Comcast's bandwidth throttling system that slows you down for using too much bandwidth is now fully armed and operational in all markets. Here's how it works, and how to not get stuck on the short bus.
The golden rule—which we covered a bit here—to keep in mind is this: If you use more than 70 percent of your downstream or upstream bandwidth for more than 15 minutes, you'll be slowed down for at least 15 minutes, or until you're brought back down to using 50 percent of your bandwidth. The other, more esoteric way to bring on the sludge is actually a pair of conditions: If a certain CMTS port—cable modem termination system, the hub your cable modem, along with those of up to 15,000 (but probably fewer) runs to—is congested and if you've been targeted as the asshole why.
By slowed down, it means all of your packets are assigned "Best Effort" quality of service, a lower tier than Priority Best Effort. Which means this, according to Comcast:
If there is no congestion, packets from a user in a BE state should have little trouble getting on the bus when they arrive at the bus stop. If, on the other hand, there is congestion in a particular instance, the bus may become filled by packets in a PBE state before any BE packets can get on. In that situation, the BE packets would have to wait for the next bus that is not filled by PBE packets.
This all happens in a compressed time frame though—missing 50 buses would mean being delayed by about a tenth of a second, but it is possible to have your packets dropped in "extreme cases." (See? The bus metaphor was theirs.)