SpeedTest on VDSL B5-Lite Link

I have a 2.1km link between a pair of B5-Lite. One end of the Link has a VDSL circuit which Sync’s at 65/18Mb. Currently the link is running a single 20Mhz channel running 173Mbps PHY. When I speed test the link I receive very poor results and often can’t exceed 30Mbs. I have tested speed over the Link by copying a 3Gb file between two PC’s and when running 80Mhz can get 450Mbs no problems. So the Link seems to be fine. What I have found is if I change the setting Traffic Split window two 25/74 and adjust the TDMA to 2ms I then get the full VDSL speed when i test? If I change it to 4ms it is poor.

The link to date has been really good I have been running it for 1 year and had no other problems. Really just want to understand why the Auto setting does not give me a good result when the manual settings do?

Anyone?? Just really trying to get an idea on why the change to TDMA has affected Speed Test?

Stephen, latency is a huge factor that impacts TCP through-put. Significant latency and changes in latency will negatively impact TCP through-put. So, 2 ms window will provide a lower latency link versus 4 ms. Auto TDMA often provides low latency, however Auto TDMA does not work with collocated links.