Which layer does UART belong to?
Most interfaces that are integrated on microcontrollers only cover the
  data link layer, with the physical layer being implemented externally
  (with some exceptions). In some cases one and the same data link layer
  might be combined with different physical layer implementations to
  create different interface standards.





  The oldest and still most predominant asynchronous interface is the
  UART (Universal Asynchronous Receiver Transmitter). Some
  implementations support both a synchronous and asynchronous mode -
  then it is a USART (Universal Synchronous Asynchronous Receiver
  Transmitter). The same thing with just a different name is the Serial
  Communication Interface (SCI). With reference to the OSI model, a UART
  implements the data link layer (layer 2). The physical layer (layer1)
  is covered by several driver standards that all utilize the UART data
  link layer, among the most popular are RS232C, RS485 and RS422.


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Asynchronous Interface Overview.  UART and LIN Bus