The following is slightly on and off topic. I decided to post as I think some users might be interested.
On the blueyonder newsgroups Telewest Broadband's Principle Engineer posted the following in relation to upload speed:
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The noise caused by the modems is one issue, and one we take seriously,
but it isn't the fundamental reason why cable modem networks are
asymmetric by nature (more download than upload).
To understand this, you have to know the history of cable. The return
path, which is the path for the upstream/upload path, has a much smaller
frequency spectrum than the downstream.This is because of the way the
network equipment is manufactured-the only reason we can come up with is
that in the early days, the return path was a bit of an afterthought,
and all focus was on the downstreasm/forward path, because that's where
the money was (analogue broadcast).
Then came along DOCSIS, and cable modem broadband. Cable companies
didn't fancy ripping out their new'ish networks so we had to find the
best way of leveraging what we had.
So in the downstream, we have a modulation technique called QAM64 or
now, as we are rolling out, QAM256.This can give you between 31 and
38Mb/sec theoretical throughput, shared over the users on that
"segment".The return path, uses QPSK (QAM4).This gives about 2Mb/sec or
5Mb/sec, depending on the carrier width.Again this is shared over the
"segment".A segment is a group of customers, grouped together logically
by their cable node ID's.
So you can now see that we have to manage that imbalance.We are in the
process of testing a new upstream modulation type called QAM16 which can
be driven up to 10Mb/sec over the segment, so we are increasing all the
time.You can do 2 things with the extra bandwidth-increase upstream
bandwidth on a per-user basis to existing customers or increase the
number of customers per segment, whilst retaining the incumbent upstream
bandwidth per user.
We are doing a bit of both
Hopefully this gives an idea of what it is all about-if you want o get
really technical, I highly recommend the following website
http://www.ct-magazine.com/
The archives section is a fantastic source of information and it written
by the leaders in their field
Hope this helps
Stewart Dunn
Principal Engineer
Central Operations Support Group
Telewest Broadband
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