9.2 Wavelength Division Multiplexing (DWDM and CWDM)
Wavelength division multiplexing (WDM) is a mechanism used to simultaneously transmit multiple streams of data on a single strand of fiber (or pair of fibers). Using WDM allows the customer to overcome the limitation of needing a separate fiber pair for each type of traffic that they want to transmit (unlike dark fibre). There are two different types of WDM—Dense WDM (DWDM) and Coarse WDM (CWDM).
DWDM, as its name suggests, supports more wavelengths because they are spaced together more tightly. Because the wavelength separation is so small, DWDM requires high-precision equipment that is typically very costly to purchase.
Figure 5. Wave Division Multiplexing (DWDM and CWDM)
This equipment provides filters to the light emitted from the data source, such as the switch, then multiplexes it onto the fiber strands along with the light from other sources. At the far end of the fiber strand, another multiplexer separates the signals from the different sources and forwards them to the appropriate destination equipment. Because the signals are also amplified by the DWDM system, they can travel much further than with native dark fibre. With the right hardware and media components in place, DWDM can potentially be extended to thousands of kilometers. The number of different wavelengths that can be supported by DWDM varies by vendor, but 32, 64, and even 128 are possible.
In summary, DWDM has the following key design considerations:
• Most typically used over short distances, such as 100 km to 200 km, but can extend further
• Dark fibre must be available
• Divides a single beam of light into discrete wavelengths (lamdas)
• Each signal can be carried at a different rate
• Dedicated bandwidth for each multiplexed channel. Approximately 0.4 nm spacing
• DWDM transponders can support multiple protocols and speeds (LAN, SAN, and other signals)
CWDM was introduced as a lower-cost alternative to DWDM. The functionality is almost identical to DWDM, except that the wavelengths (called channels in CWDM) are spaced further apart. The result of this is that fewer channels can fit on the fiber strands (maximum of 16 channels per fiber pair). Also, CWDM does not require as many components as DWDM, and those components are significantly less costly. However, a CWDM solution does not amplify and clean the signal as DWDM does, so the distance limitations for CWDM are significantly lower. In most implementations, the maximum distance is typically less the 100 km, but multiple signals, such as Ethernet and Fibre Channel, can be carried, giving it a significant advantage over native dark fibre.
