It's time to revisit the last mile -- or make that the last 100 meters -- of cable hybrid fiber-coaxial (HFC) networks.

Nokia Corp. (NYSE: NOK) announced today that it's demonstrated symmetrical speeds of 10 gigabits per second over a coax connection. The proof-of-concept demo, also being shown at the Internet & Television Expo in Boston, is an implementation of Full Duplex technology. That means that both downstream and upstream data signals are sharing the same spectrum frequency bands, extending cable's upstream capacity to the same maximum as its downstream capacity: 10 Gbits/s.

Calling the prototype solution XG-CABLE, Nokia says its demo delivers symmetrical 10-Gig speeds over a 100-meter length of coax using 1.2 Ghz of spectrum. It's also compatible with DOCSIS 3.1.

Full Duplex technology has been in the spotlight since CableLabs announced in February that it would examine the ins and outs of spectrum sharing as part of an "innovation phase" research project. (See CableLabs Makes Symmetrical Multi-Gig Push and What If Cable Does the Full Duplex?)

Nokia isn't working with CableLabs yet, but according to David Eckard, CTO of fixed networks, the company has engaged with several Tier-1 operators in both North America and Europe to explore the possibilities around deploying Full Duplex technology in the future. Eckard acknowledges it won't be easy.

"This Full Duplex operation is extremely challenging, with challenges both in the analog domain and also in the digital," says Eckard.

However, the concept is very similar to one that Nokia has already introduced in copper network environments with DSL. And importantly, Nokia can bring its expertise both in echo cancellation and in interference management from the DSL world into the cable realm.

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Nokia isn't the only company placing bets on Full Duplex. Kumu Networks , which is running Full Duplex trials with wireless carriers, says it was approached by CableLabs about applying the technology to cable networks about a year ago, and the California-based company is the one actively working with CableLabs today on its Full Duplex research.

Joel Brand, vice president of product management at Kumu Networks, also has a great way of describing in layman's terms just how Full Duplex works.

"[Imagine] you have a device that sits between your mouth and your ear," Brand explained in a recent interview, "and when you speak, that device applies the same principle of the noise-cancelling headset, and it cancels the noise that you're speaking such that you don't hear what you're saying. If you don't hear what you're saying, you can listen better to everything that comes from the environment."

That's how Full Duplex allows both downstream and upstream traffic to occupy the same frequency bands at the same time.

Figure 1: Illustration of Nokia Bell Labs XG-CABLE demonstration

In Nokia's case, the company can prove that Full Duplex works over coaxial cable, but it's only achieved symmetrical 10-Gig speeds in a scenario where fiber is pushed all the way to the tap, and then coax connects the last 100 meters to a single endpoint. In a more typical scenario, where fiber runs to the last amplifier in a network and then coax connects to multiple endpoints off of that amplifier, Nokia can still show significant performance improvements, but downstream speeds top out at about 8 Gbit/s, and upstream speeds max out at about 7.5 Gbits/s.

Real-world deployments of Full Duplex technology in a cable network are still years away. In addition to further hardware and software development and standardization work, operators will have to drive fiber deep into their networks in order for Full Duplex to be practical.

"This will be coupled to deep fiber and probably be coupled to some type of distributed access architecture as well," says Eckard.

Realistically, Full Duplex probably won't make its cable debut until about the year 2020. Between now and then, there's still a lot of work to be done. (See Don't Bet on Full Duplex Yet.)

— Mari Silbey, Senior Editor, Cable/Video, Light Reading