CableLabs has published physical layer specifications for a DOCSIS 4.0 enhancement that extends spectrum on hybrid fiber/coax (HFC) networks to 3GHz and pushes downstream speeds to 25 Gbit/s.

The specs, currently called the DOCSIS 4.0 "operational annex," arrive roughly a year after outgoing CableLabs CEO Phil McKinney announced the initiative at last year's SCTE TechExpo in Washington, DC. The milestone lands at a moment when cable operators are still split on how far to push their HFC plant: some are committing to DOCSIS 4.0 now, others are buying time with DOCSIS 3.1+, and a third group is jumping straight to fiber-to-the-premises.

What the numbers say

The baseline DOCSIS 4.0 specification supports spectrum up to 1.8GHz. At recent CableLabs interops, current D4.0 technology has demonstrated about 14 Gbit/s downstream as the platform is optimized for progressively higher modulations. The new annex nearly doubles the spectrum ceiling to 3GHz and targets 25 Gbit/s down.

The existing D4.0 specs offer two architectural options, and both survive in the new annex. The first, Extended Spectrum DOCSIS (ESD), keeps upstream and downstream traffic in separate, dedicated spectrum blocks. The second, championed by Comcast, lets both directions share a Full Duplex (FDX) band.

CableLabs says the 3GHz specs define downstream requirements for nodes, remote PHY equipment and cable modems, and also cover the MAC and OSS layers. The downstream channels remain 192MHz-wide OFDM carriers. The specs give CPE and network equipment suppliers — including vendors of remote PHY devices and cable modem termination systems (CMTSs) — a technical roadmap aimed at keeping multi-vendor interoperability intact.

The commercial calculus

If operators and suppliers judge the optional annex financially and technically feasible, the shift to 3GHz could extend the life of HFC networks and delay the need to overbuild parts of the plant with PON or commit to widescale fiber upgrades.

That analysis is already playing out in the early DOCSIS 4.0 era. Comcast, Spectrum and Mediacom Communications have D4.0 squarely in their sights. Others are leaning on DOCSIS 3.1+ as an interim step or going directly to FTTP. Cable One is still exploring all of the options before picking a direction.

Suppliers are not waiting for finalized silicon. Applied Optoelectronics (AOI) will show a 3GHz proof-of-concept amplifier at this week's SCTE TechExpo in Atlanta, even though 3GHz-capable products will likely take years to reach commercial readiness.

Running HFC plant at 3GHz carries specific engineering challenges. Operators may face amplifier respacing and could need booster amplifiers to manage the higher power levels required at those frequencies.

The work stems from a CableLabs DOCSIS 4.0 PHY Working Group covering amplifier hybrids, new optical nodes, RPD silicon, cable modems, taps, cables and connectors. "Using this information, the group developed cable plant models capable of supporting operation to 3GHz. This analysis helped the working group determine what is technically feasible at frequencies up to 3GHz with today's technology," Richardo Villa, lead physical layer technologies architect, and Karthik Sundaresan, distinguished technologist and director of HFC solutions, explained in a statement.

Eyes on 6GHz

The operational annex also feeds research into operations on plant upgraded to 6GHz, which could theoretically deliver downstream speeds in the neighborhood of 50 Gbit/s. Presentations at this week's show from CableLabs, Aurora Networks, Sercomm and AOI will tackle 6GHz challenges and migration strategies that would not require a full network replacement.

Mark Bridges, the incoming CableLabs CEO who currently serves as president and CTO and takes the top job in January, said in an interview ahead of the show that 6GHz work remains exploratory but could eventually give the industry another level of optionality. He noted that engineers are studying network behavior at 6GHz with a focus on new amplifier and tap designs, adding that 6GHz-capable tap development is the furthest along.