The Very Coherent Case for Coherent Networking

Carl Weinschenk

A constant – as certain as the fact that there are lots of political ads running in Ohio and Virginia – is that more bandwidth is needed. That’s true in wireless networks that connect to devices and it is true in the core of the network.

The most common approach to optical core networking is to have lasers flash on and off rapidly – very rapidly – to represent ones and zeros. Doing this on many wavelengths simultaneously has enabled the telecommunications infrastructure to keep pace with radically accelerating demands.

This approach may be reaching its limits, however. Another technique is coherent optical networking, which could be the most efficient way of transmitting at 100 Gigabits per second (Gbps), which is the state of the art (or, more accurately, the state of the speed).

The basic idea is explained in this Ciena video (the explanation starts at about the 45-second mark, though no explanation is given for the out-of-place music). Essentially, instead of turning the laser on and off, coherent enables data to be embedded at different amplitudes and phases (areas of the signal).


Lightreading looks at the technology in the context of undersea cables. The ability to radically increase capacity is especially important in this realm, since it can spell the difference between changing electronics at the end points or laying an entirely new cable.

The story quotes Mike Guess, a vice president at Infinera:

The introduction of coherent capabilities has enabled the industry to upgrade from 10-Gbit/s wavelengths, which had been the standard for more than a decade, straight to 100 Gbit/s. Infinera's Guess says the subsea industry has been wanting to upgrade capacity for a long time, "but the problem was making [new systems] reach as far as 10G." Coherent, he says, "has the performance base to make 100G practical."

The story says that coherent also is important for landlubbing networks.

There are announcements on the coherent networking front. Half a world away, for instance, Huawei announced that the Gansu Administration of Radio, Film and Television will deploy China’s first network combining 100 Gbps coherent technology with the vendor’s optical transport network (OTN) platform. The network will support video-on-demand, high-definition video, broadband and other services.

Earlier this month, Ciena announced that, partnering with LightRiver Technologies, it will deploy packet networking and packet-optical transport equipment for Sovernet Fiber Corp. The press release has the details on the deal, which will include Ciena’s coherent technology. Sovernet, a subsidiary of Atlantic Tele-Network, provides telecom service in northern New England.

As Ciena Senior Vice President of Marketing Rick Dodd says in the YouTube video, coherent technology allows the lasers to be tuned like a radio, instead of being turned on and off like a flashlight. That is an effective analogy, and one that suggests that there is a very coherent argument for coherent technology.



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