In 3G UMTS, two main transport channels have been provided 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 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 [1] 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 and 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 confirm this improvement for the case when number of TCP connections is low.
