Why Cable Needs EPON Over Coax
Docsis 3.0 hasn't run out of gas yet, but the cable industry doesn't want to get caught off-guard when it does.
That's one of the key reasons several MSOs and cable vendors are putting some weight behind EPON Over Coax (EPoC), a proposed Institute of Electrical and Electronics Engineers Inc. (IEEE) standard that aims to bring PON speeds to hybrid fiber/coax (HFC) networks. (See EPON-Over-Coax Starts Its Standards Journey and Broadcom Crafting PON-Speed HFC.)
The current generation of Docsis modems can bond eight downstream channels and four upstream channels, enough for bursts of 320Mbit/s downstream and 120Mbit/s upstream. Down the road, those configurations could reach up to 32 channels, at least in the downstream -- enough for bursts of more than 1Gbit/s. Cable has demonstrated as much in some recent demos that stretched the technical limits of Docsis 3.0. (See Virgin Widens 100-Meg Footprint, Tests 1.5-Gig , Comcast Also Thinking Big With 1-Gig, UPC Tests 1.3Gig Wideband and The Ultimate Cable Modem .)
But there might be limitations to how much more the chips can support.
"We need to get ahead of this," says Jeff Finkelstein, senior director of network architecture at Cox Communications Inc. , one of EPoC's backers. "If we just sit back and wait for things to happen, it puts us at a disadvantage with competition, with chip manufacturers, and with the needs of the customers."
EPoC could give cable a fresh look at orthogonal frequency-division multiplexing (OFDM), a modulation scheme that will be considered for the IEEE standard. OFDM has already begun to find a home with wireless as well as with Multimedia over Coax Alliance (MoCA) , a speedy coax-based home networking platform.
Some see OFDM as a next-gen modulation scheme for cable, steering the industry away from 6MHz-wide channels (EuroDocsis uses 8MHz-wide channels) and toward smaller subcarriers (40KHz to 100KHz) that can push out more bits per hertz. Today's 256 QAM technology does eight bits per hertz, or about 42Mbit/s for each 6MHz-wide channel. It's believed that OFDM could let cable pursue higher levels of modulation, such as 1,024 QAM (10 bits per hertz) or 4,096 QAM (12 bits per hertz), if you want to look way ahead.
"Will we reach 4,096?" Finkelstein asks. "I don't know. There are people who thought we'd never reach 256 QAM, and some thought 64 QAM would be hard. The reality is that we did reach it."
Most agree that EPoC's initial focus will be business services, but it could end up in residential networks as well. One advantage EPoC supporters can tout is the ability to be deployed piecemeal, wherever it's needed. And one aim is to ensure that EPoC can plug into the Converged Cable Access Platform (CCAP), a super-dense platform that will combine QAM and cable modem termination system (CMTS) capacity. (See Comcast Gets Ready for CCAP and Cable Rethinks 'Modular' CCAP .)
EPoC might not be the only approach that the cable industry is eyeing. CableLabs , for example, is believed to be taking kind of the opposite approach to EPoC: an advanced MAC/PHY that would use very wide channels, rather than small sub-carriers. (See Cable Ponders Life After Docsis.)
— Jeff Baumgartner, Site Editor, Light Reading Cable