[jsz0]

PON FTTH is a shared medium typically being split 32 or 64 ways so there is some over subscription to consider but for the most part there are no major technical limitations to offering symmetrical speeds. For cable ISPs there are a few problems: The amount usable of return/upstream spectrum on a cable plant is traditionally much lower than the amount of forward/downstream DOCSIS bandwidth. Until DOCSIS 3 there was no cost effective way to utilize all of the available spectrum anyway. DOCSIS 3 introduces upstream channel bonding and pushes forward with SCDMA modulation which, in theory, makes the lower/noisier spectrum under 20Mhz usable. Some cable ISPs are planning for more return spectrum space (50-75Mhz) which also solves the problem. The big hardware players in that space are lagging behind a bit on upstream bonding but it should be available soon. (it's already out there in limited deployments)