Ran into an interesting problem the other day.

Had a business customer that was on full speed JetStream. They were
forced to move to UBS like everyone else, but liked the speed, so
decided to go for a 2Mb/s plan.

They gave me a call the other day complaining their WWW browsing was
very poor. Assumed it was related to the ISP, but decided to
investigate further.

Turns out the problem occurs when a small number of users all decide to
send an email at the same time (they don't have an internal mail server,
so use their ISP's SMTP server). This chokes up the 128Kb/s upstream,
and was causing the TCP ACKs to be substantially delayed, causing the
observed slow down in WWW browsing.

Did a further test. I've I get several of their users to all send an
email while doing a download I found the download throughput dropped to
a poultry 64Kb/s. As soon as the emails finish speed takes off again.

There's been some talk about how the 128Kb/s upstream limits the maximum
performance. However, the problem is actually much more severe,
especially when many users sit behind such a link. Very quickly you
reach the point where even a small business can't possibly use the 2Mb/s
which they are paying for.

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