What’s the difference between CWDM and DWDM?

With the continuous enrichment of network services and high capacity demand, wavelength division multiplexing (WDM) technology has emerged.

What is WDM?

WDM is a communication technology that combines a series of optical signals with different wavelengths that carry information into one beam at the sending end and transmits them along a single fiber; at the receiving end will use the same technology to separate the optical signals of different wavelengths. This wavelength division multiplexer technology can transmit multiple signals on single fiber at the same time. Every signal is transmitted by a certain wavelength, one wavelength is called a channel.

And there are two core technologies of WDM (wavelength division multiplexing): CWDM (coarse wavelength division multiplexing) and DWDM (dense wavelength division multiplexing). The core differences are in wavelength spacing, capacity, cost, transmission distance and application scenarios. Let’s expose them in details below.

What is CWDM?

CWDM is the abbreviation of Coarse Wavelength Division Multiplexing. It has many advantages such as low cost, low power consumption, and small size. It is a low-cost WDM transmission technology for the access layer of the metropolitan area network.

Working principle of CWDM

What is DWDM?

DWDM is the abbreviation of Dense Wavelength Division Multiplexing. With single mode fiber bandwidth and low loss characteristics, it uses multiple wavelengths as carriers and transmit signals simultaneously in the fiber.

Working principle of CWDM

Difference CWDM vs DWDM

• Difference in wavelength range

According to the ITU-T standard, CWDM operating wavelength is 1270~1610nm (O, E, S, C, L bands), with 18 wavelengths (channels) and 20nm channel spacing, among which 8 bands from 1470nm to 1610nm are commonly used, especially 1550nm is more popular due to its lower fiber loss.

The operating bands of DWDM are C band 1525-1565nm and L band 1570-1610nm, and the channel spacing is 1.6nm (200GHz), 0.8nm (100GHz), 0.4nm (50GHz). Since the wavelength spacing of DWDM is relatively narrow, it can carry 40, 80, 96 or up to 160 wavelengths (channels).  Since the channel spacing of DWDM is close, more information can be carried on the same fiber, so the bandwidth and capacity of DWDM are larger.

• Different cost

CWDM’s channel spacing is 20nm, it does not enable CWDM to transmit and receive up to 18 channels through a pair of fibers, but also greatly simplify the structure of the multiplexer and demultiplexer.

Furthermore, the number of filter coating layers is reduced, and it is ok as long as the the center wavelength drift does not exceed 6.5nm, which improves the yield and reduces the cost, it no need cooled DFB lasers, small size, and low power consumption and lower cost.

But in DWDM systems, the working wavelengths are relatively dense, and the wavelength interval is generally only a few nm to a few tenths of a nm. Therefore, the wavelength of the laser used in the DWDM system must be accurate and have good stability. Since the wavelength offset is small, high-cost cooled DFB lasers must be used.

• Different application scenarios

CWDM is usually used for lower-cost and shorter-distance applications like 5G, base station metropolitan area network, access to remote networks and local area network like campus, banks, and enterprises. Since CWDM has a 20nm channel spacing, it is usually deployed on fiber spans of up to 80km or less, because optical amplifiers cannot be used with large-spaced channels.

100G DWDM channel spacing is 0.8nm, it can allow more wavelengths to be packed onto the same fiber, so it can transmit large amounts of data through a single fiber. With the EDFA amplifier, DWDM can operate over a range of thousands of kilometers.

DWDM adopt two technologies: AWG and TFF. Due to its high wavelength density and concentration on C band and L band, it is generally used in long-distance and ultra-long-distance signal transmission scenarios, such as DCI (Data Center Interconnect), DC-CN (Core Network), long-distance, large-capacity long-distance trunk networks, or ultra-large-capacity metropolitan area network core nodes.

For better understanding, let’s make a summary as below table:

Technology integration trend

However, the two are not irreconcilable. Sometimes they can be flexibly applied to achieve better cost performance. Currently, the following 3 solutions are commonly used:

• Hybrid WDM: Hybrid networking of CWDM+DWDM, CWDM for the access layer, and DWDM for the core layer to achieve cost optimization and technical reliability.
• In order to lower the DWDM cost, we can use tunable laser which support dynamic wavelength allocation.
• Integrate AWG and modulators into chips to promote the miniaturization and low cost of WDM devices.

Conclusion

CWDM is the king of cost-effectiveness, and DWDM is the king of capacity. The actual networking needs to be weighed based on the three factors of distance, cost, and scalability.

If the budget is limited and the transmission distance is <40km, but rapid deployment is required, such as 5G fronthaul, or the channel requirement is ≤18 waves for branch office interconnection, recommend to use CWDM;

 if the long-distance transmission is >80km, and the capacity requirement is >1Tbps, and the future needs to smoothly expand the capacity, such as upgrading from 40 channels to 80 channels or 160 channels for the core layer of the network, recommend to choose DWDM technology for government-owned networks, which is more reliable.

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