ol PDU. If an RLC entity receives RLC SDUs from the upper layer, it receives them through a single SAP between the RLC and upper layer, and after forming RLC data PDUs from the received RLC SDUs, the RLC entity delivers the RLC data PDUs to a lower layer through a single logical channel. If an RLC entity receives RLC data PDUs from a lower layer, it receives them through a single logical channel, and after forming RLC SDUs from the received RLC data PDUs, the RLC entity delivers the RLC SDUs to an upper layer through a single SAP between the RLC and the upper layer. When an RLC entity delivers RLC control PDUs to a lower layer, it delivers them via the same logical channel through which the RLC data PDUs are delivered. An RLC entity can be configured to perform data transfer in one of the following three modes: Transparent Mode (TM); Unacknowledged Mode (UM); or Acknowledged Mode (AM). Conse- quently, an RLC entity is categorized as a TM RLC entity, a UM RLC entity, or an AM RLC entity, depending on the mode of data transfer that the RLC entity is configured to provide. UPPER LAYERS (RRC OR PDCP SUBLAYERS) between Upper Layers Transmitting Receiving Transmitting Receiving UM RLC UM RLC AM RLC Entity TM RLC Entity TM RLC Entity Entity Entity Logical Channel Lower Layers (MAC sublayer and Physical Layer) Radio Air-Interface Lower Layers (MAC sublayer and Physical Layer) Logical Channel Receiving Transmitting Receiving Transmitting UM RLC UM RLC AM RLC Entity TM RLC Entity TM RLC Entity Entity Entity between Upper Layers UPPER LAYER (RRC OR PDCP SUBLAYERS) FIGURE 7-22 The structure of the 3GPP RLC sub-layer 7.4 3GPP LTE radio link control and medium access control sub-layers A TM RLC entity is configured either as a transmitting TM RLC entity or a receiving TM RLC entity. The transmitting TM RLC entity receives RLC SDUs from an upper layer and sends RLC PDUs to its peer receiving TM RLC entity via the lower layers. The receiving TM RLC entity delivers RLC SDUs to the upper layer and receives RLC PDUs from its peer t