---
title: "Wavelength Selective Switches: The Component That Defines Your Metro Ring"
slug: "wavelength-selective-switch-wss-explainer"
type: deep-dive
category: "DWDM & Coherent"
tags: [WSS, ROADM, wavelength selective switch, CDC ROADM, flex-grid, colorless directionless contentionless, MEMS, LCoS]
seo_focus_keyword: "wavelength selective switch WSS ROADM CDC"
---

The Wavelength Selective Switch is the optical component that makes a modern ROADM function, and understanding its properties — particularly its degree count and switching architecture — is what determines whether a metro ring design will have the flexibility you actually need or the flexibility that sounded good in a vendor presentation. The gap between those two things can be significant.

## What a WSS Does at the Component Level

A Wavelength Selective Switch is an optical cross-connect element that can route individual wavelengths independently between its input and output ports. The "1x9 WSS" designation means one common port (typically connected to the fiber line) and nine wavelength ports. The WSS can route any of the 96 C-band channels (at 50GHz spacing) to any of its nine ports, including routing different wavelengths to different output ports simultaneously, and can also perform per-wavelength attenuation.

The physical implementation is typically either MEMS-based (micro-electromechanical mirrors that steer wavelengths optically) or LCoS-based (Liquid Crystal on Silicon, which uses a diffraction grating and programmable liquid crystal cell array). LCoS implementations dominate in modern ROADM equipment because they support programmable wavelength bandwidth — the "flex-grid" capability — while MEMS approaches are typically fixed to the ITU grid.

Commercially, Lumentum (which absorbed JDSU and inherited their photonics IP) and II-VI/Coherent supply the majority of WSS modules to ROADM equipment manufacturers. Perle, Finisar/Coherent, and Huawei subsidiary HiSilicon supply the Chinese market. The WSS subsystem is essentially a commodity component that ROADM OEMs (Ciena, Infinera, Nokia, Fujitsu, Huawei) integrate into their platforms. When a Ciena 6500 node has different WSS degree options than a Fujitsu FLASHWAVE, it's usually reflecting different WSS module selections rather than fundamentally different optical architectures.

## Colorless, Directionless, Contentionless: What Each Actually Means

These three attributes are frequently listed together as CDC-ROADM but they're independent capabilities that add cost incrementally. It's worth understanding what each one buys operationally.

Colorless means a local add/drop port can accept or originate any wavelength. Without colorless capability, an add/drop port is fixed to a specific ITU channel, which means a transponder plugged into that port can only transmit on that pre-assigned wavelength. With colorless ports, the transponder's tunable transmitter can be assigned any wavelength in the C-band, and the ROADM routes it appropriately. This is the fundamental requirement for automation-friendly metro deployments and is now standard on any modern ROADM equipment. The WSS provides colorless add/drop by connecting the add/drop modules to the WSS common port side.

Directionless means a local add/drop port can connect to any line direction without being pre-assigned to a specific fiber pair. In a 4-degree node (where four fiber routes converge), a non-directionless architecture has specific transponder slots pre-cabled to specific degrees. A directionless architecture adds an optical switch fabric between the add/drop modules and the directional WSS ports, allowing any transponder to connect to any degree. This is expensive — the optical switch fabric is additional hardware — but essential for automated wavelength restoration, where a failed route needs to be re-routed through a different degree without physical recabling.

Contentionless means multiple add/drop ports can be assigned the same wavelength simultaneously. This is the most commonly misunderstood attribute. In a CDC-ROADM without contentionless, only one add/drop port can use wavelength λ1 at a node, even if the ROADM could technically route it from multiple sources. Contentionless capability, implemented through additional WSS stages or coherent multicasting elements, allows multiple transponders at the same node to use the same wavelength on different routes. This matters for high-capacity nodes that are provisioning many 100G or 400G wavelengths toward the same destinations.

## 1x9 vs. 1x20 WSS Degree

The "degree" of a WSS describes how many output ports it has. A 1x9 WSS can connect its common port to any of nine wavelength ports; a 1x20 WSS can connect to any of twenty. In ROADM context, the degree determines how many network directions (fiber routes) the node can connect to.

A 4-degree ROADM node — common in ring topologies — can use 1x9 WSS modules and have plenty of ports to spare. A large mesh node with 8 or 12 network directions requires 1x9 WSS modules deployed in multi-stage configurations or, more commonly, 1x20 WSS modules to achieve the necessary port count without increasing stage count (which adds insertion loss).

Each additional WSS stage adds approximately 4 to 6 dB of insertion loss. For a metro network where power budgets are often running within 3 to 5 dB of the sensitivity threshold, adding a WSS stage to achieve higher degree count can force a decision between adding an optical amplifier (cost and complexity) or reducing the span lengths (not always possible). This is the practical reason why metro ring topologies are often limited to 4 or 6 degrees even when more fiber routes exist — the optical power budget constraint makes 8+ degree nodes expensive.

## Flex-Grid: What It Costs in Practice

Traditional DWDM uses the ITU-T 50GHz fixed grid: 96 channels spaced at exactly 50GHz intervals across the C-band, each 50GHz wide. Flex-grid extends this to variable channel widths: channels can be assigned widths of 12.5GHz, 25GHz, 37.5GHz, 50GHz, 75GHz, 100GHz, or more in multiples of 12.5GHz.

The motivation for flex-grid is accommodating super-channels — wide coherent signals produced by multi-carrier transponders that span 150GHz or 200GHz. An Infinera ICE6 super-channel might span 750GHz of optical bandwidth. On a fixed 50GHz grid, you can't allocate this efficiently; on a flex-grid system, you allocate exactly the bandwidth needed.

In practice, flex-grid deployment requires LCoS-based WSS (which is now universal in modern ROADMs), network management software that understands variable spectral assignments, and coherent modems that can operate correctly at non-standard channel spacings. All of these are available from major vendors. The cost overhead is not in the WSS hardware itself but in the planning and management complexity: a flex-grid spectrum assignment database is more complex to manage than a simple 50GHz channel number, and wavelength conflict resolution in dynamic wavelength assignment algorithms becomes harder when channels are variable width.

## Why Port Count Constrains Your Metro Ring

The critical operational consequence of WSS port count is that it limits how many circuits you can add/drop at a node simultaneously. A 1x9 WSS with 4 ports used for network degrees (in a 4-degree node) has 5 ports remaining for local add/drop. Each add/drop port can handle one wavelength (or wavelength band, if branching stages are added). With 5 add/drop ports and 96 channels possible across the C-band, you cannot add/drop more than 5 wavelengths at this node unless you cascade additional WSS stages.

This sounds abstract until you're planning a 400G coherent deployment where a single customer circuit is one wavelength and you have 15 customers to add/drop at the same node. Suddenly the WSS port budget is your primary design constraint, more than fiber capacity or optical power. The upgrade path is a new node design with higher-degree WSS — which typically means replacing the WSS modules, redesigning the optical cabling within the chassis, and repricing the node.

The WSS degree and port count decisions made in the initial ROADM deployment are difficult to reverse without hardware replacement. This is the constraint that deserves more attention in metro ring planning discussions than it typically receives.
