 resides on the BS and communicates directly to the air interface. The RLC layer functions (in AM mode) mainly include the following: Segmentation, concatenation, and reassembly: RLC service data units (SDUs) that are received from the upper layer are segmented or concat- enated to RLC protocol data units (PDUs) of a certain size. Each PDU is assigned a sequence number (SN). On the receiver side, the PDUs are reas- sembled to the same RLC SDUs and delivered to the upper layer. Error correction by ARQ: the RLC AM mode basically realizes the selec- tive-repeat ARQ protocol. The ARQ is realized by the mechanisms of PDU SN, polling, and status report. Polling is sent by the RLC sender by mark- ing a polling bit in the PDU, which orders the RLC receiver to send a new status report indicating which RLC PDUs have been received and which have not. The status report is triggered by the polling or self-triggered at the receiver. By examining the status report, the sender RLC retransmits the lost PDUs. In-sequence delivery of RLC SDUs: this can be configured for the RLC layer to deliver SDUs in its original order. Various studies (e.g., Teyeb et al. [3]) have suggested that in-sequence delivery could benefit the telephone communication power (TCP) throughput. Hence, it is normally config- ured as the default for the data transmission over the RLC AM mode. Flow control: this provides a mechanism for the RLC receiver to control the sending rate of the RLC sender. While the RLC receiver maintains fixed-length reception window, it orders the sender to change the transmis- sion window size if necessary. © 2009 by Taylor & Francis Group, LLC User Plane Protocol Design for LTE System Duplicate detection: this ensures that the RLC SDUs are delivered only once to the upper layer. Ciphering function: this provides protection of privacy. Because our study is on the RLC layer performance issue, the focus is necessar- ily on the RLC ARQ function. The standard [1] defines various triggers of sending the polling at the sender and sending 