Docsis Flirts With 5Gbit/s

While Docsis vendors work on refreshing the technology to reach 10Gbit/s downstream speeds, a recent field trial shows that current gear can get almost halfway there.

Ahead of next week's ANGA Cable Show in Cologne, Kabel Deutschland GmbH and Arris Group Inc. (Nasdaq: ARRS) demonstrated downstream speeds of 4.7Gbit/s on live Docsis 3.0 plant connected to a school in Schwerin, Germany.

It's considered a new Docsis land speed record, achieved by bonding 96 8MHz-wide EuroDocsis channels (that's 768MHz total spectrum). The trial used 12 Arris cable modems, each capable of bonding eight downstream channels (the latest generation of Docsis 3.0 modems can bond up to 24). (See Intel's New Docsis 3.0 Chip Guns for 1-Gig .)

To complete the kluge setup, Arris and the operator inserted a Layer 2/3 switch and aggregated and multiplexed those 96 channels into a 10Gbit/s feed linked to a PC. Out on the network, the trial was fed by Arris's flagship CMTS, the C4, which was outfitted with four 24-downstream port line cards (plus one upstream card with 12 ports for good measure).

Some good timing also played a part. KD and Arris conducted the trial just as the operator was about to open up a new fiber node, so there was no other traffic running on that portion of the recently upgraded 862MHz plant. "For a short window of time, they (KD) saw an opportunity to try something," says Arris CTO Tom Cloonan.

The 4.7Gbit/s demo isn't anything near practical, but it does show what the current equipment generation can do. "We think this is just the starting point," Cloonan says, noting that cable modems capable of bonding 32 or 48 channels could be the next possible step, and maybe 64 after that.

The road to Docsis 4.0?
Converting most or all of cable's spectrum into one giant Docsis IP pipe is the sort of thing that Arris, Cisco Systems Inc. (Nasdaq: CSCO) and Motorola Mobility LLC are proposing with a next-generation Docsis that they claim could support 10Gibt/s downstream and 2Gbit/s upstream. But with the number of tweaks and additions on the table, such as spectrum upgrades to possibly 1.7GHz, it amounts to a quantum leap for the technology. (See Does Docsis Have a 10-Gig Future? )

Other ideas aim for nitty-gritty efficiency boosts, squeezing out more bits per Hertz. For example, the Docsis vendors are keen on OFDM (orthogonal frequency-division multiplexing), a modulation scheme already popular in mobile world, that would essentially take today's 6MHz channels (for North American Docsis) or 8MHz channels (for EuroDocsis) and chop them up into much narrower, 10KHz-wide channels. The thinking is that the smaller widths would let cable operators utilize noisy pockets that are otherwise unusable.

Another idea being pushed is low density parity-check (LDPC), a new forward-error correction flavor that would aid data transmissions in noisy plant conditions by reducing the amount of bandwidth overhead required for the current technique, called Reed-Solomon. The combo of LDPC and OFDM could improve spectral efficiency by 18 percent to 24 percent, some believe.

Docsis vs. EPoC
Those techniques are also being considered for EPON Protocol Over Coax (EPoC), an emerging Institute of Electrical and Electronics Engineers Inc. (IEEE) standard that some view as a competitor to a next-generation Docsis platform. (See Huawei Looks Beyond Docsis.)

"There's not a reason to pick one over the other in terms of performance; they're virtually identical," Cloonan says. "But cable will need to look at it from other attributes, other than performance."

Cloonan says backwards compatibility is one area where a next-gen Docsis platform would have an edge over EPoC. Of course, Arris, like Cisco and Motorola, has other good reasons to favor Docsis -- they own the bulk of the CMTS market and have some sizable turf to protect.

But Cloonan doesn't completely discount EPoC as a long-term option. EPoC, he said, could serve as a "stepping stone" for cable's eventual move to fiber-to-the-home, "if MSOs want to prepare for a 2030 timeframe."

Still, Docsis might also be made to play a role in that FTTP transition. In one possible scenario, a new version of Docsis could be paired with Radio Frequency Over Glass (RFoG), a cable industry standard that lets operators pull fiber to the premises while preserving their headends, backoffice and provisioning systems, and cable modems and set-tops.

Cloonan believes most cable operators will go with a next-gen Docsis system "if it's defined." That's still an open question, as CableLabs has yet to make any decisions about what comes after Docsis 3.0. (See The Docsis Addendum and Costs Could Keep RFoG a Niche Player .)

And those decisions may depend on geography. Cable operators in China and India are already using Ethernet-over-coax technologies to serve high-density areas, so a standard like EPoC could cater to those markets, says Infonetics Research Inc. analyst Jeff Heynen.

Heynen does agree that EPoC makes sense as an FTTP transition technology for cable but still thinks Docsis will reign for the foreseeable future. "We're at least ten years away from Docsis exhaust," he says. "And maybe that's conservative."

— Jeff Baumgartner, Site Editor, Light Reading Cable

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VictorRBlake 12/5/2012 | 5:30:44 PM
re: Docsis Flirts With 5Gbit/s

Actually my mistake. It's 2E24-1 (16M), not 2E8 LLID.

I won't dispute the performance hit to large scale polling, I think the likely uses are small numbers of business customers that will work very well for the EPON MAC exactly as it is today. But even if we were to ponder residential, think about it. Typical DOCSIS networks today, as a reslt of node-splits and smaller serving group sizes are in the same range (tens to hundreds of statations per SG).

As to performance, I think it's hard to argue that somehow the DOCSIS schedulers perform better than the EPON MAC scheudlers. It is literally no comparison (thus the reason EPON is well suited for business SLAs). The EPON scheduler is deterministic (not probabilitistic) and in practice the latencey (and variation in latency -- aka jitter) is FAR lower on EPON networks than in DOCSIS. There are a number of studies both by vendors and which I've conducted in the field (myself) that prove out the latency and jitter measurements on both DOCSIS and EPON.

So despite the truth of the theoretical problems of polling, in practice it (a) is not a problem for the scale of typical deployments and (b) in practice performs better than the alternative in terms of performance.

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