The transmission lines are half-duplex on a channel by channel basis. At any given time you can either use a channel for upload or download, but not both. And

The transmission lines are half-duplex on a channel by channel basis. At any given time you can either use a channel for upload or download, but not both. And usage is very asymmetric. It's more efficient to thus allocate more channels for download. Otherwise, you'll have unused upload channels while the download channels are saturated. It's mostly about the connection from the local node to your house, not the backbone.