same framework as intro- duced in Section 9.2, we now analyze the appropriate RLC configurations. First, let us look at the RLC receiver; we surely have to send the status report when receiving the poll. Also, it should be better to enable the missing PDU indicator. No polling mechanism itself could instantly discover a transmission failure at the MAC layer, and, although the missing PDU should be recovered as soon as pos- sible, the autonomous status report by detecting "holes" in the received sequence generally makes the error recovery faster. Given the presence of the missing PDU indication, the status prohibit timer becomes unnecessary unless excessive poll- ing comes from the RLC sender. As to the timer-based status transfer, previous study [5] has suggested a setting of slightly higher than one RLCRTT. The RTT here is the time between issuing a status report and receiving the retransmission from the sender. However, such an RTT-based timer has an inherent weakness because, in the broadband RAN, the RTT may vary a lot more than in the WCDMA case. Because WCDMA assumes a dedicated code channel with determined bit rate, the RTT variation is quite small. In the shared physical channel structure assumed by HSDPA and the future OFDMA-based system, the RTT becomes highly variable, depending on the scheduling and the current traffic load. The negative side of such timer-based status reporting is the difficulty of accurately estimating the RTT and figuring out appropriate timer settings. In addition, the timer-based status transfer issues a status report regardless of the current PDU error rate; thus, an unnecessary status report would be sent. This not only wastes uplink resources but also triggers spurious retransmissions from the sender. © 2009 by Taylor & Francis Group, LLC Long Term Evolution: 3GPP LTE Radio and Cellular Technology On the other hand, let us look at the triggers of polling at the sender side. First, polling should always be sent with the last PDU in the RLC transmission buffer or the retransmiss