---
title: "Coherent DSP Power Consumption Reality: What 400G ZR Does to Your Switch"
slug: "coherent-dsp-power-consumption"
type: analysis
category: "Coherent Optics"
tags: [coherent, DSP, power-consumption, 400G-ZR, CFP2-DCO, QSFP-DD, thermal, ZR-plus]
seo_focus_keyword: "coherent DSP power consumption 400G ZR"
---

The first time someone inserts a 400G ZR QSFP-DD module into a router and checks the chassis power draw, the reaction is typically surprise. The module draws 15–20W. The 100G SR4 QSFP28 it's replacing drew 3.5W. That's a 4–5x increase in power per port, and in a 32-port switch, the implications for cooling, power infrastructure, and total cost of ownership are significant enough to require explicit planning.

## Why Coherent DSP Consumes So Much Power

A 400G ZR pluggable contains a complete coherent transceiver in a QSFP-DD housing. The coherent DSP (Digital Signal Processor) handles: advanced modulation (16QAM or higher), soft-decision FEC with overhead of roughly 15–27%, chromatic dispersion compensation across hundreds of km of accumulated fiber dispersion, polarization mode dispersion tracking, and nonlinear impairment compensation. This is genuinely massive computational work happening in real time.

The DSP chip in a 400G ZR module — implementations from companies like Acacia (Cisco), Lumentum, and Coherent (formerly II-VI) — runs at roughly 7 nm or 5 nm CMOS process nodes to keep power manageable. Even so, the DSP alone typically consumes 8–12W. The coherent optical engine — tunable laser, modulator, coherent receiver — adds another 6–8W. Total: 14–20W depending on implementation quality and operating margins.

Compare to 400G SR4 (short-reach, intensity modulation): four VCSELs at roughly 200 mW each = 0.8W for transmitters, four photodiodes and TIAs at roughly 0.5W each = 2W for receivers, plus CDR/equalization DSP at roughly 1W. Total: 3.5–4W. The coherent module is burning 4–5x more power to achieve something fundamentally different — not just 400 Gbps in a box, but 400 Gbps across 1000+ km of unrepeatered fiber.

The ZR+ variants (OpenZR+, vendor-specific ZR+ implementations targeting up to 2,000 km reach or 800G capacity) push power consumption higher still — 18–22W is typical for current-generation 400G ZR+ implementations. Higher modulation order and additional reach margin require more DSP computation.

## Switch Port Density Implications

A Cisco Nexus 9364C-GX has 64 QSFP-DD ports. Configured for 400G SR4, the optics contribution to switch thermal load is approximately 64 × 3.5W = 224W. Configured with a mix of 400G ZR pluggables, the optics thermal load jumps to 64 × 17W = 1,088W — nearly a kilowatt of additional heat in the same 2RU chassis, beyond what the switch ASIC itself generates.

Most datacenter switches are designed with a thermal budget that assumes pluggable optics at 3–5W per port. The switch chassis airflow, heat sink design, and power supply rating are based on that assumption. Fully populating a switch with high-power coherent pluggables can exceed the designed thermal envelope for the chassis.

Vendors have responded in different ways. Cisco Nexus 9300-GX2 explicitly supports high-power QSFP-DD up to 20W per port in all 64 slots, with enhanced fan speed control and a revised thermal design. Arista 7800R3 series supports per-port power class negotiation via CMIS 4.0, which allows the switch to negotiate with the module about its power consumption before allowing high-power operation. A module requesting Class 8 (20W) in a switch that can only allocate Class 6 (12W) to that slot will either operate in a reduced-performance mode or fail to come up.

The practical recommendation: if you're planning to deploy coherent ZR pluggables in an existing switch fleet, check the per-port and per-system power budget explicitly against the chassis specification. Don't assume that because QSFP-DD is physically compatible, the thermal design supports high-power coherent modules.

## CFP2-DCO vs. QSFP-DD-ZR: The Right Tool

Before QSFP-DD 400G ZR became viable, the standard coherent pluggable form factor was CFP2-DCO. CFP2 is larger (approximately 86 × 39 × 9.5 mm vs. QSFP-DD's 18 × 8.5 × 12 mm) and supports higher power — up to 28W or more. CFP2-DCO modules achieve longer reach and higher baud rates precisely because they have more thermal headroom.

For ultra-long-haul applications — trans-continental or submarine links, metro rings with total span loss above 25 dB — CFP2-DCO remains the appropriate form factor. The Nokia PSI-M and Ciena WaveLogic 5e both offer CFP2-DCO options for these use cases. The QSFP-DD physical constraints limit the DSP design space, and for the most demanding optical paths, that matters.

For DCI at metro distances (50–500 km), QSFP-DD 400G ZR is now the standard choice. The reach is sufficient, the form factor allows standard router integration without separate transponder hardware, and the economics are compelling. A Flexoptix or Acacia 400G ZR QSFP-DD module is $3,000–5,000 versus $15,000–25,000 for a proprietary coherent transponder card achieving similar performance. On a network with 40 wavelengths, that's $480,000–$800,000 in hardware savings.

The open ZR standard (IEEE 802.3ct for 400ZR, OpenZR+ MSA for enhanced reach) has created genuine multi-vendor interoperability for the first time in coherent optical transport. Two 400ZR modules from different vendors should interoperate without vendor-specific configuration — this has been demonstrated at plugfests and is deployed in production. The ZR+ extended profiles are less well standardized and may require vendor-matching for the extended-reach variants.

## The Case for Line-Side Amplification

One approach to reducing coherent pluggable count and therefore managing power consumption at the system level: use DWDM line-side amplification to serve more router ports from fewer coherent optics.

In a 400G ZR design without amplification, each router port needs its own coherent pluggable, and the metro link capacity is limited to a single wavelength per fiber pair. With an OLS (Open Line System) providing DWDM multiplexing and amplification, 40–96 wavelengths share the same fiber pair, and the aggregation routers at each end use far fewer coherent pluggables.

The tradeoff: DWDM OLS equipment (ROADMs, amplifiers, multiplexers) costs more than dark fiber plus ZR pluggables at low channel counts. The crossover point where DWDM becomes economically favorable is typically around 8–10 wavelengths on the same fiber pair. Below that, P2P ZR pluggables on individual fiber pairs are cheaper. Above that, DWDM equipment pays for itself in fiber cost savings.

This amplification-versus-pluggable-count calculation also directly affects total power consumption. A 40-wavelength DWDM system using two EDFA amplifiers per site (8W each) consumes 16W for amplification per site, shared across all 40 wavelengths. That's 0.4W per wavelength for the amplification function — compared to 17W for a dedicated ZR pluggable per wavelength in a P2P dark fiber design. The DWDM approach consumes less total power once you're above 8–10 channels.

Coherent optics power consumption is not a reason to avoid them — the value they deliver in spectral efficiency and reach makes them indispensable for DCI. But the power numbers are real and need to be incorporated into facility planning, not discovered after installation.
