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Cable Tech

Full Duplex DOCSIS amplifier chatter heats up

Full Duplex DOCSIS (FDX) is one of two DOCSIS 4.0 flavors available to cable operators, with Extended Spectrum DOCSIS, or ESD, being the other one. But the original vision for FDX, which calls for a fiber-deep HFC network with zero amplifiers between the node and the home, has made it a non-starter for the vast majority of cable operators.

FDX is attractive in part because it runs in the same spectrum allocation of 1.2GHz that DOCSIS 3.1 networks use today. But pivoting to a node+0 architecture is an expensive proposition. And while Comcast is the most widely known advocate of FDX with N+0, other options are developing that would enable operators to deploy FDX in N+1 or N+2 architectures.

One such option being placed under the microscope is the FDX Amplifier. Rather than having critical echo cancellation technology that allows upstream and upstream traffic to run in the same block of spectrum packaged into the node, that interference-cleaning technology would instead be housed in a new, more powerful class of amplifier tailored to an FDX system.

Cable tech vendors have been relatively quiet about FDX Amplifier development activity (Cisco had been an advocate of it, but halted investment in FDX technology in 2019). But the concept continues to be a major point of discussion among industry engineers. It came up again at the recent "LiveLearning" webinar hosted by Light Reading in conjunction with SCTE.

David Whitehead, senior director of cable solutions at Harmonic, acknowledged that the concept is circulating, as it would mean FDX would no longer have to be tied solely to N+0 architectures. That, he said, could make FDX increasingly attractive to cable operators.

Jan Ariesen, CTO of Technetix, explored the concept further, holding that the use of FDX Amplifiers would enable communications to flow in both the upstream and downstream direction on the same frequency.

Great! But is it real?

But is there a product under development? Stay tuned. "We are checking out what you can do with echo cancellers … It's technology that's being researched at the moment," Ariesen said.

This month's SCTE Cable-Tec Expo would seem a good spot for suppliers to tout what they are doing in the field of FDX Amplifiers. Analyst Jeff Heynen, vice president of broadband access and home networking at Dell'Oro Group, suggests that multiple suppliers are trying to come up with an FDX Amplifier solution.

But they'll all face a significant technical challenge to pull it off. "I don't think the integration of echo cancellation is a trivial task, not only because all the elements in the outside plant have to be able to support it, but also because the cable modems have to do it, as well," Heynen explained in an email exchange.

The good news is that there's time for FDX Amplifier technology to develop, he added, noting that operators are just now getting started on upgrading their taps and passives for the "first stage" of their DOCSIS 4.0 prep work.

An industry source familiar with DOCSIS technology and product roadmaps told Light Reading that the FDX Amplifier is a true initiative. That source said Comcast has connected with Broadcom to develop the silicon for it after the operator initially attempted FDX using an FPGA-based solution, but later concluded that an FPGA approach would not meet all of its power and space requirements.

Ariesen said during the webinar that there are other options being explored that don't rely on echo cancellers. One is the use of small, Direction Neutral Amplifiers (DNAs) that don't use filters. Technetix, he said, already has a DNA that could suit an FDX environment. "If people wanted to test [that], the smaller amplifiers already exist," he said.

Weighing the options

The FDX Amplifier discussion feeds into a broader debate about the technical direction for DOCSIS 4.0, a new set of specs that will enable cable operators to deliver multi-gigabit speeds on their widely deployed HFC networks. DOCSIS 4.0 requires operators to migrate to a distributed access architecture (DAA), but there's still the option to go with either FDX or the Extended Spectrum DOCSIS (ESD) flavor of DOCSIS 4.0.

Each approach has its pros and cons.

The use of echo cancellation for FDX adds complexities, but the approach uses the same 1.2GHz allocation out there today, so many of the HFC's existing components can be used. "That means we can deploy Full Duplex DOCSIS without changing out the existing taps in the HFC infrastructure," Harmonic's Whitehead said.

Extended Spectrum DOCSIS (ESD), also referred to as Frequency Division Duplex (FDD), keeps the upstream and downstream traffic separate, enabling more capacity to be dedicated to the upstream. It's generally viewed as the simpler option, since it continues to keep upstream and downstream spectrum separated. But ESD also envisions a spectrum upgrade to 1.8GHz, so that means operators will need to deploy new taps and amps.

ESD will also drive higher power consumption as services are moved higher in the spectrum, Ariesen explained.

The use of higher power amps is one possible remedy, but it's considered too power hungry to be feasible, he said. One idea he advocated is the use of a Distributed Gain Architecture (DGA), which adds in smaller amplifiers on the line. But Ariesen warned that one downside is that cable operators will have to "recalculate" the network.

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— Jeff Baumgartner, Senior Editor, Light Reading

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