upload tests be aware are often limited by the send buffer, rather than the isp.
also if you have a HG modem, it is defaulted to enable QoS which shaves around 10% of the upload throughput, it can be disabled if you unlock the modem, ECI modems dont have this problem.
A good idea of the upload performance is using the NL based ookla server on speedtest.net. I dont know how high they set the send buffer but its high enough to get full throughput even when interleaved so seems significantly higher than what the BTw tester and all the uk speedtest.net sites use.
To give you an idea of the impact of the send buffer if I send a large email using outlook 2007, I get just get around 2mbit or so of throughput, in 2007 when they designed that app I suspect typical upload speeds were much lower so they probably hard coded a low buffer size into it. however if upload a file via ftp to the same server I host my email on (after configuring a high tcp buffer in ftp client), I get my full 20mbit sec speeds.
The amount of tcp buffer required for full throughput depends on the latency, so eg. a interleaved line will need more buffering for full speed than a fast path line. Many legacy apps eg. use a 4k or 8k bufer which is very inadequate for a high speed FTTC connection. LAN based transfers dont suffer due to the extremely low latency.
also to add if you have eg. windows vista/win7/win8 and disable the tcp auto tuning, this also puts a cap on the upload buffer size.
BT Infinity 2 Since Dec 2012
Edited by Chrysalis (Mon 03-Jun-13 16:25:30)