NCTA Tech Papers Part 1: The Whale
by Leslie Ellis // June 04 2012
My stash of printed NCTA Tech Papers dates back to 1988, and most of the 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: 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: Three competing vendor companies and a giant, household-name chip company co-wrote it: Arris (Mike Emmendorfer), Cisco (John Chapman), and Motorola Mobility (Robert Howald). The chip biggie: 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 thread associated with the limitations of today’s 5-42 MHz reverse path. “
Here’s how the whale breaks down. If you’ve wondered about the trade-offs about going “higher,” spectrally – widening the upstream path – go no further. The first third of the piece details the three main choices, with pros/cons of each. Also what’s sitting there, spectrally, that would need to be moved or mitigated.
Short version: The spectral ranges go by “sub-split,” which is where things sit now: Between 5-42 MHz. After that, the “mid-split,” which inches the upper boundary of the upstream to 85 MHz. Today’s DOCSIS 3.0-based gear already goes there; it’s not 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 up 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 MHz and 85 MHz, 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 88-108 MHz. Potential issue: Interference.
Then there’s Aeronautical Mobile and Radio Navigation, between 108-137 MHz. Potential issue: Signal leakage. And don’t forget legacy out-of-band signaling, used by today’s set-tops and modems to move things like guide data, and command-and-control information.
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 “OFDMA,” for the impressively nerdy “Orthogonal Frequency Division Multiple Access.”
Then there’s talk of improving how transit errors are corrected. More tech talk: “LDPC,” for Low Density Parity Check code. Turns out that using LDPC with OFDM creates a 20% improvement over the tried-and-true, installed method of FEC, known as Reed-Solomon.
And that’s all that can be wedged from a 182-page whale into a 450-word column. We will revisit this topic…I promise.
This column originally appeared in the Platforms section of Multichannel News.