 upper layer and receives RLC PDUs from its peer transmitting TM RLC entity via lower layers. An UM RLC entity is configured either as a transmitting UM RLC entity or a receiving UM RLC entity. The transmitting UM RLC entity receives RLC SDUs from upper layer and sends RLC PDUs to its peer receiving UM RLC entity via lower layers. The receiving UM RLC entity delivers RLC SDUs to upper layer and receives RLC PDUs from its peer transmitting UM RLC entity via lower layers. An AM RLC entity consists of a transmitting side and a receiving side. The transmitting side of an AM RLC entity receives RLC SDUs from upper layer and sends RLC PDUs to its peer AM RLC entity via lower layers. The receiving side of an AM RLC entity delivers RLC SDUs to upper layer and receives RLC PDUs from its peer AM RLC entity via lower layers. The following applies to all RLC entity types (i.e., TM, UM, and AM): RLC SDUs of variable sizes which are byte aligned (i.e., multiple of 8 bits) are supported RLC PDUs are formed only when a transmission opportunity has been notified by lower layer (i.e., by MAC) and are then delivered to lower layer. 12 SDU is a set of data that is sent by a user of the services of a given layer, and is transmitted semantically unchanged to a peer service user. Basically the SDU is the data that a certain layer will pass to the layer below. It differs from the PDU in that the PDU specifies the data that will be sent to the peer protocol layer at the receiving end. 3GPP Evolved Packet System (EPS) Uplink Logical channels Uplink Transport channels UL-SCH Figure 3.15 Mapping between uplink logical channels and uplink transport channels. Reproduced with permission C 2009 3GPP [13] Downlink Logical channels Downlink Transport channels DL-SCH Figure 3.16 Mapping between downlink logical channels and downlink transport channels. Reproduced with per- mission from 2009 3GPP [13] upper layer (i.e. RRC layer or PDCP sublayer) SAP between upper layers transmitting receiving transmitting receiving TM RLC entity TM RLC entity UM R