A simulation study of TCP performance over UMTS downlink

Data transfer on the UMTS downlink can be done either through dedicated channels (DCH) or shared channel (FACH). The data transfers on the DCHs can receive very high throughputs. However, a setup time of the order of 250 ms is required before data transfer can begin on the DCH. For very short transfers it may be better to use the shared channel. However, for long data sessions it might be better to allocate a dedicated channel. In this paper, we propose a threshold policy to determine which sessions should use the dedicated channels. Initially, we use the FACH for a given connection. Then if we get an indication that the current burst might be long (for example, we observe a long queue from that source), then beyond some threshold we shall try to allocate a DCH to that connection (if there is one available). We also present and evaluate the performance of algorithms in which a timer is set when the queue size falls below a threshold and the connection is on DCH. The use of timer allows a connection to remain for a longer duration on DCH and thus improve performance. We use ns-2 simulations to obtain the performance measures for the threshold and timer based policies.