Patent ID: 8462953

Claim:
A communication system for transmitting data of a first mobile station to a second mobile station, comprising: a base station, for configuring an uplink and a downlink of the data to correspond to a first connection ID and a second connection ID respectively, and for transmitting a traffic encryption key to the first and second mobile stations such that the first and second mobile stations share the traffic encryption key; and a relay device, coupled to the base station and the first and second mobile stations, for receiving the data encrypted by the traffic encryption key and transferring the data of the first mobile station to the second mobile station according to the first and second connection IDs without going via the base station; wherein the base station periodically updates the traffic encryption key shared by the first and second mobile stations; and, when one of the first and second mobile stations notifies the base station that the traffic encryption key is about to expire, the base station does not update the traffic encryption key until the other of the first and second mobile stations notifies that base station that the traffic encryption key is about to expire.