick DWDM when…You need lots of channels (≥40–96 today, C+L tomorrow). You’re designing amplified spans or long haul. You want future capacity headroom without pulling new fiber.
- Pick a hybrid (DWDM over CWDM) when…You already have CWDM and need more capacity fast. The C-band at your CWDM nodes supports upgrade filters. You’ve validated mux/demux insertion loss and OSNR budgets.
Example channel plans and parts
- 96-channel C-band @ 50 GHz: widely available mux/demux and monitoring tools (e.g., 96-ch OCMs) that implement ITU-T G.694.1 tables.
- 80-channel C-band: a long-standing staple in optical platforms and deployment guides (e.g., Cisco ONS series).
- Tunable DWDM transceivers (50 GHz, 96 channels): off-the-shelf SFP+ tunables can cover the entire C-band grid for 10 GbE ZR-class links (80 km).
Key takeaways
- CWDM = 18 channels, 20 nm spacing, low cost for short/medium links. Know your fiber water-peak limits.
- DWDM = 80–96 channels in C-band at 50 GHz today; C+L expansion can exceed 160. Plan with EDFA/Raman for distance.
- If you’re already amplifying, you’re effectively in DWDM country—lean into it.
- Hybrid overlays (DWDM inside CWDM) are a smart way to scale an installed base.
Conclusion
Choosing between CWDM and DWDM is less about buzzwords and more about distance × capacity × budget × growth. If you need quick capacity at low cost with unamplified spans, CWDM is your friend.
If you need big channel counts and long reach or you want the most headroom per fiber. DWDM is the sensible investment. And if you need to bridge from one to the other, DWDM-over-CWDM overlays provide a clean migration path when planned with the right filters and budgets.
If you’d like a second set of eyes on your channel plan, or you need pre-validated CWDM/DWDM gear that plays well together, the team at Network-Switch.com can help.
We’re an authorized distributor for Cisco, Huawei, and Ruijie, and we build our own line of fiber cables and optical transceivers. Our CCIE, HCIE, and RCNP engineers can tailor a CWDM or DWDM design to your routes, recommend the right EDFAs and filters, and ship a labeled, plug-ready bill of materials.
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