New cross-layer channel switching policy for TCP transmission on 3G UMTS downlink

In 3G UMTS, two main transport channels have been provided at the layer-2 (MAC) for downlink data transmission: a common FACH channel and a dedicated DCH channel. The performance of TCP in UMTS depends much on the channel switching policy used. In this paper, we first propose and analyze three new basic threshold-based channel switching policies for UMTS that we name as QS (Queue Size), FS (Flow Size) and QSFS (QS & FS combined) policy. These policies significantly improve over a ´modified threshold policy´ in (B. J. Prabhu et al., 2003) by about 17% in response time metrics. We further propose and evaluate a new improved switching policy that we call FS-DCH (at-least flow-size threshold on DCH) policy. This policy is biased towards short TCP flows of few packets. It is thus a cross-layer policy that improves the performance of TCP by giving priority to the initial few packets of a flow on the fast DCH channel. Extensive simulation results show that FS-DCH policy improves over others by about 30% to 36% in response time metrics for a particular case.