The Impact of RLC Delivery Sequence on FTP Performance in UMTS

The Radio Link Control (RLC) protocol of Universal Mobile Telecommunication System (UMTS) provides an option for in- or out-of-sequence delivery of Service Data Units (SDUs) to upper layers. In this paper, the impact of this setting on the performance of File Transport Protocol (FTP) sessions is investigated. The investigations are carried out using a real-time emulation platform, and Transmission Control Protocol (TCP) with and without Selective Acknowledgement/Forward Acknowledgment (SACK/FACK) options. The results show that out-of-sequence delivery has a negative impact at higher bandwidths and TCP FACK exacerbates the problem. This is mainly due to the aggressive retransmission strategy employed by FACK, which considers the gaps in a SACK report caused by reordering as lost segments and retransmits them immediately