Quote:
Originally Posted by spiderplant
Please can you run us through the simple math? I think it may be too simple.
I did a quick test with a 20Mbps modem earlier. Doing a single FTP, I achieved a 46:1 downstream:upstream ratio, and that was without any tweaking of the receiving system.
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That DOES sound a bit simple ... at what layer were you measuring this bandwidth? From 1:46, I'd assume you're just measuring IP packets (40/1500 would be 1.15:46).
But, you probably performed this test over ethernet. On top of your IP packet, you have an ethernet frame plus additional protocol overheads. This would normally add up to 38 bytes (ethernet addresses, packet length, checksum, guard space, etc.)
38 bytes on top of a 1500 byte data packet is 2.5% extra. 38 bytes on top of a 40 byte ack packet is 95% extra, almost double.
So, for ethernet your measured ratio would be 1.95:49.025, assuming everything above; that's close enough to 2:50.
Now, there are two BIG caveats, and one little one.
Firstly, the cable modem is NOT ethernet. I really don't know enough about it to know what sort of overheads there are, but you can be sure there's a source and destination address. The fact that the cable side has MAC addresses which look just like ethernet addresses would suggest that they could be comparable.
Secondly, I've no idea at which protocol level the bandwidth limit is at. If it's discarding the lower-layer protocols before calculating the speed, then 1.33Mbps would be sufficient (although NOT enough to you to make best use of the bandwidth).
If the bandwidth limit INCLUDES the additional overheads (whatever they may be) then it's unlikely that 1.33Mbps would be enough: even 12 bytes (two mac addresses) is enough to increase the bandwidth requirement to 1.77M.
Thirdly, delayed-ack could reduce the ack traffic by half. It will for some people ... but from your calculations above, not you.
Of course, this is all academic if you want to do anything except downloading large files. Most people will be want do to things like check for e-mails, send e-mails, read web pages, have Windows doing updates, NTP updates, weather updates, RSS feeds.
More upload would be beneficial, but then again it's more than you'd get with 20Mbps so it's a move forward.