A serial interface and the T1 controller are involved in sending physical signals over the wire so they are involved in layer 1 of the OSI model. The serial interface more so than the T1 controller are involved in encapsulating the data for transmission over the wire so they are involved in layer 2 of the OSI model. The serial interface and the T1 controller do not do any processing (or have any understanding)of IP addressing so they are not involved in layer 3 or any other higher layers of the OSI model.
HTH
Rick
HTH
Rick