Take Comcast’s announcement today that it successfully completed a live field trial of 1 terabit per second (Tbps) optical transmission over a distance of nearly 1,000 kilometers. Unlike with previous optical transport technology, that 1 Tbps connection did not fit into a single wavelength based on the wavelength grid that traditionally has underpinned DWDM transmission.
Instead the field trial, which used equipment from Ciena, was based on a flexible grid. With this approach, network operators can vary the distance between wavelengths, now known as “channels” or “carriers,” and combine multiple channels together to create a single logical path known as a “super channel” or “super carrier.” You could think of it as a really advanced form of bonding for wavelengths.
In an email to Telecompetitor, a Comcast spokeswoman said the field trial used “a 200 GHz super channel utilizing five flexgrid channels.” She also noted that “this is 250% more spectrally efficient than 100 Gbps is today.” And she confirmed that “there was no regeneration – we utilized variable gain EDFAs and Raman amplification on this path.”
Ciena isn’t the only company to use the flexible grid concept to reach data rates above 100 Gbps. Other companies that have been exploring this concept include Bell Labs, Huawei, Infinera, NEC, and probably others as well. In laboratory tests and demonstrations, companies such as these have reached speeds of 1 Tbps using various super channel options over a range of distances.
But Comcast and Ciena’s terabit trial is rather unique in that it carried live data traffic over an existing commercial network that also carried customer traffic over 10G, 40G and 100G wavelengths. According to a press release issued jointly by Comcast and Ciena today, all of the wavelengths co-existed on a mix of flexible and 50 GHz-spaced fixed grid channels.
Tweaking the wavelength grid as Ciena and others are doing also may require rethinking the reconfigurable optical add/drop multiplexers (ROADMs) that play a key role in directing optical network traffic. Comcast’s trial used a flexible grid ROADM.
As the Comcast spokeswoman explained, “Standard ROADMs are set at a fixed ITU grid spacing of 50 GHz. The flexible grid ROADMs allow you to adjust the spacing to accommodate variable sized wavelengths down to 12.5 GHz. You get flexibility between distance and speed.”
It’s also worth noting that the Comcast deployment used ELEAF fiber. Today’s announcement did not indicate how widely deployed this fiber is or how the use of a different type of fiber might impact results.
Comcast has no specific plans to deploy 1 Tbps service, the spokeswoman said. But considering how fast data network traffic is growing, it’s a smart move for the company to begin to explore options such as these before the situation becomes critical.