The Care and Feeding of Upstream 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 domain.

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