On Jan 25, 2008 7:49 PM, Jim Popovitch <yahoo@jimpop.com> wrote:
I see the *exact* same problem with Comcast at home. I get about 30 seconds of the 6.6Mbps provisioned rate then the drop kicks in and down to 43kbps it goes.
I suspect this is just bursting/clamping, as you suspect, but you may also want to investigate traffic shaping at your end. I've found I get much better *receive* throughput if I limit my *transmit* rate to less than nominal maximum. Presumably, this has to do with the fact that the feed is asymmetric; I can receive much faster than I can send, and so the send channel becomes congested and that impacts TCP ACK or other protocol control messages.
In talking with Comcast engineers privately, I've learned that the "provisioned" rates should no longer be considered as sustainable, only initial.
In all fairness, Comcast has made this very clear from day one. Heck, they've advertised it extensively on TV, under the brand name "SpeedBoost". They give you a higher initial rate on a given connection, and then reduce it after a few seconds. This makes anything that consists of irregular short requests a lot faster -- web browsing being the obvious intent. Whether you call it "bursting" (initial) or "clamping" (eventual) is semantics. :)
Part of me wonders if this isn't an effort to push "business" class services.
We've got Comcast Workplace at the office for our web browsing (cheap, disposable bandwidth) and the same "SpeedBoost" terms apply. They do offer a symmetric feed option, but it's only 1.5 megabit/sec and costs $300/month. -- Ben