---
title: "100GHz vs 50GHz DWDM Channel Plans: The C-Band Math and Why Your Old Gear Limits You More Than You Think"
slug: "dwdm-channel-plan-100ghz-vs-50ghz-c-band"
type: deep-dive
category: "DWDM & Coherent"
tags: ["DWDM", "channel plan", "C-band", "100GHz", "50GHz", "flex-grid", "EDFA", "optical amplifier"]
seo_focus_keyword: "DWDM channel plan 100GHz 50GHz C-band"
---

The C-band (conventional amplification band) in optical communications spans roughly 1530 nm to 1565 nm—the wavelength range over which Erbium-Doped Fiber Amplifiers (EDFAs) provide practical gain. The ITU-T has divided this spectrum into channels with two dominant spacings that remain relevant in deployed networks today: 100 GHz spacing (ITU-T G.694.1 fixed grid) and 50 GHz spacing (also G.694.1, but the denser variant). Understanding which works in your network and which doesn't requires clarity on the math, the equipment constraints, and where the real bottleneck usually lives.

## The C-Band Arithmetic

The ITU-T 100 GHz channel grid defines center frequencies at 193.1 THz + n×100 GHz, where n is an integer (positive, negative, or zero). In wavelength terms, the reference is 1552.52 nm (193.1 THz), and channels are separated by approximately 0.8 nm. The full C-band at 100 GHz spacing provides approximately 40 usable channels from C17 (196.1 THz, 1528.77 nm) to C61 (192.1 THz, 1560.61 nm), depending on EDFA bandwidth and system design margin.

At 50 GHz spacing, you double the channel count to approximately 80 channels in the same spectral region. Each channel occupies half the spectral width, which has direct implications for the modulation format—a 50 GHz channel occupies a spectral slot that's much tighter, requiring narrower optical filter passbands and modulation formats with lower spectral occupancy. For 10G and 25G per channel, this is manageable. For 100G per channel over 50 GHz spacing with legacy modulation formats (NRZ OOK), the spectral efficiency requirements are extremely tight and filter narrowing starts to impair signal integrity.

The flex-grid standard (ITU-T G.694.1 amendment) moves away from fixed channel positions entirely, defining spectrum as a series of 12.5 GHz slots that can be allocated in any combination. Flex-grid is the native habitat of modern coherent DWDM—a 100G DP-QPSK signal needs roughly a 37.5 GHz slot (3 × 12.5 GHz), while a 400G DP-16QAM signal needs approximately 75 GHz. Flex-grid lets you mix and match channel widths, which maximizes spectral efficiency across a mixed-rate DWDM system.

## Why 100 GHz DWDM Equipment Limits Expansion

The ROADM (Reconfigurable Optical Add-Drop Multiplexer) generation determines what's possible in your optical network. ROADMs from the 2005–2012 era were designed around fixed 100 GHz channel plans using thin-film filter (TFF) technology. TFF filters have a fixed passband of approximately 80 GHz FWHM (Full Width at Half Maximum) at the specified center frequency. They literally cannot pass a 50 GHz-spaced channel—the adjacent channel falls within the filter's stopband.

WSS (Wavelength Selective Switch) based ROADMs from the 2012–2018 period use liquid crystal on silicon (LCoS) technology with programmable filter shapes, but the first-generation WSS designs (Finisar WSS-1×9, JDSU/II-VI equivalents) typically have a minimum achievable passband of around 37.5 GHz and were characterized for 50 GHz channel spacing. These can support 50 GHz channels but not true flex-grid fractional slots.

Second-generation WSS ROADMs with flex-grid capability (available from Lumentum, Finisar, and II-VI from around 2015 onward) support 12.5 GHz granularity. These are what coherent 400G systems require, and if your ROADM nodes predate 2015, the answer to "can we deploy 400G ZR on our DWDM network" is probably "not without node upgrades."

The EDFA gain flatness profile is the second constraint. C-band EDFAs have a gain spectrum that is inherently not flat—the gain is higher around 1530–1535 nm and lower around 1555–1560 nm. Gain flattening filters (GFFs) embedded in EDFA amplifier units compensate for this, but GFFs are designed for a specific channel loading scenario. An EDFA designed for 40-channel × 100 GHz loading with a specific tilt compensation in the GFF will have a different residual gain tilt when loaded with 80-channel × 50 GHz operation. This isn't a catastrophic failure, but it means the optical power levels per channel shift, and your system's OSNR (Optical Signal-to-Noise Ratio) margin calculations change.

## Wideband Amplifiers and the L-Band Option

Standard C-band EDFAs cover 1530–1565 nm. Wideband C+L amplifiers extend coverage to include the L-band (1565–1625 nm), effectively doubling the available spectrum. This is the capacity expansion path for systems that have fully loaded the C-band and can't reduce channel spacing further.

The practical implication of adding L-band is cost and complexity: C+L amplification requires separate amplifier paths for C and L bands (combined into a single module in modern designs but still requiring separate pump lasers and gain media stages), and the ROADM nodes require WSS elements characterized for the full C+L spectral range. Not all existing ROADM node designs have an L-band upgrade path.

For networks that are capacity-constrained on existing C-band infrastructure, the evaluation path is: first, can channel spacing be reduced from 100 GHz to 50 GHz? (Requires WSS-capable ROADMs with sub-50 GHz filter granularity and coherent transceivers with adequate spectral efficiency.) Second, can flex-grid allocation improve spectral efficiency by right-sizing channels? (Requires second-generation WSS ROADMs.) Third, if C-band is fully exploited, is C+L upgrade viable? (Requires assessment of every ROADM node in the path.) In most cases, the bottleneck in the first two assessments turns out to be equipment generation, not fiber capacity.

## The Coherent Modulation Connection

The 100 GHz vs. 50 GHz question has a direct dependency on which modulation format your transponders use. Legacy 10G DWDM systems used OOK (On-Off Keying) with optical duobinary or NRZ modulation, occupying 10–20 GHz of spectrum per channel—easily accommodated in 100 GHz spacing with margin to spare. 100G DP-QPSK occupies roughly 37.5 GHz in the OIF-100G-SR specification, fitting into a 50 GHz channel with 12.5 GHz guard band. 100G DP-16QAM (used in high-capacity short-haul systems) occupies approximately 25 GHz, fitting into a 50 GHz channel with more margin.

The 400G case is where spacing starts to bite: 400G DP-16QAM at 64 GBaud occupies approximately 75 GHz of spectrum. At 100 GHz channel spacing, a 400G channel fits with 25 GHz guard band. At 50 GHz spacing, a 400G channel won't fit. This is why networks designed for 50 GHz channel spacing have limited 400G capacity if they can't migrate to flex-grid operation.

## What You Should Actually Plan For

The architecture guidance that applies most broadly: any new DWDM infrastructure investment should be flex-grid capable from the outset. The incremental cost of flex-grid WSS hardware over fixed-grid hardware is modest—typically under 10% of the WSS node cost—and it's the difference between a system that can accommodate 400G and beyond versus one that's locked into 100G channel rates.

For existing 100 GHz infrastructure, the practical capacity expansion options before a full ROADM replacement are: migration to coherent 100G on existing channels (replacing legacy 10G OOK transponders with coherent 100G, which doesn't increase channel count but multiplies per-channel capacity by 10), and evaluation of whether WSS-capable ROADMs in the network can support 50 GHz re-spacing. If even one legacy TFF-based ROADM node exists in the path, 50 GHz migration requires that node to be upgraded first.

The 10-year-old DWDM gear constraint is real and specific: TFF-based amplified spontaneous emission (ASE) levels, fixed filter passbands, and non-flex-grid WSS elements are not software upgradeable. The bottleneck is the optical hardware, and identifying exactly which nodes in a multi-span DWDM network are the limiting element is the prerequisite for any capacity planning discussion.
