CWDM vs DWDM: Wavelength Division Multiplexing Explained
CWDM vs DWDM comes down to channel spacing: CWDM uses 20 nm spacing for up to 18 wavelengths between 1270 nm and 1610 nm, while DWDM packs 40 or more channels at 100 GHz or 50 GHz spacing into the C-band around 1550 nm. CWDM is cheaper and simpler for short metro and access links; DWDM carries far more capacity over longer distances and can be amplified.
Both are forms of wavelength division multiplexing (WDM), which sends several signals over one fibre by giving each its own colour of light. If you lease dark fibre, run mobile backhaul or connect data centers, WDM is often cheaper than laying or renting more fibre. This guide explains the grids, reach, amplification, passive equipment and the optics you need for each.
How Wavelength Division Multiplexing Works
Each transceiver in a WDM system transmits on a fixed, precise wavelength. A passive multiplexer (mux) combines those wavelengths onto one fibre, and a demultiplexer (demux) at the far end separates them again and sends each to the matching receiver. Every channel is an independent link, so one fibre pair can carry 10G, 25G and 100G services side by side.
The coloured optics are standard pluggables, such as a CWDM SFP or a DWDM SFP+, plugged into normal switch or router ports. The mux and demux are unpowered filter units, usually in a 1RU chassis or a cassette. There is no protocol conversion; the WDM layer only handles light.
CWDM: Coarse Wavelength Division Multiplexing
CWDM is defined by ITU-T G.694.2. It uses 18 channels spaced 20 nm apart, from 1270 nm to 1610 nm (the exact centre wavelengths are 1271 nm to 1611 nm, but most vendors label them by the rounded figure). The wide spacing allows uncooled lasers and simple thin-film filters, which keeps both optics and mux units inexpensive and low power.
CWDM limits
- No practical amplification. The 18 channels spread across 340 nm, far wider than any common optical amplifier covers. CWDM reach is set by the optics’ power budget and the loss of the fibre plus mux and demux.
- Water peak. Older single-mode fibre has high attenuation around 1383 nm. On such fibre the 1370 nm to 1430 nm channels may be unusable, so many systems use only the 8 channels from 1470 nm to 1610 nm. Low-water-peak fibre (ITU-T G.652.D) supports all 18.
- Capacity. 18 channels is the ceiling, and often fewer in practice.
Typical CWDM SFP and SFP+ optics are offered in power-budget classes, commonly sold for 20 km, 40 km and up to 80 km at 1G, with shorter reach typical at 10G. The real figure depends on fibre loss, splices, connectors and mux insertion loss, so always work out the link budget.
DWDM: Dense Wavelength Division Multiplexing
DWDM is defined by ITU-T G.694.1. Channels sit on a frequency grid anchored at 193.1 THz, mainly in the C-band from about 1530 nm to 1565 nm. At 100 GHz spacing (about 0.8 nm) a system typically carries 40 channels; at 50 GHz spacing (about 0.4 nm) it carries 80 or more. Newer flexible-grid systems allocate spectrum in 12.5 GHz slices to fit high-speed coherent signals.
Because the channels are so close, DWDM lasers must be temperature-controlled to hold their wavelength. That makes DWDM optics more expensive and slightly more power-hungry than CWDM, but it also lets the whole system sit inside the band where optical amplifiers work.
Amplification with EDFAs
An Erbium-Doped Fibre Amplifier (EDFA) boosts every C-band channel at once, without converting the signal to electrical. Boosters, in-line amplifiers and pre-amplifiers let a DWDM system span far longer distances than an unamplified link. At 10G and above, chromatic dispersion also becomes a limit on direct-detect optics, so longer links use dispersion compensation or coherent optics, which correct dispersion digitally.
CWDM vs DWDM Comparison
| Feature | CWDM | DWDM |
|---|---|---|
| ITU-T standard | G.694.2 | G.694.1 |
| Channel spacing | 20 nm | 100 GHz (~0.8 nm) or 50 GHz (~0.4 nm) |
| Wavelength range | 1270 to 1610 nm | Mainly C-band, ~1530 to 1565 nm |
| Channel count | Up to 18 | 40 at 100 GHz, 80+ at 50 GHz |
| Lasers | Uncooled | Cooled, wavelength-stable |
| Amplification | Not practical | EDFA |
| Typical reach | Tens of km, budget-limited | Tens of km unamplified, hundreds with amplifiers |
| Cost per channel | Low | Higher |
| Best fit | Access, enterprise campus, backhaul | Metro core, DCI, high-capacity leased fibre |
Passive Mux and Demux Equipment
Most enterprise and operator WDM links are fully passive: coloured optics in the switches and a mux/demux at each end. Common formats include 4, 8, 16 and 18-channel CWDM units and 8, 16, 40 or 48-channel DWDM units.
- Dual-fibre vs single-fibre. Dual-fibre units use one fibre per direction. Single-fibre designs use different wavelengths in each direction, halving fibre use at the cost of fewer services.
- OADM. An optical add/drop multiplexer drops one or two wavelengths at an intermediate site and passes the rest through, useful on ring or chain topologies.
- Expansion and 1310 nm ports. Many units have an upgrade port to add a DWDM layer later, or a 1310 nm port that carries an existing grey optic alongside the coloured channels.
- Insertion loss. Each mux and demux adds a few dB. Include both ends, plus patch panels and splices, in the budget.
Mux units usually mount in a fibre panel or enclosure, connected with short LC duplex patch cords. Label every channel: a swapped patch cord on a WDM system is a common cause of a dead link.
CWDM and DWDM Transceivers
SFP, SFP+ and SFP28
Coloured optics exist at 1G (CWDM and DWDM SFP), 10G (CWDM and DWDM SFP+) and 25G (DWDM and CWDM SFP28, mainly for mobile fronthaul and backhaul). Each part number is tied to one channel, for example a 1550 nm CWDM SFP+ or a channel 34 DWDM SFP+ on the 100 GHz grid. Tunable DWDM SFP+ modules can be set to any C-band channel, which reduces the number of spare part types you hold. See our 10G SFP+ transceivers for CWDM and DWDM options.
QSFP28 at 100G
Be careful with naming. A 100G CWDM4 QSFP28 uses four CWDM wavelengths (1271, 1291, 1311 and 1331 nm) inside the module and runs 2 km over duplex single-mode fibre. It is a grey optic: you cannot put it through an external CWDM mux. For 100G over a DWDM system, there are 100G PAM4 DWDM QSFP28 modules for data center interconnect, typically used up to around 80 km with external amplification and dispersion compensation. Browse 100G QSFP28 transceivers for both types.
Coherent 400ZR
400ZR is an OIF implementation agreement for 400G coherent optics in pluggable form, typically QSFP-DD or OSFP. It is designed for amplified point-to-point DWDM links of around 120 km between data centers, plugged straight into a router or switch. ZR+ variants extend reach and add more modulation options. Coherent pluggables have removed the need for separate transponder shelves in many DCI designs. See our 400G transceivers.
Use Cases: Where Each Fits
- Enterprise campus and leased fibre. A company leasing one fibre pair between two buildings can run 8 CWDM channels for LAN, storage and CCTV traffic instead of renting more fibre.
- Mobile backhaul and fronthaul. Operators use CWDM and DWDM SFP+ and SFP28 to carry several radio sites or sectors over shared fibre.
- Metro networks. DWDM rings with OADMs serve many sites with high capacity and room to grow.
- Data center interconnect. DWDM with 100G PAM4 or 400ZR coherent optics links data halls across a city, a common design for cloud and colocation builds in Dubai and other GCC cities.
Operators planning WDM rollouts can see our telecom operator solutions.
How to Choose Between CWDM and DWDM
Start with distance, channel count and growth. CWDM is usually right for up to 8 or 16 channels over unamplified spans, especially at 1G and 10G. DWDM is right when you need more channels, longer distance, amplification or 100G and above per wavelength.
- Fibre type, age and whether it is low-water-peak G.652.D.
- Measured span loss (OTDR or light-source and power-meter test), not just distance on a map.
- Number of services now and in three to five years.
- Data rate per channel: 1G, 10G, 25G, 100G or 400G.
- Topology: point-to-point, chain or ring with add/drop sites.
- Host platform and coding for every coloured optic.
- Spares strategy: fixed-channel optics per site, or tunable DWDM.
WDM systems are unforgiving of dirty connectors and wrong patching, so plan testing with proper fibre testers. For the fibre basics, read our guide on single-mode vs multimode fibre, and for coding questions see compatible vs OEM transceivers.
Frequently Asked Questions
What is the main difference between CWDM and DWDM?
CWDM spaces channels 20 nm apart for up to 18 wavelengths from 1270 to 1610 nm. DWDM spaces channels at 100 GHz or 50 GHz in the C-band, giving 40 or more channels that can be amplified for long distances.
Can CWDM signals be amplified?
Not in practice. CWDM channels spread across about 340 nm, far wider than common optical amplifiers cover, so CWDM reach depends on the optics’ power budget and the loss of the fibre and mux units.
Is 100G CWDM4 the same as a CWDM system?
No. A 100G CWDM4 QSFP28 uses four CWDM wavelengths inside the module and outputs on a duplex single-mode link up to 2 km. It cannot be connected to an external CWDM mux as a single coloured channel.
How many channels does a DWDM system support?
A typical 100 GHz DWDM system carries 40 channels in the C-band, and a 50 GHz system carries 80 or more. Passive mux units are commonly sold in 8, 16, 40 and 48-channel versions.
What is 400ZR used for?
400ZR is a coherent 400G pluggable optic, usually QSFP-DD or OSFP, designed for amplified DWDM links of around 120 km. It is mainly used for data center interconnect without separate transponder equipment.
Talk to TIFNEX
Planning a CWDM or DWDM link? Send us your switch or router models, fibre type, span lengths or loss measurements and the number of services, and our engineers will propose matching coloured optics, mux units and patch cords. We deliver across the UAE, Egypt and the GCC. Request a quote and we will reply within 24 hours.
