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Comcast Full Duplex DOCSIS trial pumps out 4-Gig symmetrical speeds

Offering proof that Full Duplex DOCSIS (FDX) and DOCSIS 4.0 technologies have taken a big step closer to prime time, Comcast said recent lab trials powered by a new system-on-chip (SoC) from Broadcom delivered symmetrical speeds of more than 4 Gbit/s.

That technical achievement also puts Comcast, and possibly other cable operators, a step closer to DOCSIS 4.0, a new CableLabs specification that will be capable of delivering up to 10 Gbit/s downstream and about 6 Gbit/s in the upstream. DOCSIS 4.0 also ties into "10G," an industry-led set of tools focused on supporting symmetrical 10-Gig speeds on HFC, FTTP and wireless access networks along with advanced security and low-latency capabilities.

Comcast's lab trial, conducted in Denver, is an early proof point that FDX has emerged as a viable option for DOCSIS 4.0. FDX, a technology with roots going back to DOCSIS 3.1, allows cable operators to run both upstream and downstream traffic simultaneously in the same block of spectrum using echo cancellation, a technique that mitigates interference when the network makes use of the same spectrum at the same time for both upstream and downstream transmissions. FDX also paves the way for multiple sub-bands (one OFDM downstream channel and one or two upstream OFDMA channels) to occupy an "FDX band" that resides at 108MHz-684MHz.

Another D4.0 tech option, called Extended Spectrum DOCSIS (ESD), also aims to expand the amount of capacity used for the upstream, but uses dedicated spectrum for upstream and downstream traffic and requires relatively large guard bands.

And while Full Duplex DOCSIS tends to focus on hybrid fiber/coax (HFC) plant built out to 1.2GHz, ESD is more focused on a move to 1.8GHz.

Comcast will of course have access to both options for DOCSIS 4.0, but it has definitely emerged as the primary cable operator champion and driver for FDX.

"The advantage of FDX is that it is very efficient from a bandwidth standpoint," said Elad Nafshi, Comcast's SVP of next generation access networks. "It gives us a lot of flexibility in terms of how we best utilize the spectrum that we have out there."

Bringing FDX to life

The successful lab test trial and the utilizing of the new DOCSIS 4.0-facing SoC from Broadcom is the result of many months of effort.

"Today is a kind of kind of a culmination of all that work, in partnership with Broadcom, to put the concepts in the DOCSIS 4.0 specs into action and start to bring FDX to life in a big way," Nafshi said, calling the lab trial a "meaningful milestone" for FDX and DOCSIS 4.0. "The concepts here are laying out the foundation of the technology delivery."

Comcast said the lab demo was based on a typical Comcast digital network built to 1.2GHz. That setup included support for a an upstream band of 5MHz-85MHz while also validating symmetrical DOCSIS 4.0 operation across a full FDX band residing in a 108MHz-684MHz block of spectrum, and a full downstream signal load present and extending up to 1.2GHz.

Comcast also characterized the trial as a "foundational step" toward validating FDX technology implementation against the DOCSIS 4.0 specs. Early on, FDX has been limited to HFC plant with an N+0 architecture (a deep fiber network, with zero amplifiers between the home and the node). But Comcast stressed that it will continue to test and develop FDX for multiple HFC topologies, including support for HFC networks with amplifiers.

"This is just the beginning," Nafshi said of the performance seen in the lab trial. "We believe we can deliver even greater speeds as we continue to mature the technology."

Broadcom's involvement is a big deal

Word of the trial also confirms that Broadcom has put its weight behind DOCSIS 4.0. Broadcom's commitment to D4.0 was once a big question mark. But that changed, according to multiple industry sources, when Broadcom agreed to a multi-million-dollar non-recurring engineering (NRE) deal to take on the DOCSIS 4.0 project. Comcast declined to comment on the existence of an NRE agreement with the chipmaker. Broadcom has previously ignored inquiries from Light Reading about the purported NRE agreement.

The Broadcom chip used for the lab test is an SoC for a DOCSIS 4.0 remote PHY node. It's not clear yet how far along Broadcom is with a chip for DOCSIS 4.0 consumer premises equipment (CPE). MaxLinear, which last year struck a deal to acquire Intel's Home Gateway Platform Division, is also expected to play a big role in DOCIS 4.0 silicon development for both the network and CPE.

Nafshi said the Broadcom chip used in the FDX trial is the real deal, and not some proof-of-concept gizmo.

"This is not vaporware. This is the actual SoC," he said, noting that Comcast will continue to work "in close partnership" with Broadcom to help mature the platform.

DOCSIS 3.1 still has legs

Comcast is not yet talking next steps with FDX or hazarding a public guess on when the technology might be ready for commercial deployment, or when it might initiate DOCSIS 4.0 network upgrades.

Meanwhile, Comcast and other cable operators are still stretching the limits of their widely deployed DOCSIS 3.1 networks. Last fall, Comcast tested the delivery of symmetrical 1.25Gbit/s speeds on a live D3.1 network in Jacksonville, Florida. That trial tested D3.1 in tandem with remote PHY nodes, Comcast's virtual cable modem termination system (vCMTS) and a "high-split" that expanded the upstream spectrum to the range of 5MHz-204MHz.

And just this week, Virgin Media announced a DOCSIS 3.1 trial that delivered downstream speeds up to 2.2 Gbit/s in Southampton and Manchester using a new Hub 4 router that comes standard with the operator's 1-Gig offering.

"As we're working to build and mature DOCSIS 4.0, DOCSIS 3.1 still has tremendous benefits," Nafshi said.

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— Jeff Baumgartner, Senior Editor, Light Reading

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