MAC protocol layer exists in UE & eNodeb, It is part of LTE air interface control and user planes.

The main services and functions of the MAC sublayer include:

• Mapping between logical channels and transport channels;
• Multiplexing/demultiplexing of MAC SDUs belonging to one or different logical channels into/from transport blocks (TB) delivered to/from the physical layer on transport channels;
• scheduling information reporting;
• Error correction through HARQ;
• Priority handling between logical channels of one UE;
• Priority handling between UEs by means of dynamic scheduling;
• Transport format selection;
• Padding.

Protocol specification

3GPP TS 36.321 - Evolved Universal Terrestrial Radio Access (E-UTRA); Medium Access Control (MAC) protocol specification