Abstract:
A wireless communication system to support real time service and handoff method thereof. A plurality of mobile management units having a plurality of visitor management units perform a handoff based on the movement of a mobile communication unit. One or more visitor management unit is a common visitor management unit that is included in a network constructed by the plurality of mobile management units. When the mobile communication unit moves to another visitor management unit of the same mobile management unit, the mobile management unit including the mobile communication unit updates the mobility information to provide a real time service path between the information providing unit and the mobile communication unit. When the mobile communication unit moves to a different mobile management unit, the common visitor management unit requests another mobile management unit for registration of the mobile communication unit, thereby making a reservation for a real time service path.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    This invention relates to a wireless communication system to support real time service and a handoff method thereof, and more particularly to a wireless communication system to support real time service and a handoff method thereof using Resource Reservation Protocol (RSVP) in a mobile Internet Protocol environment. The present application is based on Korean Patent Application No. 2001-66476, which is incorporated herein by reference.  
           [0003]    2. Description of the Related Art  
           [0004]    Recently, due to the rapid spread of mobile communication and the Internet, wireless Internet service, and, furthermore, high quality wireless Internet service, such as that in a wire network are required. As a result, a lot of services, such as Multicast service and real time service, that are used in a wire environment are provided in a wireless environment. Therefore, study of real time service such as Video Conference, Video on Demand (VOD), Interactive TV, and Voice over Internet Phone (VoIP) is continuously in progress.  
           [0005]    The standard of mobile Internet Protocol (IP) for offering real time service in a wireless environment was suggested in 1996 by the Internet Engineering Task Force (IETF). However, the suggested mobile EP standard has many limitations and problems in supporting substantial real time service, including ineffective transmission path by triangle routing, cancellation of a higher rank protocol by frequent registration information renewals by a mobile host, transmission delay of a datagram and increase of blocking probability by handoff, and difficulty in transmission path set up by encapsulation and tunneling.  
           [0006]    In order to support mobility of a mobile host and secure continuous real time service, the prompt renewal of registration information and the priority of a real time service transmission path should be guaranteed when a handoff occurs. In the suggested mobile IP standard, all datagrams always pass through a home agent (HA), and, therefore, a path message of a resource reservation protocol cannot be sent to the mobile host that is moved. Even though the path message is sent to the mobile host using the method of tunneling, still there is a problem that the path for real time service should always include a home agent.  
           [0007]    The above resource reservation method is called triangle routing and is discussed in “IP Mobility Supports” by Perkins published in IETF RFC  2002 , October 1996, “Path Optimization in Mobile IP” by Johnson and Perkins published in IETF Mobile-IP Working Draft 5th edition, November 1996, and “Locality-based Path Optimization in Mobile IP” by Kang-il Lee et al. published in 4th international mobile multimedia communication workshop proceedings (September˜October 1997).  
           [0008]    [0008]FIG. 1 shows a datagram transmission process of mobile IP by conventional triangle routing. Referring to FIG. 1, a mobile host  140  requests registration of its own IP address by searching for a foreign agent  130  when it is located outside of an area covered by the home agent  120 . The foreign agent  130  that received a request for registration sends a message requesting registration of the mobile host  140  to the home agent  120 . Through the process of registration and renewal of registration information between the mobile host  140 , the foreign agent  130 , and the home agent  120 , the home agent  120  can identify the exact location of the mobile host  140 , no matter where the mobile host  140  moves.  
           [0009]    After the location of the mobile host  140  is registered in the home agent  120 , all datagrams sent from a correspondent host  110  to the mobile host  140  are encapsulated in the home agent  120  and sent through the tunnel formed by the foreign agent  130 . By decapsulation, the foreign agent  130  that received the encapsulated datagram sends the datagram which has an IP header used for tunneling to the mobile host  140 . The foreign agent  130  operates as a Care-of-Address for tunneling. According to FIG. 1, as path set up of the mobile host  140  is always done through the home agent  120 , if the mobile host becomes distant from the home network, it will cause more delay in the renewal of registration information by triangle routing.  
           [0010]    A scheme for overcoming this problem is presented in the theses titled “Implementing Resource Reservation for Mobile Hosts in the Internet Using RSVP and Mobile IP” by Chua K. C. et al. and “Mobility Management in IP Networks Providing Real-time Service” by Palemo et al., both published in 2000.  
           [0011]    The method presented in these theses, which uses a path change message, is that when the mobile host moves into the foreign network, the home agent sends a path change message about the path message received from the correspondent host and the correspondent host again sends the path message to the foreign agent of the foreign network into which the mobile host has moved. However, this method does not include a clear solution for providing continuous real time service when the mobile host moves from one foreign agent to another foreign agent, in other words, when a handoff occurs between the foreign agents.  
           [0012]    [0012]FIG. 2 shows a process for implementing resource reservation protocol using a path change message in the mobile IP environment. When the correspondent host  110  sends a path message to the mobile host  140 , the home agent  120  sends the changed information on location of the mobile host  140  via a path change message to the correspondent host  110 . The correspondent host, which received a path change message, sends the path change message again to the foreign agent  130  to where the mobile host is moved. The mobile host  140  can have a transmission path for real time service between the correspondent host  110 , the foreign agent  130 , and the mobile host  140  secured by sending a reservation message through the foreign agent  130 .  
           [0013]    However, this method does not have a clear solution in case the mobile host  140  moves to another foreign agent. In other words, if a handoff occurs as the mobile host moves into an external network, a new path set up process has to be done from the start. At this point, all data being transferred are blocked from the existing foreign agent and cause a problem of increasing the traffic of the node and the link.  
           [0014]    Meanwhile, RSVP for supporting real time service in a wire network has a lot of limitations and problems for applying wireless data communication in a mobile IP environment. If the mobile host is moved from the home network in which it is already registered and registered in an external network, it cannot receive a path message sent from the correspondent host, and, therefore, cannot send a reservation message for real time service. As a result, resource reservation protocol of the wire network cannot support the mobility of the mobile host. Accordingly, in order to support the mobility of the mobile host and secure continuous real time service, RSVP structure that is suitable for the wireless network and able to adapt itself in case of a handoff is necessary.  
           [0015]    As explained referring to FIG. 1, the path message of RSVP cannot be sent to the mobile host in the conventional triangle routing method as all datagrams always pass through the home agent. Even if the path message is transferred to the mobile host using the tunneling method, substantial real time service cannot be possible, because the real time service path always includes the foreign agent and the home agent.  
           [0016]    Also, when the mobile host is moved to another foreign agent, that is, when a handoff occurs between the foreign agents, there is no clear solution for continuous real time service in the method using a path change message explained referring to FIG. 2. That is to say, with this method the registered network cannot secure the mobility of a continuously changing wireless environment. Therefore, a structure suitable for a handoff scheme in a mobile IP environment should be added to the existing RSVP.  
           [0017]    [0017]FIG. 3 shows an object class structure of the conventional path change message. Referring to FIG. 3, the object class of the conventional path change message includes the addresses of the home agent and the Care-of-Address. However, the path change message does not have information for providing continuous real time service when the mobile host moves to another foreign agent. Therefore, if a handoff occurs as the mobile host moves into an external network, a new path set up process has to be done from the start.  
         SUMMARY OF THE INVENTION  
         [0018]    The present invention has been made to overcome the above-identified problems. Accordingly, it is an object of the present invention to provide a wireless communication system and a handoff method used by the same, which is capable of providing a real time service under a mobile IP environment that can prevent datagram transmission delay and occurrence of blocking phenomenon due to handoff.  
           [0019]    The above object is accomplished by a wireless communication system for real time service according to the present invention, including multiple mobile management means for forming a network comprised of multiple visitor management means and managing mobile information of a mobile communication device; member management means for receiving the mobile information from the mobile management means and managing location renewal status of the mobile communication device; and information provision means for producing a datagram provided to the mobile communication device according to the mobile information of the mobile communication device,  
           [0020]    The visitor management means forms a respective separate sub-network and provides a real time service path for providing the datagram to the mobile communication device, and at least one of the visitor management means is the common visitor management means included in the network formed by the multiple mobile management means.  
           [0021]    The mobile management means renews the mobile information and provides the real time service path between the information provision means and the mobile communication device when the mobile communication device moves to the sub-network formed by the other visitor management means within a network formed by the same mobile management means.  
           [0022]    The mobile management means, forming the sub-network in which the mobile communication device is located, sends a reservation message to the information provision means when a path message is received from the information provision means and provides the real time service path between the information means and the mobile communication device.  
           [0023]    The visitor management means, in which the mobile communication device is located, sends a change message to the mobile management means forming the sub-network in which the mobile communication device is located and changes the mobile information.  
           [0024]    When the mobile communication device is located in the sub-network forming the common visitor management means, the common visitor management means requests registration of the mobile communication device to the other mobile management means which does not manage the mobile information of the mobile communication device and reserves a real time service path.  
           [0025]    The mobile management means in which the mobile communication device is located, sends a path change message to the information provision means and transmits information about the mobile management means forming the sub-network to where the mobile communication device will be moved; the mobile management means, to where the mobile communication device will be moved, receives a path message of the mobile communication device from the information provision means, sends a reservation message to the information provision means, and secures a real time service path; and the member management means renews registration of the location of the mobile communication device.  
           [0026]    The common visitor management means transmits an information request message to the close mobile management means; and the mobile management means corresponding to the information request message transmits an information respondent message to the common visitor management means, transmits a path change message to the information provision means, and receives a path message from the information provision means.  
           [0027]    The above object is also accomplished by a wireless communication method for real time service, comprising the steps of: receiving a path message from an information provision means by a mobile management means in which a mobile communication device is located; identifying a visitor management means managing mobile information of the mobile communication device by the mobile management means in which the mobile communication device is located; and requesting registration of the mobile communication device to a mobile management means close to the mobile management means in which the mobile communication device is located by the common visitor management means when the visitor management means is the common visitor management means located in multiple mobile management means.  
           [0028]    The multiple mobile management means form a network comprised of the multiple visitor management means and manages mobile information of the mobile communication device.  
           [0029]    The visitor management means forms a respective sub-network and provides a real time service path in order to supply a datagram to the mobile communication device.  
           [0030]    In a non-limiting, exemplary embodiment of the present invention, the registration request step comprises the steps of: sending a path change message to the information provision means providing a datagram by the mobile management means in which the mobile communication device is located; sending the path message to the mobile management means by the information provision means, the mobile management means being moved to the mobile communication device; and sending a reservation message to the information provision means by the mobile management means to where the mobile communication device is to be moved.  
           [0031]    In another non-limiting, exemplary embodiment of the present invention, the registration request step comprises the steps of: sending an information request message to the mobile management means located in the mobile communication device and to the close mobile management means by the common visitor management means; sending an information respondent message to the common visitor management means by the mobile management means corresponding to the information request message; and sending a path change message to the information provision means and receiving the path change message from the information provision means.  
           [0032]    The path change message comprises addresses of the mobile communication device, the common visitor management means in which the mobile communication device is located, the mobile management means in which the mobile communication device is located, and the mobile management means corresponding to the information request message.  
           [0033]    In the case that the visitor management means including the mobile information of the mobile communication device is not the common visitor management means, the mobile management means in which the mobile communication device is located sends a reservation message to the information provision means when a path message is received from the information provision means and provides a real time service path to the information provision means and the mobile communication device.  
           [0034]    In the case that the visitor management means including the mobile information of the mobile communication device is not the common visitor management means, the visitor management means in which the mobile communication device is located sends a change message to the mobile management means and requests the mobile management means that a path be changed to the visitor management means.  
           [0035]    The change message comprises addresses of the visitor management means to be moved and the mobile communication device.  
           [0036]    Accordingly, the real time services are supported in the mobile IP environment, while the possibility of having datagram transmission delay and blocking phenomenon due to handoff is decreased. Also, repeated renewals of registration information via the home agent according to the movement of the moving host are prevented, as a plurality of foreign agents construct one cluster and one cluster agent represents the cluster. Furthermore, by positioning an overlapped agent shared by the clusters, a path of the next cluster is established in advance, and accordingly, problems like communication cut-off during the handoff between the clusters can be improved. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0037]    The aforementioned object and characteristics of the present invention will be more apparent by describing a preferred embodiment of the present invention in greater detail with reference to accompanied drawings, in which:  
         [0038]    [0038]FIG. 1 is a view showing a process of transmitting a mobile IP datagram by using a conventional triangle routing;  
         [0039]    [0039]FIG. 2 is a view showing a process of establishing an RSVP by using a path change message under the conventional mobile IP environment;  
         [0040]    [0040]FIG. 3 is a view showing an object class structure of the conventional path change message;  
         [0041]    [0041]FIG. 4 is a view showing the structure of a wireless communication system for real time service according to the preferred embodiment of the present invention;  
         [0042]    [0042]FIG. 5 is a view showing the structure of the datagram transmitted from a correspondent host to the home agent and the structure of a datagram encapsulated in the home agent;  
         [0043]    [0043]FIG. 6 is a view showing a general handoff process according to the present invention;  
         [0044]    [0044]FIG. 7 is a view showing a local handoff process according to the present invention;  
         [0045]    [0045]FIG. 8 is a view showing the structure of information request/response message being transmitted from an overlapped foreign agent to a cluster agent;  
         [0046]    [0046]FIG. 9 is a view showing the structure of the path change message being transmitted from the cluster agent to the correspondent host;  
         [0047]    [0047]FIG. 10 is a view showing the structure of the path change message being transmitted from the foreign agent to the cluster agent;  
         [0048]    [0048]FIG. 11A is a flowchart of the handoff method for the wireless communication system according to the present invention;  
         [0049]    [0049]FIG. 11B is a flowchart of the general handoff method for the wireless communication system according to the present invention;  
         [0050]    [0050]FIG. 11C is a flowchart of the local handoff method for the wireless communication system according to the present invention;  
         [0051]    [0051]FIG. 12 is a view showing the real time service providing process during the handoff in the foreign agents performing the conventional routing function;  
         [0052]    [0052]FIG. 13 is a view showing the real time service providing process during the handoff in the foreign agents performing the function of the cluster agent;  
         [0053]    [0053]FIG. 14 is a view showing the real time service providing process during the handoff in the clusters, with some of the foreign agents performing the function of the cluster agent and no overlapped foreign agent available; and  
         [0054]    [0054]FIG. 15 is a view showing the real time service providing process during the handoff in the clusters, with both the cluster agent and the overlapped foreign agent available. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0055]    Hereinafter, a preferred embodiment of the present invention will be described in greater detail with reference to the accompanying drawings.  
         [0056]    [0056]FIG. 4 is a view showing the structure of a wireless communication system for providing real time services according to one preferred embodiment of the present invention.  
         [0057]    Referring to FIG. 4, the wireless communication system  400  for providing real time services according to the present invention includes a plurality of cluster agents (CA)  410 ,  420 , a plurality of foreign agents (FA)  410 - 1 ˜ 410 - n,    420 - 1 ˜ 420 - n,  an overlapped agent (OA)  430 , a home agent (HA)  460 , and a correspondent host (CH)  470 .  
         [0058]    The cluster agents CAs  410 ,  420  construct independent networks like a cluster 1   415  and a cluster 2   425 . Each cluster agent CAs  410 ,  420  has a plurality of foreign agents FAs  410 - 1 ˜ 410 - n,    420 - 1 ˜ 420 - n  that constitute an independent sub-network. The overlapped agent OA  430  is a foreign agent that is shared by the cluster 1   415  and the cluster 2   425 .  
         [0059]    The CAs  410 ,  420  are connected to the HA  460  via the Internet  450 . The HA  460  is connected with the CH  470 . Mobile hosts (MH), like mobile phone, PDA, etc., are provided with the datagram from the CH  470  via the FAs  410 - 1 ˜ 410 - n,    420 - 1 ˜ 420 - n,    430  that manage the sub-network where the MH is located in.  
         [0060]    The CH  470  identifies the location of the MH  480  by transmitting the path message to the MH  480 . The CH  470  also sends the datagram for transmission to the MH  480  to the HA  460 .  
         [0061]    The HA  460  provides the CH  470  with the location information of the MH  480 , encapsulates the datagram transmitted from the CH  470 , and transmits the encapsulated datagram to the FAs  410 - 1 ˜ 410 - n,    420 - 1 ˜ 420 - n,    430  where the MH  480  is located.  
         [0062]    [0062]FIG. 5 shows the structure of the datagram being transmitted from the CH  470  to the HA  460 , and the structure of the datagram encapsulated in the HA  460 . Referring to FIG. 5, the datagram  510 , which is transmitted from the CH  470  to the HA  460 , has an IP header and a payload. The datagram  520 , which is encapsulated in the HA  460 , has a foreign header and a foreign payload. The foreign payload includes a pre-encapsulation IP header and payload.  
         [0063]    The CA 1   410  or CA 2   420  operates as a Care-of-Address (COA) on behalf of all the FAs  410 - 1 ˜ 410 - n,    420 - 1 ˜ 420 - n,    430  that exist in the lower rank and represents one cluster. The CA 1   410  or CA 2   420  also manages the mobility information of the ML  480 , i.e., the information about the CA 1   410  or CA 2   420  where the MH  480  exists, and the information of the FA (one of  410 - 1 ˜ 410 - n,    420 - 1 ˜ 420 - n,    430 ). The CA 1   410  or CA 2   420  serves the function of a terminal endpoint of the tunneling of all the lower rank FAs  410 - 1 ˜ 410 - n,    420 - 1 ˜ 420 - n,    430 .  
         [0064]    A general handoff occurs between the CAs  410 ,  420 . The general handoff accompanies a renewal of the HA registration information. During the general handoff, the COA is changed from the CA 1   410 , where the MH  480  is located, to the CA 2   420 , to where the MH  480  is going to move. For the registration of MH  480 , a relatively long period of time is required to renew the registration information through a series of processes at the HA  460 , CA 1   410  and MH  480 .  
         [0065]    The OA  430  is the FA shared by the CAs  410 ,  420 . Accordingly, when the MH  480  enters from the FAn  410 - n  of the CA 1   410  to where the OA belongs, the OA  430  registers the MH  480  in the other CA (CA 2 ) where the OA  430  also belongs, prior to the general handoff.  
         [0066]    Meanwhile, if the MH  480  moves within the same CA, like within the CA 1   410  or CA 2   420 , from a certain sub-network to another sub-network that is managed by other FAs  410 - 1 ˜ 410 - n  or  420 - 1 ˜ 420 - n,  a local handoff occurs. In other words, the local handoff occurs within the same cluster. In the case of local handoff, the registration information of the HA  460  is not renewed, but the location of the MH  480  is registered by means of renewal of location information of the CA 1   410  or CA 2   420 . The local handoff has a short registration path between the MH  480  and the CA 1   410  or CA 2   420 .  
         [0067]    The FAs  410 - 1 ˜ 410 - n,    420 - 1 ˜ 420 - n,    430  constitute independent sub-networks, and provide the real time service path for providing the MH  480  with the datagram received from the CH  470 .  
         [0068]    In the network structure of the wireless communication system  400  as described above, the CA 1   410  and the CA 2   420  for mobility management are positioned in the clusters  1  and  2 , respectively. Each of CA 1   410  and CA 2   420  operates as a COA of all the MH  480  that are registered in the network managed by the CA 1   410  or CA 2   420 . The CA 1   410  manages all FAs of the sub-network, i.e., the FA 1   410 - 1 , FA 2   410 - 2 , . . . , FAn  410 - n,  and OA  430 . The CA 2   420  manages all FAs of the sub-network, i.e., the FA 1   420 - 1 , FA 2   420 - 2 , . . . , FAn  420 - n,  and OA  430 .  
         [0069]    The general handoff during the moving of the MH  480  from one cluster to another cluster will be described in greater detail below.  
         [0070]    [0070]FIG. 6 shows the general handoff process according to the present invention. As the MH  480  moves from the cluster  415  of the CA 1   410  to the cluster  415  of the CA 2   420 , the MH  480  moves from the FAn  410 - n  to the FA 1   420 - 1  via the OA  430 .  
         [0071]    When the MH  480  enters the OA  430  region, first, in order to obtain RSVP path information, the OA  430  sends an information request message to a neighboring CA inclusive of the CA 1   410  and the CA 2   420  ({circle over (1)}). The CA 1   410 , which has the corresponding path for the information request message, responds with an information reply message, while the other CAs disregard the information request message ({circle over (2)}).  
         [0072]    [0072]FIG. 8 shows the structure of the information request/reply message being transmitted by the OA  430 . The information request/reply message is formed of address of MH  480 , address of CA 1   410 , address of CA 2   420 , . . . , address of CAn. The addresses take 4 bytes, respectively.  
         [0073]    When the MH  480  moves from the cluster 1   415  to the cluster 2   425 , the COA is also changed from the CA 1   410  to the CA 2   420 . Accordingly, the location registration of the MH  480  with respect to the HA  460  is renewed, and the CA 1   410 , which received the information request message, reports the mobility information of the ME  480  corresponding to the CA 2   420 . The reporting of the mobility information is carried out as the CA 1   410  reports the path change message to the CH  470  and the MH  480  reports the information of the CA 2   420  ({circle over (3)}).  
         [0074]    [0074]FIG. 9 shows the structure of the path change message being transmitted from the CA to the CH. The path change message includes the address of the CA 2   420 , the address of the OA  430 , the address of the CA 1   410 , and the address of the MH  480 . The addresses take 4 bytes, respectively.  
         [0075]    The CA 2   420 , which received the information request message, receives the path message of the corresponding MH  480  from the CH  470  ({circle over (4)}) of FIG. 6, and reports the reservation message to the CH  470 , to thereby ensure the path for real time services by the MH  480  ({circle over (5)}). The OA  430 , positioned in between the clusters, is registered in the neighboring cluster agent as a guest prior to the general handoff, to reserve a real time service path.  
         [0076]    During the real time service path renewal, the MH  480  receives the continuous real time service while maintaining the path of the CA 1   410 . When the path is renewed by the process ({circle over (5)}) of FIG. 6, the path of the CA 1   410  is released, while the real time services are provided through the path of the CA 2   420 .  
         [0077]    The location handoff when the MH  480  moves to the another FA within the same cluster will be described in greater detail below.  
         [0078]    [0078]FIG. 7 shows the local handoff process according to the present invention. The local handoff occurs as the MH  480  moves within the cluster  415  of the CA 1   410 , from the FA 1   410 - 1  to the FAn  410 - n.  The mobility of the MH  480 , which moves in the same cluster, is taken care of by the cluster agent (CA 1  in FIG. 7) that manages the corresponding cluster. Accordingly, all path messages transmitted from the CH  470  are transmitted to the CA 1   410 .  
         [0079]    The MH  480 , moved to the FAn  410 - n,  requests through the FAn  410 - n  that the CA 1   410  change the path to the FAn  410 - n.  At this time, the FAn  410 - n  transmits the change message to the CA 1   410 . The structure of the change message is shown in FIG. 10. The change message includes the address (4 bytes) of the FA and the address (4 bytes) of the MH.  
         [0080]    The CA 1   410  changes the value of the FA that matches the MH  480  in the guest registration list, and transmits the renewal of the registration information to the MH  480  through the FAn  410 - n.  Also, the CA 1   410  transmits the reservation message of the path message to the CA 1   410 , to thereby ensure the path between the CH  470  and the ME  480 .  
         [0081]    Since the location renewal path by the handoff is not carried out via the HA  460 , it takes a relatively short time. As described above, the local handoff process is not a process of registration via the HA  460 , and accordingly, the value of the COA is not changed, and there is no need for the HA  460  and the CH  470  to obtain the information about the changed path.  
         [0082]    [0082]FIG. 11A shows the process of a handoff method in the wireless communication system according to the present invention.  
         [0083]    Referring to FIG. 11A, the CA 1   410  or CA 2   420 , where the MH  480  is located, receives the path message from the CH  470  (step S 1100 ). The CA 1   410  or CA 2   420  that received the path message, checks the FAs  410 - 1 ˜ 410 - n,    420 - 1 ˜ 420 - n,  and  430  that take care of the mobility information of the MH  480  (step S 1120 ). If it is determined that the OA  430  is taking care of the mobility information of the MH  480 , a general handoff is carried out (step S 1120 ). However, if it is determined that the FAs  410 - 1 ˜ 410 - n  or  420 - 1 ˜ 420 - n  within the same cluster take care of the mobility information of the MH  480 , a local handoff is carried out (step S 1130 ).  
         [0084]    [0084]FIG. 11B is a flowchart showing the general handoff process carried out in the wireless communication system according to the present invention.  
         [0085]    Referring to FIG. 11B, when the MH  480  enters the OA  430  region, first, in order to obtain the RSVP path information, the OA  430  transmits the information request message to the neighboring cluster agent inclusive of the CA 1   410  and the CA 2   420  (step S 1122 ). Then, the CA 1   410 , which has the corresponding path for the information request message, responds with the information reply message (step S 1124 ), while the other CAs disregard the information request message.  
         [0086]    The CA 1   410 , which received the information request message, reports the mobility information of the MH  480  corresponding to the CA 2   420  (step S 1126 ). The reporting of the mobility information is carried out as the CA 1   410  reports to the CH  470  of the path change message and the MH  480  reports the information about the CA 2   420  where the MH  480  has moved.  
         [0087]    The CA 2   420  receives the path message of the corresponding MH  480  from the CH  470  (step S 1128 ). The CA 2   420  also reports to the CH  470  of the reservation message, to thereby ensure the path for real time services by the MH  480  (step S 1130 ). During the general handoff, the OA  430 , positioned in between the clusters, is registered in the neighboring cluster agent as a guest, prior to the general handoff to thereby reserve a real time service path.  
         [0088]    [0088]FIG. 11C is a flowchart showing the local handoff process carried out in the wireless communication system according to the present invention.  
         [0089]    Referring to FIG. 11C, through the FAn  410 - n,  the MH  480  requests the CA 1   410 , where the MH  480  is located, to change the path to the FAn  410 - n  (step S 1162 ). The path change request is made as the FAn  410 - n  transmits the change message to the CA 1   410 . The CA 1   410  changes the value of the FA that corresponds to the MH  480  in the guest registration list (step S 1164 ). Through the FAn  410 - n,  the CA 1   410  also transmits the renewal of the registered information to the MH  480  (step S 1166 ). The CA 1   410  transmits the reservation message to the CH  470 , to thereby ensure the path for real time services between the CH  470  and the MH  480  (step S 1168 ).  
         [0090]    [0090]FIG. 12 is a view showing the process of providing the real time services when the handoff occurs between the FAs that perform the conventional triangle routing function. Referring to FIG. 12, during the datagram transmission through the FA 1 , if the MH  480  moves to the FA 2  region, the IP registration process is carried out. Here, since the resource reservation setting process between the MH  480  and the FA 2  is carried out from the start, and since the process has to be carried out via the HA  460 , a blocking occurs for a long time.  
         [0091]    [0091]FIG. 13 shows the process of providing the real time services when the handoff occurs between the FA of the same cluster, with some of the FA performing the function of cluster agent. Referring to FIG. 13, during the real time datagram transmission through the FA 1 , if the MH  480  moves to the FA 2  region, a mobility IP registration process is carried out. Since the mobility IP registration process is carried out through the cluster agent, and not via the HA  460 , the blocking occurs only in the process of mobility IP registration. Meanwhile, since the resource reservation setting process is carried out between the FA 1  and FA 2 , the blocking does not occur during the resource reservation process.  
         [0092]    [0092]FIG. 14 shows the process of providing the real time services when the handoff occurs between the clusters, with some FAs performing the function of cluster agent, but with no OA available. Referring to FIG. 14, when the handoff occurs among the FAs within the same CA, since the process does not have to be carried out via the HA, the cluster handoff is carried out in the same way as described with reference to FIG. 13. Accordingly, the blocking occurs only during the mobility IP registration process and during the resource reservation setting process that is carried out by the CAs and the HA.  
         [0093]    [0093]FIG. 15 shows the process of providing the real time service when the handoff occurs between the clusters, with all the CAs and the OA available. Referring to FIG. 15, when the handoff occurs between the CAs, the COA is also changed prior to the handoff via the FA. Accordingly, the blocking does not occur only during the resource reservation setting process that is carried out by the CAs and the HA.  
         [0094]    As described above, the wireless communication system for providing real time services and the handoff method according to the present invention can overcome the problems of the conventional technology in relation to the handoff. According to the present invention, a network is constructed in a cluster basis for path reservation and handoff support, while an overlapped foreign agent is placed approximately at a boundary between the clusters. According to the present invention, also, a plurality of foreign agents are defined in one cluster, and the handoff among the foreign agents of the same cluster has a short path renewal by the local handoff, while the handoff between the clusters has a relatively longer path renewal by the general handoff.  
         [0095]    With the wireless communication system for real time services and the handoff method according to the present invention, by constructing one cluster with a plurality of foreign agents, and having one cluster agent that represents the cluster, unnecessarily repeated registration information renewals can be prevented. According to the present invention, problems like communication cut-off during the handoff can also be improved, since an overlapped foreign agent is positioned between the clusters and a path of the next cluster is established in advance during the handoff.  
         [0096]    Especially, the wireless communication system for real time service and the handoff method according to the present invention is very effective for the mobile communication data transmission environment, in which handoff occurs often due to fast movement, and for the real time service with respect to a moving mobile host. Furthermore, by adding the information request/reply message, path change message and change message to the messages of the RSVP Protocol, the handoff, which occurs during the real time service provision in the mobile IP environment, can be effectively performed.  
         [0097]    While the invention has been shown and described with reference to the preferred embodiment thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the sprit and scope of the invention as defined by the appended claims.