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The random access channel (RACH) is an uplink transport channel defined in UMTS for initial channel access to the network as well as for sending short data bursts. The RACH structure and procedure as defined by 3GPP are not directly applicable for satellite-UMTS (S-UMTS) due to the different characteristics of the satellite channel. Based on the optimized schemes proposed for S-UMTS RACH, this paper investigates the issue of assigning services with different quality of service (QoS) requirements to different access service classes (ASCs). Such service differentiation is required in third generation wireless systems where multiple service classes with different QoS parameters are to be supported. Via simulations, it is shown that service differentiation is possible through appropriate RACH resource allocation among these different ASCs.