This may be a stupid question but it seems to me due to TCP behavior.
If we have 1Mbps WAN Link ( 1Mbps Pipe, with any QoS) and i start a file transfer and get the dpeed of 500Kbps. With first Transfer running i start the second file transfer and get the totoal speed or WAN utlization of 900Kbps.
Question is that why the first file did not make the full usage of available bandwidth ?
When there is no QoS on the WAN links how the apps make use of the bandwidth.
Any Assistance grealy appriciated.
"..are you indicating the Window Size in TCP negotiation ?"
Yes. Although you normally don't see this issue at 1 Mbps unless your end-to-end latency is very high. Also, some host stack's default receive window size is larger if LAN is running at gig (I have in mind Windows host pre-Vista) which tends to avoid this from happening.
The other common TCP issue that will cause thoughput to be lower than expected are packet loss conditions that keep driving the TCP session back to slow start. (It's unusual for you to see this with one flow vs. two, I believe, since multiple TCP flows often will "sync" their loses.)
Lastly, have you provided enough time for TCP to fully ramp up? Two flows will ramp up faster than one flow.