FOADM vs TOADM vs ROADM: What's the Difference?

What is OADM (Optical Add Drop Multiplexer)? OADM is a device used in WDM networks. It can be divided into CWDM OADM and DWDM OADM according to their specific applications. In most cases, OADM is deployed with WDM Mux Demux. Its main function is to couple two or more wavelengths into the same optical fiber to increase the total bandwidth between two points.

Traditional OADM employs fixed-wavelength lasers/filters. With the continuous advancement of OADM technology, the market has seen the emergence of wavelength-tunable OADMs – TOADM and ROADM. What are the differences among them? What advantages does ROADM have over FOADM and TOADM? In this article, we will provide answers to help you understand it better.

Traditional OADM-FOADM

Fixed optical add drop multiplexer, as the name suggests, the number of wavelengths is fixed and the direction of light is fixed. The FOADM can only add and drop channels with specified wavelengths, and cannot dynamically adjust the settings to add and drop other channels.

"Add" refers to the device's ability to add one or more new wavelength channels to an existing multi-wavelength WDM signal, while "Drop" refers to the downloading or removal of one or more channels to the local signal, transfer these signals to another network path, and do not affect the transmission of other existing wavelength channels according to the original route.

Below is a simple diagram of FOADM. First, the CWDM Demux separates all the wavelengths, routing the designated wavelengths to the transmission site while the remaining wavelengths pass through the node. At the same time, another designated wavelength is added and transmitted through the CWDM Mux to the next site.

Dynamic OADMs -TOADM vs ROADM

What Is TOADM？

Compared to FOADM, the tunable optical add drop multiplexer (TOADM) can dynamically adjust the wavelength of channels at runtime without interrupting the entire system. This allows network administrators to make real-time adjustments to optical signals as needed, adapting to different transmission requirements or network configurations. Consequently, TOADM demonstrates greater advantages in terms of adaptability and flexibility over traditional fixed-wavelength devices. However, due to its relatively simple functionality and certain limitations, it cannot fully meet people's dynamic management requirements for wavelengths, which has promoted the emergence of ROADM.

What Is ROADM？

Reconfigurable optical add drop multiplexer is a device used in a DWDM network, facilitating dynamic addition or removal of service wavelengths through remote reconfiguration. That is to say, in the middle of the line, the wavelength of the up and down services can be assigned arbitrarily according to the needs, so as to realize the flexible scheduling of the services.

With a ROADM, the user can send a command to the hardware and tell it to change what colors are added or dropped at any location. By combining multiple sets of hardware together, ROADMs can connect multiple locations very flexibly and create a network that is easily modified as network requirements change.

In fact, ROADMs have evolved through three main generations. Today, ROADM technology is undergoing a transition to its fourth generation, marked by the introduction of four new functions collectively known as CDC-ROADM: colorless, directionless, and contention-less. The advantage of CDC-ROADM lies in its ability to upload and download any frequency band at any port, simultaneously addressing the issue of the same frequency band not being able to be placed on the same path. This significantly reduces the difficulty and improves the efficiency of fault recovery. ROADM nodes typically consist of a wavelength-selective switch (WSS) and other modules, with the CDC (Colorless, Directionless, and Contention-less) functionality dependent on the structure of the ROADM node. The flexible bandwidth feature depends on the key module, WSS.

The Difference Between TOADM and ROADM

Compared to the automation and dynamic wavelength configuration of ROADM, TOADM is relatively more basic. It typically requires manual operation by operators or the selection of specific wavelengths through preset configurations. Due to its simpler structure and functionality, TOADM is more cost-effective than ROADM.

In comparison to the more advanced ROADM, TOADM has certain limitations in its applications. Because TOADM is not as flexible in configuration as ROADM, TOADM is better suited for smaller and medium-sized networks or environments with relatively static network requirements that do not necessitate frequent wavelength adjustments. As tunable technologies in optical networks advance, TOADM has evolved into variants with some degree of automation, but generally, its functionalities and flexibility still fall short of those offered by ROADM.

FOADM vs ROADM：What Are the Advantages of ROADM

1. Simplify Transport Network Design In ROADM systems, signal switching doesn’t need optical-to-electric and electric-to-optical conversions just like FOADM does. The planning of entire bandwidth assignment does not need to be carried out during initial deployment of a system. It provides full flexibility of delivering any wavelength to any node throughout the ring infrastructure.

2. Quickly Respond to New Bandwidth Demands FOADM solves capacity problem, but it is inflexible for spontaneous bandwidth needs. On the contrary, ROADM-based networks are appropriate when demand is unpredictable, growing, or expected to be moved regularly.

3. Extend Wavelength Transport ROADM-based networks are enabling an automated optical layer with dynamic multipoint connectivity, independent wavelength add-drop, remote bandwidth allocation that has been enhanced power management capabilities.

4. Remote Reconfiguration ROADM allows wavelength add/drops at a node point to be remotely reconfigured, rather than requiring a technician to install add/drop lasers/filters with specific wavelengths into the WDM system node, and equalize the power levels of wavelengths so they can co-operate on the same fiber.

5. Better Service on Network Upgradability ROADM automates the optical layer to remove error-prone service provisioning, and equalizes signal loss across all wavelengths, reducing the need for expensive signal boosting equipment. So it can reduce the costs of networks.

6. Reduce OPEX & CAPEX By comparison between ROADM vs FOADM, ROADM can reduce time and labor associated with the manual provisioning needed for fixed OADM sites.

Conclusion

With the development of technology, FOADM first provided basic fixed wavelength processing capabilities, and then TOADM added some adjustment functions, while ROADM brought dynamic and flexible wavelength configuration capabilities, greatly improving the flexibility and efficiency of optical networks.

Based on cost considerations, the existing metropolitan area network is primarily built upon CWDM and FOADM technology. In order to upgrade the network, the DWDM and ROADM technologies previously applied in the backbone network are also expected to extend to the metropolitan area network. Currently, ROADM is advancing towards efficiency, intelligence, and openness, which will aid in the comprehensive upgrade of the network from electrical nodes to optical nodes. This breakthrough will overcome network node capacity bottlenecks and achieve full optical automatic scheduling.