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NCTA Tech Papers,Part 1: The Whale

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.

Stumped by gibberish? Visit Leslie Ellis at