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Published on November 20, 2013

Overlaying denser technologies... A handy overview and guide

The need for bandwidth is very well known; however, providing this extra bandwidth remains a challenge. The easy answer is to install more fiber. In 2011, over 19 million miles of fiber was installed in the USA alone! To add to the confusion, higher speeds such as 10G, 40G and 100G are pushing more data per wavelength, and 400G in now in the planning stage.

Unfortunately, not everyone can afford new fiber. Not only that, they don’t need to if the available fiber has not been fully utilized. What’s more, ultra high speeds are only necessary when all other options have been exhausted.

Cable operators transmitting CWDM have access to an even cheaper way of gaining a lot of bandwidth. CWDM is characterized by unamplified short links (typically less than 80 km), and is low cost because each laser is 20 nm apart, and characterized by uncooled lasers with limited wavelength accuracy. Going from monowavelength to 8 CWDM wavelengths multiplies the bandwidth eightfold, but does not produce the same cost increment ratio. CWDM can be upgraded accordingly to 16 wavelengths (even though the lower eight will have higher attenuation, limited reach, and nonuniform attenuation, which may contribute to complexity costs). So, once the pipe contains all 16 wavelengths, what would be the best way to gain significant bandwidth for a comparatively small incremental cost?

DWDM over CWDM is a technology that is gaining a lot of ground. Because each CWDM wavelength is 20 nm apart (7 of those 20 nm are guardband to avoid overlap, meaning that each wavelength has a 13 nm window), and because the cheapest DWDM technology has approximately 0.8 nm wavelength spacing, this means that 16 DWDM wavelengths can easily fit into the bandwidth of a single CWDM wavelength. So scarifying one CWDM wavelength provides room for 16 new ones. As such, an eight-wavelength CWDM system would become a 23-wavelength CWDM system (7 +16), thereby multiplying the available bandwidth by almost three. Of course, this can be repeated, meaning that a second CWDM channel can be scarified to provide another 16 channels. Since DWDM technology is massively (and therefore cheaply) available in the C-Band area, this can be done with the 1530 nm, 1550 nm and 1570 nm CWDM wavelengths.

The central common fiber still serves CWDM, with two CWDM muxes gating this fiber, and DWDM feeding into the CWDM mux, while respecting the CWDM wavelength limits. Depending on the topology, some 40 or more customers can be served with a single CWDM fiber.

This network topology and technology will increase your revenues by maximizing the number of channels on your CWDM network, but as is the case with any fiber network, fiber characterization as set out in the ITU-T  G.650.3 standard applies to connector cleanliness, splices, connectors, end-to-end loss, continuity validation and optical return loss. With the proper testing strategy, you’ll have less downtime, less customer impact and faster time-to-revenue.

For additional information on this technology, check out our on-demand How to Deploy and Optimize Hybrid DWDM over CWDM Networks webcast.

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