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Cisco's Optical Concept: The 1-Gig Cable Modem

LOS ANGELES -- It's probably not coming to a cable system near you anytime soon, but high-speed junkies take note: Cisco Systems has worked up a way to emulate a DOCSIS 3.0 cable modem positively screaming at 1 gigabit per second -- and even faster -- by delivering the data over an optical network.

Cisco software architect Alon Bernstein, in a presentation at the SCTE's Emerging Technologies conference here Tuesday, showed a screen shot of a command-line interface for a cable modem delivering upstream throughput of 974,723,228 bits per second.

Practically speaking, it's a 1-Gbps cable modem. "This is something no cable modem can do today," Bernstein said. It's more than 25 times the throughput available with DOCSIS 2.0 cable modems.

Bernstein's presentation was titled "A Proposal for DOCSIS 4.0" -- a cable engineer's joke, because a fourth major iteration of the cable modem spec doesn't exist today.

His point was to suggest what's next, after the advent of DOCSIS 3.0, which kicks up the potential throughput speeds by providing channel-bonding to combine multiple 6-megahertz channels in existing RF networks.

But while the industry is using the 3.0 spec to target downloads of between 100 and 160 Mbps, Cisco envisions much higher speeds -- as much as 10 Gbps in the future, according to Bernstein -- using a passive optical network (or "PON") as the underlying transport mechanism.

The combination of DOCSIS and PON, which Cisco has dubbed "DPON," essentially fools existing back-office systems for billing and provisioning into thinking regular DOCSIS modems are talking over an RF network, when in fact they're flinging bits over the fiber network. This is accomplished with translation software (a "virtual cable modem") in the headend and in the optical node at the customer premises.

In this setup, "we can use existing CMTS [cable modem termination system] software to manage subscribers, but we don’t need a CMTS as part of the solution," Bernstein said.

How complex is it to do this? "We have the building blocks existing already," Bernstein said.

Note that Cisco's DPON is different from the RF over glass, or "RFoG", concept some other vendors have floated. RFoG encapsulates existing RF traffic and pipes it over the fiber; in that architecture, the same spectral limitations cable operators face are still there. DPON runs at a higher level, taking full advantage of the fiber-optic network.

The advantage of RFoG is that an operator can use existing CMTSs and cable modems, whereas with DPON, new equipment would be required on both ends of the connection. Right now no standards exist for a DPON-style implementation, though presumably these will evolve over time.

How does video get delivering in a DPON environment? Bernstein said "the most natural way" to deliver video over PON is using Internet Protocol TV technology.

Cable could also steal a page from Verizon, Bernstein noted, with a hybrid RF/optical access network approach that would deliver data over one wavelength and RF video over another. "Verizon had to build a whole separate video headend," Bernstein pointed out. "We, of course, would not have to do that."