Abstract:
Disclosed herein are a method of controlling Hierarchical Mobile IPv6 (HMIPv6) network-based handover and an Access Router (AR) and Mobile Node (MN) therefor. The method include the steps of a first AR, to which a MN is connected, receiving an L3 handover initiation message, including a Media Access Control (MAC) address of the MN and the ID of a target Base Station (BS); the first AR creating a Local Care-of Address (LCoA) based on the MAC address of the MN and the ID of the target BS, and performing Binding Update (BU) on a Mobility Anchor Point (MAP) using the created LCoA; when an L2 handover completion message is received from the target BS of the MN, a second AR creating an LCoA and transmitting the LCoA to the MN; and the MN receiving the LCoA from the second AR and configuring the received LCoA as its own LCoA.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of Invention 
         [0002]    The present invention relates generally to a method of controlling Hierarchical Mobile IPv6 (HMIPv6) network-based handover, and an Access Router (AR) and Mobile Node (MN) therefor, and, more particularly, to a handover method based on a network using HMIPv6 in an IEEE 802.16e network. 
         [0003]    2. Description of Related Art 
         [0004]    With the continuous development and diversification of wireless access technology, users desire to receive network service even in a mobile environment. In order to provide seamless service, a handover technology in the network layer is required. Mobile IPv6 (MIPv6) is a protocol that is proposed by the Internet Engineering Task Force (IETF). The protocol provides a mobile management function in the network layer. 
         [0005]    In MIPv6, a handover process can be divided into processes, such as a movement detection process, a Care-of-Address (CoA) configuration process and a Binding Update (BU) process. While handover is being performed through the processes, an MN is unable to transmit or receive data. This period of time is called handover delay time. 
         [0006]    In order to reduce the handover delay time generated when a handover scheme proposed in the MIPv6 protocol is used, the IETF proposed Fast MIPv6 (FMIPv6) and HMIPv6, which are based on MIPv6. 
         [0007]    Of them, HMIPv6 differs from general MIPv6 as follows. During the handover process of general MIPv6, an MN performs BU on a Home Agent (HA) and a Correspondent Node (CN). 
         [0008]    HMIPv6 is configured such that an MN performs BU on a Mobility Anchor Point (MAP), unlike MIPv6. HMIPv6 can have an advantage in that it can reduce the handover delay time through this mechanism as compared with the general MIPv6. The operation of the general HMIPv6 is described below. 
         [0009]      FIG. 1  is a diagram showing the configuration of a general HMIPv6 network. 
         [0010]    As shown in  FIG. 1 , an HMIPv6 network may include an HA  10 , a CN  20 , a MAP  100 , a plurality of ARs  201  and  202 , a plurality of Base Stations (BSs)  300 , and at least one MN  400 . 
         [0011]    The Previous Access Router (PAR)  201  of the ARs corresponds to an AR that has accessed a network layer before the MN  400  performs handover. The Next Access Router (NAR)  202  of the ARs corresponds to an AR that will access the network layer after the MN  400  performs handover. 
         [0012]    The Serving Base Station (SBS)  301  of the BSs  300  corresponds to a BS which has accessed a data link layer before the MN  400  performs handover. The Target Base Station (TBS)  302  of the BSs  300  corresponds to a BS which will access the data link layer after the MN  400  performs handover. 
         [0013]    Each of the HA  10  and the CN  20  maps the permanent IP address of the MN  400  to a CoA pertinent to the permanent IP address, and transmits packets to a network to which the MN  400  belongs. 
         [0014]    A handover method in the HMIPv6 network including the elements is described below. 
         [0015]    In the case where the MN  400  has moved and handover has to be performed, the MN  400  selectively performs two types of BU processes depending on the situation. 
         [0016]    First, in the case where the MN  400  has moved between the domains of MAPs  100 , the MN  400  performs global BU (or inter-MAP). Second, in the case where the MN  400  has moved within the domain of an MAP  100 , the MN  400  performs local BU (intra-MAP). 
         [0017]    In an HMIPv6 environment, the MN  400  has two types of CoAs which are called a Regional CoA (RCoA) and a Local CoA (LCoA). The RCoA is a temporary address commonly used within the domain of the MAP  100 , and a process of notifying the MAP  100  and the CN  20  of the relationship between RCoAs corresponds to global BU. The LCoA corresponds to the same address as a CoA in the existing MIPv6. A process of notifying the MAP  100  of the relationship between the newly created LCoA and RCoA corresponds to the local BU. 
         [0018]    If the MN  400  moves and enters the domain of a new MAP  100 , the MN  400  reconfigures two types of RCoA and LCoA. Accordingly, the MN  400  performs both global BU and local BU. 
         [0019]    However, in the case where the MN  400  performs handover within the domain of the MAP  100 , the MN  400  performs a position registration procedure by performing a local BU process only on the MAP  100 . Next, the MAP  100  operates like an HA in the domain that the MN  400  has accessed and performs position management of the MN  400 . A method of the MN  400  performing handover within the domain of the MAP  100  is described in more detail. 
         [0020]      FIG. 2  is a diagram showing a handover procedure within the domain of a MAP in HMIPv6 over an IEEE 802.16e network. 
         [0021]    Referring to  FIG. 2 , when the MN  400  moves, it terminates a connection to the SBS  301  to which the MN  400  is connected and attempts to set up a new connection to the TBS  302 . 
         [0022]    The data link layer  402 , that is, the MN Layer 2, of the MN  400  performs a handover process from the SBS  301  to the TBS  302  in the data link layer, through processes from a MOB_NBR-ADV transmission process S 201  to a DSA-ACK transmission process S 206  in accordance with the IEEE 802.16e standard. This handover in the data link layer is also referred to as Layer 2 or L2 handover. 
         [0023]    After the handover in the data link layer or L2 handover is completed, the network layer  401 , that is, the MN Layer 3, of the MN  400  starts handover in the network layer by receiving a router advertisement message at step S 207 , including information of MAP  100 , from the NAR  202 . The handover in the network layer is also referred to as Layer 3 or L3 handover. 
         [0024]    The network layer of the MN can acquire the global IP address and network prefix of the MAP  100  from the router advertisement message received at step S 207 . The MN  400  can determine whether it performs handover within the domain of a MAP  100  or performs handover between the domains of MAPs  100 , or handover in the network layer is not necessary based on the global IP address and network prefix of the MAP  100 . 
         [0025]    In the case where handover in the network layer is not necessary, the MN  400  has only to maintain a current LCoA and RCoA configuration, so that a detailed description thereof is omitted here. 
         [0026]    The L3 layer  401  of the MN  400  creates an LCoA based on the network prefix of the NAR  202  which can be acquired from the router advertisement message received at step S 207 . 
         [0027]    In the case where global BU is necessary, the MN  400  creates an RCoA based on the network prefix of the MAP  100 . Here, an IPv6 stateless address auto configuration may be used. 
         [0028]    In  FIG. 2 , the case where global BU is not necessary and only local BU is necessary is assumed. This assumption corresponds to the case where the MN  400  performs handover within the domain of the MAP  100 . In this case, the network layer  401  of the MN  400  performs only local BU along with the MAP  100  at step S 208 . 
         [0029]    After local BU is completed, the MAP  100  creates a tunnel between the network layer  401  of the MN  400  and the MAP  100 . The MAP  100  then transmits packets, having the MN  400  as a destination, to the NAR  202  to which the MN  400  will belong through the tunnel using a binding cache at step S 210 . Finally, the NAR  202  transmits the packets, which have been received through the tunnel, to the MN  400  at step S 211 . 
         [0030]    The time taken for the above-described handover (that is, the handover delay time) basically includes movement detection time, CoA configuration time, and BU time. 
         [0031]    In general HMIPv6, in the case where handover is generated within the domain of the MAP  100 , the time taken for the BU time, which belongs to the entire handover delay time, can be reduced, thereby reducing the entire handover delay time. 
         [0032]    However, in general HMIPv6, handover in the L3 layer  401  of the MN  400  starts after handover in the L2 layer  402  has been terminated. In other words, handover in the network layer starts with the process S 207  of receiving the router advertisement message from a new AR (NAR). The network layer  401  of the MN  400  performs movement detection through the router advertisement message. 
         [0033]    In this case, the time taken for the MN  400  to wait for the reception of the router advertisement message varies depending on a router advertisement message time interval set in the AR. Accordingly, after handover in the data link layer has been completed, the MN  400  may have to wait for a long time depending on the setting of the AR. 
         [0034]    If handover delay is long as described above, the MN  400  inevitably experiences great packet loss. Although packet lossless is guaranteed in an upper layer using a mechanism, such as the Transmission Control Protocol (TCP), there is a problem in that a long handover delay time may lead to a low packet throughput in the MN  400  because of the characteristics of TCP. 
       SUMMARY OF THE INVENTION 
       [0035]    Accordingly, the present invention has been made keeping in mind the above problems occurring in the prior art, and an object of the present invention is to provide a method of controlling handover, which overlappingly performs handovers in the data link layer and the network layer in order to reduce packet loss and improve the packet throughput by reducing the handover delay time in HMIPv6, and an AR and MN therefor. 
         [0036]    In order to achieve the above object, accordoring to an aspect of the present invention, there is provided a method of controlling handover in an HMIPv6-based network, including the steps of: (a) a first Access Router (AR), to which a Mobile Node (MN) is being connected, receiving an L3 handover initiation message, including a Media Access Control (MAC) address of the MN and an identifier (ID) of a target Base Station (BS); (b) the first AR creating a Local Care-of Address (LCoA) based on the MAC address of the MN and the ID of the target BS, and performing Binding Update (BU) on a Mobility Anchor Point (MAP) using the created LCoA; (c) when an L2 handover completion message is received from the target BS of the MN, a second AR creating an LCoA and transmitting the created LCoA to the MN; and (d) the MN receiving the LCoA from the second AR and configuring the received LCoA as its own LCoA. 
         [0037]    The method may further include the steps of: (e) after the BU has been performed at the step (b), the MAP transmitting a buffering request message, including the LCoA on the BU has been performed, to the second AR, receiving packets having the LCoA as a destination, and transmitting the packets to the second AR; and (f) the second AR, which has received the buffering request message, buffering the packets having the LCoA, included in the buffering request message, as a destination. 
         [0038]    The method may further include the step (g) of, after the LCoA of the MN at the step (d) has been configured, the second AR transmitting the packets buffered at the step (f) to the MN. 
         [0039]    The method may further include the step (h) of, after the BU has been performed at the step (b), the MAP notifying the first AR of the completion of the BU. 
         [0040]    In this case, the L2 handover completion message at the step (c) may include the MAC address of the MN. 
         [0041]    Meanwhile, at the step (c), the second AR may create the LCoA of the MN based on its own prefix information and the MAC address of the MN included in the L2 handover completion message. 
         [0042]    At the step (c), the second AR may insert the generated LCoA into an L3 handover completion notification message and transmit the L3 handover completion notification message to the MN. 
         [0043]    The step (d) may include the steps of a Layer 2 processing unit of the MN determining whether the L3 handover completion notification message has been received; if, as a result of the determination, the L3 handover completion notification message is determined to have been received, the Layer 2 processing unit extracting the LCoA from the L3 handover completion notification message and transmitting the extracted LCoA to a Layer 3 processing unit; and the Layer 3 processing unit configuring the received LCoA as its own LCoA. 
         [0044]    Finally, the method may further include, after the step (a), the steps of: (i) the first AR determining whether the ID of the target BS, included in the L3 handover initiation message, exists in its own topology table; and (j) if, as a result of the determination at the step (i), the ID of the target BS is determined to exist in the topology table, the first AR performing the steps subsequent to the step (b), and if, as a result of the determination at the step (i), the ID of the target BS is determined not to exist in the topology table, the AR instructing the MN to directly perform BU along with the MAP. 
         [0045]    Accordoring to another aspect of the present invention, there is provided an AR, including at least one interface for performing data transmission/reception; and a control unit for extracting an MAC address of an MN and an ID of a target BS from an L3 handover initiation message received via the interface, creating an LCoA of the MN based on the MAC address and the ID, and performing BU along with a MAP. 
         [0046]    The AR may further include a topology database (DB) for managing information about neighboring ARs and BSs managed by the neighboring ARs. 
         [0047]    If the ID of the target BS extracted from the L3 handover initiation message exists in the topology DB, the control unit may create the LCoA of the MN based on the ID of the target BS and perform the BU along with an MAP. 
         [0048]    If the ID of the target BS extracted from the L3 handover initiation message exists in the topology DB, the control unit may instruct the MN to directly perform the BU along with the MAP. 
         [0049]    When a buffering request message is received from the MAP, the control unit may store packets each having an address, included in the buffering request message, as a destination in a storage device. 
         [0050]    When an L2 handover completion message is received from the target BS of the MN, the control unit may create the LCoA of the MN and transmit the created LCoA to the MN. 
         [0051]    The control unit may create the LCoA of the MN based on the MAC address of the MN, included in the L2 handover completion message, and its own prefix information. 
         [0052]    The control unit may insert the generated LCoA into an L3 handover completion message and transmit the L3 handover completion message to the MN. 
         [0053]    Accordoring to another aspect of the present invention, there is provided an MN, including a Radio Frequency (RF) communication unit for performing data communication using a radio signal; and a control unit for, when performance of L2 and L3 handovers is desired, transmitting a handover initiation message to an AR to which the control unit belongs and performing the L2 handover, and, when an L3 handover completion message is received via the RF communication unit, configuring an LCoA, included in the L3 handover completion message, as its own LCoA. 
         [0054]    The control unit may include an L2 processing unit for processing messages of a data link layer and, when the L3 handover completion message is received, extracting the LCoA from the L3 handover completion message; and an L3 processing unit for processing messages of a network layer and configuring the LCoA, extracted by the L2 processing unit, as an LCoA of the MN. 
         [0055]    The control unit may further include a local BU unit for directly performing BU along with a MAP at the request of an AR to which the MN is connected. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0056]    The above and other objects, features and advantages of present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which: 
           [0057]      FIG. 1  is a diagram showing the configuration of a general HMIPv6 network; 
           [0058]      FIG. 2  is a diagram showing a handover procedure within the domain of a MAP in HMIPv6 over an IEEE 802.16e network; 
           [0059]      FIG. 3  is a diagram showing an HMIPv6 network-based handover method according to an embodiment of the present invention; 
           [0060]      FIG. 4  is a diagram showing the block construction of an AR according to another embodiment of the present invention; 
           [0061]      FIG. 5  is a diagram showing a BU process that the AR of  FIG. 4  performs as a PAR; 
           [0062]      FIG. 6  is a diagram showing a buffering operation that the AR of  FIG. 4  performs as an NAR; and 
           [0063]      FIG. 7  is a diagram showing the block construction of the MN according to still another embodiment of the present invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0064]    A method of controlling HMIPv6 network-based handover, and an AR and MN therefor according to the present invention are described below in detail with reference to the accompanying drawings. 
         [0065]      FIG. 3  is a diagram showing the HMIPv6 network-based handover method according to an embodiment of the present invention. 
         [0066]    Referring to  FIG. 3 , a system relating to handover according to the present invention includes the L3 layer  401  and L2 layer  402  of an MN  400 , an SBS  301 , a PAR  201 , a MAP  100 , a TBS  302 , and an NAR  202 . 
         [0067]    The L3 layer  401  of the MN  400  is the network layer of the MN  400 , and is configured to communicate with the PAR  201  or the NAR  202 . The L2 layer  402  of the MN  400  is the data link layer of the MN  400 , and is configured to communicate with the SBS  301  or the TBS  302 . 
         [0068]    As described above, the SBS  301  is a BS to which the MN  400  has already been connected before the MN  400  performs handover in the data link layer. The TBS  302  is a BS to which the MN  400  will be connected after the MN  400  performs handover in the data link layer. 
         [0069]    The PAR  201  is an AR to which the MN  400  is connected in the network layer before handover is performed in the network layer. In a similar way, the NAR  202  is an AR to which the MN  400  will be connected in the network layer after handover has been performed in the network layer. 
         [0070]    The MAP  100  functions as a local HA for the L3 layer  401  of the MN  400 . 
         [0071]    In the present invention, each of the ARs  201  and  202  has information about neighboring ARs, which belong to the same domain. Each of the ARs  201  and  202  also stores information about the IDs of BSs, which belong to neighboring ARs. A handover process according to the embodiment of the present invention is described below in more detail. 
         [0072]    When the MN  400  moves, the L2 layer  402  of the MN  400  performs handover in the data link layer. The L2 layer  402  of the MN  400  exchanges messages, such as MOB_NBR-ADV, MOB_MSHO-REQ and MOB_BSHO-RSP, with the SBS  301  at steps S 301  to S 303 . 
         [0073]    The L2 layer  402  of the MN  400  determines the TBS  302  to be a final handover subject based on the exchanged messages. Next, the L2 layer  402  of the MN  400  transmits a MOB_HO-IND message, including the Base Station ID (BSID) of the TBS  302 , to the SBS  301  at step S 304 . The SBS  301  receives the MOB_HO-IND message from the L2 layer  402  and extracts the BSID of the TBS  302  and the Media Access Control (MAC) address of the MN  400  from the MOB_HO-IND message. The SBS  301  creates an L3_HO-Initiate message including the BSID of the TBS  302  and the MAC address of the MN  400 , and transmits the created L3_HO-Initiate message to the MAP  100  connected thereto at step S 305 . Here, the L3_HO-Initiate message corresponds to a message initiating L3 handover. 
         [0074]    As described above, each of the ARs  201  and  20  stores information about neighboring ARs belonging to the same domain, and information about BSs belonging to neighboring ARs. Accordingly, the PAR  201  can determine the AR  201  or  202  to which the TBS  302  belongs based on the BSID of the TBS  302  included in the L3_HO-Initiate message. 
         [0075]    If, as a result of the determination, the BSID of the TBS  302  does not exist in a database (DB) managed by the PAR  201 , it corresponds to the case where the MN  400  has to perform handover between the domains of the MAP  100  or the case where handover in the network layer is not necessary. In this case, the PAR  201  instructs the L3 layer  401  of the MN  400  to perform a handover process between the domains of the MAP  100  in general HMIPv6 or to omit a network layer handover process. 
         [0076]    However, if, as a result of the determination, the TBS  302  is determined to belong to the NAR  202 , the PAR  201  performs a handover process within the domain of the MAP  100  in HMIPv6 instead of the MN  400 . 
         [0077]    That is, the MAP  100  creates an LCoA to be used by the MN  400  based on the network prefix of the NAR  202 , which is found based on the BSID of the TBS  302 , and the MAC address of the MN  400 , which is included in the L3_HO-Initiate message, at step S 306 . 
         [0078]    As the router advertisement message may include information about the MAP  100  depending on the HMIPv6 specification, each of the ARs  201  and  202  also stores information about the MAP  100  placed at the upper position in the network architecture. Accordingly, the MAP  100  may also create an RCoA for the MN  400  by combining the network prefix of the MAP  100  and the MAC address of the MN  400  at step S 306 . 
         [0079]    The PAR  201  then performs a local BU process along with the MAP  100  using the created LCoA and RCoA instead of the MN  400  at step S 307 . The MAP  100  updates a binding cache for the MN  400  to a new LCoA by performing the local BU along with the PAR  201 . 
         [0080]    In response to local BU ACK from the NAR  202  at step S 308 , the MAP  100  creates an L3_Buffer-Initiate message (a buffering request message) and transmits it to the NAR  202  at step S 309 . The MAP  100  then tunnels packets, received at the RCoA, to the new LCoA at step S 310 . 
         [0081]    The L3_Buffer-Initiate message includes the LCoA of the MN. The NAR  202  that has received the L3_Buffer-Initiate message buffers packets having the corresponding LCoA as a target IP address at step S 311 . 
         [0082]    Meanwhile, the data link layer L2 of the MN  400  performs handover in the data link layer simultaneously when the handover in the network layer L3 of the MN  400  has been performed by the ARs  201  and  202  and the BSs  301  and  302 . The L2 layer  402  of the MN  400  performs the handover in the data link layer of the MN  400  through a re-entry procedure (S 312 ) along with the TBS  302 . Finally, the L2 layer  402  of the MN  400  transmits a DSA-ACK message to the TBS  302 , so the handover in the data link layer is completed at step S 313 . 
         [0083]    When the DSA-ACK message is received at step S 313 , the TBS  302  transmits an L2_HO-Complete message (an L2 handover completion message), including the MAC address of the MN  400 , to the NAR  202  connected thereto, thereby providing notification that the handover in the data link layer of the MN  400  has been completed at step S 314 . The NAR  202  creates the LCoA of the MN  400  based on the MAC address of the MN  400  included in the received the L2_HO-Complete message and transmits an L3_HO-Complete message including the created LCoA, to the MN  400  at step S 315 . 
         [0084]    The L2 layer  402  of the MN  400  extracts the LCoA from the received L3_HO-Complete message, and transmits the extracted LCoA to the L3 layer  401 . The L3 layer  401  of the MN  400  configures the LCoA included in the L3_HO-Complete message at step S 316 , and performs subsequent communication using the LCoA. 
         [0085]    The L3 layer  401  of the MN  400  transmits an L3_HO-Complete ACK message to the NAR  202  at step S 317 . When the L3_HO-Complete ACK message is received, the NAR  202  transmits the packets buffered for the MN  400  to the MN  400 , thereby completing the entire handover process at step S 318 . 
         [0086]      FIG. 4  is a diagram showing the block construction of the AR according to another embodiment of the present invention. 
         [0087]    The AR  200  for performing the handover process of  FIG. 3  may include an interface  210 , a control unit  220 , and a memory unit  230 . The control unit  220  may include a message determination unit  221 , an MOB-HO-IND processing unit  222 , a Buffer-Initiate processing unit  223 , and an L2_HO-Complete processing unit  224 . 
         [0088]    The interface  210  corresponds to a port for performing data communication with the AR  200 , the BS  300 , or the MN  400 . 
         [0089]    The memory unit  230  includes a topology DB  231  and an MN buffer  232 . The topology DB  231  corresponds to a DB in which information about neighboring ARs belonging to the same domain as the AR  200  and information about BSs below the neighboring ARs are stored. The MN buffer  232  corresponds to storage space for buffering packets which are received at a specific LCoA from the MAP. 
         [0090]    The control unit  220  is responsible for the overall control of the AR  200 . In the control unit  200  of  FIG. 4 , a detailed construction responsible for a general control function is omitted and only the block construction for performing the handover process of  FIG. 3  is shown. 
         [0091]    The message determination unit  221  of the control unit  200  determines the type of message received via the interface  210 , and transmits the message to an element for processing the message depending on the type of message. For example, when an MOB-HO-IND message is received, the message determination unit  221  transmits the message to the MOB-HO-IND processing unit  222 . In contrast, when a Buffer-Initiate message is received, the message determination unit  221  transmits the message to the Buffer-Initiate processing unit  223 . 
         [0092]    The MOB-HO-IND processing unit  222  extracts the BSID of the TBS  302  and the MAC address of the MN  400  from the received MOB-HO-IND message. The MOB-HO-IND processing unit  222  then determines whether the BSID of the TBS  302  exists in the topology DB  231  of the memory unit  230 . 
         [0093]    If, as a result of the determination, the BSID of the TBS  302  is determined to exist in the topology DB  231  of the memory unit  230 , the MOB-HO-IND processing unit  222  transmits the BSID of the TBS  302  and the MAC address of the MN  400  to the local BU unit  225 . The MOB-HO-IND processing unit  222  controls the local BU unit  225  so that the local BU unit  225  performs local BU along with the MAP  100 . 
         [0094]    The Buffer-Initiate processing unit  223  operates in response to the L3_Buffer-Initiate message received from the MAP  100 . When the L3_Buffer-Initiate message is received, the Buffer-Initiate processing unit  223  extracts the LCoA of the MN  400  from the L3_Buffer-Initiate message. The Buffer-Initiate processing unit  223  then configures packets transmitted to the LCoA so that the packets are buffered in the MN buffer  232  of the memory unit  230 . 
         [0095]    After the handover in the data link layer between the MN  400  and the TBS  302  is completed, the L2-HO-Complete processing unit  224  processes the L2_HO-Complete message received from the TBS  302 . The L2_HO-Complete processing unit  224  extracts the MAC address of the MN  400  from the L2_HO-Complete message. Next, the L2_HO-Complete processing unit  224  creates an LCoA to be used by the MN  400  based on network prefix information of the AR  200  and the MAC address of the MN  400 , and transmits an L3_HO-Complete message including the created LCoA to the MN  400 . 
         [0096]      FIG. 5  is a diagram showing a BU process that the AR of  FIG. 4  performs as the PAR. 
         [0097]    First, the AR stores information about neighboring ARs belonging to the same domain as the AR and information about lower BSs belonging to the neighboring ARs. A DB in which the pieces of information are stored corresponds to the topology DB at step S 501 . 
         [0098]    An MN performs a handover process in the data link layer along with an SBS, and transmits the resulting MOB_HO-IND message to the SBS. When the MOB_HO-IND message is received, the SBS transmits an L3_HO-Complete message, including the MAC address of the MN and the BSID of a TBS, to the AR (that is, the PAR). 
         [0099]    The AR, which is the PAR, determines whether an L3_HO-Initiate message has been received from the lower BSs managed by it at step S 502 . If, as a result of the determination at step S 502 , the L3_HO-Initiate message is determined to have been received, the PAR extracts the BSID of the TBS from the L3_HO-Initiate message at step S 503 . 
         [0100]    Thereafter, the PAR determines whether the extracted BSID of the TBS exists in the topology DB at step S 504 . If, as a result of the determination at step S 504 , the extracted BSID of the TBS is determined not to exist in the topology DB, the PAR instructs the MN to perform handover using a general HMIPv6 mechanism at step S 508 . 
         [0101]    Meanwhile, if, as a result of the determination at step S 504 , the extracted BSID of the TBS is determined to exist in the topology DB, the PAR creates an RCoA and an LCoA based on the MAC address of the MN, the BSID of the TBS, and network prefix information of a MAP which are included in the L3_HO-Initiate message at step S 505 . 
         [0102]    Thereafter, the PAR transmits a local BU message to the MAP based on the created RCoA and LCoA so that position registration of the MN, that is, local BU, is performed at step S 506 . The AR, which is the PAR, determines whether a local BU ACK message has been received from the MAP at step S 507 . 
         [0103]    If, as a result of the determination at step S 507 , the local BU ACK message is determined not to have been received from the MAP, the AR, which is the PAR, may continue to wait for the reception of the local BU ACK message (‘No’ at step S 507 ). If the local BU ACK message is not received from the MAP for a specific period of time, the AR may consider performing an operation for retransmitting the local BU message to the MAP. 
         [0104]    If, as a result of the determination at step S 507 , the local BU ACK message is determined to have been received from the MAP, the AR, which is the PAR, terminates the operation (‘Yes’ at step S 507 ). 
         [0105]      FIG. 6  is a diagram showing a buffering operation that the AR of  FIG. 4  performs as the NAR. 
         [0106]    A MAP performs local BU using the local BU message received at step S 506  of  FIG. 5 . After the local BU has been performed, the MAP transmits an L3_Buffer-Initiate message to the NAR. In response thereto, the AR, which is the NAR, determines whether the L3_Buffer-Initiate message has been received from the MAP at step S 601 . 
         [0107]    If, as a result of the determination at step S 601 , the L3_Buffer-Initiate message is determined not to have been received from the MAP (‘No’ at step S 601 ), the AR does not perform any operation and returns to step S 601 . However, if, as a result of the determination at step S 601 , the L3_Buffer-Initiate message is determined to have been received from the MAP (‘Yes’ at step S 601 ), the AR, which is the NAR, extracts the LCoA of the MN from the L3_Buffer-Initiate message at step S 602 . Next, the AR, which is the NAR, performs an operation of buffering packets, which are received at the extracted LCoA, in the memory unit at step S 603 . 
         [0108]    Meanwhile, while the packets are buffered, the MN performs handover in the data link layer along with a TBS. After the handover in the data link layer has been completed, the TBS transmits the L2_HO-Complete message, including the MAC address of the MN which has performed the handover, to the NAR (S 314  of  FIG. 3 ). 
         [0109]    In response thereto, the AR, which is the NAR, determines whether the L2_HO-Complete message has been received from a BS, such as a TBS at step S 604 . If, as a result of the determination at step S 604 , the L2_HO-Complete message is determined not to have been received (‘No’ at step S 604 ), the AR, which is the NAR, continues to perform the buffering operation at step S 603 . 
         [0110]    However, if, as a result of the determination at step S 604 , the L2_HO-Complete message is determined to have been received (‘Yes’ at step S 604 ), the AR, which is the NAR, creates an LCoA to be used by the MN in the L3 layer based on the MAC address of the MN, included in the L2_HO-Complete message, and its own network prefix information at step S 605 . 
         [0111]    Thereafter, the AR, which is the NAR, transmits an L3_HO-Complete message, including the generated LCoA of the MN, to the MN at step S 606 . When the MN transmits an ACK message providing notification of the reception of the L3_HO-Complete message, the NAR transmits the packets, which have been buffered at step S 603 , to the MN at step S 607 . 
         [0112]      FIG. 7  is a diagram showing the block construction of the MN according to still another embodiment of the present invention. 
         [0113]    The MN  400  for performing the handover process of  FIG. 3  may include a Radio Frequency (RF) communication unit  410 , a control unit  420 , a memory unit  430 , a display unit  440 , and an input unit  450 . From among these, the display unit  440  and the input unit  450  are similar to those of an existing MN, so that detailed descriptions thereof are omitted here. 
         [0114]    If the performance of L3 handover is desired, the control unit  420  transmits the MOB_HO-IND message (step S 304 ) to the BS (SBS) to which the control unit  420  is connected, and continues to perform L2 handover. 
         [0115]    If an L3 handover completion message, that is, the L3_HO-Complete message at step S 315 , is received from the NAR  202  to which handover will be performed by the MN  400 , the control unit  420  functions to configure an LCoA, included in the L3_HO-Complete message, as its own LCoA. 
         [0116]    For this handover process, the control unit  420  may include an L2_HO control unit  421  and an L3_HO control unit  424 . In the case where the MN  400  has moved and therefore handover is determined to be necessary, the L2_HO control unit  421  performs the handover in the data link layer along with the SBS. The L2_HO control unit  421  can acquire the BSID of a TBS to which the handover will be performed by the control unit  421  through this handover process in the data link layer. 
         [0117]    The L2_HO control unit  421  creates an MOB_HO-IND message. In particular, the MOB_HO-IND processing unit  422  of the L2_HO control unit  421  creates the MOB_HO-IND message, including the MAC address of the MN  400  and the BSID of the TBS to which handover will be performed by the MN  400 , and transmits the created MOB_HO-IND message to the SBS  301  via the RF communication unit  410 . 
         [0118]    Meanwhile, the L3_HO-Complete processing unit  423  of the L2_HO control unit  421  processes the L3 handover completion message (that is, the L3_HO-Complete message) received from the NAR  202 . The L3_HO-Complete processing unit  423  extracts the LCoA from the L3_HO-Complete message, and transmits the extracted LCoA to the LCoA configuration unit  425  of the L3_HO control unit  424 . 
         [0119]    The LCoA configuration unit  425  of the L3_HO control unit  424 , which has received the LCoA from the L3_HO-Complete processing unit  423 , configures the LCoA of the MN  400  as the LCoA received from the L3_HO-Complete processing unit  423 . After the LCoA is configured, the L3_HO-Complete processing unit  423  can transmit an L3_HO-Complete ACK message to the NAR  202  and request buffered packets. 
         [0120]    As described above, according to the present invention, in an IEEE 802.16e-based HMIPv6 network, an MAP, an AR and a BS perform handover in cooperation with each other, instead of an MN. In order to minimize packet loss generated during the handover process, the AR uses a buffer. 
         [0121]    Consequently, since the handover in the data link layer and the handover in the network layer are performed at the same time, the overall handover delay time can be reduced. Packet loss generated while handover is performed can be minimized and the packet throughput can be improved. 
         [0122]    Although the embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions, and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.