1. Field of the Invention
The present invention generally relates to a synchronization method of communication, in particular, to a synchronization method adapted to be employed in the third generation mobile communication system for decreasing the call-dropping probability thereof.
2. Description of Related Art
With the arrival of the information era, demands for mobile audio, data communication, and kinds of mobile service are continuously increasing. Limited communication channels and insufficient transmission speed deter the concurrent communication system to be further developed. Therefore, the third generation mobile (3G) mobile communication, capable of providing higher frequency band utilizing efficiency and faster transmission service, has been developed accordingly.
FIG. 1 is a schematic diagram for illustrating a 3G mobile communication network. Referring to FIG. 1, a user equipment (UE) 10, e.g., a cellular phone or one of other handheld communication devices, communicates to a universal mobile telecommunication system radio access network (UTRAN) 11 via radio signals. The UTRAN 11 includes several radio network subsystems (RNS) 121, 122. Each of the RNSs is controlled by a radio network controller (RNC) 131, 132. Each RNC is connected with a plurality of node bases (Node B) 141-143. For example, as shown in FIG. 1, the RNS 121 includes RNCs 131 and Node Bs 141, 142, and the RNS 122 includes RNCs 132 and a Node B 143. The UTRAN 11 communicates with other telecommunication network, e.g., internet, via a core network (CN) 15 to provide 3G audio and/or data transmission service.
If moved or the wireless environment is changed, the UTRAN may shift UE among different RNSs, e.g., changing from connection to the RNS 121 to connect to the RNS 122. Such a procedure is called serving radio network subsystem (SRNS) relocation. In such a procedure, the UE 10 and the UTRAN 11 have to communicate to each other by synchronously using identical setting configuration. For example, the UE 10 and the UTRAN 11 must synchronously use data cipher corresponding to a new setting configuration to encrypt or decrypt the data.
FIG. 2 is a schematic diagram for illustrating the 3G mobile communication synchronization between a UE and a UTRAN. Referring to FIGS. 1 and 2, in an SRNS relocation procedure, the UTRAN transmits a radio resource control (RRC) message containing ciphering configuration information. After receiving the RRC re-configuration message containing the ciphering configuration information, the UE changes to the corresponding new ciphering configuration and transmits RRC response message to the UTRAN. For example, the UTRAN transmits a UTRAN mobility information RRC message containing downlink counter synchronization information to the UE. After receiving the aforementioned RRC message, the UE changes to the new configuration and feeds back a UTRAN mobility information confirm RRC message to respond the reception of the RRC message.
Because of the standard of 3G mobile communication, the UE is setup to measure e.g. wireless channel, according to which the RNC performs and controls channel source management. The obtained results also depend on the adjustment of the wireless channel quality factors for maintaining quality of different services. Therefore, the UE transmits a measurement report at predetermined times to the UTRAN according to the measurement event or the standard. However, the 3G mobile communication standard does not clearly define when the UTRAN should use its corresponding new ciphering configuration. As such, the UTRAN cannot correctly use the same data cipher of the UE to encode or decode the data, which is illustrated with reference to FIG. 4 as follows.
FIG. 3 is a schematic diagram for illustrating incorrectly encrypting/decrypting due to a synchronization failure between the UE and the UTRAN of 3G mobile communication. At the beginning, the UTRAN transmits UTRAN mobility information RRC message containing downlink counter synchronization information and ciphering mode info containing new ciphering configuration to the UE. However, after receiving the aforementioned RRC message, the UE most probably sends measurement reports to the UTRAN at predetermined times triggered by measurement event or following the standard, before changing to the new ciphering configuration and feeds back a UTRAN mobility information confirm. In such a way, the UE will use the old configuration to transmit the measurement report. Unfortunately, if the UTRAN has already used the new ciphering configuration to receive the measurement report, the UE and the UTRAN is unsynchronized, thus causing incorrectly data encrypting/decrypting of the two parties. Affections of the incorrectly data encrypting/decrypting are illustrated below with reference to FIG. 4.
FIG. 4 is a schematic diagram for illustrating incorrectly data encrypting/decrypting of the UE and the UTRAN. Referring to FIG. 4, it is assumed that the measurement report transmitted by the UE is composed of protocol data units (PDUs) 401-402, while the UTRAN mobility information confirm is composed of PDUs 403-404 of the RLC entity. If the data are asynchronously encrypted/decrypted, the UTRAN would decrypt the received PUDs 405-408 into incorrect service data unit (SDU), e.g., SDU 411 and SDU 412 which would not form RRC messages. The measurement report includes measurement values of the wireless connection, and the UTRAN can select exchanging cells according to the measurement values. Most importance of all, the new configurations of data cipher between UE and UTRAN are not synchronous any more and the old configuration cannot be changed back. Therefore, the call-dropping probability is increased.