To ROADM or Not to ROADM: When Does a FOADM Make Sense in Your Optical Network?

Watch XKL’s Chief Architect Chad Lamb present at NANOG 92, with a side-by-side comparison of ROADMs with their predecessor, the FOADM, Fixed Optical Add Drop Multiplexers. What are some tradeoffs between these two technologies, advantages and disadvantages. When does a FOADM make sense in your network?

Moderator:
I would like to introduce Chad Lamb, who will be presenting “To ROADM or Not to ROADM: When Does a FOADM Make Sense in Your Optical Network?” Chad is the Chief Systems Architect at XKL and has traveled all the way from the Seattle area to join us today. This is his first time presenting at NANOG, and we are pleased to have him speaking with us. Welcome, Chad.

Chad Lamb:
Thank you, and thank you to the program committee for letting me present to you today. Congratulations—you’ve made it to the last talk of NANOG 92! Good job.

I know many of you have traveled long distances to get here, and just as your bodies are adjusting to this time zone, you’re going to head right back. Sorry about that!

Introduction

Today, we’re going to discuss ROADM and FOADM. For those unfamiliar with these terms, I’ll provide a quick primer to get us started. I only have 30 minutes, and I can get a bit wordy, so I’ll move quickly through the basics. After that, we’ll focus on what we hear from customers—because, after all, this is about solving your problems.

From there, I’ll dive into topologies where using a ROADM or FOADM might make sense and provide a comparison of these technologies. Finally, we’ll summarize when it makes sense to use a ROADM and when a FOADM is the better choice.

Key Acronyms and Terms

Let’s set the stage with a few key acronyms:

  • WSS: Wavelength Selective Switch.
  • CDC-F: Contentionless, Directionless, Colorless, Flex-Grid.

These terms will come up frequently, so keep them in mind as we go.

Customer Challenges

We always want to address what you, the customer, are experiencing. Here are some common challenges we hear about regarding ROADM and FOADM deployments:

For ROADM Deployments:

  1. Software Updates and Fixes: These can be disruptive, especially at Layer 1, where uptime is critical.
  2. Hardware Failures: While technology has improved, components like the WSS still face issues, and replacing them is complex.
  3. Vendor Lock-In: The WSS is proprietary across vendors, which makes interoperability difficult.
  4. Steep Learning Curve: Managing ROADM software and interfaces can be challenging for teams unfamiliar with the tools.
  5. Cost and Complexity: Balancing cost versus functionality is a significant consideration.

For FOADM Deployments:

  1. Lack of Channel Balancing: Equalizing power levels across DWDM channels can be tricky.
  2. Cabling and Channel Pre-Planning: FOADM requires precise pre-planning and manual patching.
  3. On-Site Visits: Any changes—like adding or dropping channels—require a physical presence.

Where Do FOADMs and ROADMs Fit?

FOADMs and ROADMs are suitable for any network requiring add/drop functionality, except for simple point-to-point connections.

  • Point-to-Point: No need for an OADM.
  • Ring Topologies: ROADMs or FOADMs allow channels to express through intermediate sites without unnecessary equipment.
  • Multi-Ring Networks: At interconnection points, like a site joining two rings, a ROADM is particularly useful to prevent single points of failure.

Architectural Comparison: FOADM vs. ROADM

FOADM

  • Structure: Fixed filters (AWG, thin-film, etc.) determine channel routing.
  • Channels: Add/drop channels require manual patching; once configured, changes are difficult.
  • Pros: Simple, cost-effective, low power consumption.
  • Cons: No remote reconfiguration, no channel balancing without external devices.

ROADM

  • Structure: Replaces fixed filters with Wavelength Selective Switches (WSS).
  • Functionality: Software-programmable to add/drop or express channels. Supports colorless (any wavelength), directionless (east/west routing), and contentionless (no wavelength conflicts).
  • Pros: Remote management, dynamic adjustments, and channel balancing.
  • Cons: Higher cost, complexity, and power consumption.

Key Considerations

  1. Channel Equalization:
    • Equalizing power levels across channels ensures they meet transceiver power specifications.
    • ROADMs handle this inherently via WSS, while FOADMs require external equalizers.
  2. Noise:
    • Noise originates from amplifiers, filters, and nonlinear effects in the fiber.
    • ROADMs tend to have higher insertion loss but allow finer control.
  3. Adjacent Channel Interference:
    • FOADM filters generally offer better isolation (28–40 dB) than WSS in ROADMs (~25 dB).
  4. Power Consumption and Failures:
    • FOADMs have no powered components, leading to fewer failures and lower OPEX.
    • ROADMs consume more power and have active components that can fail. 

Conclusion:
FOADM is a simple, cost-effective solution for networks with static configurations. It is ideal for environments where manual adjustments are manageable, and cost is a primary concern.

ROADM, on the other hand, excels in dynamic environments requiring remote reconfiguration, channel balancing, and advanced protection mechanisms.

Ultimately, the choice between ROADM and FOADM depends on your network’s complexity, operational needs, and budget.

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About XKL

XKL helps businesses of all sizes expand and reimagine their networks in innovative ways with built-to-order, cost-effective and easy-to-manage DWDM solutions. Backed by our proven customer-centric, consultative approach and 30+ years of experience, XKL’s Layer 1 optical networking solutions deliver the capabilities and reliability required to meet even the most challenging network demands.

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