In a method of operating a mobile radio network user data is transmitted from a first network entity to a second network entity the user data is combined prior to transmission into at least one packet data unit in a first convergence protocol layer protocol unit of the first network entity; and the at least one packet data unit is transmitted from a first radio link control layer protocol unit of the first network entity to a second radio link control layer protocol unit of the second network entity. If the transmission of the at least one packet data unit fails, the first radio link control layer protocol unit transmits an error message to the first convergence protocol layer protocol unit after receiving a confirmation message confirming the failure from the second radio link control layer protocol unit.