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Bandwidth limit

Recently we upgraded our internet bandwidth from 8 to 34Mbps. When we try to download a huge file from sites where there is no bandwidth limitation, we are not able to cross more than 200Kbytes per sec, eventhough the link utilisation is at 5 Mbps. Our argument with the service provider is when bandwidth is free, why we are not getting more download speed. Is there any setting on our router to be modified. We are using Cisco 3725 router IOS 12.3(14)T7. To check, we directly connected one lapto to the ISP's equipment and still we are not able to cross 200KBps. Pl advise why bandwidth is getting limited.




Re: Bandwidth limit


are you sure you have the proper client settings to achieve more than 200 KBps?

TCP throughput = (window size)/RTT

With 16 KB window size (default in some operating systems) and .8 sec RTT you end up with 200 KBps per TCP session. This is then a TCP limit and you could have 10 Gbps end-to-end and would not cross it.

So make sure you have Window scaling turned on and the window size large enough.

Check your client settings first and also make sure there is no download limit applied by the server side.

As you can get download rates of 5 Mbps (combined) I would not assume there is any configured limit implied by the SP.

Hope this helps!

Regards, Martin


Re: Bandwidth limit


I believe that you have a problem with TCP window size. Never do a performance test with file transfer. You have tested the the TCP/IP stack of your laptop and maybe of the remote server. Windows use only a window size of 64k. I recommend that you test the performance with UDP traffic, because there is no neeed for ACK for the data packets. And by the way, file transfer with FTP is very bad, because most of the FTP programs are very old and simple.

I hope this helps.

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