Docsis 3.1 Will Change Cable's Data Channels
Much more about Docsis 3.1 will be shared here on Thursday, but the new channel environment and anticipated PHY-layer change was one of the key items engineers from Broadcom Corp. (Nasdaq: BRCM) and Motorola Mobility discussed on Tuesday during a technical session about the CableLabs initiative. (See Docsis 3.1 to Be Smarter, Faster & Cheaper and Docsis 3.1 to Be Revealed at Cable-Tec Expo.)
North American Docsis currently uses 6MHz-wide channels that each pump out about 40Mbit/s, while EuroDocsis using 8MHz channels that yield about 50Mbit/s. With Docsis 3.1, the cable industry hopes to squeeze out more bits per hertz by chopping 200MHz-wide blocks into thousands of small subcarriers and combining them using the orthogonal frequency-division multiplexing (OFDM) modulation scheme. It's believed that this approach can help to improve spectral frequency by at least 25 percent.
Utilizing one of those blocks would enable cable to produce a data path in the neighborhood of 1Gbit/s. Cable later bond together 200MHz-wide channels together to create multi-gigabit capacities and give cable the ability to keep up with fiber-to-the-premises (FTTP) speeds without having to pull fiber all the way to the home.
Motorola Mobility, a vendor that's participating in the development of the specs, is recommending a Docsis 3.1 capacity target of 10Gbit/s downstream and 2.5 in the upstream, said Robert Howald, fellow of the technical staff at the company.
But Niki Pantelias, associate technical director for Broadcom's broadband technology group, suggested that it would be unwise to design a Docsis 3.1 modem that can bond multiple 200MHz-wide channels and approach speeds of 5Gbit/s or more right out of the chute.
That's partly because Broadcom believes the cost of such a modem would be prohibitive, at least in the early going. Plus, it's unlikely that cable will even need to deliver capacities of more than 1Gbit/s to residential customers in the near-term. She instead sees Docsis 3.1 modems starting off with single-channel designs and later evolving to bond together multiple channels as chip processing performance and pricing improve, and customers actually need those bigger capacities.
Because Docsis 3.0 and 3.1 will use different modulation schemes, it's anticipated that cable will have to carve out separate spectrum for each platform for awhile, producing a temporary coexistence model.
But to maintain backward-compatibility at the modem layer, Dr. Richard Prodan, the CTO of Broadcom's broadband technology group, suggests that the first generation of Docsis 3.1 devices use a special piece of hybrid silicon that support both Docsis 3.0 and 3.1 receivers. Cable could then later cut over to 3.1-only modems as operators phase out 6MHz channels in favor of the new broadband platform. But that transition is expected to take several years.
— Jeff Baumgartner, Site Editor, Light Reading Cable