What are ROADMs?

The reconfigurable optical add/drop multiplexer (ROADM) is an element in DWDM ring and mesh networks, which enables dynamic and flexible wavelength routing. The device has an important role in extending and modifying existing WDM networks, as it enables adding new wavelengths and changing the path of existing wavelengths within the network. The role of the ROADM is becoming more important as optical networks are growing and compose of more sites and become more complex.

The ROADM supports colorless and directionless features, and enables automatic power balancing of the wavelengths across the network, critical in links with many EDFAs (Erbium-doped fiber amplifier) and multiple channels.

What is the ROADM’s role in the optical network?

WDM networks can have as many as 96 wavelengths, each one with a different route. The challenges of network managers include changing the path of wavelengths, keeping track of the wavelengths, monitoring their operation and deploying new wavelengths easily.

DWDM network expansion entails adding wavelengths, ROADMs enable to do this easily and cost-effectively, using software-based provisioning of wavelengths through an NMS.

The ROADM automatically balances the optical power of the wavelengths across the network, a critical task in ensuring signal continuity and integrity.

What are the components in an ROADM?

The ROADM includes the following components:

  • Wavelength selective switch (WSS) - an active component that performs the actual wavelength switching. The WSS enables users to dynamically route any wavelength to and from any port and then seamlessly change connectivity as needed by blocking or passing wavelengths.
  • Optical channel monitoring (OCM) - monitors the optical power of each wavelength.
  • Variable optic attenuator (VOA) - configures the attenuation of optical power in each wavelength for power balancing between the different channels, and spectral flatness.

A ROADM node comprises several ROADM devices connected to each other using input ports. The following figure shows the structure of a typical ROADM device with four input ports. Each device is connected to a different ROADM switching direction, called degree.

Wavelength Provisioning Using 4-degree ROADM

Wavelength Provisioning Using 4-degree ROADM

What are the advantages of ROADM networks?

One of the main advantages of ROADM networks is the provisioning of wavelengths from a remote NMS (network management system) without the need to make major network changes, redesign or the need to send a technical person to every site along the path. The NMS automatically discovers the network topology, finds the optical paths between the nodes, determines the relevant configuration for all ROADMs in the path, and performs the provisioning for the required wavelength.

The NMS also enables to easily route wavelengths or group of wavelengths from any site to any site, and allows the network administrator to select the path and assign a protection path.

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