Three Levels of Data Link Layer Complexity

• Simplex connectionless - The sender simply sends its frames without waiting for the receiver to acknowledge them. No attempt is made to detect or re-transmit lost frames. Most modern LAN technologies (such as Ethernet) use this method and leave error resolution to higher layers.

  This is also termed an unacknowledged connectionless service.

• Half-duplex connectionless - each frame sent is individually acknowledged. Frames which are lost or garbled are retransmitted if the receiver requests them (again) or after a suitable timeout.

  This is also termed an acknowledged connectionless service.

• Full-duplex connection-oriented - each frame is individually numbered and is guaranteed by the data link layer to be received once and only once and in the right order. The result is that the data link layer presents a reliable frame stream to the network layer.

  This is also termed an acknowledged connected service.