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DOCSIS

Cable's new upstream tech must break through the noise

CABLE NEXT-GEN EUROPE DIGITAL SYMPOSIUM – Deploying DOCSIS 3.1 channels that employ OFDMA (orthogonal frequency-division multiple access) modulation presents cable operators with the proverbial opportunity and challenge. They can take advantage of OFDMA's spectrum efficiency to rapidly beef up the capacity in the upstream, but they must also be wary of noise, particularly in lower parts of the spectrum, that can impair performance.

OFDMA, which relies on blocks of tiny subcarriers, is more efficient than older, DOCSIS 3.0-era technologies that bond together multiple 6MHz-wide channels. OFDMA represents potential improvements and enhancements to the D3.1 upstream, but it's a technique that is still being ironed out.

OFDMA has emerged as a problematic area, as it is susceptible to noise, partly because DOCSIS 3.1 modems are capable of changing modulation levels and sometimes hop into lower portions of the spectrum where noise can present issues, Brady Volpe, president and founder of The Volpe Firm, a tech consulting firm for cable ops and telcos, explained here Tuesday on the "DOCSIS Maximus" panel discussion.

That can "lock up" a modem or cause other issues, particularly in modems where the code base is not fully developed for OFDMA, Volpe said. "But we're getting better in this area" as that code continues to be hardened, he added.

The early lesson, he said, is to utilize OFDMA up higher in the return path spectrum, and away from the range of 5MHz-20MHz, "where it's really noisy."

That scenario also illustrates the need for cable operators to keep their plant clean and as free of noise as possible so they can squeeze everything they can out of that valuable spectrum. "Clean up your plant, and you'll get the speed you want," said Robert Flask, senior product line manager at Viavi Solutions.

Those challenges have not stopped some operators from testing or deploying OFDMA upstream spectrum on their DOCSIS 3.1 networks.

Portugal's NOS started to test OFDMA last year and has since extended support for it across the network. Utilizing OFDMA was a "quick way" to get more upstream capacity to customers that needed it during pandemic-fueled lockdown periods, said Paulo Jorge Rosaria, NOS's head of fixed-access technology.

About 35% of NOS customers have a DOCSIS 3.1 gateway capable of taking advantage of the OFDMA spectrum, he added.

New mindset needed for OFDMA

Rosaria suggested that cable operators will need to alter their mindset with OFDMA, as it behaves and needs to be managed a lot differently than what operators have been doing with older DOCSIS 3.0-based upstream tech. "OFDMA is completely different," he said, noting that engineers will need to get comfortable with the fact that it's "normal" for a D3.1 modem to change its modulation profile – sometimes higher and sometimes lower – based on evolving network conditions.

Jeff Heynen, VP of broadband access and home networking at Dell'Oro Group, also pointed out that another issue operators face with OFDMA is their sometimes still-large installed base of DOCSIS 3.0 modems that use ATDMA (Advanced Time Division Multiple Access). Those modems will need to be cleared out and replaced with D3.1 devices to help justify the cost of adding OFDMA channels, he explained.

Panelists also discussed when or if European cable ops would pursue DOCSIS 4.0, a new set of CableLabs specs aimed at delivering up to 10 Gbit/s downstream and multi-gig upstream speeds, along with enhanced security and lower latencies, on the HFC network.

Alexander Adams, managing director and CSO, Adams Network Engineering GmbH & Co.KG, said he is increasingly convinced that more operators in Europe will go for DOCSIS 4.0, pointing to Vodafone in Germany as an operator that is at least weighing the idea.

But a challenge on the horizon is the need for those operators to switch in new taps and passives that can handle the higher frequency range – up to 1.8GHz – supported by DOCSIS 4.0, an expense and logistical task that some operators could balk at, he said.

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— Jeff Baumgartner, Senior Editor, Light Reading

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