Data-link layer.

The data-link layer is the hardware/electronic/bit level layer of
networking.

Responsible for :

  converting data bits into electronic signals.

  converting electronic signals into electrical signals,
    radio waves, or light pulses.

  lowest level data reliability.

  embedding timing data in data stream.

  embedding data onto a reliable transmission encoding.

  providing a static hardware level node indenfication 
    Media access control or MAC