WHAT IS WDM? – How wavelength division multiplexing works and when to use it

How do you increase capacity between sites without adding more fiber? Wavelength division multiplexing (WDM) addresses this by allowing multiple data streams to be transmitted over a single optical fiber. This makes it possible to scale capacity cost-effectively by using existing infrastructure more efficiently. Learn when to use WDM, how it works, and how open solutions help maintain flexibility as networks scale.

Why WDM matters today

Capacity demand between sites is increasing as applications and services are distributed across multiple locations. Data that was previously handled within a single site now moves continuously between data centers, exchange points, and regional network nodes.

At the same time, expanding fiber infrastructure is not always feasible due to cost, availability, or lead times. This shifts the focus from adding new fiber to using existing fiber more efficiently. WDM addresses this by increasing the amount of traffic that can be carried over the same fiber footprint, allowing capacity to scale in line with demand.

What is WDM?

WDM stands for wavelength division multiplexing. It is a method for combining multiple data signals onto a single optical fiber by assigning each data stream a distinct light wavelength. This is often compared to using a fiber as a single-lane road, where each service requires its own path. With WDM, multiple wavelengths travel in parallel, like lanes on a multi-lane highway, each carrying its own traffic without interference.

Key points in brief:

• Each wavelength acts as an independent channel, carrying its own data stream without interfering with others
• Multiple wavelengths share the same fiber, increasing capacity per link
• At the receiving end, a demultiplexer separates the wavelengths back into individual data streams
• WDM supports a mix of services, including IP, storage, voice, and video, on the same fiber

WDM is typically implemented as either coarse WDM (CWDM) or dense WDM (DWDM) depending on channel spacing and capacity requirements, supporting services from 100G to 400G and 800G.

How WDM optimizes the use of optical fiber

Without WDM, each service typically requires its own fiber. As capacity grows, the number of fibers increases in line with demand, driving higher infrastructure and leasing costs.

With WDM, multiple wavelengths are transmitted over the same fiber. Each wavelength carries an independent data stream, increasing the total capacity of the link. WDM can support up to 96 channels on a 100 GHz grid, depending on the configuration.

This allows existing fiber infrastructure to carry significantly more traffic, delaying or eliminating the need for additional fiber deployment.

When to use WDM

WDM is used when capacity between sites is increasing, but adding more fiber is costly, slow, or not feasible. Some examples are when metro networks face permitting constraints for additional fiber deployment, when enterprise data center interconnects need to improve cost efficiency per fiber pair, or when internet exchange networks need to upgrade legacy infrastructure and scale capacity without disruption.

See how NDIX seamlessly upgraded capacity to 100G using an open line system interoperable with its existing WDM infrastructure

Consider WDM when:

• Capacity between sites is growing faster than new fiber can be deployed
• Incremental capacity upgrades are needed without disrupting existing traffic
• Fiber is leased or limited, requiring efficient use of each fiber pair
• Future capacity expansion should be decoupled from optical infrastructure upgrades

How WDM systems are built

In its simplest form, a WDM system consists of four elements that work together to transmit multiple wavelengths over a single fiber:

How transceivers support WDM by transmitting data as light

Transceivers are wavelength-specific lasers that convert data signals from SAN and IP switches to optical signals that can be transmitted over fiber. Each data stream is converted into a signal with a light wavelength assigned a unique color, allowing multiple independent channels to operate in parallel on the same link. Because each wavelength operates independently, additional channels can be added without affecting existing traffic. 

Each channel is transparent to the speed and type of data, allowing any mix of SAN, WAN, voice and video services to be transported simultaneously over a single fiber or fiber pair. 

How multiplexers optimize the use of fiber channels for WDM

Passive multiplexers and OADMs are used to combine, separate, and manage wavelengths across a WDM system. 

The WDM multiplexer, often referred to as a passive mux, combines multiple optical signals onto a single fiber. At the receiving end, a demultiplexer separates them back into individual channels. Multiplexers are typically deployed at the endpoints of a link, with one unit at each site in a point-to-point connection. 

In networks with additional sites, optical add-drop multiplexers (OADMs) are used to insert or extract specific wavelengths at intermediate points, allowing some channels to be accessed locally while others continue through the network. This enables versatile ring, distribution, and multi-site network topologies. 

Modern WDM systems can support up to 96 channels on a 100 GHz grid, depending on the configuration. 

How patch cords connect transceivers and multiplexers

Patch cords serve as the glue connecting transceivers and multiplexers, linking the active components of the WDM system. LC patch cords are commonly used to connect the optical output of the transceiver to the input of the multiplexer. 

How WDM uses dark fiber with fiber pairs or single fiber

Access to dark fiber is a requirement for any WDM solution. The most common deployment uses a fiber pair, with one fiber for transmitting data and the other for receiving it.  

In some cases, only a single fiber is available. WDM can still be used by assigning different wavelengths for transmit and receive, allowing bidirectional communication over the same fiber.

Open WDM and open line systems

WDM systems have traditionally been built as closed platforms, where optical components, transponders, and management systems are tightly integrated within a single vendor solution. While this can simplify initial deployment, it can make upgrades more complex and harder to predict as network requirements change. 

Open WDM, often implemented as an open line system, separates the optical transport layer from the services running over it. Transceivers, bit rates, and network equipment can be upgraded without replacing the underlying line system. 

This makes it easier to introduce new hardware, increase capacity, or extend the network without redesigning the optical layer.

Build and scale your WDM network with Smartoptics

Whether you are upgrading an existing WDM network or deploying new capacity, Smartoptics open line systems enable you to scale efficiently without unnecessary complexity. Built on an open approach, our solutions allow you to break unwanted vendor lock-in, remain flexible, and minimize costs as your network grows. 

Explore our full range of open optical networking solutions to make the most of the latest WDM technology.