Expanding spectrum – particularly in the upstream direction – is one of the hallmarks of DOCSIS 4.0, a new set of specs that will deliver multiple-gigabit speeds alongside support for enhanced security and lower latencies.

But operators have a lot of optionality to noodle on as they decide where to eventually set the "split" that determines how much spectrum is dedicated to the upstream and the downstream. Today, most cable networks set aside a band of 5MHz to 42MHz for the upstream, though several operators are starting to push ahead with a "mid-split" that raises that ceiling to 85MHz, or a "high-split" that elevates it to 204MHz.

But, as DOCSIS 4.0 enters the picture, operators will also have the option to raise the upstream bar to 300MHz, 396MHz, 792MHz or even 984MHz. DOCSIS 4.0 deployments are still far off on the horizon, but many are already starting to consider where they might set that future split.

For many operators, the potential "sweet spot" for a future split falls somewhere in the middle of that range – at about 396MHz – Tom Cloonan, CTO-network solutions at CommScope, said on a DOCSIS 4.0-focused panel at this week's all-digital Anga Com conference.

Full Duplex DOCSIS vs. Extended Spectrum DOCSIS

Cloonan estimates that a 396MHz split would deliver about 3 Gbit/s in the upstream and roughly 9 Gbit/s in the downstream on a hybrid fiber/coax (HFC) network that utilizes Full Duplex DOCSIS (FDX), a flavor of DOCSIS 4.0 that envisions a plant built out to 1.2GHz and allows upstream and downstream traffic to occupy the same block of spectrum.

Meanwhile, the Extended Spectrum DOCSIS (ESD) flavor of DOCSIS 4.0, which keeps the upstream and downstream traffic separate and envisions a plant capacity upgrade to 1.8GHz, would deliver about 3 Gbit/s upstream and 10 Gbit/s downstream, Cloonan noted.

So, for those keeping score, DOCSIS 4.0 with ESD does provide a small downstream advantage to DOCSIS 4.0 with FDX, when the split is set at 396MHz. Cloonan also presented a chart that illustrates the range of throughputs supported by both ESD and FDX at various split levels:

Figure 1: Permissible Throughputs with DOCSIS 4.0 FDX & ESD Click here for a larger version of this image.
(Source: CommScope/Anga Com)

"They are similar, but different, and both are useful technologies that will be used differently by different operators," Cloonan said of FDX and ESD.

Most cable operators are looking at the ESD flavor of DOCSIS 4.0. An exception is Comcast, which has long been a champion of Full Duplex DOCSIS. Cloonan allowed that "maybe a few others" are also looking at Full Duplex DOCSIS.

When will operators need DOCSIS 4.0?

Where the future split of a DOCSIS 4.0 network will reside is a crystal ball scenario. When cable operators will actually start to deploy, or even need, DOCSIS 4.0 is another predictive exercise.

MSOs are expected to "exhaust" the capabilities of their DOCSIS 3.1 networks before pushing hard on DOCSIS 4.0, Jay Lee, chief technology and strategy officer, broadband access, at ATX Networks, said.

That D3.1 network exhaustion could take shape in many ways. It could mean a near-term focus on mid-split or high-split upstream augmentation upgrades, the activation of more efficient OFDMA upstream channels that allow more bits to be pumped through the available spectrum, the launch of new Low Latency DOCSIS (LLD) capabilities, and progress with the emerging distributed access architecture (DAA) for hybrid fiber/coax (HFC) networks.

Cloonan said demand for billboard downstream speeds could hit 4 Gbit/s by 2028, presenting a possible point in time when operators will need to start turning up DOCSIS 4.0 functions such as a 1.8GHz downstream paired with an upstream split of at least 300MHz.

However, the "plumbing" of the network in preparation for DOCSIS 4.0, which requires DAA, will need to get underway years earlier. Depending on the operator, it's possible that some will start to deploy DOCSIS 4.0-capable taps as early as next year, with installations of new nodes and amplifiers set to follow.

A move to DAA, which will move key electronics toward the edge of the network and allow for more efficient digital optics, represents "a big piece" of the DOCSIS 4.0 network evolution, Lee said.

"DOCSIS 4.0 features might sit latent in the network for a few years, but they'll be out there waiting for the time for when they are needed," Cloonan said.

The good news, Lee said, is that the DOCSIS 4.0 specs are out, providing the means for chip makers and vendors to develop the "endpoints" – the modems and the nodes – for DOCSIS 4.0.

"The stage is set for us to move forward and start to look forward to deploying DOCSIS 4.0," he said.

Lee also shed some light on how the future DOCSIS 4.0 modem will take shape. Rather than a traditional DOCSIS CPE that sits inside the home and lives behind a splitter, DOCSIS 4.0 CPE will likely serve as a gateway at the point-of-entry to the home and eliminate some complexities and RF loss concerns, he said.

"That's actually a positive for the HFC network and some of the challenges that designers need to face," Lee added.

A 3GHz future?

Cloonan also noted that there's work underway for a potential future DOCSIS specification, or perhaps an annex to today's DOCSIS 4.0 specs, that would support HFC networks built out to 3GHz.

If and when cable gets to that point, it's possible that operators could move the upstream split to 1GHz and be in position to deliver 10 Gbit/s downstream and up to 7 Gbit/s in the upstream, he said.

But a move to 3GHz is likely way out in the future, possibly 15 years or more, Lee said. He's not yet convinced that 3GHz will play a big role in cable's future, holding that a move to fiber-to-the-premises (FTTP) is a more likely step after DOCSIS 4.0.

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— Jeff Baumgartner, Senior Editor, Light Reading

A version of this story first appeared on Broadband World News.