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How to choose CWDM-DWDM equipment in optical transmission network

June 20, 2023

In the construction of urban metro networks (especially for long-distance OTN optical transmission network), WDM transmission equipment is especially important. DWDM(dense wavelength division multiplexing) equipment offers long-distance and high bandwidth transmission capability, while CWDM(coarse wavelength division multiplexing) equipment is more cost-effective. Therefore, selecting the appropriate WDM equipment is essential for building a cost-effective optical transmission system.

Difference between CWDM WDM equipment and DWDM WDM equipment

Current effective methods to address the increasing bandwidth of information transmission are the use of CWDM WDM devices and DWDM WDM devices, but they differ in many ways.

1. CWDM WDM equipment and DWDM WDM equipment channel spacing

The channel spacing is defined as the difference in nominal carrier frequency between two adjacent optical channels and is generally used to prevent inter-channel interference. CWDM WDM devices have a wider spacing than DWDM WDM devices and can transmit 18 wavelengths in a spectral grid from 1270 nm to 1610 nm with a channel spacing of 20 nm. Even more wavelengths can be transmitted with a channel spacing of 0.8 nm (1.6 nm and 0.4 nm are also available).

CWDM and DWDM channel spacing

2. The transmission distance of CWDM WDM equipment and DWDM WDM equipment

Since the wavelength of dense wavelength division multiplexing (DWDM) equipment being within the fiber optic range, CWDM WDM devices are currently unable to achieve unlimited distance transmission. They have a maximum transmission distance of only 160km, while DWDM WDM devices can transmit much longer distances than CWDM WDM devices.

3. CWDM WDM equipment and DWDM WDM equipment modulation laser

The system of CWDM WDM equipment has lower requirements on the technical specifications of the laser and generally use uncooled lasers. On the other hand, the system of DWDM WDM equipment needs to use cooled lasers, which utilize temperature regulation to ensure that the DWDM system has better performance, higher safety and longer service life. As a result, the DWDM WDM equipment consumes more energy than the CWDM WDM equipment that use uncooled lasers. Therefore, DWDM WDM devices consume more energy than CWDM WDM devices that use uncooled lasers.

4. Cost of CWDM WDM equipment and DWDM WDM equipment

The use of cooling laser technique to regulate the temperature in the system of DWDM WDM equipment increases the cost of using the system, as the temperature distribution is not uniform over a wide range of wavelengths. In addition, DWDM WDM systems are typically four to five times more expensive than CWDM WDM systems. However, with the increasing popularity of dense wavelength division multiplexers (DWDM), the price of DWDM optical modules is nearly 30-35% lower than the price of CWDM optical modules.

CWDM / DWDM wavelength division multiplexing equipment use, transmission comparison

1. CWDM / DWDM wavelength division multiplexing equipment use

Dense Wavelength Division Multiplexing (DWDM) devices are gaining popularity in the market due to the increasing demand for bandwidth and the great advancements in cost reduction. However, coarse wavelength division multiplexing (CWDM) equipment still maintains a high price advantage in connection rates below 10G and in short-range transmission scenarios. It is still the most viable device available for network construction at low data rates.

2. CWDM / DWDM wavelength division multiplexing equipment for hybrid transmission

CWDM WDM and DWDM WDM have their own advantages in OTN networks. CWDM WDM has the advantage of using lower-cost distributed feedback lasers without the need for cooling, as well as inexpensive passive filters. Therefore, CWDM WDM systems are now widely used in DWDM WDM systems. Although using CWDM technology in DWDM systems allows for cheaper optical modules, the larger channel spacing in CWDM channel limits the number of available wavelengths  in the system. The reduction in wavelengths also restricts the transmission capacity of the system, making it unable to fully match the performance of non-hybrid DWDM wavelength division multiplexing equipment.

The above analysis shows that in the future CWDM WDM and DWDM WDM devices will complement each other rather than replace each other.

How to set up CWDM/DWDM WDM equipment network case study

1. User needs

(1) Expansion and upgrade: connect 40ch DWDM WDM equipment to the 1550nm channel of 8ch CWDM WDM equipment to realize channel combination.

(2) Node requirements: the link is 70km long, and two nodes need to be added at 25km and 55km.

2. Program design

(1) Due to the limitation of bandwidth, it is not possible to expand with 40 DWDM channels , so Feiyu’s engineers use 8-channel C27-C34 DWDM WDM equipment to meet the application requirements of the customer.

(2)Due to the WDM equipment, the OADM(Optical Add-drop Multiplexer) and total fiber loss of ≤ 24dB, the output of a single channel can be amplified up to 8db using an amplifier, thus optimizing the link signal quality and reducing errors.

(3) Add 1ch OADM to the node to download and upload signals, respectively.

CWDM and DWDM program design


Until now, if a user’s WDM network required more channels, it must be converted to using DWDM WDM equipment. Because of the smaller wavelength spacing of DWDM WDM equipment, it allows a large increase in the number of channels, but the cost per channel also increases significantly. Therefore, users need to evaluate their future growth of their service capacity and decide whether to install less flexible CWDM WDM equipment at a lower initial cost or more flexible DWDM WDM equipment with higher initial costs.