The upstream connection must be a CSMA/CD network, almost exactly like an ethernet exceot with slightly tighter bandwidth requirements so that you don't step on everyone else's MTV with your outgoing data. Your box is competing with the house down the street, and the simplest way to do that is CSMA/CD, which is going to limit efficiency, especially under heavy upstream load. Some networks cannot do any upstream services (they have amplifiers pointing the wrong way in between the home and the head end where the data-to-RF convertor sits); these systems typically send the upstream data over a phone line.
A report on the Glenview network:
This looks like the precursor to the network being set up here. They connected schools, libraries, and government groups through a secondary cable plant in a Chicago suburb in 1993. This report is also pointed to by the Zenith HomeWorks page. They make cable modems but have a really boring page. There should be a rule that says page design consultants can't use background gifs and variable link colors until they first provide useful content.