It depends on the layer 1 medium. In a shared RF environment (dialup, cable, wifi, p2p RF) you normally have a shared bandwidth space. This is typically "channelized" and you can pick how many channels you want for "up" and how many for "down". This isn't normally dynamic, it needs to be fixed by the standard or at the very least by the head end equipment. It's more expensive in the above RF spaces because cabling is expensive the more home runs you want to do. With fiber it's a little easier, 1:32 pon splits still give a _lot_ of bandwidth for upstream because it's easier to isolate the adjacent wavelengths (you can pack them in tighter), and the normal noise floor is lower. With 1:1 fiber DIA it's a total non issue and you can do what ever is the limit of the noise of the fiber and the limits of the transceivers (typically optical packages in sfp/sfp+ packaging these days)