CWDM

Course Wave Division Multiplexing

From: Storage Networking Protocol Fundamentals, by James Long

The operating principles of CWDM are essentially the same as DWDM, but the two are quite different from an implementation perspective. CWDM spaces wavelengths farther apart than DWDM. This characteristic leads to many factors (discussed in the following paragraphs) that lower CWDM costs by an order of magnitude. CWDM requires no special skill sets for deployment, operation, or support. Although some CWDM devices support non-transparent service,transparent CWDM devices are more common.

Transparent CWDM involves the use of specialized gigabit interface converters (GBIC) or small form-factor pluggable GBICs (SFP). These are called coloredGBICs and SFPs because each lambda represents a different color in the spectrum. The native GBIC or SFP in the client device is replaced with a colored GBIC or SFP. The electrical interface in the client passes signals to the colored GBIC/SFP in the usual manner. The colored GBIC/SFP converts the electrical signal to an optical wavelength in the WDM window instead of the optical wavelength natively associated with the client protocol (typically 850 nm or 1310 nm). The client device is connected to a transparent interface in the CWDM device, and the optical signal is multiplexed without being shifted. The colored GBIC/SFP negates the need to perform wavelength shifting in the CWDM device. The network administrator must plan the optical wavelength grid manually before procuring the colored GBICs/SFPs, and the colored GBICs/SFPs must be installed according to the wavelength plan to avoid conflicts in the CWDM device.

To the extent that client devices are unaware of the CWDM system, and all link-level operations are conducted end-to-end, transparent CWDM service is essentially the same as transparent DWDM service. Transparent CWDM mux/demux equipment is typically passive (not powered). Passive devices cannot generate or repeat optical signals. Additionally, IOAs operate in a small wavelength range that overlaps only three CWDM signals. Some CWDM signals are unaffected by IOAs, so each CWDM span must terminate at a distance determined by the unamplified signals. Therefore, no benefit is realized by amplifying any of the CWDM signals. This means that all optical signal loss introduced by CWDM mux/demux equipment, splices, connectors, and the fiber must be subtracted from the launch power of the colored GBIC/SFP installed in the client. Thus, the client GBIC/SFP determines the theoretical maximum distance that can be traversed. Colored GBICs/SFPs typically are limited to 80 km in a point-to-point configuration, but may reach up to 120 km under ideal conditions. Signal monitoring typically is not implemented in CWDM devices.

Tip

IOAs may be used with CWDM if only three signals (1530 nm, 1550 nm, and 1570 nm) are multiplexed onto the fiber.