1. Field of the Invention
The present invention relates to a radio network controller.
2. Description of the Related Art
In recent years, rapid progress has been made in the research and development of CDMA (Code Division Multiple Access) communications systems, so that not only audio and still images but also a broadband CDMA system (W-CDMA: Wideband CDMA) has emerged. 3GPP (3rd Generation Partnership Project [http://www.3gpp.org/]), which is a standardization group for a 3rd Generation Mobile System, aims to realize a system that can provide high-quality services, developing various W-CDMA communications system standards.
FIG. 1 is a schematic diagram of a conventional 3GPP communications system 100. A core network 102, which is connected to any network, includes a switch 104 between this system and another system. This switch 104 is connected to radio network controllers 106 and 108, which are connected to multiple base stations 110 and 112 and multiple base stations 114 and 116, respectively. These base stations can communicate with a mobile terminal 118 through a radio link in the W-CDMA. The nodes of the switch 104, the radio network controllers 106 and 108, and the base stations 110 through 116 are connected with ATM transmission lines that enable asynchronous transfer mode (ATM) communications.
FIG. 2 shows a main protocol stack employed in such a 3GPP communications system. In the drawing, the left column indicates layers, and the right column indicates protocols. The graphically illustrated layers are, in order from the lowest, the Physical Layer (PHY), the Transport Network Layer (TNL), and the Radio Network Layer (RNL). In the Transport Network Layer (TNL), ATM and AAL2 (ATM Adaptation Layer 2) are employed. In the Radio Network Layer (RNL), a frame protocol (FP), Medium Access Control (MAC), Radio Link Control (RLC), and Radio Resource Control (RRC) are employed. The 3GPP communications system has a data retransmission function for ensuring data, which is realized by the RLC of the Radio Network Layer (RNL) in particular. The part from ATM to RLC corresponds to the Layer 2 (Data Link Layer) that ensures data in the OSI model.
The radio network controllers 106 and 108 (the transmission side) transmit a signal to the mobile terminal 118. The mobile terminal 118, receiving a predetermined signal (data in which a polling bit is set), returns a response message for acknowledgement. The mobile terminal returns an ACK (Acknowledgement) response when receiving the signal (data) normally, and when not, returns an NACK (Non-Acknowledgement) response. The transmission side transmits the next signal when receiving the ACK response, and retransmits the data that has not been received normally by the mobile terminal when receiving the NACK signal. In principle, the transmission side is on standby without transmitting the next signal until the ACK or NACK response is returned. However, if the transmission side is always to wait until receiving the response message, the transmission side ends up waiting continuously when the data of the transmission side disappears in a transmission line before reaching the mobile terminal or the response from the mobile terminal disappears. Accordingly, the transmission side includes a timer function. If the response message is not received before a predetermined period (a timeout period) expires, the transmission side considers that transmitted data is not received normally, and retransmits the transmitted data. In the 3GPP communications system, internode communications are performed through ATM transmission lines that are dedicated lines enabling extremely high-speed communications. Accordingly, the response period and the delay period of the mobile terminal per communications line (connection or call) are substantially constant. Therefore, the predetermined period is set to a constant value characteristic of the system.
By the way, if it is possible to realize a communication system into which the conventional 3GPP communications system and a communication system using the Internet are combined, it is expected that connection fees could be controlled, and that providable services would improve greatly in both quality and amount.
FIG. 3 is a schematic diagram of such a next generation communications system 200. A core network 202 includes a switch 204 between this system and another system. This switch 204 is connected to radio network controllers 206 and 208, which are connected to multiple base stations 210 and 212 and multiple base stations 214 and 216, respectively. These base stations can communicate with a mobile terminal 218 through a radio link in the W-CDMA. The system is thus far the same as that shown in FIG. 1. However, in the communications system shown in FIG. 3, while the switch 204, the radio network controller 208, and the base stations 212 and 216 are ATM nodes that perform ATM communications, the radio network controller 206 and the base stations 210 and 214 are IP nodes that operate based on the Internet Protocol (IP). Of the graphically illustrated transmission lines, the ATM transmission lines are indicated by solid lines, and the IP transmission lines are indicated by broken lines. Converters 205, 207, 211, and 215 for protocol conversion are provided between the ATM nodes and IP nodes that communicate with each other. The Internet Protocol (IP) can be related to the ATM and AAL2 parts of the Transport Network Layer (TNL) in the protocol stack of FIG. 2.
As described above, since the ATM transmission lines are extremely high-speed dedicated transmission lines, the difference in response period between connections is negligible. However, the IP transmission lines are a public network different from dedicated lines. Accordingly, the difference in response period between connections may vary extremely greatly. The network topology of IP transmission lines is not strictly fixed, and communications channel length and devices to go through differ between connections. Further, these are difficult to predict. When the radio network controller 206 and the base station 210 communicate, an IP transmission line 209 is used. However, an increase in the number of nodes to go through causes an increase in transmission delay. Further, when the IP node network controller 206 and the ATM node base station 212 communicate, it is necessary to perform protocol conversion in the converter 211 in the middle of the communications channel. This conversion may cause an increase in transmission delay. These transmission delays due to IP transmission lines are not constant, and may vary constantly depending on the communications environment.
When a signal retransmission procedure as performed in the communications system of FIG. 1 is performed in this communications system 200, some connections (communications channels) may require a response period (a period of time required to go back and forth between the radio network controller and the mobile terminal) longer than a fixed predetermined period (timeout period). In this case, even if the data of the transmission side is normally received, the response message is not returned before the predetermined period expires. Accordingly, retransmission is performed. The retransmission of the normally received signal is an unnecessary retransmission of a signal, so that there is a risk of degradation of transmission efficiency.
To the contrary, other connections may only require a response period much shorter than the predetermined period. In this case also, retransmission should be held up until the timeout period expires. This results in a long period of data transmission although high-speed communications with a short response period are performable. In this case, transmission efficiency is also degraded. Accordingly, it is desired that the timeout period for retransmission be slightly longer than the response period, and a timeout period longer or shorter than this may degrade transmission efficiency.
From this viewpoint, it may be considered to understand all communications channels and set an appropriate timeout period for each communications channel so as to use a different timeout period for each connection. However, as described above, the network topology in IP transmission lines is not strictly fixed. Accordingly, understanding all the communications channels and managing multiple timeout periods appropriately result in a complicated system and higher apparatus prices, thus not being a practical solution.