Why are my downloads up to ~1500KByte/sec only, when the ADSL connection locks at 13611Kbit/s?

Question

No uploading is going on other than the overhead of downloading which appears to be not high for the abilities of the connection: Only about 30-40KByte/s when the router locks at 1012Kb/s and other direct uploads or uploading overheads can reach more than 100KByte/sec so I don't think it's a congestion at uploading that is doing it.

Is there something I miss? Because I assume 13611Kbit/s should be ~1701Kbyte/sec.

Is it an overheard at the ADSL level I don't understand? Could it be the ISP doing it?

If it's active throttling it can't be on single connections since 2 high speed connections still go up to ~1500KByte/sec.

It's not an example on torrents or other complex situations.

The tests were on Ethernet, but I doubt the results would be different on wireless.

I wonder if the settings of those connections at my end could be doing it, e.g. MTU settings, though I haven't touched the defaults of a common Realtek NIC.

Answer 1

The speed of your ADSL connection is a maximum that in practice you will very rarely achieve. There are numerous reasons for this.

All protocols, including TCP/IP, HTTP, FTP, etc introduce some overhead to the process. So if you're downloading a file, and measuring the rate at which you're receiving the file (in KBytes/sec), you need to add the TCP/IP and layer 2 overheads, as well as the overhead of any higherlevel protocol in play (HTTP, FTP, etc) in order to get to the actual bit-rate.

Your download may also be slower due to congested pipes upstream of your ISP, or even within your ISP's network. If your ISP has 200 clients connected at 10Mbps, but only has a 1Gbps uplink to the internet then if all the clients are on at one time they'll only be able to use half their nominal bandwidth before the ISP's uplink is saturated. In practice this kind of oversubscription generally works well, because you usuallu don't have everyone on at the same time,but the effect can be noticeable during peak times.

Another consideration is the speed at which the server your downloading from is connected to the Internet. If you've got a 10Mbps connection but the server at the other end only has 5Mbps you'll never achieve more than 5Mbps (and probably less) downloading from that server unless there's some caching involved in between. And the same congestion consideration is at play as well if it's a popular server that hasn't sized its connections (or caching) well for the level of traffic it gets.

There are other factors as well, almost all of which are out of your control.

And having said all that, the numbers you're quoting mean you're hitting about 89% of your nominal bandwidth without taking any protocol overhead into consideration. That's a pretty good figure.