Cable Tech

Comcast forges 'network to modem' DOCSIS 4.0 connection

Comcast is laying claim to a "10G" milestone with word that it has completed a "network to modem" connection of DOCSIS 4.0 technology. That connection, made via a live lab trial, was the result of Comcast's virtual cable modem termination system (vCMTS) speaking to DOCSIS cable modems configured to operate in the Full Duplex DOCSIS (FDX) band.

Comcast announced the tech demo in tandem with this week's all-virtual SCTE Cable-Tec Expo. The cable operator said the demonstration shows it can upgrade its existing vCMTS platform via a software upgrade to FDX and thus be in position to support symmetrical multi-gigabit speeds on hybrid fiber/coax (HFC) networks.

On the premises end of the network, Comcast said DOCSIS 3.1 modems configured to operate in the FDX band were used for the test. The FDX band, a swath of RF spectrum operating in the range of 108MHz to 684MHz, allows for both upstream and downstream traffic to run in the same block of spectrum.

Comcast tells Light Reading it developed special test modems with DOCSIS 3.1 silicon to enable the operator to test FDX network elements. Comcast did that to complete the 10G connection, but does not view it as a way to deliver FDX at scale. For that, Comcast will use FDX modems/gateways.

This diagram illustrates a sample allocation of HFC network spectrum, including an FDX band that allows for upstream and downstream traffic to be overlapped in a block of spectrum occupying 108MHz-684MHz.  
Click here for a larger version of this image.  
(Source: Comcast/SCTE Cable-Tec Expo 2021)
This diagram illustrates a sample allocation of HFC network spectrum, including an FDX band that allows for upstream and downstream traffic to be overlapped in a block of spectrum occupying 108MHz-684MHz.
Click here for a larger version of this image.
(Source: Comcast/SCTE Cable-Tec Expo 2021)

FDX, originally developed as an annex to DOCSIS 3.1, has been carried over to DOCSIS 4.0 and envisions the cable plant being built out to 1.2GHz. Extended Spectrum DOCSIS (ESD), another option for DOCSIS 4.0, keeps the upstream and downstream traffic separate and targets upgrading the network to 1.8GHz. Comcast has been the primary champion of FDX. Most other operators mulling future upgrades to DOCSIS 4.0 have focused on the ESD flavor of the specifications.

DOCSIS 4.0 fits in with 10G, a cable industry initiative designed to deliver symmetrical speeds of 10 Gbit/s, low latencies and enhanced security for multiple types of access networks, including HFC, fiber-to-the-premises and wireless.

Comcast's latest DOCSIS 4.0 lab demo was conducted by company technologists in Philadelphia and Denver, Elad Nafshi, Comcast's SVP of next-generation access networks, explained in a blog post.

"In the live lab demonstration, we were able to connect all of the key network elements necessary to deliver 10G signals from our network to the neighborhoods we serve … and all the way into our customers' homes," Nafshi wrote.

Comcast, he added, believes the live lab demo marks "the world's first test of a 10G connection all the way from our network to a modem," and the first to be powered by FDX. Nafshi said work continues toward the development of "a full 10G" cable modem, but did not reveal a projected timeline. However, the plan is to start with fully-capable FDX modem prototype that the operator can test, Comcast tells Light Reading.

This week's demo followed one in April 2021 in which Comcast announced that FDX lab trials featuring a chipset from Broadcom delivered symmetrical speeds of more than 4 Gbit/s. That, in turn, followed a trial last year in which Comcast produced symmetrical speeds of 1.25 Gbit/s over a live production network in Jacksonville, Florida.

Nafshi's post also made reference to "groundbreaking work underway" with tech partners to develop an FDX Amplifier. That device, being worked on by Harmonic and others, should enable Comcast to deploy FDX across a large portion of its network, not just to a subset of fiber-deep "node+0" areas where it does not have any amplifiers deployed between the home and the node.

Other FDX action

Thursday's announcement from Comcast followed a wave of other info the company shared this week about its work on FDX and DOCSIS 4.0.

Speaking at the event on Monday, Comcast engineering fellow Rob Howald said that the operator is "making a lot of good progress" with the technology in the labs. He noted that integration work is in full swing amid ongoing lab trials and that Comcast plans to eventually shift to some field trial work. He also noted that Comcast's vCMTS now "speaks" DOCSIS 4.0.

Rich Prodan, another Comcast fellow, expanded on Comcast's FDX-facing work this week at Cable-Tec Expo. In a white paper presentation, Prodan said lab tests of echo cancellation – a technique that resolves interference issues caused by having upstream and downstream traffic operate in the same block of spectrum – found that the technology performed better than expected.

That lab test used a reference node for FDX with a remote PHY device powered by an ASIC to handle the echo cancellation and demodulation. The result produced a total of 4.2 Gbit/s of upstream bandwidth – 400 Mbit/s from the non-FDX legacy upstream band (5MHz-85MHz) and 3.9 Gbit/s from the FDX band upstream. The downstream end was able to generate about 7.6 Gbit/s – 3 Gbit/s from the legacy downstream spectrum and 4.3 Gbit/s from the downstream piece of the FDX band. Together, that would put total capacity of the network past 10 Gbit/s, and enable Comcast to allocate a sizable chunk of upstream and downstream capacity dynamically, as needed, in the FDX band.

"The performance was greater than we expected," Prodan said, noting that Comcast's lab tests of echo cancellation achieved "error-free" 1024 QAM modulation of OFMDA upstream channels. That exceeded an expected modulation of 64 QAM, resulting in a 67% capacity increase over the baseline requirements of the DOCSIS 4.0 specs for FDX, he said. Higher modulations, such as 1024 QAM, are more spectrally efficient, enabling the cable network to pass through more bits per hertz.

The result, Prodan added, showed that Comcast would be able to support multi-gigabit upstream signals via the FDX systems.

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— Jeff Baumgartner, Senior Editor, Light Reading

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