PACKET DATA SERVICE IN RADIO COMMUNICATION SYSTEM

Disclosed are a method and a device for offering a packet data service during to a handover of a user terminal from one radio network controller to another. To avoid the loss of data during SRNS relocation, there is provided a method for checking the validity of the next expected receive PDCP sequence number sent from a receiver PDCP layer with the send PDCP sequence number of the first transmitted but not yet acknowledged PDCP SDU and the send PDCP sequence number first unsent PDCP SDU of the sender PDCP layer. A PDCP protocol structure is reconstructed to support a lossless SRNS relocation in the packet service domain, and control information and operational procedure therefore are newly defined. As a result, the lossless SRNS relocation is achieved in the packet service domain and the mobility of data communication is ensured.