Many Wavelengths to Serve You

What happens when you max out the capacity of your network but still need more bandwidth? You have to go with a faster network, right? You may have to go with faster NIC (Network Interface Cards) and Ethernet switches for every node. What you don't want to do is run out of capacity on your lines. When you've got all you are going to get out of your structured wiring system, it gets expensive, really expensive, to upgrade that network. But what if you could just add some more signals into those same cables and multiply the bandwidth 32 or even 160 times? If your network is fiber optic based, you can do just that.

The magic is in a process called WDM or Wavelength Division Multiplexing. We think of fiber optic networks as having a beam of light at one end and a receiver at the other. The transmission electronics modulates a laser beam with our data stream and a photodiode at the other end converts the modulated light back into electrical pulses. What WDM does is send multiple beams down the same fiber optic cable in parallel. Two beams gives you 2x the capacity, four beams gives 4x, and so on. This process is so well developed that you can send 16 to 160 separate beams simultaneously to support enormous bandwidths.

Fiber optic networks are much faster than copper-based networks to begin with. Gigabit Ethernet over copper can be a challenge within an office environment. Forget long haul. T3 is the highest of the copper T-Carrier systems at 45 Mbps and is usually only deployed over a few miles. T1 lines can reach out to just about any distance using regenerators, but the bandwidth is limited to 1.5 Mbps. Compare that to typical long haul fiber bandwidths of OC-48 at 2.5 Gbps or OC-192 at 10 Gbps. Some carriers are upgrading their core rates to the OC-768 level of 40 Gbps.

In the world of copper transmission lines, frequency division multiplexing (FDM) is the equivalent of fiber optic WDM. FDM is what Cable TV companies use. Each channel has its own carrier and occupies its own narrow band of frequencies in the spectrum. More than 100 standard TV channels of 6 MHz each can be carried on a typical cable system. Some of these channels are assigned to broadband Internet services rather than television transmissions.

Wavelengths are also different frequencies which, in the optical (including infrared) parts the spectrum, represent different colors. They are also called lambdas after the Greek letter representing wavelength. Light split from a prism demonstrates that many different colors can exist simultaneously in a single beam without interference.

There a two general categories of WDM. CWDM or Coarse Wavelength Division Multiplexing uses up to 16 different wavelengths within an individual single mode fiber strand. DWDM or Dense Wavelength Division Multiplexing uses up to 160 wavelengths in a fiber strand and requires more precisely controlled lasers. But at 10 Gbps capacity per wavelength, a system with 160 wavelengths offers well over a Terabit/second bandwidth.

WDM schemes are found in long haul carrier core networks where the cost of installing hundreds or thousands of miles of fiber optic cable far exceeds the cost of new electronics to increase capacity by adding more wavelengths on the same fiber. WDM or CWDM is also installed on dark fiber when customers need the extra capacity in metropolitan (MAN) networks.

Thanks to WDM and competitive carrier build-outs, fiber optic bandwidth is more affordable than ever. That's especially true for the new Ethernet over fiber services available to businesses in major metropolitan areas.