Abstract:
There is provided a hybrid communication terminal which can overcome a drawback that, in performing switching a system between a 3G network and a WiMAX/LTE is performed under control of a mobile unit, when an IP address allocated to a mobile unit from the network differs before and after system switching, an active session of communication application is disconnected so that it is impossible to continue the communication. In automatically switching connection between different systems during communication, in order to conceal from the communication application that an IP address assigned by the network is changed, an exchange of user data with the communication network is performed via a virtual network device where the IP address is always the same.

Description:
CLAIM OF PRIORITY 
     The present application claims priority from Japanese patent application serial no. 2010-270299, filed on Dec. 3, 2010, the content of which is hereby incorporated by reference into this application. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to a hybrid communication terminal which is connected to a computer, and more particularly to a hybrid communication terminal which receives the supply of power from the computer and can perform the handover while continuing the communication. 
     As a mobile communication system, there has been known a broadband mobile service which is based on an existing mobile network such as a High Speed Packet Access (HSPA) and 1xEVDO which are regarded as technology of 3G, particularly 3.5G of recent years. Further, an environment has been created where a user can make use of various networks which includes novel broadband mobile services such as WiMAX which is regarded as the next generation technology which realizes the wider broadband communication and LTE which will soon commercially available. 
     For this end, user contents are no longer limited to voice communication services, a mobile body dedicated internet with small capacity for mobile and an exchange of e-mails. That is, WiMAX and LTE allow users to sufficiently make use of broadband contents represented by motion picture streaming even under a mobile environment even when the lowering of a communication speed generated by a Doppler effect which occurs when the moving body moves at a high speed is taken into consideration. Further, with the advent of a low latency network represented by LTE, an environment where a VOIP service becomes fully available at a portable terminal has been created. 
     WiMAX and LTE which are novel broadband mobile services are excellent in high speed property compared to the 3G network which uses an existing mobile body network as a base. On the other hand, WiMAX and LTE are provided in a spotted manner with respect to service areas in an initial stage of practice. As a result, there is not yet provided a situation where WiMAX or LTE can be used in a single form anywhere and any place. Accordingly, a need has been increasing for a terminal which can automatically switch a communication mode in such a manner that the communication using WiMAX or LTE is conducted in an environment where WiMAX or LTE is available, and an existing 3G network having a wide service area is conducted in an area outside ranges of these high speed networks. 
     Further, in the connection switching between the 3G network and WiMAX/LTE, the maintenance of the continuity of communication is an important requisite in terms of providing services not only at the time of using VOIP but also at the time of using contents where data are continuously exchanged such as streaming service. Assuming a case where the connection switching between the existing network and WiMAX/LTE is possible in a situation where the continuity of communication is not maintained, the contents which a user can use are limited eventually. This becomes an obstacle for a communication provider and an Application Service Provider (ASP) who provide various and versatile services and hence, there has been a strong demand for the maintenance of the continuity of communication at the time of switching the network. 
     The technique disclosed in JP-A-2008-236079 can perform the communication of the same data simultaneously between a communication terminal provided with a Media Independent Handover (MIH) function and two different network systems. The communication terminal starts the connection with the first network, and monitors radio communication quality between the communication terminal and the first network. The communication terminal, when a monitor result is lower than a preset threshold value of radio communication quality, performs the connection with the second network. The communication terminal starts communication between the communication terminal and the second network using data equal to data used between the communication terminal and the first network. The communication terminal monitors a radio communication quality state between the communication terminal and the second network in the same manner. The communication terminal, when it is confirmed that the communication can be performed with radio communication quality between the communication terminal and the second network exceeding a certain threshold value, disconnects the connection with the first network so that the handover is executed. 
     In JP-A-2009-049875, a communication terminal performs communication simultaneously with two different networks or with either one of these networks which is communicable. The communication terminal connects with the first network and acquires information such as communication quality between the communication terminal and the first network, power consumption when the communication is performed between the communication terminal and the first network, a communication speed between the communication terminal and the first network, and communication application to be used. In the same manner, the communication terminal can also acquire the similar information between the communication terminal and the second network. The communication terminal uses these monitor values as parameters of handover, and selects the network on a side where power saving can be realized within a range where communication application to be used can be used without troubles. That is, the communication terminal executes a new connection or the handover with the side where power saving can be realized. 
     &lt;Task to be Solved in Suppressing the Increasing of Cost and in Enabling Early Start of Providing Services in Mounting Functions Under Control of Network&gt; 
     A conventional handover system between different networks requires dedicated facility for realizing a handover which maintains the continuity of communication not only on a communication terminal but also on respective network sides. 
     The communication terminal requires a dedicated circuit and software for controlling switching networks. On the other hand, the networks require the provision of a dedicated facility in respective network systems. Accordingly, the initial facility investment necessary for a telecommunication provider is increased leading to a high cost. 
     Further, to realize the handover which maintains the continuity of communication, with respect to a dedicated facility which is added to a network, it is necessary to take into account easiness of future system migration, roaming or the like. For this ends, generally used is a dedicated facility which supports a method fixed by the standards organization, to be more specific, 3GPP and 3GPP2 in the case of 3G network or LTE, and IEEE and WiMAX forum in case of WiMAX. However, it takes time for review before these standards organizations fix the handover specification between WiMAX or LTE and the existing network. Accordingly, the telecommunication provider faces an obstacle in a handover service between the WiMAX or LTE and a 3G network from the beginning of providing services using WiMAX or LTE in a single form. 
     With respect to WiMAX and LTE, at the time of starting providing services, a service area is narrow. Accordingly, in general, adopted is a method where the propagation of the service is promoted by maintaining the continuity of communication with the existing network. However, there arises a drawback that the maintenance of the continuity of communication cannot be acquired in an early stage. 
     &lt;Task to be Solved for Realizing Maintenance of Continuity of Communication&gt; 
     In performing switching of the system between 3G network and WiMAX/LTE during communication under control of a mobile unit, on a network system where there is totally no exchange between both networks, an IP address before switching of the system and an IP address after switching of the system differ from each other. Accordingly, a change of an IP address is detected by an OS of a posterior terminal (communication terminal to which the mobile unit is connected) such as a PC resulting in the disconnection of active session of communication application. 
     Even when an interwork is provided between both networks so that the communication is performed on a network system where an IP address is not changed before and after system switching, it is impossible to register two same IP addresses on a PC at system switching timing from a viewpoint of an IP network system. Accordingly, there is timing where IP address is not present before and after system switching. When the communication application performs the exchange of data at this timing, an active session of the communication application is disconnected in the same manner as the above-mentioned task. 
     &lt;Technique Disclosed in Prior Art Document&gt; 
     JP-A-2008-236079 discloses the invention relating to a handover method which uses an MIH technique. Usually, to realize the maintenance of the continuity of communication at the time of switching the system in the network system which uses the MIH through an MIH technique, an agent server of the MIH is arranged on a network side. In general, the management of a session including IP addresses of different network systems is controlled on a network side using the agent server of the MIH. 
     JP-A-2008-236079 completely fails to disclose a method where an IP address is relayed under control of a terminal side. JP-A-2008-236079 also does not refer to as an MIH server. However, it is at least considered that JP-A-2008-236079 discloses a technique which depends on a network side function with respect to the maintenance of continuity of communication at the time of switching the system between the 3G network and the WiMAX/LTE. 
     JP-A-2008-236079 also discloses an example where a hand-set-type mobile unit is used. In this case, a collective control can be performed on a mobile unit side including the management of an IP session and the management of communication application. Even when an IP address is changed or timing at which IP address is not present at system switching timing, the control of communication application by concealing such a state can be performed relatively easily because of a closed platform. On the other hand, in a case such as a mobile data card where the continuity of communication at the time of switching the system is maintained using a function provided on a platform under an environment of an independent open platform such as a PC, it is necessary to conceal a change of an IP address or the presence of timing at which there is no IP address in the OS or the communication application. However, in JP-A-2008-236079, there is no description corresponding to these techniques. 
     In JP-A-2009-049875, when the connection terminal is connected to the different kinds of networks, power consumptions, throughputs and the like of the respective networks are measured, and the switching to the different kinds of system is performed by taking into account the characteristic of application to be used and also the power consumption. However, in the same manner as JP-A-2008-236079, in JP-A-2009-049875, there is no description on the constitution which maintains the continuity of communication at the time of switching the system under control of the terminal. 
     SUMMARY OF THE INVENTION 
     To overcome the above-mentioned drawbacks, according to the present invention, communication application always performs communication through a virtual network device where an IP address is fixed thus realizing the maintenance of the continuity of communication. 
     To be more specific, when an inter-system Hand Over (HO) control user interface receives a connection request from a user, the user interface confirms a radio environment or the like of a network where the user sets priority connection, and transmits a connection request to a network device corresponding to a first network corresponding to a priority network when the connection request exceeds a certain threshold value, and the network device corresponding to the first network which has received the request requests connection processing to a mobile unit through a device driver corresponding to the first network, the mobile unit and the first network execute connection processing, and when all processing are normally completed, an IP address is assigned to the mobile unit by a Home Agent (HA) connected to the first network. 
     Next, with respect to the allocation of the IP address, under an environment where the IP address is changed before and after switching of the network, an IP address allocated from the network is set in the network device which is actually connected to the first network in a final step of the above-mentioned connection processing with the first network. 
     Next, a dummy IP address is set in a virtual network device which performs data communication with a communication application through the inter-system HO control user interface. At this point of time, the connection of the mobile unit with the first network is completed so that the communication application can perform the communication through the mobile unit. 
     Next, with respect to the exchange of the user data, the first network expects that an IP packet where an IP address allocated by the HA is set to a source IP address reaches from the mobile unit, and the communication application can also perform the communication when an IP packet where an IP address of a virtual network device with which the communication application actually performs an exchange is set to an target IP address reaches the communication application. 
     Accordingly, the virtual network device substitutes a source IP address in the IP packet transmitted from the communication application with the IP address allocated from the first network, and transfers the IP packet to the device driver corresponding to the first network. 
     When the IP address transmitted from the first network reaches the device driver corresponding to the first network, a Target IP address of the IP packet is rewritten with a dummy IP address allocated to the virtual network device, and the dummy IP address is transferred to a virtual network device driver, and the data reaches the communication application. 
     Next, at the time of performing switching between the first network and the second network, an IP address allocated from the second network is not allocated to virtual network device which actually transacts with the communication application but is allocated to a network device which corresponds to a posterior second network and hence, a change of the IP address cannot be recognized from the communication application. As a result, the continuity of communication at the time of switching the system can be maintained. 
     On the other hand, in an environment where an IP address is not changed before and after switching the network, the continuity of communication at the time of switching the system can be maintained also by a method where a dummy IP address is set in the network device which is actually connected to the first network, and an IP address allocated by the first network is set in the virtual network device which performs the data communication with the communication application. 
     In this case, when the first network is a 3G network and also is a modem, the acquisition of an IP address is performed within Internet Protocol Control Protocol (IPCP) processing in the PPP authentication. In this processing, the driver corresponding to the first network converts an IP address which is allocated from the first network into a dummy IP address, and transmits the dummy IP address to a PPP in the OS and hence, the dummy IP address is set in the first network device. 
     Next, inter-system handover control user interface reads an IP address which is actually allocated from the first network from a driver corresponding to the first network and, thereafter, sets the IP address in the virtual network device. At this point of time, the connection processing of the mobile unit with the first network is completed so that the communication application can perform communication through the mobile unit. 
     Next, with respect to the exchange of the user data, in this method, an IP address allocated from the first network is set in the virtual network device which performs exchange of data with the communication application. Accordingly, the IP address allocated from the first network is set as a source IP address of an IP packet which reaches the first network from the communication application, and an IP address of the virtual network device is set as a Target IP address of an IP packet which reaches the virtual network device from the first network and hence, substitution processing of an IP address during communication is unnecessary. 
     Next, at the time of switching the network between the first network and the second network, when the second network is WiMAX, the acquisition of an IP address is performed in Dynamic Host Configuration Protocol (DHCP) processing. Here, the driver corresponding to the second network converts an IP address allocated from the second network into a dummy IP address, and transmits the dummy IP address to the DHCP in the OS so that the dummy IP address is set in the network device corresponding to the second network. 
     Next, the inter-system handover control user interface reads an IP address actually allocated from the second network from the driver corresponding to the second network, and confirms that the IP address allocated from the first network and the IP address allocated from the second network are not changed so that switching of the network is completed. 
     Also with respect to the exchange of the user data after switching the system, the IP address allocated from the first network is set in the virtual network device which performs the exchange of data with the communication application. Accordingly, the IP address allocated from the first network is set as a source IP address of an IP packet which reaches the first network from the communication application, and the IP address of the virtual network device is set as the Target IP address of the IP packet which reaches the virtual network device from the first network and hence, the substitution processing of the IP address during communication is unnecessary. 
     Accordingly, also in this IP address substitution processing, the IP address of the virtual network device which performs the exchange of data with the communication application is not changed at system switching timing and hence, the continuity of communication at the time of switching the system can be maintained under control of the terminal. 
     Accordingly, assuming that an end user side facility is constituted of: the inter-system handover control user interface; the Hybrid communication terminal which is independently communicable with two or more kinds of networks; the device driver corresponding to the first network represented by the 3G network; the device driver corresponding to the second network represented by the WiMAX/LTE; and the virtual network device which plays a role of transferring data between the communication application and the device drivers corresponding to the respective network systems during data communication, all of which are mounted in the inside of the PC, using only such an end user side facility, then the communication terminal is automatically connected to the network in a non-connection state when it is determined that radio communication quality of one of the communication terminal and the network which is used is insufficient to maintain the current connection while the communication terminal is connected with one of the different networks. The function which can ensure the maintenance of continuity of communication at the time of switching the network can also be realized. Accordingly, it is possible to provide services without requiring the installation of a dedicated facility to the respective networks or with the installation of a minimum scale of the dedicated facility. Accordingly, the increase of cost in providing functions under control of networks can be prevented. Further, services can be provided sooner. 
     The maintenance of continuity of communication under control of a mobile unit side can be realized so that it is unnecessary to additionally install a dedicated facility to both networks or a scale of the dedicated facility can be minimized. Accordingly, a burden imposed on a telecommunication provider due to the introduction of facility can be reduced. 
     Further, even when user data in real time is generated at system switching timing, it is necessary to conceal a change of an IP address or the presence of timing at which there is no IP address for a fixed time from the communication application. As a result, the session of the communication application is not disconnected. Accordingly, both an end user and a telecommunication provider can make use of various and versatile services. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Preferred embodiments of the present invention will now be described in conjunction with the accompanying drawings, in which; 
         FIG. 1  is a block diagram of a radio network; 
         FIG. 2  is a functional block diagram of a Hybrid communication terminal and also is a hardware block diagram of a PC; 
         FIG. 3  is a functional block diagram of the PC; 
         FIG. 4  is a view showing a GUI screen of a user interface; 
         FIG. 5  is a functional block diagram showing the flow of data at the time of performing connection processing to a 3G network and at a standby time; 
         FIG. 6  is a functional block diagram showing the flow of data at the time of performing connection processing to a WiMAX network and at a standby time; 
         FIG. 7  is a functional block diagram showing the flow of data during communication with the 3G network; 
         FIG. 8  is a functional block diagram showing the flow of data during communication with the WiMAX network; 
         FIG. 9  is a sequence diagram at the time of newly connecting the communication terminal to the 3G network; 
         FIG. 10  is a sequence diagram at the time of newly connecting the communication terminal to the WiMAX network; 
         FIG. 11  is a sequence diagram at the time of switching the connection to the 3G network during communication with the WiMAX network; 
         FIG. 12  is a sequence diagram at the time of switching the connection to the WiMAX network during communication with the 3G network; 
         FIG. 13  is a sequence diagram for explaining the IP address substitution at the time of newly connecting the communication terminal to the WiMAX network or at the time of switching the connection to the WiMAX network during communication with the 3G network (a case where an IP address is changed before and after switching the system); 
         FIG. 14  is a sequence diagram for explaining the IP address substitution at the time of newly connecting the communication terminal to the 3G network or at the time of switching the connection to the 3G network during communication with the WiMAX network (a case where an IP address is changed before and after switching the system); 
         FIG. 15  is a sequence diagram for explaining the IP address substitution at the time of newly connecting the communication terminal to the WiMAX network or at the time of switching the connection to the WiMAX network during communication with the 3G network (a case where an IP address is not changed before and after switching the system); and 
         FIG. 16  is a sequence diagram for explaining the IP address substitution at the time of newly connecting the communication terminal to the 3G network or at the time of switching the connection to the 3G network during communication with the WiMAX network (a case where an IP address is not changed before and after switching the system). 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, embodiments will be explained in detail in conjunction with drawings. Parts which are substantially identical are given the same number and the explanation of the parts is not repeated. Further, although the explanation is made hereinafter using 3G and WiMAX as two kinds of radio networks, two kinds of radio networks are not limited to these networks. 
     Embodiment 1 
     Firstly, the constitution of a radio network  1000  will be explained in conjunction with  FIG. 1 . In  FIG. 1 , the radio network  1000  is constituted of a WiMAX/3G_Hybrid communication terminal  300  connected to a Personal Computer (PC)  400 , a 3G network  100 , and a WiMAX network  200 . When the WiMAX/3 G_Hybrid communication terminal  300  is connected to the PC  400  for the first time, the PC  400  performs the zero-installing of an inter-system handover control user interface  500 . The 3G network  100  includes a 3G base station  110 . The WiMAX network  200  includes a WiMAX base station  210 . 
     Hereinafter, the WiMAX/3G_Hybrid communication terminal  300  is simply referred to as a Hybrid communication terminal  300 . The inter-system handover control user interface  500  is also simply referred to as a control user interface  500 . Due to such abbreviation, the increase of the volume of the description of specification can be prevented. 
     &lt;Summary of Operation at the Time of Initial Communication Connection&gt; 
     When the Hybrid communication terminal  300  is mounted on the PC  400 , the Hybrid communication terminal  300  receives the supply of electricity from the PC  400 . Upon receiving the supply of electricity, the Hybrid communication terminal  300  performs initial setting with the PC  400  so that the Hybrid communication terminal  300  is recognized as a network device corresponding to the 3G network  100  or a network device corresponding to the WiMAX network. A virtual network device is registered in the PC at the time of zero-installing the control user interface  500 . The virtual network device is always recognized on the OS at the time of completion of starting the OS irrelevant to whether or not the Hybrid communication terminal  300  is connected to the PC  400 . 
     The Hybrid communication terminal  300  receives an acquisition instruction of the priority connection network which a user set in the control user interface  500  in advance from the control user interface  500 . The Hybrid communication terminal  300  is shifted to a standby state through the priority network when the Hybrid communication terminal  300  succeeds in the pilot acquisition of the priority network. 
     The control user interface  500 , when the Hybrid communication terminal  300  is normally recognized by the PC  400  and the control user interface  500  receives a standby completion report of the priority network from the Hybrid communication terminal  300 , performs a predetermined power control. Next, the control user interface  500  transmits a command requesting the Hybrid communication terminal  300  to periodically report a standby state of the Hybrid communication terminal  300  with the network which the Hybrid communication terminal  300  stands by and a radio state of the network which the Hybrid communication terminal  300  does not stand by. Thereafter, the control user interface  500  monitors radio states of the respective networks  100 ,  200  and a state between the Hybrid communication terminal  300  and the network which the Hybrid communication terminal  300  stands by in real time. The control user interface  500  controls the Hybrid communication terminal  300  in accordance with a priority connection mode which the user sets and waits for a connection instruction from the user. 
     When the priority network is out of a range, the control user interface  500  performs a predetermined power control and subsequently transmits an instruction to the Hybrid communication terminal  300  such that the Hybrid communication terminal  300  stands by the non-priority network. 
     When both networks are outside the range, the control user interface  500  performs a predetermined power control and subsequently transmits an instruction to the Hybrid communication terminal  300  such that the Hybrid communication terminal  300  periodically reports network radio states of both the WiMAX network  200  and the 3G network  100 . 
     When the user performs a communication connection operation, the control user interface  500  applies a predetermined power control to the Hybrid communication terminal  300 . Next, the control user interface  500  transmits an instruction to the Hybrid communication terminal  300  so that the Hybrid communication terminal  300  is connected to the network in a standby state. 
     The Hybrid communication terminal  300 , upon receiving a connection instruction command from the control user interface  500 , establishes a session of the network and a radio layer on a standby side. The Hybrid communication terminal  300  performs authentication processing by making use of a Point to Point Protocol (PPP) or a Dynamic Host Configuration Protocol (DHCP). 
     The control user interface  500 , in a step of acquiring an IP address from the network, sets a predetermined network IP address or a dummy IP address in the network device corresponding to the network which the control user interface  500  is going to connect with and the virtual network device which directly performs data communication processing with the communication application. After confirming that the predetermined IP address is correctly set in the respective network devices, the control user interface  500  notifies the user of the completion of the connection. The dummy IP address is selected from private IP addresses. The dummy IP address may be also selected from an Automatic Private IP Addressing (APIPA) address. 
     A connection mode which a user sets in the control user interface  500  in advance is selected from a group consisting of a mode where the Hybrid communication terminal  300  is connected to the 3G network  100  with priority, a mode where the Hybrid communication terminal  300  is connected to the WiMAX network  200  with priority, and a mode where the Hybrid communication terminal  300  is connected to one of the 3G network  100  and the WiMAX network  200 . However, the processing of the mode where the Hybrid communication terminal  300  is connected to one of the 3G network  100  and the WiMAX network  200  is not relevant to this embodiment and hence, the explanation of the processing of the mode is not explained. 
     &lt;Operation at the Time of Handover&gt; 
     Under following conditions the control user interface  500  starts switching processing from the network to which the Hybrid communication terminal  300  is currently connected to the network different from the currently connected network. 
     Condition 1. The Hybrid communication terminal  300  is currently connected to the priority network and the communication radio quality of the currently connected network is degraded so that the condition for starting the execution of the handover is satisfied.
 
Condition 2. The Hybrid communication terminal  300  is currently connected to the non-priority network and a state of the network is in a non-communication state so that the Hybrid communication terminal  300  is a standby state.
 
     To prevent the inter-system handover from falling into an infinite loop, when the handover from the non-priority network to the priority network is made, the handover during communication is not performed, and switching processing to the priority network is started in response to a shift of state of the non-priority network to a non-communication state so that the Hybrid communication terminal  300  is brought into a standby state. 
     The Hybrid communication terminal  300 , upon receiving an instruction from the control user interface  500 , reports the radio communication quality information of the WiMAX network  200  or the 3G network  100  which is the handover transmission destination to the control user interface  500 . 
     The control user interface  500  determines whether or not the received radio communication quality satisfies the condition for starting the handover. When the control user interface  500  determines that the received radio communication quality satisfies the condition for starting the handover, the control user interface  500  transmits a handover execution command to the Hybrid communication terminal  300 . 
     The Hybrid communication terminal  300 , upon receiving a handover execution command, establishes the connection of the Hybrid communication terminal  300  with the WiMAX network  200  or the 3G network  100 . The Hybrid communication terminal  300  notifies the control user interface  500  of the completion of predetermined IP address substitution processing. 
     The control user interface  500 , upon receiving, from the Hybrid communication terminal  300 , a report on the completion of connection with the network which is the handover transmission source, transmits a request for clearing the information on session with the WiMAX network  200  or the 3G network  100  which is the handover transmission source to the Hybrid communication terminal  300 . The control user interface  500  displays that the connection network is changed on a user interface  510  thereof. The control user interface  500  continues monitoring of a state of the network under communication connection. 
     When the connection is not established, the Hybrid communication terminal  300  tries the reconnection with the WiMAX network  200  or the 3G network  100  which is the handover transmission source and continues the communication when the Hybrid communication terminal  300  is succeeded in the reconnection. When the Hybrid communication terminal  300  fails in the reconnection, the Hybrid communication terminal  300  readily performs standby processing with the priority network. When the standby processing is completed, the control user interface  500  displays a state indicative of the completion of the standby state to a user. 
     The constitution of the Hybrid communication terminal  300  and the PC  400  will be explained in conjunction with  FIG. 2 . The Hybrid communication terminal  300  is constituted of a HUB  310 , a 3G function part  320 , and a WiMAX function part  330 . The 3G function part  320  is constituted of a 3G power source management part  321 , a 3G memory part  322 , a 3G posterior IF control part  323 , a 3G supervisory control part  324 , a 3G modem part  325 , a 3G RF band part  326 , and a 3G antenna  327 . The WiMAX function part  330  is constituted of a WiMAX power source management part  331 , a WiMAX memory part  332 , a WiMAX posterior IF control part  333 , a WiMAX supervisory control part  334 , a WiMAX modem part  335 , a WiMAX RF band part  336 , and a WiMAX antenna  337 . 
     The HUB  310  connects the 3G function part  320  and the WiMAX function part  330  to the PC  400 . 
     The 3G supervisory control part  324 , the 3G modem part  325  and the 3G RF band part  326  of the 3G function part  320  are divided into a transmission part and a reception part, and the supply of electricity to the respective parts is individually controlled by the power management part  321 . 
     In the same manner, the WiMAX supervisory control part  334 , the WiMAX modem part  335 , and the WiMAX RF band part  336  of the WiMAX function part  330  are divided into a transmission part and a reception part, and the supply of electricity to the respective parts is individually controlled by the power management part  331 . 
     The PC  400  is constituted of a USB IF part  460 , a CPU  465 , a memory  470 , a display part  475 , a hard disc  480 , an operation part  485 , and an internal communication line  490  which connects these parts. 
     Function blocks of the PC  400  will be explained in conjunction with  FIG. 3 . In  FIG. 3 , the PC  400  is constituted of a communication application  410  (to be more specific, an internet browser and a mailer), a connection network state display table  420 , a TCP/UDP/IP part  430 , a device driver  440 , a device manager  450 , the inter-system handover control user interface  500 , a PPP  405 , and a DHCP  415 . 
     The device driver  440  is constituted of a WiMAX device driver  442 , and a virtual network device driver  443 . 
     The device manager  450  is constituted of a 3G modem  451 , a WiMAX network adaptor  452 , and a virtual network device  453 . 
     The inter-system handover control user interface  500  is constituted of a user interface  510 , a user interface Application Program interface (API) group  520 , a handover/user interface control part  530 , a 3G network state monitoring part  540 , a WiMAX network state monitoring part  560 , a 3G API control part  570 , a WiMAX API control part  580 , an virtual network API control part  575 , a 3G API group  590 , a WiMAX API group  550 , and a virtual network API group  585 . 
     The device manager  450  recognizes the 3G function part  320  as a modem and recognizes the WiMAX function part  330  as a network adaptor based on a communication connection mode of the 3G function part  320  and a communication connection mode of the WiMAX function part  330 . The virtual network device  453  which actually performs communication with the communication application  410  is recognized as a network adaptor. Here, when the Hybrid communication terminal  300  terminates PPP, the 3G function part  320  can be recognized as a network adaptor. 
     Here, the virtual network driver  443  which performs actual processing under control of the virtual network device includes a 3G modem driver  441  which performs actual processing under control of the 3G modem  320 , a WiMAX device driver  442  which performs actual processing under control of the WiMAX network adaptor  452 , and an interface which performs the transfer of user data. 
     The user interface  510  which a user operates notifies a connection/disconnection request from the user to the handover/user interface control part  530 . The user interface  510  displays a result of determination whether or not the communication connection is allowed in response to the request, a state of the WiMAX network  200  and a state of the 3G network  100 , and various set contents. The user interface  510  performs change setting of the various set contents. Individual functions of the user interface  510  correspond to the respective API of the user interface API group  520 . 
     The handover/user interface control part  530  has an interface function of connecting the user interface API group  520  with the 3G API group  590 , the WiMAX API group  550 , and the virtual network API group. The handover/user interface control part  530  decides the execution of handover. The handover/user interface control part  530  determines the network to which the Hybrid communication terminal  300  is to be connected at the time of newly connecting the Hybrid communication terminal  300  or a route along which the Hybrid communication terminal  300  is to be controlled at the time of performing actual connection/disconnection processing, during data communication or at the time of switching the system. 
     The 3G network state monitoring part  540  monitors radio communication qualities of the respective systems and a state (connection/disconnection/standby/under Pilot acquisition or the like) of the 3G function part  320  based on response messages from the 3G API group  590 . When a state is changed, the 3G network state monitoring part  540  notifies the handover/user interface control part  530  of the change. 
     The WiMAX network state monitoring part  560  monitors radio communication qualities of respective systems and a state (connection, disconnection, standby, pilot acquisition or the like) of the WiMAX function part  330  based on response messages from the WiMAX API group  550 . When the state changes, the WiMAX network state monitoring part  560  notifies the handover/user interface control part  530  of the change. 
     The 3G API control part  570  executes the 3G API group  590  based on a command from the handover/user interface control part  530 . As a result, the 3G API control part  570  controls the 3G function part  320 . 
     The WiMAX API control part  580  executes the WiMAX API group  550  based on a command from the handover/user interface control part  530 . As a result, the WiMAX API control part  580  controls the WiMAX function part  330 . 
     The virtual network API control part  575  executes the virtual network API group  550  based on a command from the handover/user interface control part  530 . As a result, the virtual network API control part  575  executes IP address substitution processing between the 3G modem  451  and the virtual network device  453 . The virtual network API control part  575  executes IP address substitution processing between the WiMAX network adaptor  452  and the virtual network device  453 . The virtual network API control part  575  executes a user data transfer control between the 3G modem  451  and the virtual network device  453 . The virtual network API control part  575  executes a user data control transfer control between the WiMAX network adaptor  452  and the virtual network device  453 . 
     As can be understood from a comparison between the block diagram shown in  FIG. 2  and the block diagram shown in  FIG. 3 , the device driver  440 , the device manager  450  and the inter-system handover control user interface  500  realizes programs on the memory  470  as the CPU  465  executes the program. 
     A GUI screen of the user interface  510  will be explained in conjunction with  FIG. 4 . As shown in  FIG. 4 , the user interface  510  is constituted of a state display window  511 , a transmission/reception data counter  512 , a radio wave display window  513 , a connection/disconnection button  514 , and priority connection setting part  515 . 
     The state display window  511  is a window for showing which network is in a standby state or in the midst of connection. The transmission/reception data counter  512  is a window which shows transmission/reception amounts of respective data communicated among the WiMAX network  200 , the 3G network  100  and the WiMAX/3G handover corresponding terminal  300 . The radio wave display window  513  displays communication radio quality of the WiMAX network  200  or the 3G network  100  during a standby state or a connection state. The connection/disconnection button  514  is a button which controls connection and disconnection of the WiMAX network  200  or the 3G network  100 . The priority connection setting part  515  can set a mode where the terminal  300  is connected to the WiMAX with priority, a mode where the terminal  300  is connected to the 3G with priority, a mode where the terminal  300  is connected to only the WiMAX, or a mode where the terminal  300  is connected to only the 3G. 
     All of these functions correspond to the respective API of the user interface API group  520  in a one-to-one basis. 
     Next, the flow of data during the connection processing and the flow of data during date communication will be explained in conjunction with  FIG. 5  to  FIG. 8 . 
     &lt;Flow of Data at the Time of Standing by 3G Network or at the Time of Connecting Terminal to 3G Network&gt; 
     The flow of data at the time of standing by 3G network  100  or at the time of connecting to 3G network  100  will be explained in conjunction with  FIG. 5 . In  FIG. 5 , a communication application  410  and an inter-system handover control user interface  500  are positioned in an application layer. A PPP  405 , a TCP/UDP/IP  430  and a DHCP  415  are positioned in a communication protocol layer. A 3G modem  451 , a virtual network device  453  and a WiMAX network adaptor  452  are positioned in a network device interface layer. A 3G modem driver  441 , a WiMAX network adaptor driver  442  and a virtual network device driver  443  are positioned in a device driver layer. A 3G function part  320  and a WiMAX function part  330  are positioned in a WiMMAX_Hybrid communication terminal layer. A 3G network  100  and a WiMAX network  200  are positioned in a network layer. 
     In a standby state, the control interface  500  collects information network information including pilot radio intensity which is currently acquired by the 3G function part  320  through the 3G network  100  and standby state information by polling for every fixed cycle. The control interface  500  displays such a state on a user interface  510 . This processing is only state acquisition processing and hence, this processing is controlled without via the PPP  405  (indicated by a broken line in  FIG. 5 ). 
     Next, at the time of performing connection processing to the 3G network  100 , the control interface  500  notifies the 3G function part  320  of a radio session establishment request via the 3G modem  451  and the 3G modem driver  441 . Upon receiving the completion report of the radio session establishment, next, the control interface  500  controls the PPP  405  and notifies the PPP session establishment request. After receiving the completion report of the PPP session establishment request, the control interface  500  performs the IP address setting processing at the respective network devices. After this processing is all normally completed, the control interface  500  performs a connection completion display on the user interface  510 . 
     &lt;Flow of Data at the Time of Standing by WiMAX Network and at the Time of Performing Connection Processing to WiMAX Network&gt; 
       FIG. 6  shows the flow of data at the time of standing by the WiMAX network  200  and at the time of connection processing to WiMAX network  200 . 
     In a standby state, the control interface  500  collects information network information including pilot radio intensity which is currently acquired by the WiMAX function part  300  through the WiMAX network  200  and standby state information by polling for every fixed cycle. The control interface  500  displays such a state on the user interface  510 . This processing is only state acquisition processing and hence, this processing is controlled without via the DHCP  415  (indicated by a broken line in  FIG. 6 ). 
     Next, at the time of performing connection processing to the WiMAX network  200 , the control interface  500  notifies the WiMAX function part  330  of a radio session establishment request via WiMAX network adaptor  452  and the WiMAX network adaptor driver  442 . Upon receiving the completion report of the radio session establishment, the control interface  500  notifies similarly the WiMAX function part  330  of an EAP session establishment request. Upon receiving the completion report of the EAP session establishment, the control interface  500  controls the DHCP  415 , and performs IP address acquisition processing. Upon receiving the completion report of the IP address acquisition processing, the control interface  500  applies the IP address setting processing to the respective network devices. When this processing is normally completed, the control interface  500  performs a connection completion display on the user interface  510 . 
     &lt;Flow of Data During Communication Through 3G Network&gt; 
       FIG. 7  shows the flow of data in a state where the communication application  410  in the midst of communication with the 3G network  100 . In  FIG. 7 , the virtual network device  453  transmits the user data from the communication application  410  to the virtual network device  443 . The virtual network device  443  transfers the user data to the 3G modem driver  441  during communication connection. Accordingly, the user data can be transferred to the 3G network  100  during communication connection via the 3G function part  320 . 
     Next, a 3G modem driver  441  receives data from the 3G network  100  via the 3G function part  320 . The 3G modem driver  441  transfers the data to the virtual network device  443 . Accordingly, the data from the 3G network  100  can be transferred to the communication application  410  via the virtual network device  453 . 
     &lt;Flow of Data in the Midst of Communication Through WiMAX Network&gt; 
       FIG. 8  shows the flow of data when the communication application  410  performs the communication with the WiMAX network  200 . In  FIG. 8 , the virtual network device  453  transmits the user data from the communication application  410  to the virtual network device  443 . The virtual network device  443  transfers user data to the WiMAX device driver  442  during the communication connection. Accordingly, the user data can be transferred to the WiMAX network  200  during the communication connection via the WiMAX function part  330 . 
     Next, a WiMAX network adaptor driver  442  receives data from the WiMAX network  200  via the WiMAX function part  330 . The WiMAX network adaptor driver  442  transfers the data to the virtual network device  443 . Accordingly, the data from the WiMAX network  200  can be transferred to the communication application via the virtual network device  453 . 
     As one method for enabling the communication application to surely transfer the user data to the virtual network device during data communication, there is a method which makes use of a default gateway which can be set on the OS of the PC during the communication connection processing. To be more specific, in setting a default gateway of a virtual network device, when an IP address set in the virtual network device is 119.105.100.100, the gateway is also set to 119.105.100.100 in the same manner. Accordingly, the default gateway can transfer the user data of communication application to the virtual network device. 
     In the same manner, with respect to setting of a route between the virtual network driver  453  and the 3G modem driver  441  and a route between the virtual network driver  453  and the 3G modem driver  441 , an IP address in a network device in the higher order of these device drivers is set during communication connection processing and hence, such route setting can be controlled by providing a routing table where the IP address can be set on the OS of the PS during the communication connection processing along the route contents described above. It is needless to say that a method which controls the flow of data by providing an interface for user table transfer to respective data drivers can be used. 
     Next, the operation sequence at the time of newly connecting the communication terminal to the 3G network  100  and the WiMAX network  200  and at the time of disconnecting the connection, and operation sequence at the time of switching the system between both networks  100 ,  200  will be explained in conjunction with  FIG. 9  to FIG.  12 . 
     &lt;Operation at the Time of Newly Connecting Communication Terminal to WiMAX Network and at the Time of Disconnecting Connection&gt; 
     Processing operations at the time of newly connecting the communication terminal to the WiMAX network  200  and at the time of disconnecting the connection will be explained in conjunction with  FIG. 9 . In  FIG. 9 , the communication application  410  transmits a connection request to the control user interface (SHO control UI)  500  (S 101 ). The control user interface (SHO control UI)  500  executes API corresponding to the connection request. To be more specific, the control user interface (SHO control UI)  500  transmits a radio layer establishment request to the WiMAX network adaptor driver  442  (S 102 ). The WiMAX network adaptor  442  transmits a radio layer establishment request to a WiMAX function part  330  (S 103 ). The WiMAX function part  330 , upon receiving the request signal, executes WiMAX radio layer establishment processing with the WiMAX network  200  (S 104 ). When the processing is completed, the WiMAX function part  330  transmits radio layer establishment completion notification to the WiMAX network adaptor driver  442  (S 105 ). The WiMAX network adaptor driver  442 , upon receiving the radio layer establishment completion notification, transmits the radio layer establishment completion notification to the control user interface  500  (S 106 ). 
     Next, control user interface  500 , to execute the EAP authentication, executes API corresponding to the EAP authentication. To be more specific, the control user interface  500  transmits an authentication processing request to the WiMAX network adaptor driver  442  (S 107 ). The WiMAX network adaptor  442  transfers the authentication processing request to the WiMAX function part  330  (S 108 ). The WiMAX function part  330 , upon receiving the request signal, executes WiMAX EAP processing with the WiMAX network  200  (S 109 ). When the processing is completed, the WiMAX function part  330  transmits the authentication processing completion notification to the WiMAX network adaptor driver  442  (S 110 ). The WiMAX network adaptor driver  442  transmits the authentication processing completion notification to the control user interface  500  (S 111 ). 
     Next, the control user interface  500 , to execute the DHCP, executes the API corresponding to the DHCP. To be more specific, the control user interface  500  transmits a DHCP processing request to the DHCP  415  which is a function on the OS (S 112 ). The DHCP  415 , upon receiving the request, executes DHCP processing with the WiMAX network  200  (S 113 ). When the processing is normally completed, the DHCP  415  transmits a DHCP completion notification to the control user interface  500  (S 114 ). 
     Upon receiving the completion notification, the control user interface  500  applies IP address setting processing to the virtual network device  453  and the WiMAX network adaptor  452  (S 115 ). Upon completion of normal setting processing, the control user interface  500  displays connection completion on a user interface  510  (S 116 ). Through these steps, the communication application  410  and the WiMAX network  200  are brought into a communication state. 
     In such a communication state, the control user interface  500  detects a disconnection event (S 117 ). The control user interface  500  transmits a disconnection processing request to the WiMAX network adaptor driver  442  (S 118 ). The WiMAX network adaptor driver  442 , upon receiving the request, transmits a disconnection processing request to the WiMAX function part  330  (S 119 ). The WiMAX function part  330 , upon receiving the request, executes WiMAX disconnection processing in association with the DHCP  415  (S 120 ). When the disconnection processing is completed, the WiMAX function part  330  transmits disconnection processing completion notification to the WiMAX network adaptor driver  442  (S 121 ). The WiMAX network adaptor driver  442 , upon receiving the completion notification, transmits the disconnection processing completion notification to the control user interface  500  (S 122 ). 
     The control user interface  500 , upon receiving the completion notification, transmits an IP address release request to the virtual network device  453  (S 123 ). The virtual network device  453  executes IP address release processing (S 124 ). The virtual network device  453  transmits an IP address release completion message to the control user interface  500  (S 125 ). The control user interface  500  confirms whether or not release processing is properly executed and, thereafter, confirms that the priority network is shifted to a standby state, and displays the standby state on the user interface  510  (S 126 ). 
     &lt;Operation at the Time of Newly Connecting Communication Terminal to 3G Network and at the Time of Disconnecting the Connection&gt; 
     Processing operations at the time of newly connecting the communication terminal to the 3G network  100  and at the time of disconnecting the connection will be explained in conjunction with  FIG. 10 . In  FIG. 10 , the communication application  410  transmits a user connection request to the inter-system handover control user interface (SHO control UI)  500  (S 131 ). Upon receiving the connection request S 101  from the communication application  410 , the control user interface (SHO control UI)  500  executes API corresponding to the connection request S 101 . To be more specific, the control user interface  500  transmits a radio layer establishment request to the 3G modem driver  441  (S 132 ). The 3G modem driver  441  transmits a radio layer establishment request to the 3G function part  320  (S 133 ). The 3G function part  320 , upon receiving the request signal, executes 3G radio layer establishment processing with the 3G network  100  (S 134 ). The 3G function part  320 , when processing is completed, transmits the radio layer establishment completion notification to the 3G modem driver  441  (S 136 ). The 3G modem driver  441 , upon receiving the completion notification, transmits the radio layer establishment completion notification to the control user interface  500  (S 137 ). 
     The control user interface  500 , to execute the PPP processing, executes API corresponding to the PPP processing. To be more specific, the control user interface  500  transmits a PPP layer establishment request to the PPP  405  which is a function on the OS (S 138 ). The PPP  405 , upon receiving the request, executes PPP processing with the 3G network  100  (S 139 ). The PPP  405 , when the processing is normally completed, transmits a PPP completion notification to the control user interface  500  (S 141 ). 
     Next, the control user interface  500  applies IP address setting processing S 135  to the virtual network device  453  and the 3G modem  451  (S 142 ). When the setting processing is normally completed, the control user interface  500  displays connection completion on the user interface  510  (S 143 ). Through these steps, the communication application  410  and the 3G network  100  are brought into a communication state. 
     The control user interface  500  detects a disconnection event from the user or the communication application  410  (S 144 ). The control user interface  500  transmits a disconnection processing request to the 3G modem driver  441  (S 146 ). The 3G modem driver  441 , upon receiving the request, transmits a disconnection processing request to the 3G function part  320  (S 147 ). The 3G function part  320 , upon receiving the request, executes 3G disconnection processing in association with the PPP  405  (S 148 ). The 3G function part  320 , when the disconnection processing is completed, transmits a disconnection processing completion notification to the 3G modem driver  441  (S 149 ). The 3G modem driver  441 , upon receiving the completion notification, transmits a disconnection processing completion notification to the control user interface  500  (S 151 ). 
     The control user interface  500 , upon receiving a completion notification, transmits an IP address release request to the virtual network device  453  (S 152 ). The virtual network device  453  executes IP address release processing (S 153 ). The virtual network device  453  transmits an IP address release completion notification to the control user interface  500  (S 154 ). The control user interface  500  confirms that the priority network is shifted to a standby state, and displays the standby state on the user interface  510  (S 156 ). 
     &lt;Connection Switching Operation from WiMAX Network to 3G Network&gt; 
     Connection switching processing from the WiMAX network  200  to the 3G network  100  will be explained in conjunction with  FIG. 11 . In  FIG. 11 , the communication application  410  is in a communication state with the WiMAX network  200 . In such a state, the control user interface  500  detects a network switching event corresponding to a preset priority connection setting (S 161 ). The control user interface  500  transmits a disconnection processing request to the WiMAX network adaptor driver  442  (S 162 ). Next, the control user interface  500  transmits a user data holding request to the virtual network device driver  443  (S 163 ). The virtual network device driver  443 , when the processing is started, transmits a user data holding start notification to the control user interface  500  (S 164 ). In a network switching period, buffering is performed between the communication application  410  and the virtual network device  443  for a fixed period. However, when a network switching is not completed within a certain period, buffering of user data is stopped, and user data which is subjected to the buffering until then is discarded. Further, the virtual network device  453  holds an IP address. 
     The WiMAX network adaptor driver  442 , upon receiving a disconnection processing request signal, transmits a disconnection processing request to the WiMAX function part  330  (S 166 ). The WiMAX function part  330 , upon receiving the request, executes WiMAX disconnection processing in association with the DHCP  415  (S 167 ). The WiMAX function part  330 , when disconnection processing is completed, transmits a disconnection processing completion notification to the WiMAX network adaptor driver  442  (S 168 ). The WiMAX network adaptor driver  442 , upon receiving a completion notification, transmits a disconnection processing completion notification to the control user interface  500  (S 169 ). 
     The control user interface  500 , upon receiving a completion notification, executes API corresponding to the completion notification. To be more specific, the control user interface  500  transmits a radio layer establishment request to the 3G modem driver  441  (S 170 ). The 3G modem driver  441  transmits a radio layer establishment request to the 3G function part  320  (S 171 ). The 3G function part  320 , upon receiving the request signal, executes 3G radio layer establishment processing with the 3G network  100  (S 172 ). The 3G function part  320 , when the processing is completed, transmits a radio layer establishment completion notification to the 3G modem driver  441  (S 173 ). The 3G modem driver  441 , upon receiving a completion notification, transmits a radio layer establishment completion notification to the control user interface  500  (S 174 ). 
     The control user interface  500 , to execute the PPP processing, executes API corresponding to the PPP processing. To be more specific, the control user interface  500  transmits a PPP layer establishment request to the PPP  405  which is a function on the OS (S 176 ). The PPP  405 , upon receiving the request, executes PPP processing with the 3G network  100  (S 177 ). The PPP  405 , when the processing is normally completed, transmits a PPP completion notification to the control user interface  500  (S 178 ). 
     The control user interface  500 , upon receiving a completion notification, applies IP address setting processing to the virtual network device  453  and the 3G modem  451  (S 179 ). The control user interface  500 , when the setting processing is normally completed, transmits a user data holding completion request to the virtual network device driver (S 181 ). The virtual network device driver stops buffering of user data, and transmits a user data holding completion notification to the control user interface  500  (S 182 ). Due to the above-mentioned operations, user data from the communication application  410  is brought into a communication state with the 3G network  100 . 
     &lt;Connection Switching Operation from 3G Network to WiMAX Network&gt; 
     Connection switching processing from the 3G network  100  to the WiMAX network  200  will be explained in conjunction with  FIG. 12 . In  FIG. 12 , the communication application  410  is in a communication state with the 3G network  100 . In such a state, the control user interface  500  detects a network switching event corresponding to a preset priority connection setting (S 191 ). The control user interface  500  transmits a disconnection processing request to the 3G modem driver  441  (S 192 ). Next, the control user interface  500  transmits a user data holding request to the virtual network device driver  443  (S 193 ). The virtual network device driver  443 , when the processing is started, transmits a user data holding start notification to the control user interface  500  (S 194 ). In a network switching period, buffering is performed between the communication application  410  and the virtual network device  443  for a fixed period. However, when the network switching is not completed within a certain period, the buffering of user data is stopped, and user data which is subjected to the buffering until then is discarded. Further, the virtual network device  453  holds an IP address. 
     The 3G modem driver  441 , upon receiving a disconnection processing request, transmits a disconnection processing request to the 3G function part  320  (S 196 ). The 3G function part  320 , upon receiving the request, executes 3G disconnection processing in association with the PPP  405  (S 197 ). The 3G function part  320 , when the disconnection processing is completed, transmits a disconnection processing completion notification to the 3G modem driver  441  (S 198 ). The 3G modem driver  441 , upon receiving the completion notification, transmits a disconnection processing completion notification to the control user interface  500  (S 199 ). 
     The control user interface  500 , upon receiving a completion notification, executes API corresponding to the completion notification. To be more specific, the control user interface  500  transmits a radio layer establishment request to the WiMAX network adaptor driver  442  (S 201 ). The WiMAX network adaptor  442  transmits a radio layer establishment request to the WiMAX function part  330  (S 202 ). The WiMAX function part  330 , upon receiving the request signal, executes WiMAX radio layer establishment processing with the WiMAX network  200  ( 203 ). The WiMAX function part  330 , when the processing is completed, transmits a radio layer establishment completion notification to the WiMAX network adaptor driver  442  (S 204 ). The WiMAX network adaptor driver  442 , upon receiving the radio layer establishment completion notification, transmits a radio layer establishment completion notification to the control user interface  500  (S 206 ). 
     The control user interface  500 , to execute the EAP authentication, executes the API corresponding to the EAP authentication. To be more specific, the control user interface  500  transmits an authentication processing request to the WiMAX network adaptor driver  442  (S 208 ). The WiMAX network adaptor  442  transmits the authentication processing request to the WiMAX function part  330  (S 209 ). The WiMAX function part  330 , upon receiving the request signal, executes WiMAX EAP processing with the WiMAX network  200  (S 209 ). The WiMAX function part  330 , when the processing is completed, transmits an authentication processing completion notification to the WiMAX network adaptor driver (S 211 ). The WiMAX network adaptor driver  442  transmits the authentication processing completion notification to the control user interface  500  (S 212 ). 
     The control user interface  500 , to execute the DHCP, executes the API corresponding to the DHCP. To be more specific, the control user interface  500  transmits a DHCP processing request to the DHCP  415  which is a function on the OS (S 213 ). The DHCP  415 , upon receiving the request, executes the DHCP processing with the WiMAX network  200  (S 214 ). The DHCP  415 , when the processing is completed, transmits a DHCP completion notification to the control user interface  500  (S 216 ). 
     The control user interface  500 , upon receiving a completion notification, applies IP address setting processing to the virtual network device  453  and the WiMAX network adaptor  452  (S 217 ). When the setting processing is normally completed, the control user interface  500  transmits a user data holding completion request to the virtual network device driver  443  (S 218 ). The virtual network device driver  443  stops buffering of user data and transmits a user data holding completion notification to the control user interface  500  (S 19 ). Due to the above-mentioned operations, the communication application  410  is brought into a communication state with the WiMAX network  200 . 
     Next, IP address substitution processing and an IP address setting method at the time of newly connecting the communication terminal to the 3G network  100  and the WiMAX network  200 , and IP address substitution processing and an IP address setting method at the time of switching the system between the 3G network  100  and the WiMAX network  200  will be explained in conjunction with  FIG. 13  to  FIG. 16 . 
     &lt;IP Address Substitution Processing Method at the Time of Connecting Communication Terminal to WiMAX Network: (1)&gt; 
     The IP address substitution processing at the time of newly connecting the communication terminal to the WiMAX network  200  or when the connection of communication terminal is switched to the WiMAX network  200  during the communication of the communication terminal with the 3G network  100  in a network environment where an IP address is changed before and after switching the system will be explained in conjunction with  FIG. 13 . In  FIG. 13 , the WiMAX network  200  transmits a DHCP ACK (network (NW)-IP address) to the WiMAX network adaptor driver  442  (S 231 ). The WiMAX network adaptor driver  442  transmits a DHCP ACK (NW-IP address) having the same contents to the DHCP  415  (S 232 ). The DHCP  415 , upon receiving an ACK signal, transmits a DHCP processing completion to the control user interface  500  (S 233 ). The DHCP  415  executes setting processing of an IP address assigned to the WiMAX network adaptor  452  by DHCP processing through the WiMAX network  100  (S 234 ). 
     The control user interface  500  transmits a request for setting a dummy IP address to the virtual network device  453  to the TCP/UDP/IP  430  on the OS (S 236 ). The TCP/UDP/IP  430 , upon receiving the request signal, sets a dummy IP address to the virtual network device  453  (S 237 ). 
     Here, at the time of switching the system, a series of dummy IP address setting (S 236  and S 237 ) to the virtual network adaptor is unnecessary. 
     The control user interface  500  transmits a signal which requires a content of a network IP address which is assigned by the WiMAX network  200  to the WiMAX network adaptor driver  442  (S 238 ). The WiMAX network adaptor driver  442 , upon receiving the request signal, transmits a network IP address response signal to the control user interface  500  (S 239 ). The control user interface  500 , upon receiving the response signal, notifies the content of the network IP address to the virtual network device  443  (S 241 ). Next, the control user interface  500  notifies the content of the dummy IP address to the WiMAX network adaptor driver  442  (S 242 ). The control user interface  500  confirms that a network IP address is set to the WiMAX network adaptor  442  and a dummy IP address is set to the virtual network device using the TCP/UDP/IP  430  (S 243 ). The control user interface  500  displays a connection completion on the user interface  510  (S 244 ). 
     Here, the reason the content of the network IP address is notified to the virtual network device driver  443  and the content of the dummy IP address is notified to the WiMAX network adaptor driver is to rewrite an IP address in an IP packet of user data described later. 
     Next, the communication application  410  transmits transmission data designated to a dummy IP address to the virtual network device  453  (S 246 ). The virtual network device  453 , upon receiving transmission data to which a dummy IP address set to the virtual network device  453  per se is set as a transmission source IP address from the communication application  410 , transmits transmission data to the virtual network device driver  443  in the same manner (S 247 ). The virtual network device driver  443 , upon receiving transmission data, executes rewriting processing of a transmission source IP address from a dummy IP address to a network IP address which is notified in step  241  (S 248 ). The virtual network device driver  443  transmits transmission data where an IP address assigned by the network is set as a transmission source IP address to the WiMAX network adaptor driver  442  (S 249 ). The WiMAX network adaptor driver  442 , upon receiving transmission data, transmits transmission data to the WiMAX network  200  via the WiMAX function part  330  (S 251 ). Due to the above-mentioned operations, it is possible to transmit user data from the communication application  410  to the WiMAX network  200 . 
     Next, the WiMAX network  200  transmits reception data to the WiMAX network adaptor driver  442  (S 252 ). The WiMAX network adaptor driver  442 , upon receiving reception data from the WiMAX network  200 , rewrites a designation IP address in reception data set to the WiMAX network adapter  452  to a dummy IP address set to virtual network device  453  (S 253 ) which is notified in step  242 . Thereafter, the WiMAX network adaptor driver  442  transmits reception data to the virtual network device driver  443  (S 254 ). The virtual network device driver  443 , upon receiving data, transmits the reception data to the virtual network device  453  (S 256 ). The virtual network device  453 , upon receiving data, transmits the reception data to the communication application  410  (S 257 ). 
     Due to the above-mentioned IP address substitution processing and IP address setting, it is possible to transmit data from the WiMAX network  200  to the communication application  410 . 
     &lt;IP Address Substitution Processing Method at the Time of Connecting Communication Terminal to 3G Network: (1)&gt; 
     The IP address substitution processing at the time of newly connecting the communication terminal to the 3G network  100  or when the connection of communication terminal is switched to the 3G network  100  during the communication of the communication terminal with the WiMAX network  200  in a network environment where an IP address is changed before and after switching the system will be explained in conjunction with  FIG. 14 . In  FIG. 14 , the 3G network  100  transmits an IPCP_NAC (NW-IP address) to the 3G driver  441  in accordance with generally-used IPCP processing in the PPP (S 261 ). The 3G driver  441 , upon receiving the IPCP_NAC (NW-IP address) in which an IP address assigned by the 3G network  100  is stored from the 3G network  100 , transmits the IPCP_NAC (NW-IP address) to the PPP  405  in the same manner (S 262 ). The PPP  405 , upon receiving a NAC signal, transmits an IPCP request (NW-IP address) to the 3G modem driver  441  (S 263 ). The 3G modem driver  441 , upon receiving a signal, transmits the IPCP request (NW-IP address) to the 3G network  100  through the 3G function part  320  in the same manner (S 264 ). The 3G network  100 , upon receiving the signal, transmits an IPCP_ACK (NW-IP address) to the 3G modem driver  441  (S 266 ). The 3G modem driver  441 , upon receiving the signal, transmits the IPCP_ACK (NW-IP address) to the PPP  405  in the same manner (S 267 ). The PPP  405 , upon receiving the signal, transmits a PPP layer establishment completion S 134  to the control user interface  500  (S 268 ). The PPP  405  executes setting processing of an IP address assigned to the 3G modem  451  by IPCP processing through 3G network  100  (S 269 ). 
     The control user interface  500  transmits a request for setting a dummy IP address to the virtual network device  453  to the TCP/UDP/IP  430  on the OS (S 271 ). The TCP/UDP/IP  430 , upon receiving the request signal, sets a dummy IP address to the virtual network device  453  (S 272 ). 
     Here, at the time of switching the system, a series of dummy IP address setting (S 236  and S 237 ) to the virtual network adaptor is unnecessary. 
     The control user interface  500  transmits a signal which requests a content of a network IP address which is assigned by the NW to the 3G modem driver  441  (S 273 ). The 3G modem driver  441 , upon receiving the request signal, transmits a network response signal to the control user interface  500  (S 274 ). The control user interface  500 , upon receiving the response signal, notifies the content of the network IP address to the virtual network device  443  (S 276 ). Next, the control user interface  500  notifies a content of a dummy IP address to the 3G modem driver  441  (S 277 ). The control user interface  500  confirms that the network IP address is set to the 3G modem driver  441  and the dummy IP address is set to the virtual network device  453  using the TCP/UDP/IP  430  (S 278 ). The control user interface  500  displays the connection completion on the user interface  510  (S 279 ). 
     Here, the reason the content of the network IP address is notified to the virtual network device driver  443  and the content of the dummy IP address is notified to the 3G modem driver  441  is to rewrite an IP address in an IP packet of user data described later. 
     Next, the communication application  410  transmits transmission data to the virtual network device  453  (S 281 ). The virtual network device  453 , upon receiving transmission data to which a dummy IP address set to the virtual network device  453  per se is set as a transmission source IP address from the communication application  410 , transmits transmission data to the virtual network device driver  443  in the same manner (S 282 ). The virtual network device driver  443 , upon receiving transmission data, executes rewriting processing of a transmission source IP address from a dummy IP address to a network IP address which is notified in step  276  (S 283 ). Thereafter, the virtual network device driver  443  transmits transmission data where an IP address assigned by the network is set as a transmission source IP address to the 3G modem driver  441  (S 284 ). The 3G modem driver  441 , upon receiving transmission data, transmits transmission data to the 3G network  100  through the 3G function part (S 286 ). Due to the above-mentioned operations, it is possible to transmit user data from the communication application  410  to the 3G network  100 . 
     Next, the 3G network  100  transmits reception data to the 3G modem driver  441  (S 287 ). The 3G modem driver  441 , upon receiving reception data from the 3G network  100 , rewrites a designation IP address in reception data set to the 3G modem driver  441  to a dummy IP address set to the virtual network device  453  which is notified in step  277  (S 288 ). Thereafter, the 3G modem driver  441  transmits reception data to the virtual network device driver  443  (S 289 ). The virtual network device driver  443 , upon receiving data, transmits same reception data to the virtual network device  453  (S 291 ). The virtual network device  453 , upon receiving data, transmits same reception data to the communication application  410  (S 292 ). Due to the above-mentioned IP address substitution processing and IP address setting, it is possible to transmit data from the 3G network  100  to the communication application  410 . 
     Embodiment 2 
     &lt;IP Address Substitution Processing Method at the Time of Connecting Communication Terminal to WiMAX Network (2)&gt; 
     The IP address substitution processing at the time of newly connecting the communication terminal to the WiMAX network  200  or when the connection of communication terminal is switched to the WiMAX network  200  during the communication of the communication terminal with the 3G network  100  in a network environment where an IP address is not changed before and after switching the system will be explained in conjunction with  FIG. 15 . In  FIG. 15 , the WiMAX network  200  transmits a DHCP ACK (NW-IP address) to the WiMAX driver  442  in accordance with generally-used DHCP processing (S 301 ). The WiMAX network adaptor driver  442 , upon receiving a DHCP ACK (NW-IP address) from the WiMAX network  200 , rewrites a network IP address assigned by the WiMAX network  200  stored in a DHCP ACK message to a dummy IP address which is notified from the control user interface  500  in a step before DHCP processing is executed (S 302 ). Thereafter, the WiMAX network adaptor driver  442  transmits the rewritten DHCP ACK (dummy IP address) to the DHCP  415  (S 303 ). The DHCP  415 , upon receiving the ACK signal, transmits DHCP processing completion to the control user interface  500  (S 304 ). The DHCP  415  executes setting processing of an IP address which is notified to the WiMAX network adaptor S 452  from the control user interface  500  (S 306 ). 
     The control user interface  500  transmits a signal which requests a content of a network IP address which is assigned by the WiMAX network  200  to the WiMAX network adaptor driver  442  (S 307 ). The WiMAX network adaptor driver  442 , upon receiving the request signal, transmits the content of the network IP address which is assigned by the WiMAX network  200  to the control user interface  500  (S 308 ). The control user interface  500  transmits a request for setting the network IP address to the virtual network device  453  to the TCP/UDP/IP  430  on the OS (S 309 ). The TCP/UDP/IP  430 , upon receiving the request signal, sets the network IP address to the virtual network device  453  (S 311 ). The control user interface  500  confirms that the dummy IP address is set in the WiMAX network adaptor  442  and the network IP address is set in the virtual network device  453  using the TCP/UDP/IP  430  (S 312 ). The control user interface  500  displays the connection completion on the user interface  510  (S 313 ). 
     Here, at the time of switching the system, a series of dummy IP address setting to the virtual network adaptor is unnecessary and hence, step  309  and step  311  are unnecessary. 
     In following step  314  to step  323 , the flow of data transmission from the communication application  410  to the WiMAX network  200  and the flow of the data reception from the WiMAX network  200  to the communication application  410  will be explained. This processing, however, is flows of generally-executed processing when data communication is performed in a state where an IP address is not substituted and a mobile unit is connected to a PC. These steps are provided as reference information for comparison with the &lt;IP address substitution processing method at the time of connecting communication terminal to WiMAX network (1)&gt; and hence, the detailed explanation of these steps is omitted. 
     &lt;IP Address Substitution Processing Method at the Time of Connecting Communication Terminal to 3G Network (2)&gt; 
     The IP address substitution processing at the time of newly connecting the communication terminal to the 3G network  100  or when the connection of communication terminal is switched to the 3G network  100  during the communication of the communication terminal with the WiMAX network  200  in a network environment where an IP address is not changed before and after switching the system will be explained in conjunction with  FIG. 16 . In  FIG. 16 , the 3G network  100  transmits an IPCP_NAC (NW-IP address) in which the assigned IP address is stored to the 3G modem driver  441  in accordance with generally-used IPCP processing steps in PPP (S 331 ). The 3G modem driver  441 , upon receiving the IPCP_NAC (NW-IP address) from the 3G network  100 , rewrites the network IP address which is stored in the IPCP_NAC message and is assigned by the 3G network  100  to a dummy IP address which is notified by the control user interface  500  in a step before PPP processing is executed (S 332 ). Thereafter, the 3G modem driver  441  transmits the rewritten IPCP_NAC (dummy IP address) to the PPP  405  (S 333 ). The PPP  405 , upon receiving the NAC signal, transmits an IPCP request (dummy IP address) to the 3G modem driver  441  (S 334 ). The 3G modem driver  441 , upon receiving the signal, rewrites the dummy IP address which is stored in the IPCP request to a network IP address (S 336 ). The 3G modem driver  441  transmits the IPCP request (NW-IP address) to the 3G network  100  through the 3G function part  320  in the same manner (S 337 ). The 3G network  100 , upon receiving the signal, transmits an IPCP_ACK (NW-IP address) to the 3G modem driver  441  (S 338 ). The 3G modem driver  441 , upon receiving the signal, rewrites a network IP address which is stored in the IPCP_ACK message to a dummy IP address (S 339 ). The 3G modem driver  441  transmits the rewritten IPCP_ACK (dummy IP address) to the PPP  405  (S 341 ). The PPP  405 , upon receiving the signal, transmits the PPP layer establishment completion to the control user interface  500  (S 342 ). The PPP  405  applies dummy IP address setting processing to the 3G modem  451  (S 343 ). 
     The control user interface  500  transmits a signal which requests a content of a network IP address which is assigned by the 3G network  100  to the 3G modem driver  441  (S 344 ). The 3G modem driver  441 , upon receiving the request signal, transmits a network response signal to the control user interface  500  (S 346 ). The control user interface  500 , upon receiving the response signal, transmits a request for setting a network IP address to the virtual network device  453  to the TCP/UDP/IP  430  on the OS (S 347 ). The TCP/UDP/IP  430 , upon receiving the request signal, sets a network IP address to the virtual network device  453  (S 348 ). The control user interface  500  confirms the TCP/UDP/IP  430  that the dummy IP address is set to the 3G adaptor  442  and the network IP address is set to the virtual network device  453  (S 349 ). The control user interface  500  displays connection completion on the user interface  510  (S 351 ). 
     At the time of switching the system, a series of network IP address setting to the virtual network adaptor is unnecessary basically under an environment where an IP address is not changed before and after switching the system and hence, such network IP address setting is unnecessary whereby step  347  and step  348  are unnecessary. 
     In following step  352  to step  361 , the flow of data transmission from the communication application  410  to the 3G network  100  and the flow of the data reception from the 3G network  100  to the communication application  410  are explained. This processing, however, are flows of generally-executed processing when data communication is performed in a state where an IP address is not substituted and a mobile unit is connected to a PC. These steps are provided as reference information for comparison with the &lt;IP address substitution processing method at the time of connecting communication terminal to 3G network (1)&gt; and hence, the detailed explanation of these steps is omitted.