[nycbug-talk] Traffic shaping a cable internet connection

Chris Buechler nycbug at chrisbuechler.com
Fri Jan 12 15:59:48 EST 2007

Jonathan Stewart wrote:
> I am working on setting up bandwidth limiting for my cable modem 
> connection to help keep latency down but I don't know my actual upload 
> limit. iftop has peaked as high as 1mb/s but the average seems to be 
> closer to 500-700kb/s. I'm not sure if I would be better off setting the 
> limit a little low like 600 and sacrificing a little bandwidth to 
> hopefully reduce latency or if I should just set it to 1mb/s or so and 
> hope it works out right that way? Anyone know anywhere better to ask?

It's essentially impossible to shape a connection with varying bandwidth 
(it's theoretically possible, but very difficult to implement and I 
don't know of any such implementations). If you set it to 1 Mb, but 
you're capped at 700 Kb, it won't help at all. Your modem will start 
queuing and your latency will go through the roof.  If you set it to 700 
Kb and you actually have 1 Mb, you'll be limited to the slower speed, 
but it should work as intended. 

Your cable modem should have a static upload cap (unless you have a 
really, really strange ISP). If you don't know what it is, I'd suggest 
asking your ISP. If your modem's cap is 1 Mb up, you may "max out" at 
500-700 Kbps speeds at times (maybe a limit between you and whatever 
remote site, maybe your ISP employs some sort of traffic shaping on 
their network) but your modem shouldn't be queuing at that point so your 
latency should still be low. If at times you're using 500-700 Kb up and 
your cap is supposed to be 1 Mb, but your latency is through the roof at 
those lower speeds, you have an issue you should address with your ISP 
(assuming nothing on your network is causing the slow downs).


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