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NCTA Tech Papers, Part 1: The Whale

6/04/2012 12:06 PM

My stash of printed NCTA tech papers dates to 1988 and most recent editions are on CD-ROM, not paper, but still: The fattest one I can find dates back to 1997. Page count: 442.

In this year’s 687-page techfest, the whale is a 182-page monster, replete with 83 figures, 42 tables, 10 recommendations and seven areas of further study. Title: Mission Is Possible: An Evolutionary Approach to Gigabit-Class DOCSIS.

Another reason to lift an eyebrow: Executives from three competing vendors and a householdname chip company co-wrote it: Arris (Mike Emmendorfer), Cisco (John Chapman), Motorola Mobility (Robert Howald) and Intel (Shaul Shulman).

What on Earth could foster such a surfeit of collaborative (digital) ink? As a self-professed junkie for the subject of cable’s skinny upstream signal path, imagine my delight when reading this line, early on page 1: “Cable operators are facing a rising threat associated with the limitations of today’s 5-42 MHz reverse path.” Yippee!

Here’s how the whale breaks down. If you’ve wondered about the tradeoffs about going “higher,” spectrally - widening the upstream path - go no further. The first third of the piece details the three main choices, with the pros and cons of each, as well as what’s sitting there, spectrally, that would need to be moved or mitigated.

Short version: The spectral ranges go by “subsplit,” which is where things sit now: 5-42 MHz. After that, the “mid-split,” with an 85 MHz reverse path. Today’s DOCSIS 3.0-based gear already goes there; it hasn’t been activated anywhere yet.

Then there’s the “high-split,” which stretches the upstream to 200 MHz, and the “top-split,” which piles upstream bandwidth on top of today’s downstream spectrum, above 1 GHz.

The “pro” of a mid-, top- or high-split is the extra bandwidth; the “cons” always start with some variation of “this touches the plant in a rather big way.”

Then there’s the matter of what’s already sitting between 42 and 85 MHz, up to 200 MHz and above 1 GHz. In the lower regions, it’s things like analog channels, which will go digital anyway. And there’s the FM band, which sits between 88 and 108 MHz. Potential issue: interference.

Aeronautical Mobile and Radio Navigation is between 108 and 137 MHz. Potential issue: signal leakage. And don’t forget legacy out-of-band signaling, used by today’s set-tops and modems for things like guide data .

Other ways to “manufacture” upstream bandwidth without touching the plant in a rather big way involve going to a different form of advanced modulation, called “OFDM,” for the impressively nerdy “orthogonal frequency-division multiplexing.”

There’s some talk of improving how transit errors are corrected. More tech talk: “LDPC,” for “lowdensity parity-check” code. Turns out that using LDPC with OFDM creates a 20% improvement over DOCSIS’s tried-and-true, installed method of forward error correction, known as Reed-Solomon.

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