1.  Field of the Invention
The present invention generally relates to a mobile communications system for performing mobile communications by means of switching, and in particular to a system and method for connecting the packet mobile communications system to the Internet. 
2.  Description of the Related Art
In general, to connect a mobile terminal to the Internet through a radio mobile communications network, the mobile terminal has an unique global IP (Internet Protocol) address for the Internet and an identification (ID) number for the mobile communication network assigned thereto.  Further, since the mobile terminal moves from one location to another, it is necessary to identify the current location of the mobile terminal.  To accomplish this, a location register is provided in the mobile communication system.  When the mobile terminal moves to a new location, that is, a new cell, the location of the mobile terminal is registered in the location register through the new cell base station. 
In the case where a packet addressed to the mobile terminal is received from the Internet, the location register is also used to transfer the received packet to the mobile terminal, resulting in increased load on the location register. 
There has been disclosed a connection system for connecting a mobile unit to a network operating according to TCP/IP protocol in Japanese Patent Unexamined Publication No.  4-227149.  More specifically, the system is provided with a global gateway to which a plurality of IP addresses are assigned.  Depending on demand from a mobile unit, the global gateway dynamically assigns an available one of the IP addresses to the mobile unit.  Therefore, the mobile unit temporarily possesses the IP address as necessary. 
This system has an advantage that it is not necessary to assign an unique IP address to each mobile terminal.  However, when receiving a packet addressed to the mobile unit from the Internet, the location register will be also used to identify where the mobile unit is currently located.  Therefore, the load on the location register is increased. 
An object of the present invention is to provide an internet communication system and method which can achieve the decreased number of IP addresses which are required in the Internet. 
Another object of the present invention is to provide an internet communication system and method which can achieve the decreased load on connection control. 
According to the present invention, in a communication method between a mobile station and a network which requires a unique network address for each terminal, the mobile station having an identification number assigned thereto in a mobile communications system including a plurality of mobile communication switching devices, the method comprising the steps of a) instructing the mobile station which has transmitted a communication request signal to a mobile communication switching device to use a private address as a source address of an outgoing signal, wherein the private address is selected from a group of private addresses uniquely assigned to the mobile communication switching device; b) converting the private address to a unique network address as a source address to transmit the outgoing signal to the network when receiving the outgoing signal having the private address as the source address; and c) storing a first correspondence of the private address and the identification number of the mobile station and a second correspondence of the private address and the unique network address so that the first and second correspondences are used to transfer an incoming signal from the network to the mobile station. 
When transmitting an outgoing signal to the network, a private address is assigned to the mobile station which has transmitted the outgoing signal to a mobile communication switching device, wherein the private address is selected from a group of private addresses uniquely assigned to the mobile communication switching device.  A first correspondence of the private address and the identification number of the mobile station is stored.  After converting the private address to a unique network address as a source address, a second correspondence of the unique network address and the private address is stored. 
When receiving an incoming signal from the network, the incoming signal having the unique network address as a destination address, the unique network address is converted to the private address according to the second correspondence, and the incoming signal is transferred to the mobile station through a mobile communication switching device based on the first correspondence.  A first mobile communication switching device corresponding to the private address may be selected according to the first correspondence and the incoming signal is transferred to the first mobile communication switching device. 
Subsequently, it is determined whether the mobile station can communicate with the first mobile communication switching device.  When it is determined that the mobile station can communicate with the first mobile communication switching device, the incoming signal is transferred to the mobile station through the first mobile communication switching device. 
When it is determined that the mobile station cannot communicate with the first mobile communication switching device, a second mobile communication switching device which can currently communicate with the mobile station is searched for.  The incoming signal is transferred from the first mobile communication switching device to the second mobile communication switching device, and then is transferred from the second mobile communication switching device to the mobile station. 
Since the private address is assigned to the mobile station when the communication is started, the decreased number of unique network addresses is achieved, because all the mobile stations hardly perform concurrent communications at a time. 
The searching for the mobile station is performed over the mobile communication switching devices when it is determined that the mobile station is not located in the relatively wide service area of the mobile communication switching device.  Therefore, the number of times a location register is accessed is dramatically reduced.