CWDM, DWDM, O-Band — How They Are Alike, How They Differ, and How Not to Choose Wrong

What All WDM Variants Have in Common

Every WDM system works on the same principle: a single fibre carries multiple independent data streams, each on a different wavelength of light. The key passive component is a MUX and DEMUX pair. The multiplexer at the input gathers signals from different channels and feeds them into one fibre, and the demultiplexer at the output separates them back out.

Both devices are passive, have no moving parts, need no power, and generate no heat. A good MUX and DEMUX pair works for twenty or thirty years without intervention. You buy once. That is where the similarities end, because the differences that matter lie in channel density, range, power requirements, and cost.

CWDM — a Good Start for Enterprise and Metro Networks

CWDM operates in the band from 1271 to 1611 nm with 20 nm spacing between channels, giving eighteen nominal channels. The wide spacing means the lasers do not have to be precisely temperature-stabilised, and that translates directly into lower module cost and a simpler system design.

Range without amplifiers reaches from 40 to 80 km, depending on fibre quality and the number of passive elements in the path. CWDM is by definition an unamplified system, because EDFA amplifiers do not work efficiently outside the C-band, and CWDM extends far beyond that band. It is a solution for campus connections, inter-building networks, first metro deployments, and any organisation looking for a fast and inexpensive capacity increase over distances up to a few dozen kilometres.

There is one rule when buying a MUX and DEMUX pair for CWDM that many organisations regret ignoring after the fact: always buy with an expansion port. The price difference is a few hundred złoty, but the value at expansion time is enormous, because it lets you add DWDM modules into existing CWDM channels without replacing the whole device.

DWDM — When Scale and Range Matter

DWDM operates in the C-band from 1528 to 1565 nm with 100 GHz (0.8 nm) or 50 GHz (0.4 nm) spacing between channels. At the standard 100 GHz grid this gives forty channels, at 50 GHz eighty, and with the L-band added over one hundred and sixty.

The key difference from CWDM is that DWDM works with EDFA amplifiers, which regenerate the signal in the optical domain without converting it to an electrical signal. This opens the way to building links over hundreds and thousands of kilometres, on the same fibre. The higher cost of DWDM follows directly from the engineering, because the lasers have to hit a very narrow wavelength window, with a tolerance of around 0.1 nm versus 2 to 3 nm for CWDM, and require temperature stabilisation by a Peltier element. This is not the manufacturer’s margin, but physics and manufacturing precision. DWDM is a solution for operators with backbone networks, for DCI connections between data centres in different cities, and for any organisation that needs dozens of channels or distances exceeding the capabilities of CWDM.

O-Band — a New Option Worth Knowing

O-Band (Original Band, from 1260 to 1360 nm) is one of the first telecommunications bands. Over time, long-haul transmission was taken over by the C and L bands, because they have lower attenuation in silica fibre and EDFA amplifiers work in them. O-Band does have higher attenuation, but it is coming back today for a specific reason that was always its advantage.

That property is its insensitivity to chromatic dispersion. For G.652 cable, the type most commonly used in Poland and Europe, chromatic dispersion in the O-band is close to zero. This means you can transmit 100G over a distance of up to 30 km without dispersion compensators and without optical amplifiers.

This translates into concrete numbers. An O-band-based solution saves nearly 900 kWh of energy per year on a single 8×100 Gbit link, lowers the network build cost by around 50 percent, and reduces the total cost of ownership, including five years of maintenance, by around 80 percent. Salumanus was one of the first companies in the world to introduce complete O-band-based solutions for optical networks. They include passive multiplexers and optical modules in the QSFP28 interface form factor, installed directly in network devices. The modules operate with PAM4 modulation and have a broadband receiver. This is a solution for 5G operators and internet providers based today mainly on 10G connections, who want to build an access and aggregation network at minimal infrastructure cost and minimal energy consumption. It is also an excellent entry point into 100G technology for organisations that until now had no reason to invest in coherent DWDM systems.

CWDM and DWDM Hybrid — the Best of Both Worlds on Existing Infrastructure

If you have a working CWDM and you are facing a capacity wall, you do not have to replace the whole infrastructure. Each CWDM channel occupies a window with 20 nm spacing, and within that window several narrow DWDM channels fit. Physically, you add DWDM modules as a layer into existing CWDM channels, without replacing cables, without replacing the main MUX and DEMUX, without interrupting network operation.‍

This is a solution for organisations with a CWDM investment that want to multiply capacity without starting from scratch. The condition is one and it returns like a refrain: the CWDM MUX and DEMUX pair must have an expansion port.

What Really Distinguishes Coherent Modules, and Why 0 dBm Matters

When choosing modules for a DWDM or O-Band system, one parameter matters disproportionately in practice: output power. Most DWDM systems are calibrated for a signal power of 0 dBm at the multiplexer input, while most coherent modules on the market transmit a signal at minus 10 dBm. The effect is that at minus 10 dBm you have to add an EDFA amplifier between the module and the multiplexer, meaning an additional device, additional cost, and worse OSNR.

Manufacturers who try to work around this problem bolt a miniature EDFA amplifier onto the module. The result is that the module transmits 0 dBm, but has a significantly higher power draw and worse OSNR than a solution designed from the ground up for 0 dBm. GBC Photonics 400G OpenZR+ modules are designed with 0 dBm output power natively, without an additional EDFA. They plug directly into existing DWDM systems without modification. The transmitter OSNR is 43 dB, the power draw drops below 22 W, and the module is fully tunable across the C-band, with a channel change in 10 seconds.

The DSP Market Landscape — What Is Worth Knowing Before Buying

The coherent optical module market currently has four companies with their own DSP chips: Acacia (acquired by Cisco), Infinera, Ciena, and Marvell. They effectively set the technological direction. Module manufacturers, including GBC Photonics, buy DSPs from these suppliers and integrate them with their own optics. This is worth knowing, because the DSP determines the module’s capabilities: range, support for modulation modes, PCS capabilities, and integration with management systems. You buy a physical module, but you choose a DSP ecosystem. From the perspective of an operator in Poland and CEE, the conclusion is simple: when choosing a module supplier, always ask which DSP is inside and what its capabilities are in the context of your specific route requirements.

A Short Decision Guide

A few locations, distances up to 50 km, from a few to eighteen channels — that is CWDM. Simple, cheap, works, always with an expansion port in the MUX and DEMUX. A 5G or internet provider access and aggregation network, distances up to 30 km, 100G transmissions — that is O-Band, meaning minimal infrastructure cost, no amplifiers, and an 80 percent lower total cost of ownership. You have CWDM and need more capacity without replacing infrastructure — that is the CWDM and DWDM hybrid. Dozens or hundreds of channels, long distances, and a network backbone — that is DWDM, the only option in this class.

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