- Harmonic touted a 14 Gbps download record in DOCSIS 4.0 interop test
- Cable operators will probably be able to offer those high speeds, but not without some limitations
- A single customer device may struggle to handle that throughput, said a CableLabs exec
As operators hop aboard the DOCSIS 4.0 train, Harmonic thinks its gear is just what cable needs to get on fiber’s level and obtain faster speeds. Naturally, experts told us it won't come easy.
Harmonic said its virtual cable modem termination system (vCMTS) platform delivered 14 Gbps of downstream throughput in a recent CableLabs interoperability event, surpassing the 10 Gbps benchmark target for the DOCSIS 4.0. specification. The test was done with the help of Harmonic’s DOCSIS 4.0 Unified Remote-PHY device as well as two third-party DOCSIS 4.0 cable modems that were used “in parallel” to reach the 14-gig aggregate throughput, a Harmonic spokesperson told Fierce.
It’s not just about touting higher download speeds. If operators aim only for 10 Gbps, that “would leave significant spectrum capacity unused,” Harmonic said. The vendor's Remote-PHY device was configured with eight OFDM channels (basically, different "lanes" of transmitting data) utilizing up to 1.8 GHz of spectrum.
"With DOCSIS 4.0, delivering downstream speeds above 10 Gbps is technically viable," said the Harmonic rep. "Scaling it widely will depend on network upgrades, spectrum availability and customer demand."
Dell'Oro Group VP Jeff Heynen noted operators that allocate more channels to the downstream side can “easily” push bandwidth up to 14 Gbps, but not without a cost.
“You then sacrifice the upstream bandwidth," he said.
DOCSIS 4.0 modems can theoretically support upload speeds of up to 6 Gbps. Both wired and wireless carriers are increasingly focusing on upstream capacity as consumers stream more video, use more AI applications and the like.
The DOCSIS 4.0 lowdown
Operators typically have two DOCSIS 4.0 flavors to choose from, extended spectrum (ESD) and full-duplex (FDX). ESD, favored by Charter and Cox, increases the spectrum pipe to 1.8 GHz but keeps the traditional approach using dedicated chunks of spectrum for the upstream and downstream traffic flows.
FDX, which Comcast is pursuing, sticks with 1.2 GHz spectrum but uses noise cancellation to allow upstream and downstream traffic to be transmitted over the same spectrum.
But what if you didn’t have to choose between one or the other? Harmonic touts its gear is compatible with both ESD and FDX, otherwise known as “unified” DOCSIS 4.0. The cable industry has been pushing the unified approach to give operators more flexibility in their plant upgrades. Broadcom, for instance, is working with Charter and Comcast to create chipsets that could be used for a combined deployment method.
“Full duplex will more than likely be used to deliver symmetric service tiers to compete directly with symmetric fiber offerings,” Heynen said. “But you could see a scenario where a user might want to have a much faster downstream connection than upstream.”
Deployment challenges
Getting to 10G and beyond is of course easier said than done. Today’s cable modem may not be able to process a higher downstream capacity – at least not by itself.
Curtis Knittle, VP of wired technologies at CableLabs, said different modems may support a different number of OFDM channels. Even if a vCMTS or outside plant is capable of 14 Gbps download speeds, that doesn’t mean the modem can “consume all that capacity individually.”
However, operators could get around that and divide and conquer the bandwidth, so to speak.
“The total aggregate downstream capacity can be shared by multiple CPE on the cable network,” Knittle explained. “For example, if the aggregate downstream capacity from the vCMTS/RPD is 14 Gbps in total, 9 Gbps could be received by one CPE while the other 5 Gbps is simultaneously received by another CPE.”
Adding more spectrum is one thing. Another challenge in deploying DOCSIS 4.0 is figuring out how to improve spectrum at higher frequencies, he noted.
“Spectrum at higher frequencies is not always as ‘clean’ as it is at lower frequencies,” said Knittle. “So an OFDM channel located at a higher frequency may not be able to carry as many bits as an OFDM channel at a lower frequency.”