tocol stack and interfaces (replicated from 3GPP 38.300). error correction through retransmission in AM sequence numbering segmentation/resegmentation RLC layer flows are set up, reestablished, and released by RRC layer commands and flows are unique per radio bearer/channel (and obviously, per UE). The RLC transfer modes are defined by the type of channels that they are used for and the traffic characteristics associated with them. TM is associated with BCCH, CCCH, and PCCH. TM processing is effectively a bypass mode-there are no headers added and the RLC functionality is nil. UM processing supports fragmentation and reassembly. This func- tion is needed to map the variable (IP packets after all) size RLC SDUs to the MAC PDU size that is determined by time/frequency resources that are dictated by Physical Layer lim- itations and allocated by the MAC scheduler algorithm. To enable out-of-order processing Chapter 2 System components, requirements, and interfaces 61 on the receiving side, the UM RLC protocol adds a sequence number and offset field and a couple of segmentation indicator bits in an RLC header that is attached to the PDU. AM operation is the same as UM operation but includes an acknowledgment mechanism through status reports that allows retransmission of dropped packets and thus reliable packet delivery to upper layers at the cost of increased latency associated with retransmission. Selection between AM and UM operation is done during channel establishment and done based on the target QoS characteristics. UM is used when reliability is not required-for example, for voice and video traffic. AM is used when reliability is required, for example, for TCP-based flows such as web traffic. Per 3GPP 38.321, MAC functions include: map between logical channels and transport channels and as such present logical channels to the RLC and upper layers multiplexing/demultiplexing of RLC PDUs onto MAC PDUs or TBs that are transmitted and received by the Physical Layer scheduling and priority handling error correction t