In physical layer, we already have methods to share a common link by dividing into channels using FDM etc.
Then why do we need to think about multiple access at data link layer? I.e why csma, csma/cd are needed? We shouldn't be seeing any collision at all when I'm using a different channel for communication right? Do these protocols come into play only when i'm using a less bandwidth link due to which I can't share them by dividing into frequency bands at all?