I'm not DrDrew either (and he and others more expert on this can correct/clarify what I say), and while "news @ videotron.ca" has touched on a lot of this already, here's my shot at this anyway:
The asymmetry between the much higher download speeds vs. the lower upload speeds is basically due to both the asymmetrical bandwidths and SNRs of the downstream (or "forward") path vs. the upstream (or "reverse") path.
Because of the original broadcast TV channel assignments in the US, the upstream path from the customer back to the cable headend has been limited to between 5 to 42 MHz (vs. bandwidths of 50 MHz or more that are currently being allocated to HSI service in the downstream).
This bandwidth-limited, low-frequency, upstream path is also susceptible to an number of impairments such as impulse noise, the "ingress" of interfering signals from outside the system (such as from CB and ham radio transmissions), and from noise and interference originating from inside subscribers' homes. This results in SNRs that are typically 6 dB or more worse than those obtained on the downstream path, so that only lower-order modulations that can tolerate the lower SNRs, like 16- or 64-QAM, can be used (vs. 256-QAM in the downstream).
Here are some recent threads in this forum that have discussed the limitations of the upstream path:8 downstream 8 upstream modem? -
»8 downstream 8 upstream modem?4 Upstream Channels -
»4 Upstream Channels
Here are also some other articles that discuss the issues with the upstream path and some things that can be done to reduce the asymmetry:Why Is Cable's Upstream Path So Darned Ugly? -
»www.translation-please.com/colum···umnid=97 Cable's Upstream Gap -
»www.lightreading.com/blog.asp?bl···lr_cable Will Anyone Switch On Cable's Upstream Booster -
EDIT: BTW - Here are the other parts of Leslie Ellis's early articles about Cable's upstream path (and still fairly applicable for being over 10 years old!):Peer-to-Peer Meets Cable's Upstream Path -
»www.translation-please.com/colum···umnid=98How Cable Gets More Stuff Upstream -