Optimum Number of RLC Retransmissions for Best TCP Performance in UTRAN

As far as transfer reliability is concerned, the wireless segment of UTRAN is the weakest part of an Internet session. To overcome this imperfection, the RLC protocol features a link-level retransmission mechanism. In this paper, we show by analytical means and computer simulations that for a given Block Error Rate (BLER), the maximum number of RLC transmissions has a very important impact on TCP performance, both Round Trip Time (RTT) and throughput. We extend the link level Automatic Repeat Request model proposed by Peisa et al. (2001) to be able to set a maximum number of RLC transmissions, and with the help of Padhye's TCP throughput model (2000), we determine the corresponding throughput analytically. We also show that, knowing the current BLER, a network operator may tune the maximum number of retransmissions either to reduce the RTT or to increase the throughput.