SCTE CABLE-TEC EXPO - As cable operators chart a course into multi-gig territory, fiber rivals have begun highlighting symmetrical capabilities as a key differentiator for their broadband services. But according to CableLabs VP of wired technologies Curtis Knittle, there’s no fundamental reason DOCSIS can’t become a symmetrical technology itself. In fact, the only thing really stopping that from happening at this point is economics, he said.
A bit of background is required to understand why DOCSIS is asymmetrical today. Knittle explained that back when CableLabs was formulating the DOCSIS spec, the upstream channel was vying for spectrum with traditional TV services. Thus, there was limited room to work with and, given the industry already knew downstream traffic would outpace upstream flows, an asymmetrical spec was born to reflect those factors.
But Knittle noted “there’s no fundamental limitations for why cable can’t be symmetric just like PON.” Theoretically, that means the DOCSIS specification could become symmetric in the future.
DOCSIS 4.0 is already taking a step in that direction, he added. By expanding the available spectrum on HFC to 1.8 GHz in the extended spectrum version of the spec, DOCSIS 4.0 allows operators to enable faster upstream speeds which close the gap with downlink rates that have historically been much higher. Compared with DOCSIS 3.1, which maxes out at 10 Gbps downstream and 1-2 Gbps upstream, DOCSIS 4.0’s support for 10 Gbps down and 6 Gbps up makes it much closer to a symmetrical service.
It seems even more spectrum could be made available for a theoretical symmetrical spec, given that HFC research over the past several years has focused on a shift to 25G and even 50G. Knittle confirmed research to take things beyond 1.8GHz is already underway, though did not name who is doing the work.
The CableLabs exec declined to comment on what might be included in any future DOCSIS specs. That said, he reiterated it is “certainly possible” for HFC architecture to become truly symmetric.
Spec aside, Knittle said the primary hurdle to such a move would be the economics of making the required upgrades to the network. “If you wanted to make the upstream spectrum and the downstream spectrum completely equal, you’d have to switch out the components like amps and taps just like they’re doing with DOCSIS 4.0,” he noted.
The other key question is whether there’s truly a need for symmetry. While Knittle acknowledged “there’s going to be a marketing challenge” around this question, he argued thus far there’s no use case that truly demands symmetrical speeds.
It’s true that upstream usage is growing but he concluded “it’s at a spot where a gig symmetric should be able to satisfy the overwhelming majority of consumer demand truly for the next ten years or so. And that can be done with high split DOCSIS 3.1.”