Just as the cable industry is moving ahead with a next-gen access platform that packs the edge QAM and cable modem termination system (CMTS) functions into a device called the Converged Cable Access Platform, or CCAP, talk has already begun to spread about another approach that aims to break it apart again and distribute that capacity toward the edges of the network.
The concept is called “remote PHY,” and it’s a topic that is already stirring up a debate in cable engineering circles. The latest public forum on the topic took place last month at the SCTE Cable-Tec Expo in Atlanta during a session aptly titled “Remote PHY: Close to Our Heart.”
The general notion of remote PHY is to place elements of the headend out to the edges of the network and, therefore, keep the signals in digital format longer before they have to be converted to analog.
MORE BITS PER HERTZ
By extending the reach of those digital signals, operators can support higher modulations and, therefore, pack more bits per hertz onto the hybrid fiber/coax (HFC) plant. That makes the network more spectrally efficient, but also faster, enabling it to handle the kinds of capacities envisioned by DOCSIS 3.1, which is being designed to scale up to 10 Gigabits per second in the downstream, and between 1 Gbps and 2 Gbps in the upstream.
“You want to get as many modems running at the highest modulation as possible,” Jorge Salinger, vice president of access architecture at Comcast Cable, said, noting that the three primary goals of remote PHY are performance, simplicity and scalability.
While the current implementation of CCAP “will get us very far down the road,” remote PHY and the idea of “virtualizing” the elements of that architecture will essentially explode it out, break it into its constituent parts out on the network, and set the table for a much more streamlined headend, Jeff Finkelstein, executive director, strategic architectures, Cox Communications, said.
Today’s approach with CCAP aims to reduce power consumption and use port densities that also save on physical headend space. But cable engineers acknowledge they already see a day when the capacities they’ll require will eventually outstrip the power- and space-saving benefits of CCAP.
Virtualizing CCAP and managing those remote network elements via a common control plane could go a long way toward solving those future concerns. But there’s more than one way to tackle it, which is why the industry is trying to get a grip on it now.
If done right, remote PHY should scale. “It’s the same software; the ports are just remote,” John Chapman, Cisco Systems fellow and chief technology officer of the vendor’s cable access business unit, said. “You manage it like it’s the same CMTS.”
But remote PHY also marks an evolution of the current market, so standards and specifications will need to be created.
Discussions about remote PHY are already underway with Cable-Labs, Chapman said. “You have to maintain that [product] interoperability,” he added “Interoperability is a must.”
There are many paths toward achieving digital fiber, Finkelstein said, adding, “But the path to get there is with open standards that allow that interoperability.”
“Nothing is cast in stone” with respect to remote PHY, Salinger said. “But the ball is starting to move in that direction.”