The Care and Feeding ofUpstream Channel Bonding
LOTS OF ACTION LATELY AROUND THE
topic of upstream channel bonding,
especially as more and larger digital
stuff wants transit across that
slender spectral area.
A brief review: Cable spectrum
comes in two stripes: down and
up. All the bits of online video,
Netflix streaming, Skype calls,
HDTV, video on demand — stuff
heading into your house — that’s the downstream
signal path. Most (95%) of an operator’s total
available bandwidth is pointed downstream.
The upstream signal path is the tiny sliver of
spectrum set aside to move the bits that leave
your house. Your part of a voice-over-Internet
protocol phone call. A click to fetch a Web page.
Pausing an on-demand movie.
Ask any engineer what’s happening in the upstream,
usage-wise, and you’ll hear a variation of
this: Average and peak consumption is on the rise.
As digital consumers, we’re not only sending more
stuff upstream, we’re sending bigger stuff. “Your
part of a VoIP call” is a whole different thing than
“your part of a video chat.” Voice moves at 64 Kilobits
per second; video in the Megabits per second.
That’s why we’re starting to hear more about
upstream channel bonding — a method that lets
cable modems transmit across multiple channels
at once. Cox made news on the topic last
week, with word that its tests of a 12-channel
bond yielded a 400-Mbps link.
(Note: Great stuff, but be careful anytime you see mention of 88 MHz as the upper boundary of the upstream. The upstream path tops out at 54 MHz. Going higher is a fine goal, but it isn't a finger snap and a "voila.")
One big difference between upstream and downstream
bonding is channel width. While downstream
channels all weigh in at 6 MHz, upstream channels
can be sized at 1.6 MHz, 3.2 MHz, and 6.4 MHz.
Why? Because the upstream signal path was
never envisioned or designed to carry broadcast
video, which was what dictated 6-MHz channelization,
back in the earliest days of analog television.
Modulation methods tend to notch down a
step, compared to the downstream, too. That’s
because it’s way noisier in that slice of spectrum
— and when you’re sending through noise,
just like when you’re flying through turbulence, it
works better if you slow down.
The fastest of the three upstream modulation
methods — 64 QAM — runs at around 7.5
Mbps on a 1.6 MHz channel width, 15 Mbps at
3.2 MHz, and 30 Mbps at 6.4 MHz. Most cable
modems running DOCSIS 3.0 are configured to
bond four upstream channels. So, four bonded
6.4 MHz channels running 64-QAM yields is
around 120 Mbps. Cox bonded 12 channels at
64 QAM to get to 400 Mbps.
That’s a quickie on upstream channel bonding
— an enormously useful tool for a spectrallystrained
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