[I_Am_Nous]

The way those cable modem systems work is essentially laying a data channel (usually several) on top of the existing coaxial cable network, similar to DSL laying data on top of the existing telephone network. However this means the cards which transmit and receive are still very much analog beasts, pumping out some incredible signal levels to as far as possible. Similar to DSL, the download centric focus is built into the design. Also, your small modem can't scream nearly as loud as the downstream signal can so some signal loss is more likely, limiting the upload channels. Finally a cable modem network is usually quite shared, with something like 8 transmission lines feeding entire neighborhoods or cities. Depending on node congestion you may not even get your advertised speeds. At least with DSL your line is basically dedicated to you lol