In light of the expected growth in wireless data communications and the commonly anticipated up/downlink asymmetry, we present a performance analysis of downlink data transfer over extsc{d}ownlink extsc{s}hared extsc{ch}annels (extsc{dsch}s), arguably the most efficient extsc{umts} transport channel for medium-to-large data transfers. It is our objective to provide qualitative insight in the different aspects that influence the data extsc{q}uality extsc{o}f extsc{s}ervice (extsc{qos}). As a most principal factor, the data traffic load affects the data extsc{qos} in two distinct manners: {em (i)} a heavier data traffic load implies a greater competition for extsc{dsch} resources and thus longer transfer delays; and {em (ii)} since each data call served on a extsc{dsch} must maintain an extsc{a}ssociated extsc{d}edicated extsc{ch}annel (extsc{a}-extsc{dch}) for signalling purposes, a heavier data traffic load implies a higher interference level, a higher frame error rate and thus a lower effective aggregate extsc{dsch} throughput: {em the greater the demand for service, the smaller the aggregate service capacity.} The latter effect is further amplified in a multicellular scenario, where a extsc{dsch} experiences additional interference from the extsc{dsch}s and extsc{a}-extsc{dch}s in surrounding cells, causing a further degradation of its effective throughput. Following an insightful two-stage performance evaluation approach, which segregates the interference aspects from the traffic dynamics, a set of numerical experiments is executed in order to demonstrate these effects and obtain qualitative insight in the impact of various system aspects on the data extsc{qos}.
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