Cable networks are already starting to match the gigabit download speeds offered by fiber-based Internet services, but they still lag far behind on upload throughput. New technology could change that situation, bringing cable's theoretical speeds up to 10Gbps for both downloads and uploads.

The cable industry R&D consortium CableLabs announced a plan for full duplex technology in February, and the Nokia-owned Bell Labs yesterday said it has achieved 10Gbps symmetrical speeds in the lab in a "world-first" demo.

"Demonstrating for the first time that the concept is valid and achievable, the XG-CABLE test used point-to-point cable topologies to deliver 10Gbps symmetric data speeds over coaxial cable using 1.2Ghz of spectrum," Nokia said in its press release yesterday. With a point-to-multipoint deployment, Nokia's results were nearly 8Gbps downstream and 7.5Gbps upstream.

The technology is still in the proof-of-concept stage and requires fiber to be built most of the way toward homes, relying on cable for the final stretch. This isn't a huge barrier because cable networks already use a lot of fiber. "By leveraging the XG-CABLE technology, operators can effectively use existing HFC [hybrid fiber-coaxial] cables over the last 200 meters to provide upstream speeds never before achievable due to the limited spectrum available," Nokia said.

The prototype doesn't support a full 10Gbps when fiber is 200 meters from homes, though. Nokia's 10Gbps transmissions were achieved over 100 meters of cable.

The famous Bell Labs was founded at AT&T in 1925 and has gone through a handful of owners, most recently being shifted to Nokia when that company acquired Alcatel-Lucent in January this year.

Nokia said its XG-CABLE implementation "can easily integrate" with the full duplex work that CableLabs is doing for the DOCSIS (Data Over Cable Service Interface Specification) 3.1 spec. This would help cable operators upgrade their networks without breaking the services offered to existing modems; customers could keep older modems until they want faster speeds only supported by new equipment.

For cable customers who feel limited by upload speeds, the full duplex aspect is what's most important here. Comcast has started offering gigabit download speeds over cable, but the service only comes with 35Mbps uploads. Comcast's only symmetrical gigabit offering uses fiber-to-the-premises, which is available to fewer homes in Comcast's footprint and is a lot more expensive than cable services.

A Nokia white paper (download link here) describes how the company is enabling symmetrical speeds. Current DOCSIS technologies use Frequency Division Duplexing to separate upstream and downstream traffic into different frequencies, with the upstream side being given less bandwidth. At most, uploads in DOCSIS 3.1 are about 10 percent as fast as downloads.

Full duplex allows the upload and downloads to use the entire available spectrum at the same time instead of being split into separate frequencies. "Using the spectral efficiency of DOCSIS 3.1 as a reference, each 100MHz of bandwidth yields approximately 800Mbps downstream and upstream, giving a total capacity of 10Gbps in each direction [when using 1.2GHz of spectrum]," Nokia said. Though the tests were in a lab, Nokia said it used "network elements and cabling comparable to a typical HFC network."

But moving to full duplex isn't a simple change. Transmissions between a customer's cable modem and a cable company's facilities create an "echo" when they're traveling in opposite directions. A large echo can exceed the strength of the useful signal, so echo cancellation is needed to make communication possible. Nokia said its echo cancellation technique "is comparable to noise cancelling headphones. XG-CABLE measures the echo experienced by the transceiver and properly compensates for it."

Full duplex technology also must account for "interference between concurrent signals from different cable modems in close proximity, e.g., cable modems served by the same tap or splitter," Nokia said. To solve this problem, a Cable Modem Termination System (CMTS) located in cable company facilities has to manage the upstream and downstream transmissions from all cable modems "using dynamic allocation of time slots, frequency slots and transmit power based on rate demands."

The white paper acknowledges that Nokia's approach requires fiber to be built deeper into networks, but it notes that "many operators are already pursuing a deep-fiber strategy."