Abstract:
The mobile station has a monitor for monitoring the communication qualities of plural wireless interfaces and selecting one of the wireless interfaces to communicate, and a memory for storing the correspondence between the address unique to the selected wireless interface and the network address unique to the mobile station. The mobile station notifies the gateway of the correspondence. The mobile station supplies power to a not operating wireless interface at constant intervals and during a constant time to monitor the communication quality.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a Continuation Application of U.S. application Ser. No. 10/929,625 filed on Aug. 31, 2004. The present application claims priority from U.S. application Ser. No. 10/929,625 filed on Aug. 31, 2004, which claims priority from Japanese application 2003-342758 filed on Oct. 1, 2003, the content of which is hereby incorporated by reference into this application. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The present invention relates to hand-over technology in wireless system. 
         [0003]    The hand-over between different types of media such as between cellular phone and wireless LAN is called vertical hand-over, which is reported as in the following paper. 
         [0004]    For example, “Dynamic Network Reconfiguration Support for Mobile Computers”, written by J. Inouye, J. Binkley and J. Watpole, Proceedings of ACM/IEEE International Conference on Mobile Computing and Networking (Mobicom &#39;97), Budapest, September 1997 (nonpatent document 1). This document describes an example of the network in which a mobile host adaptively changes the network interface and transmission route. In an experiment using this network, it is said that when the interface is changed between wired LAN and wireless LAN, the IP (Internet Protocol) address is also changed depending on the selected interface, thus making it impossible to continue an application such as telnet. If a mobile station switches from wired LAN to wireless LAN, the mobile station disconnects the communication using an IP address for wired LAN, and reconnects by using another IP address for wireless LAN. All the applications being executed must be restarted after the reconnection. 
         [0005]    In addition, another paper is given, for example, “A Flexible Multiplexing Mechanism for Supporting Quality of Service in Mobile Environments”, written by Marc Bechler, and Hartmut Ritter, Proceedings of the Hawaii International Conference on System Science, Maui, Hawaii, January 2001, volume 9 (nonpatent document 2). This document describes an example of the hand-over between different types of media such as wireless LAN and cellular phone by using mobile IP technology. In this example, the IP packet that an application is to transmit and receive is encapsulated within the IP packet having the IP address of a selected network device and transmitted. Since the IP address of the IP packet transmitted and received by the application is constant, the application can be continued even if the hand-over process is made. 
         [0006]      FIG. 15  shows an example of the conventional system. A mobile station  21  exchanges a registration request/response to a home agent  190  so that the mobile station  21  can register its position in the home agent  190 . The home agent  190  detects the network of the mobile station  21 , and registers the address in a transfer destination table. The communication from the home agent  190  to the mobile station  21  is made by “IP tunnel” in which an IP packet is encapsulated within another IP packet. When a terminal  1  transmits an IP packet to the home address of the home agent  190 , the home agent  190  encapsulates the IP packet and transmits it to the care-of-address of the mobile station  21 . The care-of-address of the mobile station  21  is changed in accordance with the change of network device. However, even if the care-of-address of the mobile station  21  is changed, the packet to the home address is given to the application of the mobile station  21 .  FIG. 16  shows the software structure of the mobile station  21 . Even if the IP address (care-of-address) of TCP/IP software  104  is changed, the mobile IP  200  gives the information to the home address to a high-order application. 
       SUMMARY OF THE INVENTION 
       [0007]    It is an objective of the invention to provide a wireless system capable of continuously operating applications before and after the wireless interfaces of a mobile station are switched for communication. 
         [0008]    It is another objective of the invention to suppress control information from increasing due to IP encapsulation. 
         [0009]    It is still another objective of the invention to reduce the consumption power of a mobile station having a plurality of wireless interfaces. 
         [0010]    A wireless system according to the invention has a mobile station having a plurality of wireless interfaces, and a gateway connected to a fixed network. This mobile station has means for monitoring the communication qualities of the wireless interfaces, means for selecting any one of the wireless interfaces to communicate according to the communication qualities, and means for making the unique address of the selected wireless interface be associated with the network address of the mobile station. The unique address of the wireless interface is, for example, a MAC address, and the network address of the mobile station is, for example, an IP address. The mobile station also has means for notifying the gateway of the correspondence between the unique address of the wireless interface and the network address of the mobile station. 
         [0011]    The mobile station according to the invention has a plurality of wireless interfaces, and means for supplying power at intervals of a constant period b of time and during a constant period a of time to any one (or ones) of the wireless interfaces that is (or are) not communicating so as to monitor the communication qualities, but for not supplying power thereto except for the monitoring time. 
         [0012]    The gateway according to the invention has means for recording the correspondence between the unique address of a wireless interface and the network address of the mobile station according to the notice from the mobile station. 
         [0013]    According to the invention, an application for TV conference or the like can be continued without rebooting it before and after the hand-over process. In addition, since the hand-over process can be performed without producing an IP tunnel like mobile IP, the overhead on the control information is not increased due to the encapsulation of IP packet. In addition, since IP address consumption is little, addresses can be effectively used. Also, the user can easily manage IP addresses. Moreover, the mobile station according to the invention has a plurality of wireless interfaces, and means for supplying power at intervals of a constant period b of time and during a constant period a of time to any one (or ones) of the wireless interfaces that is (or are) not communicating so as to monitor the communication qualities, but for not supplying power thereto except for the monitoring time. Therefore, the consumption power in the mobile station can be reduced. 
         [0014]    Other objects, features and advantages of the invention will become apparent from the following description of the embodiments of the invention taken in conjunction with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]      FIG. 1  is a diagram showing an example of the construction of a wireless system according to the invention. 
           [0016]      FIG. 2  is a diagram showing an example of the construction of a mobile station according to the invention. 
           [0017]      FIG. 3  is a diagram showing an example of the software structure in the mobile station according to the invention. 
           [0018]      FIG. 4  is a diagram showing an example of the switching process in the mobile station according to the invention. 
           [0019]      FIG. 5  is a diagram showing an example of the construction of a gateway according to the invention. 
           [0020]      FIG. 6  is a diagram showing an example of the exchange of signals in the system according to the invention. 
           [0021]      FIG. 7  is a diagram showing another example of the exchange of signals in the system according to the invention. 
           [0022]      FIG. 8  is a diagram showing another example of the construction of the system according to the invention. 
           [0023]      FIG. 9  is a diagram showing another example of the construction of the mobile station according to the invention. 
           [0024]      FIG. 10  is a diagram showing another example of the software structure in the mobile station according to the invention. 
           [0025]      FIG. 11  is a diagram showing another example of the switching process in the mobile station according to the invention. 
           [0026]      FIG. 12  is a diagram showing a still another example of the exchange of signals in the system according to the invention. 
           [0027]      FIG. 13  is a diagram showing an example of the power control method in the mobile station according to the invention. 
           [0028]      FIG. 14  is a diagram showing an example of the format of the switching request according to the invention. 
           [0029]      FIG. 15  is a diagram showing an example of the construction of the conventional wireless system. 
           [0030]      FIG. 16  is a diagram showing an example of the software structure in the conventional mobile station. 
           [0031]      FIG. 17  is a diagram showing still another example of the software structure in the mobile station according to the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0032]      FIG. 1  shows an embodiment of a system according to the invention. The terminal  1  and a gateway (GW)  3  are connected to the public network  2 . The gateway  3  receives a packet from the public network  2 , and transmits it to the access point (AP)  8  or to the cellular gateway (CGW)  12 . The gateway  3  also receives a packet from the access point  8  or from the cellular gateway  12  through the sub-network  4 , and transmits it to the public network  2 . The access point  8  has the wireless LAN interface  9  such as a wireless LAN card. The cellular gateway  12  has the cellular phone interface  13  such as a packet communication card and a cellular phone. The mobile station  21  has the wireless LAN interface  23  and the cellular phone interface  22 . Communication is made between the wireless LAN interface  9  of the access point  8  and the wireless LAN interface  23  of the mobile station  21 . Communication is made through a cellular phone infrastructure  15  between the cellular phone interface  13  and the cellular phone interface  22  of the mobile station  21 . 
         [0033]      FIG. 2  shows an example of the construction of the mobile station  21 . An RF unit  31  or  41  receives and transmits an RF signal through an antenna, and converts the frequencies of the signals inputted from and outputted to a base band (BB) unit  32  or  42 . The RF unit  31  or  41  sends the electric field intensity of the received signal to a controller  35  through the BB unit  32  or  42 , a MAC unit  33  or  43 , and an interface (I/F)  34  or  44 . The BB unit  32  or  42  makes the assembly and modulation of wireless packets from the MAC PDU (MAC Protocol Data Unit), and supplies them to the RF unit  31  or  41 . The MAC unit  33  or  43  adds a MAC header to the IP packet fed from the I/F  34  or  44  to produce the MAC PDU and supplies it to the BB unit  32  or  42 . The MAC unit  33  or  43  also analyzes control information of MAC PDU fed from the BB unit  32  or  42 , and processes the MAC PDU according to the MAC protocol. The MAC unit  33  or  43  receives the MAC PDU containing the IP packet from the BB unit  32  or  42  and sends the IP packet to the controller  35 . The I/F  34  and I/F  44  are respectively the interfaces provided in the wireless LAN interface  23  and cellular phone interface  24  to a main body  71 . These interfaces  34  and  44  play an intermediation role for the input/output information and control signal to the wireless LAN interface  23  and cellular phone interface  22 , respectively. The main body  71  has a memory  36  for storing information to be transmitted and received, a power supply  37  for supplying power, a user interface  39  and the controller  35  for executing software and controlling the hardware of mobile station  21 . The controller  35  is generally constructed by using a CPU that has a memory. The controller  35  monitors the communication status according to the electric field strength informed from the I/F  34  or  44  and selects the wireless interface to be switched to. In addition, the controller  35  processes the IP packet fed from the wireless LAN interface  23  or from cellular phone interface  22  according to IP (Internet Protocol). The memory  36  stores the interface to be switched to, and the controller  35  supplies IP packets to the wireless LAN interface  23  or cellular phone interface  22  according to the stored information. The main body  71  may be, for example, a laptop computer or PDA (Personal Digital Assistance). The user interface  39  is, for example, a display, loudspeaker, microphone or keyboard. The memory  36  is, for example, a memory or hard disk. It is assumed that the wireless LAN interface  23  has its own MAC address of MAC 1 , and that the cellular phone interface  22  has its own MAC address of MAC 2 . When communication starts, the mobile station  21  uses, for example, the wireless interface specified by the initial MAC address stored in the memory  36 . The MAC address used before the start of communication may be held in the memory  36 . In addition, the user may write MAC 1  or MAC 2  via the user interface in that region of memory  36  in which a MAC address is stored before the start of communication. 
         [0034]      FIG. 3  shows the structure of software to be executed in the controller  35 . A driver  101  is the software for controlling the cellular phone interface  22 . A driver  102  is the software for controlling the wireless LAN interface  23 . Wrapper is the generic name of pretreatment software for compatibility to be kept or security to be confirmed. A wrapper  103  monitors the quality of wireless communication, switches the interfaces  22 ,  23  according to the quality, and transmits a switching request to the gateway  3 . A TCP/IP  104  is the software for making communication according to TCP (Transmission Control Protocol) and IP (Internet Protocol). The mobile station  21  is assumed to have an IP address of MS IP. A socket  105  is the TCP/IP interface used when a TCP/TP communication program is produced. An application  106  is the program for making TCP/IP communication such as telnet. 
         [0035]      FIG. 4  shows an example of the algorithm of wrapper  103 . In step  1 , the communication quality of the wireless LAN is monitored, and in step  2 , judgment is made of whether the current communication quality meets a criterion of switching. The communication quality may be, for example, field strength or error rate. The controller  35  monitors the field strength inputted from the I/F  34 . If the field strength exceeds a threshold in step  2 , judgment is made of which interface is currently used in step  3 . If the current interface is not wireless LAN interface  23 , a switching request is transmitted to the gateway in step  4 . In step  3 , the control  35  refers to the MAC address stored in the memory  36 , and identifies the current interface. In step  5 , the wireless LAN interface  23  is switched to, and the IP address (MS IP) is made to correspond to MAC address (MAC 1 ). In step  5 , the memory  36  stores the MAC address (MAC 1 ) switched to. If the field strength of the wireless LAN does not exceed the threshold in step  2 , judgment is made of which interface is currently used in step  6 . If the current interface is the wireless LAN interface  23 , a switching request is transmitted to the gateway  3  in step  7 . In step  8 , the cellular phone interface  22  is switched to, and the IP address (MS IP) is made to correspond to MAC address (MAC 2 ). In step  8 , the memory  36  stores the MAC address (MAC 2 ) switched to. 
         [0036]      FIG. 5  shows an example of the construction of the gateway  3 . When a packet is inputted to the gateway  3 , it is stored in a buffer  171 . A table  174  contains a routing table and an IP-MAC address correspondence table (ARP table: Address Resolution Protocol table). An IP header analyzer  172  compares the destination IP address of the header to the route information of table  174  to determine the next destination. A frame controller  173  refers to the table  174  to obtain the MAC address of the destination IP address, rewrites the MAC address of the packet inputted to the gateway  3 , and transmits the packet with the address rewritten. If the packet supplied to the gateway  3  is the switching request to the gate itself (gateway  3 ), the controller  175  analyzes IP data  184 , and rewrites the table  174  according to the information switched to that is contained in the IP data. The controller  175  searches the ARP table of the table  174  for the address specified by MS IP  186  of the switching request, and rewrites the corresponding MAC address in the table  174  into MAC address  187  specified by the switching request. After rewriting the table  174 , the IP packet of the destination, MS IP  186  is transmitted to the wireless interface specified by MAC address  187  of the switching request. When the packet to the IP address of mobile station  21  is fed to the gateway  3 , but when the IP address of mobile station  21  is not registered in the ARP table of table  174  due to the communication-starting time or time-out, the gateway  3  transmits an ARP request. When receiving the ARP request, the mobile station  21  refers to the MAC address of the current wireless interface stored in the memory  36 , and sends the information to the gateway  3 . The gateway  3  causes the information sent from the mobile station  21  to be recorded on the ARP table of table  174 . After the table  174  is rewritten, the IP packet to the mobile station  21  is transmitted to the wireless interface specified by the MAC address of the current wireless interface stored in the memory  36 . When the packet to the terminal  1  is supplied to the gateway  3 , the gateway  3  transmits the packet according to the routing table of table  174 . In addition, the controller  175  sends a switching reponse  62  to the mobile station  21 , and a connection request  63  to the cellular gateway  12 . 
         [0037]      FIG. 14  shows an example of the message format of the switching request transmitted from the mobile station  21  to the gateway  3 . The switching request consists of MAC header  181  and MAC data  182 . The MAC data  182  contains IP header  183  and IP data  184 . The IP data  184  contains a command  185  indicating the switching request, IP address  186  of mobile station  21 , MAC address  187  of mobile station  21  to be switched to, and a call number  188  of mobile station  21 . 
         [0038]      FIG. 6  shows an example of the signal exchange within the system shown in  FIG. 1  in the case where the mobile station  21  switches from the wireless LAN interface  23  to the cellular phone interface  22 . The mobile station  21  transmits an association request  51  to the access point  8 , and the access point  8  responds to this request to transmit an association response  52  to the mobile station  21 , so that the mobile station  21  is associated with the access point  8 . At this time, it is assumed that the MS IP and MAC 1  are linked in the mobile station  21 . The mobile station  21  transmits and receives packets  55 ,  56  via the access point  8  to and from the terminal  1  connected to the network  2 . When the mobile station  21  detects that the quality of the communication between the wireless LAN interfaces  9  and  23  is deteriorated, the mobile station  21  transmits a switching request  61  to the gateway (GW)  3 . The switching request  61  is transmitted so that the mobile station  21  can switch from the current one of the wireless interfaces  22  and  23  to the new one. The switching request  61  contains information of MAC address of either MAC 1  or MAC 2  to which the mobile station  21  is switched. In addition, when the interface to be switched to is the cellular phone interface, the switching request  61  contains the information of the telephone number of the cellular phone. The gateway  3  transmits a switching response  62  to the mobile station  21 . The access point  8  may be disconnected from the mobile station  21  to stop the communication by the switching response  62 . The gateway  3  transmits a connection request  63  to the cellular gateway  12 . This request  63  contains the call number  188  of the cellular phone interface  22  of mobile station  21 . The cellular gateway  12  calls the mobile station  21 , and the mobile station  21  is called. The cellular gateway  12  transmits a connection response  65  to the gateway  3 . 
         [0039]    The gateway  3  generally knows the corre-spondence between MAC address and IP address of a host within a sub-network from the ARP (Address Resolution Protocol) before the IP communication, and manages this correspondence as ARP table. According to the ARP, the gateway  3  broadcasts an ARP request to the entire LAN, and the host, when receiving this ARP request, sends the ARP with its own MAC address written back to the gateway  3 . The gateway  3  receives this MAC address, and makes communication by unicast. When the host to which the ARP request is sent does not belong to the same sub-network, the MAC address to the ARP request is treated as the MAC address of default gateway (router). 
         [0040]    The gateway  3  refers to the information contained in the switching request  61  and changes the correspondence between MAC 1  and MS IP of the ARP table to that of MAC 2  and MS IP. The mobile station  21  changes the bind of MAC 1  and MS IP to that of MAC 2  and MS IP. The mobile station  21  transmits and receives packets  59 ,  60  by using the cellular phone interface  22 . 
         [0041]      FIG. 7  shows an example of the signal exchange within the system shown in  FIG. 1  in the case where the mobile station  21  switches from the cellular phone interface  22  to the wireless LAN interface  23 . The mobile station  21  transmits and receives packets  59 ,  60  to and from the terminal  1  by using the cellular phone interface  22 . At this time, it is assumed that MAC 2  and MS IP are bound. In addition, it is then assumed that the mobile station  21  can assure the quality enough to communicate by wireless LAN  23  by improving the quality of communication between the wireless LAN interfaces  9  and  23 . The mobile station  21  and access point  8  exchange an association request  51  and an association response  52  so that the mobile station  21  can be associated with the access point  8 . The mobile station  21  transmits a switching request  66  to the gateway (GW)  3 . The gateway  3  transmits a switching response  67  to the mobile station  21 . 
         [0042]    The gateway  3  refers to the information contained in the request  66  and changes the correspondence between MAC 2  and MS IP of the ARP table to that of MAC 1  and MS IP. The mobile station  21  changes the bind of MAC 2  and MS IP to the bind of MAC 1  and MS IP. The mobile station  21  transmits and receives packets  55 ,  56  to and from the terminal  1  by using the wireless LAN interface  23 . 
         [0043]    The cellular gateway  12  may have a dial-up router in place of the cellular phone interface  13  so that the cellular gateway  12  and cellular phone infrastructure  15  can be connected via land line network. In addition, the mobile station  21  may have more than three wireless interfaces. The cellular phone can be connected without being aware of location, but it is slow in communication speed as compared to the wireless LAN. The wireless LAN is fast in communication speed and at low cost as to the associated service fee, but the area where the service is offered is limited to dot-like areas called hotspot. If the user uses the cellular phone and wireless LAN by switching them, the service area can be expanded. Also, if the user uses the cellular phone and wireless LAN having different features by switching them, the user can acquire the merit to increase the freedom with which the user can select fast or more inexpensive service. The wireless interfaces of the mobile station  21  may be other than the combination of wireless LAN interface and cellular phone interface. For example, the mobile station  21  may have cellular phone interface and Bluetooth interface. In addition, the mobile station  21  may have, for example, cellular phone interface and UWB (Ultra WideBand) interface. The wireless interfaces of the mobile station  21  may be different types of interfaces from the wireless LAN interface. For example, the interfaces of standard IEEE 802.11a and standard 802.11b have no compatibility because the communication frequencies are different. Thus, by switching the wireless interfaces of usable standards, it is possible to expand the service area as compared with the case where an interface of a single standard is used. In an area where both standards can be used, the user can select a wireless interface of fast-communication-speed standard. 
         [0044]    In addition, the mobile station  21  may have a plurality of interfaces of the same type.  FIG. 8  shows another example of the construction of the system in the case where a mobile station  24  having wireless LAN interfaces  25 ,  26  makes hand-over process between the access points  8  and  10 . The terminal  1  and gateway (GW)  3  are connected to the network  2 . The gateway  3  routes packets to the access point  8  (AP 1 ) or access point  10  (AP 2 ). The access points  8  and  10  have wireless LAN interfaces  9  and  11  such as wireless LAN cards, respectively. If plural wireless LAN interfaces of the same type are provided, the status of a channel to which switching is made can be monitored in parallel by the plural wireless LAN interfaces. Also, use of plural wireless LAN interfaces of the same type will enable switching of channels such as frequencies to be fast made as compared to a mobile station having a single wireless interface. Thus, by switching wireless interfaces of the same type, it is possible to fast make hand-over processes as compared to a mobile station having a single wireless interface. 
         [0045]      FIG. 9  shows an example of the construction of the mobile station  24 . An RF unit  111  or  113  transmits and receives RF signals through an antenna, and converts the frequencies of the RF signals from and to a BB unit  112  or  142 . The BB unit  112  or  142  makes the assembly and modulation of wireless packets from MAC PDU (MAC Protocol Data Unit), and supplies them to the RF unit  111  or  131 . A MAC unit  113  or  143  supplies MAC PDU obtained by demodulating wireless packets to the BB unit  112  or  142 , analyzes the control information of the MAC PDU fed from the BB unit  112  or  142 , and processes the MAC PDU according to the MAC protocol. An I/F  114  or  144  is the interface between the wireless LAN interface  25  or  26  and the main body  71 . Each I/F plays an intermediation role for the input/output information and control signal to the wireless LAN interfaces  25  and  26 . The main body  71  has the memory  36  for storing the transmitted and received information, the power supply  37  for supplying electric power, the user interface  39  and the controller  35  for controlling the hardware of mobile station  24 . The main body  71  may be, for example, a laptop computer. The user interface  39  is, for example, a display, loudspeaker, microphone, or keyboard. The memory  36  is, for example, a memory or hard disk. 
         [0046]    It is assumed that the wireless LAN interfaces  26  and  25  have their own MAC addresses of MAC 1  and MAC 2 , respectively. 
         [0047]      FIG. 10  shows the structure of software executed by the control  35 . The driver  102  is the software for controlling the wireless LAN interfaces  25  and  26 . A wrapper  107  monitors the quality of wireless communication, switches the interfaces  25  and  26  according to the quality, and transmits a switching request to the gateway  3 . The TCP/IP  104  is the software for making communication according to TCP (Transmission Control Protocol) and IP (Internet Protocol). The mobile station  24  is assumed to have an IP address of MS IP. The socket  105  is the TCP/IP interface used when a TCP/TP communication program is produced. The application  106  is the program for making TCP/IP communication such as telnet. 
         [0048]      FIG. 11  shows an example of the algorithm of wrapper  107 . In step  21 , the communication quality of the wireless LAN  25 ,  26  is monitored, and in step  22  the communication quality of wireless LAN interface  25  is compared with that of wireless LAN interface  26 . The communication quality may be, for example, field strength or error rate. If the wireless LAN interface  26  has a better communication quality in step  22 , but is not currently used for communication, a switching request is transmitted to the gateway in step  24 . In step  25 , the wireless interface is switched to the wireless LAN interface  26 , and the IP address (MS IP) and MAC address (MAC 1 ) are bound together. If the wireless LAN interface  25  has better communication quality in step  22 , and if the wireless LAN interface  26  is currently used to communicate, a switching request is transmitted to the gateway in step  27 . In step  28 , the wireless interface is switched to the wireless LAN interface  25  from the interface  26 , and the IP address (MS IP) and MAC address (MAC 2 ) are bound together. 
         [0049]      FIG. 12  shows an example of the signal exchange within the system shown in  FIG. 8  in the case where the mobile station  24  switches from the wireless LAN interface  25  to the wireless LAN interface  26 . It is now assumed that the mobile station  24  can assure the quality enough to communicate through wireless LAN by improving the quality of the communication between the wireless LAN interfaces  9  and  26 . The mobile station  24  and the access point  8  exchange the association request  51  and the association response  52  so that the mobile station  24  can be associated with the access point  8  (AP  1 ). It is then assumed that the mobile station  24  can assure the quality enough to communicate through wireless LAN  25  by improving the quality of the communication between the wireless LAN interfaces  11  and  25 . The mobile station  24  and access point  10  (AP  2 ) exchange an association request  53  and an association response  54  so that the mobile station  24  can be associated with the access point  10 . The mobile station  24  transmits and receives packets  55 ,  56  to and from the terminal  8  by using the first associated wireless LAN interface  26 . At this time, the MS IP and MAC 1  are bound together in the mobile station  24 . 
         [0050]    It is assumed that the quality of communi-cation between the wireless LAN interfaces  9  and  26  is deteriorated as compared to that between the wireless LAN interfaces  11  and  25 . The mobile station  24  transmits a switching request  57  to the gateway (GW)  3 . The gateway  3  transmits a response  58  to the mobile station  24 . 
         [0051]    The gateway  3  refers to the information contained in the request  57  to change the corre-spondence between MAC 1  and MS IP to the correspondence between MAC 2  and MS IP in the ARP table. The mobile station  24  changes the bind between MAC 1  and MS IP to the bind between MAC 2  and MS IP. The mobile station  24  transmits and receives packets  59 ,  60  by using the wireless LAN interface  25 . 
         [0052]      FIG. 13  shows an example of the power control to be performed by the control  35 . When the mobile station  24  starts to transmit and receive packets  59 ,  60  to and from the terminal  1  by using the wireless LAN interface  25 , the control  35  shuts off the supply of power to stop the operation of the wireless LAN interface  26 . The control  35  supplies power to the wireless LAN interface  26  during intervals  160 ,  161  of duration a with a period b. For example, the duration a is a few hundreds of milliseconds, and the period is a few seconds. 
         [0053]    The mobile station generally knows the presence of a base station by receiving a beacon or probe response transmitted from the base station. A wireless LAN access point transmits a beacon at intervals of about 100 ms, and the mobile station received the beacon refers to the information contained in the beacon and registers with the base station. Alternatively, the mobile station sends a probe request and the base station catched the request transmits a probe response. The mobile station refers to the information in the probe response and registers with the base station. The wireless LAN interface  26  receives a beacon or a probe response during the intervals  160 ,  161 , and acquires the identifiers of the surrounding base stations, and field strengths from the received signal. The controller  35  makes the processing shown in  FIG. 11  by using the information obtained above. 
         [0054]    While the above example is the case where two wireless LAN interfaces are used in the mobile station  24 , the wireless mobile station  24  may have three or more wireless interfaces. In addition, the wireless interfaces may be other than the wireless LAN. Moreover, the system according to the invention may use the conventional mobile IP at the same time. In this case, a home agent is placed in the network  2 .  FIG. 17  shows an example of the software structure of the mobile station. IP software  200  is introduced above the TCP/IP software  104 . 
         [0055]    It should be further understood by those skilled in the art that although the foregoing description has been made on embodiments of the invention, the invention is not limited thereto and various changes and modifications may be made without departing from the spirit of the invention and the scope of the appended claims.