CWDM vs. DWDM: Top Differences

Aug 27, 2023

Coarse wavelength division multiplexing (CWDM) is a type of wavelength division multiplexing generally leveraged for optical transmission across shorter distances. On the other hand, dense wavelength division multiplexing (DWDM) is an optical transmission technology that leverages numerous light wavelengths to merge multiple data streams onto a single optical fiber and transmit them across longer distances.

CWDM Wavelengths vs. DWDM Wavelengths

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But what exactly is wavelength division multiplexing (WDM)? Before we dive into WDM, let us begin by understanding what a wavelength is.

The word “optics” in “fiber optics” gives us a fair idea about the mechanism used in this technology. The signaling medium used by fiber optics is light or, if we’re being more scientific, electromagnetic radiation.

Simply put, a wavelength is used to measure the distance between two photons in a solid light beam, while frequency measures the time between two signals. Think of these two terms as the two sides of a coin — a shorter wavelength indicates less time taken between signals and, thus, a higher frequency.

Thus, the wavelength or frequency of any light source can be used to gauge the physical limitation of it being used for signal processing. Signals faster than the beam frequency cannot be used, nor can we use equipment smaller than the wavelength.

Besides these factors, wavelength is also useful to explore how light interacts with an object. As fiber optic communications use lasers to transmit data over long distances, studying such interactions is important when creating fiber optics.

Wavelength division multiplexing (WDM) uses a multiplexer (also called a data selector) to combine numerous varying data streams and transmute them into wavelengths of light. These wavelengths are transmitted over fiber and then demultiplexed at the receiver’s side, where they are split back into data streams.

Simply put, WDM enables the transmission of numerous distinct signals using one fiber using varying light colors. This enhances the quantity of data that can be sent and received. WDM also supports the bidirectional transmission and receipt of information, thus allowing users to send and receive data on one fiber simultaneously.

The “varying colors of light” need not be gauged visually as they can be described using frequency and wavelength. Frequency defines the number of times a wave of light cycles in a second. On the other hand, wavelength defines the physical space between two peaks in the wave.

Differences in material can determine the speed at which light travels. In a vacuum such as outer space, light travels at a constant speed of 299,792,458 meters per second. This value is denoted by the letter “c.”

In the case of glass fiber, light travels slower at approximately 0.7 times “c.” Frequency and wavelength can be used to calculate the speed at which light travels in fiber. In real-world systems such as WDM, the data rate is not as fast as the carrier wave’s frequency.

Now that we have a basic understanding of WDM and how it works, let us learn more about CWDM and DWDM.

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As a subset of WDM, coarse wavelength division multiplexing (CWDM) transmits multiple signals over a single fiber using varying light colors.

Before 2002, CWDM referred to numerous different channel configurations. However, since then, the International Telecommunication Union (ITU) has standardized a specific channel spacing grid for CWDM. Today, CWDM specifically uses wavelengths between 1,270 nm and 1,610 nm with a 20 nm channel spacing.

In this new standard, erbium-doped fiber amplifiers (EDFAs) were limited as the signals’ spacing was not appropriate for amplification. This translates to the total CWDM optical span reaching around 60 km for a 2.5 Gbit/s signal, thus making it ideal for metropolitan applications. The optical frequency stabilization requirements were also relaxed in this standard, allowing the cost of CWDM components to reach closer to those of non-WDM components.

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Like CWDM, dense wavelength division multiplexing (DWDM) works by combining the transmission of many different signals (such as phone calls, internet data, and video streaming) into one optical fiber by using varying colors or wavelengths of light.

DWDM is a subset of WDM that typically leverages the spectrum band from 1,530 nm to 1,625 nm, or more commonly the C-band and L-band, to input 40, 88, 96, or 160 channels or wavelengths onto one strand of fiber.

The “dense” in DWDM comes from using tighter (or denser) wavelength spacing to accommodate more channels. This leaves a width of only about 0.8 nm for each channel. This is in direct contrast to CWDM, where a wider range of frequencies is used and each channel is spread farther apart. As established above, CWDM channels are typically 20 nm wide, meaning DWDM can accommodate a higher number of channels and much more information in a single fiber optic cable.

DWDM also accommodates the special EDFA, allowing it to enhance the signals traveling over the fiber. DWDM is ideal for transmitting different signals over the same fiber, and EDFA enhances this functionality by boosting multiple signals at once. EDFA works best at the 1550 nm band, which is why this band is generally used for DWDM.

Using DWDM allows enterprises to upgrade the throughput of their fiber optic networks, thereby transmitting more data without the need to replace the rest of their network hardware. EDFA plays a role here, too — as it can amplify various signals simultaneously, enterprises can add more signals to the fiber while using the same amplifier.

Ultimately, DWDM drives more efficient use of fiber networks and reduces costs by unlocking the fullest potential of existing network equipment.

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Both CWDM and DWDM support the long-distance transmission of data, video, and voice signals. Both technologies optimize network performance. However, they differ in a few key ways.

The word “coarse” in CWDM refers to the spacing of the wavelength between channels. Compared to the more tightly packed DWDM wavelengths, CWDM leverages laser signals that differ in 20 nm increments.

CWDM can support 18 channels with wavelengths from 1,610 nm to 1270 nm. One system can support up to 8 channels, each supporting data rates of up to 10 Gbps.

A key difference in how CWDM and DWDM work is “chromatic spacing.” DWDM can send and receive more data. However, the smaller differences in wavelengths decrease signal tolerance and demand a far more precise laser design. This is why DWDM cables are much costlier than CWDM cables.

The 0.4 nm wavelength spacing of DWDM means it can support denser packing of signals, hence its name.

This improved density allows data transmission at rates as high as 100 Gbps.

Thus, assuming 160 channels per fiber cable, each channel capable of carrying 100 Gbps of data, each DWDM fiber cable can essentially support a capacity of around 1.6 Tbps.

Mux/Demux is the component that combines different channels onto one outbound fiber and receives the same channels from one inbound fiber. It separates the channels into individual wavelengths and delivers each to the required local interface. Capacity expansion of existing network fiber is possible through this process.

Drop/insert provides two local interface ports. One port removes a channel of a specific wavelength from the fiber in one direction. The second port adds the same channel back to the fiber in the opposite direction. This component supports two distinct pathways traversing opposite directions. This has several use cases — for instance, it helps users ensure a viable ring topology network even in case of breaks.

Finally, drop/pass removes a single wavelength-specific channel from the fiber, enabling the other channels to pass straight through to other network nodes. When this component drops the channel, the data is sent to a local interface. Here, the same channel is sent back to the drop/pass module for transmission, thus establishing a point-to-point connection between the local interface and the other device.

These three CWDM components work together to drive more efficient network communications.

Let’s take a high-level look at the DWDM data transmission process and the function of each component within the system.

1. The data stream is received via the router and fed as input to the transponder.

2. In the transponder, the signal is mapped to a DWDM wavelength and transmitted to the Mux to consolidate the optical signal.

3. As the signal passes through the Mux, optical amplifiers enhance the signal so it can be transmitted over longer distances.

4. In transit, optical add/drop multiplexers (OADM) are responsible for adding and removing bitstreams of specific wavelengths. Additional amplifiers can also be leveraged to boost signal distance further.

5. Finally, the signal reaches the Demux and is “demultiplexed” into individual DWDM wavelengths. These wavelengths are transmitted through the transponder and converted into the corresponding signals before being routed to their final destination.

The use of CWDM is also seen in transceivers like Gigabit interface converter (GBIC) and SFP CWDM optics. These systems use standardized CWDM wavelengths for wavelength-multiplexed transport over fiber.

Passive CWDM does not use electrical power. This CWDM implementation is commonly used in fiber to the premises (FTTP) deployments along with passive optical components like prisms and bandpass filters.

Overall, CWDM applications focus on supporting efficient and cost-effective transmission of data, video, and voice signals.

The growth of CWDM has not slowed due to DWDM; in fact, it continues to evolve and be adopted for specialized applications such as optical routing and transport devices as a more economical option.

DWDM is typically leveraged in long-distance, high-bandwidth, protocol-independent, and secure applications. For instance, it is the system of choice for telecommunications and cable companies and is used extensively in carrier transport networking.

Carrier transport networks generally comprise multiple layers of aggregation known as the access network, edge network, core backbone network, and metro aggregation network. DWDM is mainly used in core backbone networks and metro aggregation networks.

In metro aggregation networks, DWDM is used to combine data from several geographic locations. Service providers today constantly strive to bring computing capabilities closer to end users, an application that DWDM is useful for due to its flexibility and ability to provide higher bandwidth aggregation. It is, therefore, used to converge more data into a single node for computing.

DWDM is also ideal for use in the core backbone network, where high-speed switching of large data volumes often occurs between major central offices across regions.

Finally, DWDM is popularly seen in high-throughput data centers such as hyperscale cloud centers and colocation data centers. This is because this technology can combine numerous services with independent tenants. Additionally, data centers are becoming more geo-distributed, making DWDM popular for data center interconnect (DCI).

CWDM systems generally use 8, 16, or 32 channels, while DWDM systems can support up to 96 channels. This might make DWDM look objectively better; however, not every application requires so many channels (and the extra costs that come with it).

CWDM also uses wide-band optical filters and uncooled distributed-feedback (DFB) lasers, which lower costs and decrease power dissipation and size. Passive CWDM goes one step ahead and uses zero electrical power, instead using passive optical components to separate wavelengths.

Even upgrading CWDM systems is simple and economical, as it only requires combining mux/demux filters without dispersion compensation modules or optical power having to be adjusted.

The “pay as you grow” architecture offered by CWDM allows users to easily expand their systems without existing connections having to undergo disruption.

Like CWDM, DWDM can also be deployed on existing fiber, which means it can increase the data transfer capacity of enterprise networks as breakthroughs are made in optical technology.

Even though DWDM is costlier than CWDM, it is still more economical to deploy a DWDM system rather than install several hundred miles of new fiber to transfer increased data volumes across several states, countries, or even oceans.

Further, DWDM is bitrate and protocol independent because data flows through individual wavelengths, and there is no interference among channels. This allows DWDM to transmit different types of data, such as video, text, and voice, over a single fiber optic cable.

This non-interference quality of DWDM also drives data integrity and enables user separation, as well as other variants of partitioning.

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Both CWDM and DWDM have unique benefits but are not necessarily rival technologies. Rather, they complement each other and play equally important, if distinct, roles in optical networking.

CWDM is flexible and can be deployed for fiber network capacity expansion. Compared to DWDM, it is compact and cost-effective. However, it is not ideal for configurations requiring spectral efficiency or data transmission for distances over 50 miles.

CWDM can leverage passive hardware components. It is deployed for several applications. For instance, it is commonly seen in enterprise networks for point-to-point topology systems and telecom access networks. Generally, CWDM is well-suited for short-range applications and configurations requiring fewer channels.

CWDM is also preferred for enterprise applications where lower cost, simpler hardware requirements, and lower power consumption are priorities. For instance, CWDM is useful for local area networks (LAN). Here, CWDM is preferred over DWDM as it typically requires less power and is more energy-efficient.

Data centers are also a good use case for CWDM as they require high-capacity, low-latency connections within the premises, typically between storage systems and servers. Here, CWDM provides the required high-capacity connectivity without being as cost-intensive and difficult to manage as DWDM. Similar advantages also make CWDM useful for industrial applications like connecting control systems to remote sensors.

Conversely, DWDM is ideal for networks requiring greater channel capacity and higher speeds and applications that need amplifiers for transmitting data across longer geographic distances. Even though DWDM hardware is not cheap, it is still more economical than deploying new optic fiber networks.

If an organization requires its capacity and service rates to be enhanced, it has two choices: spend high recurring costs for leased lines that support connectivity with higher data rates or deploy and manage its own DWDM systems on existing optical networks.

There is an increased demand for the latter as more enterprises seek to enhance network capacity using DWDM optical networking applications. These applications are also leveraged for maximizing fiber connectivity among sites. DWDM is a viable solution for enterprises seeking scalable on-demand bandwidth.

Ultimately, whether a user opts for CWDM or DWDM, they will gain access to the benefits of optical networking. They both play pivotal roles in the multilayer networks popular in today’s enterprise space. They help extend the range of traditional pluggable optics, linking data centers and tethering sites together within a business park or campus, across metropolitan regions, between cities or states, and even across national borders.

As a result, both CWDM and DWDM are popular in the public sector, healthcare, utilities, financial, corporate, and data center applications. Optical networks, in general, are also considered to be the ideal solution for mission-critical networks.

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Technical Writer