Hence each time B sends a frame to A it must stop-and-wait for an acknowledgement before sending another frame
Hence each time B sends a frame to A it must stop-and-wait for an acknowledgement before sending another frame
Since frames must now be able to travel in both directions, a channel which is capable of at least half-duplex communication is necessary
Note however that data is still only travelling in the one direction i.e. to the network layer the channel seems to be simplex