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A Comparison between CWDM and DWDM

Time:2016-10-26 Click:1200

Transporting a single data stream through an optical fiber can be costly and inefficient. Wavelength Division Multiplexing (WDM) can transmit multiple, independent data streams through a single optical fiber to better utilize the bandwidth of the given optical fiber. Each data stream adopts a different wavelength of light (also know as lambda λ) and the data streams travel through a single fiber. With WDM, multiple cloud IaaS tenants can share the same optical fiber. Closely spaced wavelengths provide a higher number of channels (or bandwidth) per fiber. According to the wavelength spacing, WDM technology can be divided into two parts: Coarse WDM (CWDM) and Dense WDM (DWDM).

For many SAN engineers, DWDM is the ideal SAN extension mechanism since it provides very high scalability, very low and predictable latency, and moderately long distances. With regeneration, the point-to-point distance for a DWDM link can be extended further. However, additional latency is incurred during the regeneration process. The maximun distance for a DWDM link is dependent on the components used along the link, such as optical amplifiers, DCUs, VOAs, OADMs, and so on.

Tips: As a rule of thumb, the maximum point-to-point distance is about 200 km with amplification only (i.e., no regeneration).

CWDM, just like DWDM, uses multiple light wavelengths to transmit signals over a single optical fiber. Nevertheless, the two technologies vary in many ways:

  • Fiberstore CWDM MUX/DeMUX1. CWDM uses a 20-nm wavelength spacing that is much wider than the 0.4-nm for DWDM. The wider wavelength spacing in CWDM means lower product development costs. This is one reason why CWDM is less costly than DWDM.

Note: Most CWDM devices (i.e., CWDM MUX/DeMUX) operate in the 1470-nm to 1610-nm range. The frequency grid for DWDM and the wavelength grid for CWDM systems are defined by the International Telecommnications Union (ITU) standards G.694.1 and G.694.2, respectively.

  • 2. CWDM provides a maximun of 8 lambdas between two CWDM multiplexers over a single fiber pair as compared to DWDM, which support up to 32 lambdas (based on 0.8-nm or 100-GHz wavelength spacing) over a single fiber pair.

Note: Some long-haul DWDM sytems can support up to 160 lambdas per fiber pair.

  • Fiberstore CWDM SFP3. Each CWDM channel uses a specialized GigaBit Interface Converter (GBIC) or small form-factor pluggable (SFP) transceivers. These specialized transceivers are commonly known as colored GBIC and SFP. Each CWDM channel uses a different “color” GBIC or SFP because each lambda represents a different color in the spectrum. In this case, the native GBIC or SFP in the client devices are substituted with a colored GBIC or SFP. You can buy the most cost-effecitve CWDM SFP from Fiberstore.
  • 4. CWDM multiplexers are usually passive (i.e., not powered) devices containing a very accurate prism to multiplex eight separate wavelengths of light along a single fiber pair. Passive devices can’t generate or repeat optical signals.
  • 5. No amplification is possible with CWDM because CWDM uses wavelengths that can’t be amplified with EDFA amplifiers. Therefore, the maximum distance for a CWDM link is approximately 100 km.

Note: The Cisco ONS 15501 EDFA, which has a wavelength range of 1530 nm to 1563 nm. can only amplify two signals (1530 nm and 1550 nm) out of the eight signals that are multiplexed onto the fiber pair.

  • 6. CWDM provides an alternative solution to DWDM for low-latency and high-bandwidth requirements associated with synchronous replication applications. However, DWDM is more scalable than CWDM. DWDM also has longer diatance capacity than CWDM because DWDM can be amplified (using a DWDM EDFA). The main benefit of CWDM is its low cost. It is a cheaper solution than DWDM. In other words, CWDM is optimized for cost, while DWDM is optimized for bandwidth. For enterprises that have access to dark fiber and have only limited scalability requirements, CWDM is a relatively inexpensive way to achieve low-latency and high-bandwidth interconnections between DCs. The CWDM implementation also results in less complex installation, configuration, and operation as compared to DWDM.
  • 7. CWDM can also be used to enable multiple ISL connections between the switched over a single fiber since it requires less fiber for interconnecting two metro sites. The same benefit applies to port channel implementation between the switched.

In short, DWDM is a solution that provides a higher number of connections and longer reach, or extension, at a much higher cost which CWDM is a more cost-effective solution for metro or campus solutions where the distance is limited.

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