Peer-to-Peer Meets Cable's Upstream Path

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In last week's electronic mail was a letter from an old friend who gleefully professed his love for his new cable modem. For him, downloading 5,000-plus hours of music from Napster Inc. before it was shuttered was retribution against the recording industry for pushing the Partridge Family's I Think I Love You
to the No. 1 slot in 1970, over Credence Clearwater Revival's Proud Mary.

Putting aside the slippery slope of copyright law, the letter gave me pause to consider a different question: What is the impact of unbridled file sharing between cable-modem customers?

On the growing list of broadband buzz phrases, this phenomenon is known as "peer-to-peer computing." (Acronym alert: "P2P.") At the heart of P2P is the fate of cable's already slender upstream path, should file swapping extend beyond compressed audio files to bandwidth-slurping video files.

Figuring out how bad it could get, unfortunately, doesn't model easily. Straight division of capacity by number of users or what they're sending doesn't quite work. Too many variables. But it does help to know how cable-modem technology arbitrates available bandwidth.

First, you need to know how much upstream (home-to-headend) capacity you really have. Capacity is a function of bandwidth (how wide) and modulation (how sent). Cable's upstream signal path lies in a very funky spectral slice, between 5 megahertz and 40 MHz. Only about 25 MHz of it is really usable, because of the noisiness below 20 MHz.

Upstream capacity, unlike the downstream, isn't carved into 6-MHz chunks. That's because it was never intended to carry video signals, which require roughly 6 MHz for an analog picture. Instead, upstream capacity is calculated on a bits-per-hertz model. The amount of bits-per-hertz, in turn, relies on the type of modulation used to imprint information on an upstream carrier.

Cable providers, at least for now, use an upstream modulation technique called "QPSK," or "quadrature phase shift key." It's the same technique used in satellite/DBS transmissions. It's different-slower and sturdier-from the downstream modulation used for digital traffic ("QAM," for quadrature-amplitude modulation), simply because the upstream band is so darned noisy. Something was needed to shuttle traffic as quickly as possible through an environment teeming with villains.

QPSK is like driving a car on a bad road. It doesn't matter how fast the engine is. On a pocked road, you slow down or you risk damaging your car.

In the cable upstream path, the potholes are the bursts of radio frequency and electrical noise that leak into the system. Noise can come from individual homes or external sources. Regardless, it gets smooshed together with the intended signal, and then with everyone else's noise and intended signal. It all gets amplified as it moves to the headend. (Engineers call this "noise funneling.")

QPSK yields a usable capacity (after overhead) of about 1.2 megabits per second/MHz. Multiplied by the 25 MHz of usable upstream bandwidth, the aggregate upstream capacity is about 30 mbps.

Next, you need to know how that capacity is divvied up among customers. As telco digital subscriber line providers have pointed out nearly to death, cable's network is shared. That means somebody has to arbitrate how much bandwidth each user gets.

That somebody is the CMTS, or Cable Modem Termination System (CMTS)-the headend part.

Current CMTS technology treats every upstream request identically, and slots available upstream capacity equally among all takers. So if 250 users on a 50 percent penetrated, 500-home node need to use the upstream path-including, say, 50 gluttons and 200 non-gluttons-the CMTS accepts the traffic on a one-by-one basis.

After the first set of packets that comprises everyone's individual needs is collected, the CMTS starts through the queue again. Whether it's a one-word reply to an instant message or a portion of a video file, it's the same: Get in line. One packet at a time. No shoving, please.

On the other hand, if one cable-modem customer happens to be the only
one online at a given moment in time, that does not equate to full access to the 30 Mbps in the upstream. Most MSOs cap the amount of upstream bandwidth to around 384 kbps.

That means bandwidth gluttons-teenagers sending games, music or video-will have to wait longer and longer to upload "free" stuff to each other. The increased traffic they're helping to generate will slow them
down more so than the casual Web surfer.

For now, the only thing you can do is hope any slowdowns they generate for themselves will make them get impatient enough to go find something else to do.

When the new, DOCSIS 1.1 gear comes out, it will be technically possible to charge more for prioritized CMTS treatment.

But by then, the on-demand stuff of Napster and Gnutella won't be as free-and free isn't nearly as fun when you have to pay for it.