Link Speed – Maximum speed per session

I work at a teleport and most of our business is related to providing Internet service to ISPs in South Asia. One of the most frequent complaints that our help desk gets is “My link is slow, you guys are not providing us the complete speed”. Usually it is a case of the customer not understanding the finesse of TCP/IP. I came across this article by Adam Grow III. He uses the following formula to calculate the maximum possible speed per session on a link.

Throughput = 8*(TCPS/PropDelay)

Throughput is measured in Kilobits per seconds (Kbps)
TCPS = Size of the TCP/IP stack in byte (Windows default is 8192)
PropDelay = propogation delay in seconds

Using the above formula, for a geostationary satellite (where the propogation delay is ~540ms), the throughput is ~120Kbps. So even if the client has a 1Mb link and he just has one session, he can only use a fraction of the link. Only multiple sessions can saturate the link.