---
title: "Metro DWDM: The Case For and Against Going Open"
slug: "metro-dwdm-open-vs-proprietary"
type: analysis
category: "DWDM & Coherent"
tags: [metro DWDM, OpenROADM, coherent pluggables, 400G ZR, disaggregation, ROADM, transponder, open line system]
seo_focus_keyword: "metro DWDM open vs proprietary OpenROADM"
---

The traditional metro DWDM architecture looks like this: a proprietary ROADM platform from Ciena, Infinera, or Fujitsu handles the optical layer, transponder cards convert between client grey optics and tunable colored wavelengths, and the whole system operates under a single vendor's network management system. It works reliably. It's also expensive, slow to provision, and vendor-locked in ways that become more uncomfortable as network capacity demands accelerate.

The alternative — disaggregated metro DWDM with coherent pluggable transceivers — has moved from architecture concept to deployable reality over the past three years, driven primarily by the 400G ZR and 400G ZR+ standards. Understanding where the disaggregated model genuinely works and where vendor integration still wins requires being clear about the technical tradeoffs.

## What 400G ZR Actually Is

The 400G ZR specification (OIF Implementation Agreement IA-400ZR) defines a single-carrier 400G coherent interface using DP-16QAM modulation, targeting up to 120km on standard G.652 SMF at -21 dBm launch power with no optical amplification. The specification was developed by the Optical Internetworking Forum and published in 2020. Unlike previous coherent interfaces that required 19-inch rack transponder equipment, 400G ZR fits in a QSFP-DD form factor — the same module used for 400G grey optics.

The implication is significant: a switch with QSFP-DD ports can, in principle, terminate a 400G coherent wavelength directly without a separate transponder shelf. Arista introduced this capability in the 7280R3 and 7800R3 series. Cisco implemented it in Nexus 9000 with appropriate line cards. The router or switch becomes its own transponder.

400G ZR+ extends this concept with a family of enhanced coherent implementations from vendors including Ciena (WaveLogic 5 Nano), Infinera (ICE-X), and Acacia/Cisco's variants. ZR+ modules typically support adaptive modulation (stepping between DP-16QAM, DP-8QAM, and DP-QPSK) to trade capacity for reach. A 400G ZR+ module might operate at 400G for 120km spans or back down to 200G to traverse a 1000km path. The tradeoff is power consumption — ZR+ QSFP-DD modules run at 15-20W, compared to 3.5W for a grey 400G SR8 — but you're eliminating an entire transponder shelf.

## The OpenROADM Promise and Delivery

OpenROADM is an industry initiative, hosted under the Linux Foundation as part of the O-RAN ecosystem, that defines vendor-neutral YANG data models for ROADM configuration and management. The stated goal is to allow operators to mix ROADM hardware from different vendors and manage the whole through a common interface. AT&T has been the primary driver since the initiative started in 2016.

In practice, OpenROADM has delivered meaningful value in two specific areas: wavelength provisioning automation and multi-vendor NMS integration. The YANG models are detailed enough to enable programmatic control of amplifier gain settings, ROADM port attenuation, and wavelength routing matrices without vendor-specific CLI. Operators who have deployed OpenROADM-compliant ROADMs from vendors including Ciena and Fujitsu report meaningful improvements in provisioning time — from days to hours for wavelength turn-up.

What OpenROADM has not delivered is true vendor interoperability at the optical layer. The ROADM hardware itself remains vendor-specific. The colorless/directionless/contentionless (CDC) ROADM architecture from Ciena is not interchangeable with Infinera's spatial switching implementation at a physical level. You can manage them with the same north-bound API, but you cannot mix ROADM chassis from different vendors in the same optical span and expect the transponders to be agnostic about what they're sending through.

The amplifier chain is the critical constraint. Coherent DSPs (like those used in ZR+ modules) perform electronic dispersion compensation and can adapt to impairments in the fiber path, but they need accurate optical power management from the ROADM to function correctly. Different ROADM vendors implement power equalization algorithms differently, and a ZR+ module optimized for a Ciena ROADM chain may not behave identically on an Infinera platform without re-validation.

## Where Disaggregation Works

The cleanest case for coherent pluggable disaggregation is the point-to-point metro ring where span lengths are 80km or less, chromatic dispersion is manageable, and the application is capacity expansion on existing dark fiber. A telco or cable operator running a 5-node ring with 40 to 80km between nodes can deploy 400G ZR modules in IP routers and eliminate transponder shelves entirely. The operational model simplifies: the IP layer directly drives the optical layer, reducing the number of devices to manage and monitor.

This scenario is exactly the use case that has driven actual deployment. At least a dozen Tier 2 and Tier 3 operators in North America and Europe have deployed 400G ZR in this configuration since 2021. The Flexoptix-compatible 400G ZR QSFP-DD portfolio covers this use case at significantly lower cost than single-vendor transponder solutions.

## Where Vendor Integration Still Wins

Long-haul and ultra-long-haul are the clearest counter-examples. Spans exceeding 1000km require Raman amplification, careful optical power budget management, and DSP algorithms tuned for the specific chromatic dispersion and polarization mode dispersion characteristics of the fiber plant. These requirements are still best addressed by integrated transponder/ROADM solutions from vendors who have co-engineered the DSP and the line system. Mixing a ZR+ pluggable with a Ciena 6500 line system on a 2000km path is theoretically possible but practically fraught — the DSP operating point assumptions in the pluggable may not match the amplifier gain tilt the ROADM produces.

High-channel-count metro core networks are another case where integration advantages persist. A 96-channel C-band deployment with high power channels, mixed modulation formats, and tight channel spacing (50GHz or 37.5GHz) benefits from ROADM-integrated optical power control that understands the full channel loading. The open line system model here requires accurate optical modeling of the entire span, which is achievable but requires sophisticated controller software that most operators don't run internally.

## Lock-in That Remains

Even in fully disaggregated deployments, some lock-in is unavoidable. The coherent DSP inside a ZR+ module is proprietary — Acacia's AC400, Marvell's Polaris, Broadcom's Orion, and Coherent/Ciena's own implementations each have different performance characteristics, operational interfaces, and tuning parameters. You can swap optical form factors (QSFP-DD to CFP2 to OSFP) more easily than you can swap DSP vendors without performance regression.

The network management layer also concentrates lock-in. The domain controller that manages a disaggregated optical layer — performing topology discovery, route computation, and optical impairment modeling — is typically proprietary software from a systems integrator or equipment vendor even when the hardware itself is multi-vendor. OpenROADM addresses the south-bound device interface but doesn't solve the optical path computation problem, which requires physics-aware software that carriers typically don't develop themselves.

The honest assessment is that metro DWDM disaggregation has delivered real value for the use cases it was designed for, reduced costs significantly in point-to-point and simple ring topologies, and created a healthy coherent pluggable market. It has not eliminated the need for integrated vendor solutions where optical span engineering complexity is high. Both architectures will coexist for at least the next decade.
