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This paper proposes overflow control scheme to support high-speed downlink packet access (HSDPA) mechanism in the Universal Mobile Telecommunication System (UMTS). To access the UMTS services, a user equipment (UE) communicates with all cells (base stations) in an active set. However, multiple links between the UE and the cells in the active set may reduce the transmission speed due to interference. 3GPP specification TR 25.950 proposes HSDPA. In HSDPA, the UE only selects one cell (referred to as the serving cell) in the active set for high-speed downlink transmission. In this mechanism, the radio network controller sends the packet frames to the cells in the active set. For the serving cell, the packet frames are forwarded to the UE. On the other hand, every non-serving cell in the active set queues the packet frames in a buffer. If the link quality between the serving cell and the UE degrades below some threshold, the UE selects the best cell in the active set as the new serving cell. Since the non-serving cells do not send packet frames to the UE, their buffers may overflow. In this paper, we propose the scheme to address the buffer overflow issue. Our scheme guarantees that when the buffer of a non-serving cell is full, the previously received packet frames in the buffer can be safely dropped, and after the UE has switched wireless link to the new serving cell, no packet frames are lost.