ANONYMOUS wrote: In practice the situation is blurred because the PL and DLL are combined in a Media Access Control (MAC) Layer, such as a single Ethernet interface card. If taken too literally - that the PL is just the wire itself, the scene where errors and collisions occur - then we must assume that the DLL has all the responsibility of decoding the signal and detecting invalid signals. But if we believe that the PL includes, not just the wire, but also circuitry to encode and decode the signals, then that circuitry 'simultaneously' detects invalid signals. IN case of wired Ethernet that same decoding circuitry might also implement signal truncation and jamming. You may find it easy to think of the PL as performing the 'physical' roles, and the DLL as the 'logical' role. As above, you can think of collision detection as a 'physical' activity, but deciding what to do about it, a 'logical' activity, a role of the DLL. The wire, the air, can't do anything except propagate and attenuate the single, but they are directly connected the circuitry that can make 'sense' of what is observed.