 practically, the status prohibit timer delays the status report and also reduces the number because only one status report would be sent upon timer expiry, even if multiple reports have been triggered. The status prohibit timer prevents too many status reports from being sent to congest the uplink. It can be easily figured out that the status prohibit timer should be set slightly higher than one RTT (round-trip time). As can be seen, the RLC protocol itself defines many ways to configure the ARQ operation. The goal is always to have timely discovery of lost PDUs and to have timely retransmission of them. Other issues have to be considered also, such as reducing the number of unnecessary status reports and reducing the number of duplicate retransmissions. It can be foreseen that the previously listed methods to trigger polling and status report should be selectively configured, in order to reduce negative interactions between each other. Some comprehensive studies of RLC configuration and performance for the WCDMA data channel (DCH) can be found in references 4 and 5; however, no comprehensive study has been found for downlink transmission with shared channel as in the case of HSDPA or the future orthogonal frequency division multiple access (OFDMA)-based system. 9.3 Impact of Broadband RAN The RLC was designed at the time WCDMA was introduced; the transmission speed of the air interface was only 384 kb/s (for one user). Also, the separation of RLC and MAC by the lub interface makes the RLC ARQ operation rather slow. The RTT easily amounts to several hundreds of milliseconds. Although the introduction of HSDPA makes the air interface speed much higher (in the order of 1 Mb/s), the lub interface still limits the RLC layer performance. What has changed significantly from WCDMA to HSDPA is that the RLC is no longer the sole mechanism for data loss recovery. Because the MAC layer HARQ (hybrid ARQ) recovers most of the errors on the air interface, the RLC layer only has to handle the residual errors after the HARQ ope