If you need to add extra bandwidth between LAN switches, the most popular way is to string another fibre between them and configure up an etherchannel link. But what if you’re short of fibres? Within a building, chances are you can get more fibres blown through the risers easily enough, but what if adding fibres is going to mean digging up the road?

You may be leasing dark fibre from a telco to connect data centres together, or to link to a backup site. If you find you need more bandwidth, it could be extremely expensive for them to provide you another few pairs.

Dense Wave Division Multiplexers (DWDM) provide the means of giving you multiple optical wavelengths off of one fibre pair—typically 32 or 64—that you can use to connect to Gigabit ports on a switch or router, ESCON or fibrechannel. But these systems tend to be horrendously expensive, and are used either by Service Providers or very large enterprises.

But there’s a smaller, cheaper alternative that might be a more realistic option if it’s going to be just too expensive or disruptive to lay in new fibres. CWDM—Coarse Wave Division Multiplexing—is the baby brother of DWDM, and it’s simpler and cheaper to install.

Technical Details
Both CWDM and DWDM use wavelength division multiplexing (WDM) technologies to carry multiple data channels, each over its own unique wavelength, over one optical fibre. The difference between the two systems is the frequency that the signals are carried at, and the spacing between the wavelengths. CWDM technology uses a 20-nanometer (nm) wavelength grid and normally combines up to 8 or 16 wavelengths. DWDM technology uses a 1-nm wavelength grid and normally provides for 32 or more wavelengths. DWDM wavelengths are spaced much closer together than CWDM ones, and therefore require more precise—and therefore more expensive—lasers within the transponders. This higher price is offset by the fact that you get more wavelengths to use and, since the frequency range used by DWDM allows for optical signal amplification, greater distances.

DWDM systems tend to be deployed as dedicated optical platforms. They’re not the simplest of devices to install, since the transponder lasers need to be tuned at each end for the correct matching wavelengths—they really are just multiplexers for light, so think TDM but frequency slots rather than time ones.

CWDM devices are a lot simpler. The driving force behind these is cost, rather than performance, so you probably won’t get as full a range of data types supported, such as ESCON or FICON. To a large extent manufacturers use GBIC/SFP presentation, so for point to point topologies, you don’t have to buy dedicated platforms in which to install them, but can just slot them into existing LAN hardware.

Deployments
CWDM is used predominantly by enterprise customers to provide additional bandwidth between two or more sites in a campus or metro environment. Remote disaster recovery sites require huge amounts of bandwidth to allow for data replication, and if it’s real-time mirroring you need, where latency is important, then Ethernet is a good choice. Extending SANs, too, lends itself to multiple Gigabit connections.

Up to a maximum of 50 miles over single mode fibre in a point to point configuration can be supported with CWDM, depending on the dB loss of the fibre and connectors. As with all optical equipment, distances are given in terms of optical budgets rather than metres and kilometres, so you do need to get your existing fibre tested out first.

Point to point is the easiest deployment model. Unlike DWDM, where you tune the transponders, with CWDM, you buy the GBICs pre-set to the wavelength--typically called colour--you want. It doesn’t really matter which wavelengths you choose, as long as they’re the same at each end and you only have one wavelength-pair per fibre. You plug them into your existing switch or router, patch up to eight at a time to a mux that has your fibre to the other end plugged in, and away you go. In many cases, the GBICs are even different colours depending on wavelength, so there’s no excuse for getting them mixed up.

If you want to create a ring, plug the GBICs into the OADMs (optical add and drop multiplexers) that are part of the CWDM portfolio—you’ll get less overall distance—and drop off a couple of wavelengths at intervening sites. Because you can’t tune or change anything, there’s no real management capabilities, but you really don’t need any—it really is just done with lights and mirrors.

Several switch and optical manufacturers produce CWDM equipment. Among these are MRV Communications, which supports both Gigabit Ethernet and FibreChannel. Cisco’s CWDM range of GBICs, which fit into most of the Catalyst switch range, in conjunction with its separate muxes, are GigE only. Nortel Network’s CWDM products fit its Optera and Passport ranges, while a href='http://www.ciena.com/products/onlineedge/onlineedge.htm/' target='_blank' >Ciena’s Online Edge portfolio is perhaps one of the more comprehensive, offering support for SDH to be carried over a wavelength in addition to the standard LAN protocols.