Abstract:
An apparatus and method for implementing a vertical handoff between a WLAN and a broadband wireless communication network are provided. The vertical handoff apparatus includes a base station (BS) of the broadband wireless network and at least one access point (AP) of the WLAN within the cell coverage area of the BS, and a switch for forwarding packets between the BS and the at least one AP and processing a handoff between the BS and the at least one AP.

Description:
PRIORITY 
     This application claims priority under 35 U.S.C. §119 to an application entitled “Apparatus and Method for Implementing Handoff between Heterogeneous Networks in a Wireless Communication System” filed in the Korean Intellectual Property Office on Jun. 23, 2005 and assigned Serial No. 2005-54416, the contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to an apparatus and a method for implementing handoff between heterogeneous networks in a wireless communication system, and in particular, to an apparatus and a method for implementing handoff between an Institute of Electrical and Electronics Engineers (IEEE) 802.11 network and an IEEE 802.16 network. 
     2. Description of the Related Art 
     The providing of services with diverse Quality of Service (QoS) levels at about 100 Mbps is an ongoing research area for a future-generation communication system, namely, a 4 th  Generation (4G) communication system. The existing 3 rd  Generation (3G) communication systems support approximately 384 kbps in a relatively poor outdoor channel environment and at most 2 Mbps in a relatively good indoor channel environment. Wireless Local Area Network (WLAN) and Wireless Broadband (WiBro) systems typically support 20 Mbps to 50 Mbps. In this context, studies have actively been conducted on guarantee of  mobility and QoS for WLAN and WiBro supporting relatively high rates in the 4G communication system. 
     One of the studies concerns handoff between heterogeneous networks, WLAN and WiBro (or broadband wireless communication system). Generally, handoff refers to homogeneous handoff. However, the IEEE 802.21 working group is developing standards to enable handoff between heterogeneous networks, this inter-technology handoff will be provided seamlessly in the 4G communication system. 
     The IEEE 802.11 Task Group (TG) f deals with support of handoff between 802.11 Access Points (APs) and the IEEE 802.11 standards which defines only handoff-associated messages. Typically, the cell coverage of an IEEE 802.11 network ranges within tens to hundreds of meters, and an IEEE 802.16 network reaches a few kilometers. Thus, it is meaningless to separate the two networks. The IEEE 802.11 APs are expected to increase the capacity of IEEE 802.16 Base Stations (BSs) or cover shadowing areas that the 802.16 BSs cannot cover. The 802.11 network will be integrated into the 802.16 network rather than configuring a network by separating them. Hereinafter, IEEE may be omitted to refer standard specification. 
     A description will be made below of a conventional handoff between a WLAN and a broadband wireless communication network. 
       FIG. 1  illustrates a conventional system model in which the 802.11 network coexists with the 802.16 network. The 802.11 network and the 802.16 network operate separately. 
     In contrast to the 802.11 standard which has not specified handoff between 802.11 APs, the 802.11f draft provides a simple definition of handoff. In order to  support a handoff between the 802.11 APs, communications should be enabled between them by the use of an item called a Wireless Switch (WS) and messages defined by the 802.11f draft. Meanwhile, an 802.16 handoff is performed in the manner defined by the 802.16e draft. 
     The terms used herein “handoff between homogeneous networks” and “horizontal handoff” are interchangeably used with the same meaning, and “handoff between heterogeneous networks” and “vertical handoff” are also interchangeably used with the same meaning. In addition, the 802.11 network and the 802.16 network are called “WLAN” and “broadband wireless communication network”, respectively. 
     Now a description will be made of a conventional handoff technique based on the above description. 
     Horizontal Handoff Between IEEE 802.11 APs 
     As described above, the IEEE 802.11 TG f specifies simple messages for handoff between APs. A Subscriber Station (SS) shall initiate a handoff procedure according to the 802.11f draft. A reassociation request message and a reassociation response message are defined to support handoff in the IEEE 802.11 standards. The reassociation request message further includes an old AP field in addition to an association request message, and the reassociation response message is identical to an association response message. 
     In the handoff procedure as provided by the 802.11f draft, the SS dissociates from an old AP and reassociates with a new AP. The 802.11f defines an Inter-Access Point Protocol (IAPP) which defines messages exchanged between APs, for handoff. Such IAPP messages include IAPP ADD-notify, IAPP MOVE-notify, and IAPP MOVE-response.  
     Table 1 below illustrates the format of the IAPP ADD-notify message. 
     
       
         
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Information 
                 Size (bytes) 
                 Notes 
               
               
                   
               
             
             
               
                 MAC Address 
                 6 
                 Address of Mobile Subscriber 
               
               
                   
                   
                 Station (MSS) 
               
               
                 Sequence Number 
                 2 
                 Sequence number of association 
               
               
                   
                   
                 request frame 
               
               
                 Timeout 
                 2 
                 Timeout period 
               
               
                   
               
             
          
         
       
     
     Table 2 illustrates the format of the IAPP MOVE-notify message. 
     
       
         
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                 Information 
                 Size (bytes) 
                 Notes 
               
               
                   
               
             
             
               
                 MAC Address 
                 6 
                 Address of Mobile Subscriber 
               
               
                   
                   
                 Station (MSS) 
               
               
                 Sequence Number 
                 2 
                 Sequence number of association 
               
               
                   
                   
                 request frame 
               
               
                 Old AP 
                 6 
                 Address of old AP 
               
               
                 Timeout 
                 2 
                 Timeout period 
               
               
                   
               
             
          
         
       
     
     Table 3 illustrates the format of the IAPP MOVE-response message. 
     
       
         
               
               
               
             
           
               
                 TABLE 3 
               
               
                   
               
               
                 Information 
                 Size (bytes) 
                 Notes 
               
               
                   
               
             
             
               
                 MAC Address 
                 6 
                 Address of Mobile Subscriber 
               
               
                   
                   
                 Station (MSS) 
               
               
                 Sequence Number 
                 2 
                 Sequence number of association 
               
               
                   
                   
                 request frame 
               
               
                 Old AP 
                 6 
                 Address of old AP 
               
               
                 New Basic Service 
                 6 or 8 
                 BSSID of new network 
               
               
                 Set Identification 
               
               
                 (BSSID) 
               
               
                   
               
             
          
         
       
     
       FIG. 2  illustrates a conventional handoff procedure in an 802.11 WLAN. 
     Referring to  FIG. 2 , an SS  280  associates with a first AP  250  by  association request and response messages in step  211 . The first AP  250  transmits an IAPP ADD-notify message to a WS  210 , notifying the entry of the S  280  into the first AP  250 . The WS  210  multicasts the IAPP ADD-notify message to APs within the same domain in step  215 . It is assumed that a second AP  270  is located in the same domain. 
     When the SS  280  moves to the second AP  270 , i.e. a handoff to the second AP  270  is requested in step  217 , it transmits a reassociation request message to the second AP  270  and receives a reassociation response message for the reassociation request from the second AP  270  in step  219 . 
     As the SS  280  has associated with the second AP  270 , the second AP  270  transmits an IAPP MOVE-notify message to the first AP  250  via the WS  210  in step  221 . The first AP  250  dissociates from the SS  280  in step  223  and transmits an IAPP MOVE-response message including information about a requested context to the second AP  270  via the WS  210  in step  225 . In this way, the handoff is completed. 
     Horizontal Handoff Between IEEE 802.16 BSs In the IEEE 802.16 network, initialization between a base station (BS) and a subscriber station (SS) (or MSS) is based on IEEE 802.16-2004 and horizontal handoff is implemented in compliance with the IEEE 802.16e draft. 
       FIG. 3  is a flowchart illustrating a conventional handoff procedure in an 802.16e broadband wireless communication network. 
     Referring to  FIG. 3 , at an initialization or when a signal is disconnected from a BS, an SS selects a cell (or BS) to camp on by downlink channel scanning in step  301 . For example, the SS scans successive downlink channels starting with the latest received channel until a valid downlink signal is received.  
     After the cell selection, the SS acquires physical synchronization using the preamble of a downlink (DL) frame received from the selected BS in step  303 . If a DL-MAP and a Downlink Channel Descriptor (DCD) are received successfully, it is considered that synchronization with the BS has been acquired. 
     The SS receives an Uplink Channel Descriptor (UCD) from the BS and acquires uplink parameters from the UCD information in step  305 . Based on the uplink parameters, if its determined that the uplink is not available to the SS, then the SS starts channel scanning for another channel. However, if the uplink is available, the SS waits for the next DL-MAP and UL-MAP and checks an initial ranging area allocated by the BS. 
     In step  307 , the SS performs ranging. Specifically, the SS transmits an RNG-REQ message to the BS according to the initial ranging area. The RNG-REQ message is formatted as follows. 
     
       
         
               
               
               
             
           
               
                 TABLE 4 
               
               
                   
               
               
                 Information 
                 Size (bytes) 
                 Notes 
               
               
                   
               
             
             
               
                 Management Type 
                 1 
                 ID (4) identifying RNG-REQ 
               
               
                 Downlink Channel ID 
                 1 
                 8-bit ID of downlink channel 
               
               
                 TLV Encoded Information 
                 variable 
                 MAC address, version, 
               
               
                   
                   
                 capability of SS 
               
               
                   
               
             
          
         
       
     
     The SS initially transmits the RNG-REQ message at a minimum power level, but if not receiving a response from the BS, it gradually increases the power level. The BS replies to the RNG-REQ message with RNG-RSP, allocates a Connection ID (CID) to the SS, and allocates an individual initial ranging area for correcting a transmission power level and a timing offset to the SS. The structure of RNG-RSP is illustrated in Table 5 below.  
     
       
         
               
               
               
             
           
               
                 TABLE 5 
               
               
                   
               
               
                 Information 
                 Size (bytes) 
                 Notes 
               
               
                   
               
             
             
               
                 Management Type 
                 1 
                 ID (5) identifying RNG-RSP 
               
               
                 Uplink Channel ID 
                 1 
                 8-bit ID of uplink channel 
               
               
                 TLV Encoded Information 
                 variable 
                 Timing control information, 
               
               
                   
                   
                 power control information, 
               
               
                   
                   
                 basic CID 
               
               
                   
               
             
          
         
       
     
     The SS then exchanges RNG-REQ and RNG-RSP with the BS through the individual initial ranging area, thereby adjusting the transmission power and timing. 
     After the ranging, the SS negotiates basic capabilities with the BS by exchanging SS Basic Capability Request (SBC-REQ) and SS Basic Capability Response (SBC-RSP) in step  309 . Table 6 below illustrates the format of SBC-REQ. 
     
       
         
               
               
               
             
           
               
                 TABLE 6 
               
               
                   
               
               
                 Information 
                 Size (bytes) 
                 Notes 
               
               
                   
               
             
             
               
                 Management Type 
                 1 
                 ID (26) identifying SBC-REQ 
               
               
                 TLV Encoded Information 
                 variable 
                 Capabilities supported by SS 
               
               
                   
               
             
          
         
       
     
     Table 7 below illustrates the format of SBC-RSP. 
     
       
         
               
               
               
             
           
               
                 TABLE 7 
               
               
                   
               
               
                 Information 
                 Size (bytes) 
                 Notes 
               
               
                   
               
             
             
               
                 Management Type 
                 1 
                 ID (27) identifying SBC-RSP 
               
               
                 TLV Encoded Information 
                 variable 
                 Capabilities supported by 
               
               
                   
                   
                 BS among SS-requested 
               
               
                   
                   
                 capabilities 
               
               
                   
               
             
          
         
       
     
     In step  311 , the SS performs authorization and exchanges keys. The SS then associates with the BS by exchanging Registration Request (REG-REQ) and  Registration Response (REG-RSP) with the BS in step  313 . 
     Table 8 below illustrates the configuration of the REG-REQ message. 
     
       
         
               
               
               
             
           
               
                 TABLE 8 
               
               
                   
               
               
                 Information 
                 Size (bytes) 
                 Notes 
               
               
                   
               
             
             
               
                 Management Type 
                 1 
                 ID (6) identifying REG-REQ 
               
               
                 TLV Encoded 
                 variable 
                 Message authentication code 
               
               
                 Information 
                   
                 (MAC), uplink CID, IP 
               
               
                   
                   
                 information, and SS 
               
               
                   
                   
                 management information 
               
               
                   
               
             
          
         
       
     
     Table 9 below illustrates the format of the REG-RSP. 
     
       
         
               
               
               
             
           
               
                 TABLE 9 
               
               
                   
               
               
                 Information 
                 Size (bytes) 
                 Notes 
               
               
                   
               
             
             
               
                 Management Type 
                 1 
                 ID (7) identifying REG-RSP 
               
               
                 Response 
                 1 
                 Indicates whether REG-REQ 
               
               
                   
                   
                 reception is successful 
               
               
                   
                   
                 based on MAC 
               
               
                 TLV Encoded Information 
                 variable 
                 Response to management 
               
               
                   
                   
                 information of REG-REQ 
               
               
                   
               
             
          
         
       
     
     Following the registration, the SS establishes an IP connection in step  315 . That is, the SS negotiates an IP version that the BS supports, and is allocated an IP address by a Dynamic Host Configuration Protocol (DHCP) mechanism, that receives date and time for the time stamp of log files. In step  317 , the SS transmits operational parameters to the BS. Steps  315  and  317  are optional. 
     When the initialization procedure is completed in this way, the SS establishes a connection in step  319  and operates normally by the connection in step  321 . During the normal operation, the SS searches neighboring BSs by  channel scanning at predetermined intervals. 
     Later, if a handoff is determined, the SS terminates the connection to the old BS (or source BS) in step  327  and selects a target BS in step  329 . 
     In steps  331 ,  333  and  335 , the SS performs a new network entry for the target BS in a similar manner to the above-described initialization procedure. The new network entry involves the process of searching for a cell offering a high Signal-to-Interference plus Noise Ratio (SINR) without association, before normal registration to the cell. Hence, the old BS does not know the movement state of the SS. 
     When finally deciding on the target BS, the SS performs re-authorization in step  337  and carries out re-registration and re-establishes service flows in step  339 . Thus, the SS associates with the target BS in step  339 . In step  341 , the SS operates in a normal manner by the connection to the new BS. 
     Meanwhile, the SS can re-establish an IP connection in step  343 . In case of a “make-before-break” handoff, the SS terminates every connection to the old BS in step  345 . 
     The IEEE 802.16e working group is working on standardizing mobile wireless MAN for supporting the mobility of the SS, while keeping backward compatibility with the 802.16 standards. To a Mobile SS (MSS), a BS broadcasts a Neighbor Advertisement (MOB_NBR-ADV) message including information about neighboring BSs to all MSSs within the cell coverage. Table 10 below illustrates the structure of the MOB_NBR-ADV message. 
     
       
         
               
               
               
             
           
               
                 TABLE 10 
               
               
                   
               
               
                 Information 
                 Size (bytes) 
                 Notes 
               
               
                   
               
             
             
               
                 Management Type 
                 1 
                 ID (53) identifying 
               
               
                   
                   
                 MOB_NBR-ADV 
               
               
                 Operator ID 
                 3 
                 Unique network ID 
               
               
                 Configuration 
                 1 
                 Increases by 1 each time 
               
               
                 Change Count 
                   
                 broadcast data is changed 
               
               
                 N_NEIGHBORS 
                 1 
                 The number of neighbor BSs 
               
               
                 Each Neighbor 
                 variable 
                 BS ID, preamble index, PHY 
               
               
                 information 
                   
                 profile ID 
               
               
                 HO Process 
                 1 
                 Information about processes 
               
               
                 Optimization 
                   
                 that can be omitted at re- 
               
               
                   
                   
                 entry 
               
               
                   
               
             
          
         
       
     
     The MSS transmits a Scanning Interval Allocation Request (MOB_SCAN-REQ) message to the serving BS to search neighboring BSs for handoff and determines a neighboring BS&#39;s scanning interval based on a MON_SCN-RSP message replied for the MON_SCN-REQ message. Without a request form the MSS, the BS can allocate a neighboring BS&#39;s scanning interval. Table 11 below illustrates the configuration of the MON_SCN-REQ. 
     
       
         
               
               
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE 11 
               
               
                   
                   
               
               
                   
                 Information 
                 Size (bytes) 
                 Notes 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Management Type 
                 1 
                 ID (54) identifying 
               
               
                   
                   
                   
                 MOB_SCN-REQ 
               
               
                   
                 Scan Duration 
                 1 
                 Scanning interval 
               
               
                   
                 Interleaving 
                 1 
                 Overlap between normal 
               
               
                   
                 Interval 
                   
                 operation interval 
               
               
                   
                   
                   
                 and scanning interval 
               
               
                   
                 Scan Iteration 
                 1 
                 The number of iterative 
               
               
                   
                   
                   
                 scannings 
               
               
                   
                 Message 
                 21 
                 MAC ensuring integrity 
               
               
                   
                 Authentication 
               
               
                   
                 Code (MAC) 
               
               
                   
                   
               
             
          
         
       
     
     The format of MOB_SCN-RSP is given in Table 12 as follows. 
     
       
         
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 12 
               
               
                   
               
               
                 Information 
                 Size (bytes) 
                 Notes 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Management Type 
                 1 
                   
                 ID (55) identifying 
               
               
                   
                   
                   
                 MOB_SCN-RSP 
               
               
                 Scan Duration 
                 1 
                   
                 Scanning interval 
               
               
                 Start Frame 
                 4 
                 bits 
                 Scanning start frame 
               
               
                 Interleaving 
                 1 
                   
                 Overlap between normal operation 
               
               
                 Interval 
                   
                   
                 interval and scanning interval 
               
               
                 Scan Iteration 
                 1 
                   
                 Number of iterative scannings 
               
               
                 Report Mode 
                 2 
                 bits 
                 Mode of reporting measurement 
               
               
                   
                   
                   
                 to BS 
               
               
                 Message 
                 21 
                   
                 MAC ensuring integrity 
               
               
                 Authentication 
               
               
                 Code (MAC) 
               
               
                   
               
             
          
         
       
     
     IEEE 802.16e will define a “break-before-make” handoff in which an old channel is released before connection of a new communication channel and a “make-before-break” handoff in which disconnection of an old channel follows connection of a new channel. Yet, at present, standardization documents are mainly focusing on the “break-before-make” handoff. 
       FIG. 4  is a diagram illustrating a signal flow for a conventional overall handoff procedure in the 802.16 broadband wireless communication network. While handoff initiation can occur in both the BS and the MSS, the MS initiates a handoff in the illustrated case of  FIG. 4 . 
     Referring to  FIG. 4 , an MSS  410  acquires neighbor BSs  470  and  490  by frequency channel scanning and determines whether to implement a handoff by measuring received signal strengths form the neighbor BSs  470  and  490 . If the MSS  410  decides on a handoff, it transmits a MOB_MSSHO-REQ message including information about the neighbor BSs  470  and  490  as candidate target BSs to a serving BS  450 . Table 13 below illustrates the MOB_MSSHO-REQ. 
     
       
         
               
               
               
               
             
               
               
               
               
               
             
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 13 
               
               
                   
                   
               
               
                   
                 Information 
                 Size (bytes) 
                 Notes 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Management Type 
                 1 
                   
                 ID (57) identifying 
               
               
                   
                   
                   
                   
                 MOB_MSSHO-REQ 
               
               
                   
                 N_Recommended 
                 4 
                 bits 
                 Number of BSs 
               
               
                   
                   
                   
                   
                 that MSS sets 
               
               
                   
                   
                   
                   
                 as candidates 
               
             
          
           
               
                   
                 Each Candidate BS 
                 variable 
                 BS ID, preamble index, 
               
               
                   
                 Information 
                   
                 SINR, etc. 
               
             
          
           
               
                   
                 Message 
                 21 
                   
                 MAC ensuring integrity 
               
               
                   
                 Authentication 
               
               
                   
                 Code (MAC) 
               
               
                   
                   
               
             
          
         
       
     
     Upon receipt of MOB_MSSHO-REQ, the serving BS  450  transmits a HO-pre-notification message to the candidates BSs  470  and  490 , notifying the handoff of the MSS  410  in steps  413  and  415 . Simultaneously, the serving BS  450  informs them of the MSS ID, connection parameter, capabilities, requested BandWidth (BW), and QoS information of the MSS  410 . In steps  417  and  419 , the candidates BSs  470  and  490  transmit an ACKnowledgement in a HO-pre-notification-response message to the serving BS  450 . 
     Meanwhile, the serving BS  450  determines a target BS based on information included in the HO-pre-notification-response message. It is assumed herein that the BS  490  is selected as the target BS. The serving BS  450  then transmits an HO-confirm message to the target BS  490  in step  421  and notifies the MSS  410  of the target BS  490  in a MOB_BSHO-RSP message in step  423 . The present 802.16e draft has not defined the message format of HO-pre-notification. Table 14 below illustrates the format of the MOB_BSHO-RSP. 
     
       
         
               
               
               
             
               
               
               
             
           
               
                 TABLE 14 
               
               
                   
               
               
                 Information 
                 Size (bytes) 
                 Notes 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Management Type 
                 1 
                 ID (58) identifying 
               
               
                   
                   
                 MOB_BSHO-RSP 
               
               
                 N_Recommended 
                 1 
                 Number of BSs that MSS sets 
               
               
                   
                   
                 as candidates 
               
               
                 Each Candidate BS 
                 variable 
                 Store BS ID, preamble index, 
               
               
                 Information 
                   
                 HO process optimization 
               
               
                   
                   
                 information in BS- 
               
               
                   
                   
                 recommended order 
               
               
                 New_BS Information 
                 variable 
                 Information about BS that 
               
               
                   
                   
                 serving BS recommends 
               
               
                   
                   
                 among BSs that MSS has not 
               
               
                   
                   
                 selected as candidates 
               
               
                 Message 
                 21 
                 MAC ensuring integrity 
               
               
                 Authentication 
               
               
                 Code (MAC) 
               
               
                   
               
             
          
         
       
     
     In step  425 , the MSS  410  notifies the serving BS  450  of normal handoff completion in MOB_HO-IND. The serving BS  450  releases resources and a connection from the MSS  410  in step  427 . The MSS  410  can cancel the handoff by a predetermined field of the MOB_HO-IND message or reject a handoff recommended by the serving BS  450 . 
     In step  429 , the MSS performs fast ranging based on known information about the target BS  490 . The MSS  410  then enters a new network in steps  431  and  433 , in the manner described with reference to  FIG. 3 . Table 15 below illustrates the configuration of the MOB_HO-IND. 
     
       
         
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 15 
               
               
                   
               
               
                 Information 
                 Size (bytes) 
                 Notes 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Management Type 
                 1 
                   
                 ID (59) identifying 
               
               
                   
                   
                   
                 MOB_HO-IND 
               
               
                 HO-IND_type 
                 2 
                 bits 
                 Indicates one of serving BS 
               
               
                   
                   
                   
                 release, HO cancel, and HO 
               
               
                   
                   
                   
                 reject 
               
               
                 Message Authentication Code 
                 21 
                   
                 MAC ensuring integrity 
               
               
                 (MAC) 
               
               
                   
               
             
          
         
       
     
     As described above, the present IEEE 802.11 and 802.16 standards define handoff between homogeneous networks, not yet handoff between heterogeneous  networks. The 802.11 APs will increase the capacity of the 802.16 BSs or cover shadowing areas that the 802.16 BSs cannot cover. The integration of the 802.11 network into the 802.16 network requires an efficient handoff technique between the 802.11 and 802.16 networks. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to substantially solve at least the above problems and/or disadvantages and to provide at least the advantages described below. Accordingly, an object of the present invention is to provide an apparatus and method for implementing handoff between heterogeneous networks in a wireless communication system. 
     Another object of the present invention is to provide an apparatus and method for implementing handoff between an IEEE 802.11 network and an IEEE 802.16 network in a wireless communication system. 
     The above objects are achieved by providing an apparatus and method for implementing a vertical handoff between a WLAN and a broadband wireless communication network. 
     According to one aspect of the present invention, an apparatus for implementing a handoff between a WLAN and a broadband wireless communication network includes a BS of the broadband wireless network and at least one AP of the WLAN within the cell coverage area of the BS, and a switch for forwarding packets between the BS and the at least one AP and processing a handoff between the BS and the at least one AP. 
     According to another aspect of the present invention, there is provided a method in a switch for processing a vertical handoff in a wireless communication  system including at least one AP within the cell coverage area of a BS of a broadband wireless network, and a switch for connecting the BS and the at least one AP, upon receipt of a vertical handoff request message from an SS associated with an AP, the vertical handoff message is converted to a handoff notification message used for the broadband wireless communication network, and transmitted to the BS. Upon receipt of a response message for the handoff notification message, a motion notification message notifying of the movement of the SS is transmitted to the AP. 
     According to a further aspect of the present invention, there is provided a method in a switch for processing a vertical handoff in a wireless communication system including at least one AP within the cell coverage of a BS of a broadband wireless network, and the switch for connecting the BS and the at least one AP, a handoff notification message notifying a handoff of an SS to an AP is received form the BS. Reception of a motion notification message notifying association of the SS with the AP is obtained from the AP, upon receipt of the handoff notification message. A response message for the handoff notification message is transmitted, upon receipt of the move notification message. 
     According to still another aspect of the present invention, there is provided an apparatus for implementing a vertical handoff between a WLAN and a broadband wireless communication network that includes a BS of the broadband wireless network and at least one AP of the WLAN within the cell coverage area of the BS, connected to the BS by a wireless link. The BS relays handoff messages between APs and processes a vertical handoff between the BS and the APs. 
     According to further still another aspect of the present invention, there is provided a method in an AP for processing a vertical handoff in a wireless communication system including at least one AP within the cell coverage area of  a BS of a broadband wireless network, upon reception of a vertical handoff request message from an SS, a vertical handoff is completed by exchanging handoff control messages used for the broadband wireless communication network with the BS. After completing the vertical handoff, a response message for the vertical handoff request message is transmitted to the SS and a connection is released from the SS. 
     According to yet still a further aspect of the present invention, there is provided a method in a BS of a broadband wireless network for processing a vertical handoff in a wireless communication system including at least one an AP within the cell coverage of the BS, upon receipt of a vertical handoff request message from an SS, where a vertical handoff is completed by exchanging handoff control messages used for the broadband wireless communication network with an AP being a handoff target. following completion of the vertical handoff, a response message for the vertical handoff request message is transmitted to the SS. Upon reception of a handoff indication message from the SS after transmitting the response message, a connection is released from the SS. 
     According to yet another aspect of the present invention, there is provided a method of processing a vertical handoff in an SS associated with an AP in a wireless communication system including at least one AP within the cell coverage of a BS of a broadband wireless communication network, a signal is acquired from a neighboring BS by scanning. It is determined whether to perform a vertical handoff based on the received signal strength of the neighboring BS. When deciding on the vertical handoff, a vertical handoff request message is transmitted to the AP. Upon reception of a response message for the vertical handoff request message, it is determined whether the response message acknowledges the vertical handoff. When the vertical handoff is acknowledged, network entry into the BS is performed.  
     According to yet another further aspect of the present invention, there is provided a method of processing a vertical handoff in an SS associated with a BS of a broadband wireless communication network in a wireless communication system including at least one AP within the cell coverage of the BS, a signal is acquired from a neighboring AP by scanning. It is determined whether to perform a vertical handoff based on the received signal strength of the neighbor AP. When deciding on the vertical handoff, a vertical handoff request message is transmitted to the BS and then the SS is associated with the AP. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings in which: 
         FIG. 1  illustrates a conventional system model in which an 802.11 network coexists with an 802.16 network; 
         FIG. 2  illustrates a conventional handoff procedure in an 802.11 WLAN; 
         FIG. 3  is a flowchart illustrating a conventional handoff procedure for an MSS in an 802.16 broadband wireless communication network; 
         FIG. 4  is a diagram illustrating a signal flow for a conventional overall handoff procedure in the 802.16 broadband wireless communication network; 
         FIG. 5  is a diagram illustrates a system model according to a first embodiment of the present invention; 
         FIG. 6  is a detailed flowchart illustrating an operation of a WS according to the first embodiment of the present invention; 
         FIG. 7  is a diagram illustrating a signal flow for a handoff procedure between a WLAN and a broadband wireless communication network according to first the embodiment of the present invention;  
         FIG. 8  is a diagram illustrating a signal flow for a handoff procedure from the broadband wireless communication network to the WLAN according to the first embodiment of the present invention; 
         FIG. 9  is a diagram illustrating a system model according to a second embodiment of the present invention; 
         FIG. 10  is a diagram illustrating an example of an AP managing an MSS connected to a WLAN in the system model illustrated in  FIG. 9 ; 
         FIG. 11  is a diagram illustrating an example of a BS of a broadband wireless communication network managing an MSS connected to the WLAN in the system model illustrated in  FIG. 9 ; 
         FIG. 12  is a diagram illustrating a signal flow for a handoff procedure from the WLAN to the broadband wireless communication network according to the second embodiment of the present invention; 
         FIG. 13  is a diagram illustrating a signal flow for a handoff procedure from the broadband wireless communication network to the WLAN according to the second embodiment of the present invention; 
         FIG. 14  is a flowchart illustrating a vertical handoff procedure from an 802.11 AP to an 802.16 BS in a dual-mode MSS according to the present invention; and 
         FIG. 15  is a flowchart illustrating a vertical handoff procedure form the 802.16 BS to the 802.11 AP in the dual-mode MSS according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Preferred embodiments of the present invention will be described herein below with reference to the accompanying drawings. In the following description, well-known functions or constructions are not described in detail since they would obscure the invention in unnecessary detail.  
     A detailed description will now be made of vertical handoff between an 802.11 network and an 802.16 network. 
       FIG. 5  is a diagram illustrating a system model according to the present invention. Referring to  FIG. 5 , a 802.16 BS  500  includes at least one 802.11 APs  520 ,  521  and  522  within its cell coverage area. A WS  510  is further provided to connect the BS  500  to the APs  520 ,  521  and  522  and a wired network (Internet)  530  by wired links. The WS  510  can be either incorporated into the BS  500  or separately configured. The WS  510  manages information about MSSs connected to the BS  500  and the APs  520 ,  521  and  522  (i.e. MAC, IP address, etc.) and controls handoff between APs or between the BS and an AP based on the information. This WS  510  includes at least three interfaces: one interfaces with the wired network  530 , a second interfaces directly with the 802.16 BS  500  via an 802.16 MAC, and a third interfaces with the 802.11 APs  520 ,  521  and  522  by wired links. If a new MSS associates with an 802.11 AP  520 ,  521  or  522 , or the 802.16 BS  500 , information about the new MSS is transferred to the WS  510 . The information may be signaled by a new defined message or by overlaying the header of an existing registration message. 
     The WS  510  can receive data from the wired network  530 , the 802.11 network, or the 802.16 network. It forwards the received packet to the 802.16 network, the 802.11 network, or the wired network  530  according to the destination address of the packet. 
     In order to support horizontal handoff between 802.11 APs, the WS  510  manages information about MSSs connected to the 802.11 network and relays handoff information. For vertical handoff, the WS  510  converts/relays handoff messages between an 802.11 AP and an 802.16 BS. The system model illustrated in  FIG. 5  offers the benefit of obviating the need for modifying the existing 802.16 protocols by adding a WS module to an 802.16 BS.  
       FIG. 6  is a detailed flowchart illustrating an operation of the WS  510  according to the present invention. 
     Referring to  FIG. 6 , the WS  510  monitors reception of a packet in step  611 . Upon receipt of the packet, the WS  510  determines whether the received packet is a data packet or a control packet in step  613 . In case of a data packet, the WS  510  determines where to forward the packet by checking the destination address of the packet in step  617 . 
     If the destination is the wired Internet, the WS  510  forwards the data packet to the wired network in step  631 . If the destination is the 802.16 network, the WS  510  forwards the data packet to an 802.16 BS in step  633 . If the destination is the 802.11 network, the WS  510  forwards the data packet to an 802.11 AP in step  635 . 
     In case of a control packet, the WS  510  checks the control type of the control packet in step  615 . If the control packet is a Vertical HandOFF (VHO) request message directed from the 802.11 AP to the 802.16 BS, the WS  510  converts the VHO request message to an 802.16 HO-notification message in step  641  and transmits the HO-notification message to the 802.16 BS in step  643 . 
     If the control packet is a HO-notification-response message directed from the 802.16 BS to the 802.11 AP, the WS  510  transmits the HO-notification-response message to the 802.16 BS in step  651  and then transmits an 802.11 IAPP MOVE-notification message to the 802.11 AP in step  653 . 
     If the control packet is an IAPP MOVE-response message directed from the 802.11 AP to the 802.16 BS, the WS  510  updates information about a corresponding MSS (e.g. position and forwarding information, etc.) in step  661 .  
     If the control packet is an HO-notification message directed from the 802.16 BS to the 802.11 AP, the WS  510  awaits reception of an IAPP MOVE-notify message from the 802.11 AP in step  671 . 
     If the control packet is an IAPP MOVE-notify message directed from the 802.11 AP to the 802.16 BS, the WS  510  converts the IAPP MOVE-notify message to an 802.16 HO-notification response message in step  681  and sends it to the 802.16 BS in step  683 . 
     If the control packet is an HO-notification-confirm message transmitted from the 802.16 BS to the 802.11 AP, the WS  510  converts the HO-notification-confirm message to an IAPP MOVE-notify-response message in step  691  and transmits it to the 802.11 AP and updates information regarding the MSS in step  693 . 
     If the control packet is a handoff message between APs, the WS  510  transmits the handoff message to the 802.11 AP and updates information about the MSS in step  695 . The horizontal handoff between the 802.11 APs is performed in compliance with the 902.11f draft described before. Notably, the message is relayed by the WS  510  in the 802.16 BS. 
     A vertical handoff procedure between the 802.11 network and the 802.16 network will be described below in detail. 
     A handoff is initiated in different manners in the 802.11 and 802.16 networks. According to the 802.11f draft, when the MSS decides on a handoff to a new AP by channel scanning, it re-associates with the new AP and the new AP transmits a handoff message to an old AP. In this way, the handoff is completed.  That is, the MSS always initiates a handoff and communicates with the new AP only, without any communications with the old AP in IEEE 802.11. 
     Alternatively, both the MSS and the BS can perform handoff initiation in IEEE 802.16. After the MSS or the BS decides on a handoff, the old BS processes a handoff request. That is, the MSS releases a connection from the old BS before establishing a connection to the new BS. Considering the technological difference, the vertical handoff should be implemented between the 802.11 and 802.16 networks. 
       FIG. 7  is a diagram illustrating a signal flow for a handoff procedure from a WLAN to a broadband wireless communication network according to the present invention. 
     Referring to  FIG. 7 , the illustrated call flow is for a vertical handoff of an MSS  710  from an 802.11 AP  750  to an 802.16 BS  790 . A new message called “VHO request” is defined for vertical handoff initiation of the MSS associated with the AP. 
     According to the conventional 802.11f draft, during a handoff between 802.11 APs, the MSS  710  transmits a handoff request to a new AP, rather than to an old AP. Compared to this handoff initiation method, the MSS requests a handoff to an old BS. Thus, the conventional 802.11 technique needs some modification for vertical handoff between the 802.11 and 802.16 networks. In this context, the vertical handoff request message is defined. The vertical handoff request message is configured based on the structure of an 802.11 MAC control frame. In a control field of the control frame, a type value of ‘00’ represents a Management type and one of reserved values, ‘0110’ is allocated as a subtype. When the MSS  710  associated with the 802.11 AP  750  is to perform a vertical handoff to the 802.16 BS  790 , it transmits this message to the old AP  750 ,  requesting the handoff. Table 16 below illustrates the format of the VHO request message. 
     
       
         
               
               
               
             
           
               
                   
                 TABLE 16 
               
               
                   
                   
               
               
                   
                 Information 
                 Notes 
               
               
                   
                   
               
             
             
               
                   
                 Capability Information 
                 Indicates request/advertised capability 
               
               
                   
                 Target BS Address 
                 MAC address of target 802.16 BS 
               
               
                   
                   
               
             
          
         
       
     
     Capability Information is the same field as Capability Information in an association request message. Since 11 Least Significant Bits (LSBs) are reserved, capability information or QoS information required for the handoff from the 802.11 network to the 802.16 network can be set in Capability Information. Target BS Address indicates a 48-bit (i.e. 6-byte) MAC address of a target 802.16 BS. 
     In operation, the MSS  710  associates with the 802.11 AP  750  by exchanging association request and response messages in step  711 . In step  713 , the 802.11 AP  750  transmits association information of the MSS  710  to the WS  770 . Meanwhile, the MSS  710  obtains a signal from the 802.16 BS  790  by channel scanning in step  715  and decides on a vertical handoff to the 802.16 BS  790  in step  717 . 
     In step  719 , the MSS  710  transmits a VHO request message to the 802.11 AP. The 802.11 AP  750  forwards the VHO request message to the WS  770  in step  721 . 
     The WS  770  converts the VHO request message to an 802.16 HO-notification message in step  723 , transmits the HO-notification message to the 802.16 BS  790  as if it were an old 802.16 BS in step  725 , receives an  HO-notification-response message from the 802.16 BS  790  in step  727 , and transmits an HO-notification-confirm message to the 802.16 BS  790  in step  729 . The messages exchanged in steps  725 ,  727  and  729  are handoff-associated messages defined by the 802.16 standards. 
     In step  731 , the WS  770  generates an 802.11 IAPP MOVE-notify message and transmits it to the AP  750 . The AP  750  transmits a VHO response message for the VHO request message to the MSS  710  in step  733 . The VHO response message is also configured based on the format of the 802.11 MAC control frame. In control fields of the control frame, a type value of ‘00’ represents a Management type and one of reserved values, ‘0111’ is allocated as a subtype. Table 17 below illustrates the VHO response message. 
     
       
         
               
               
               
             
           
               
                   
                 TABLE 17 
               
               
                   
                   
               
               
                   
                 Information 
                 Notes 
               
               
                   
                   
               
             
             
               
                   
                 Capability Information 
                 Indicates request/advertised capability 
               
               
                   
                 Status Code 
                 Success or failure of VHO request 
               
               
                   
                   
                 0: Successful 
               
               
                   
                   
                 1: Unspecified failure 
               
               
                   
                 Target BS Address 
                 MAC address of target 802.16 BS in 
               
               
                   
                   
                 case Status Code is 0 
               
               
                   
                   
               
             
          
         
       
     
     The Capability Information and Status Code fields shown in Table 17 above are defined by the IEEE standards. Other information associated with the vertical handoff can be written in a reserved area of Status Code. 
     After transmitting the VHO response message, the AP  750  dissociates from the MSS  710  by releasing allocated resources and a connection from the MSS  710  in step  735  and transmits an IAPP MOVE-response message to the WS   770  in step  737 . The WS  770  updates information about the MSS  710  (e.g., position and forwarding information), that is, updates the MSS information to the effect that the MSS  710  has associated with the BS  790  in step  739 . 
     Finally, the MSS  710  performs initial ranging with respect to the target BS  790  in step  741  and carries out a network entry in step  743 . 
       FIG. 8  is a diagram illustrating a signal flow for a handoff procedure from the broadband wireless communication network to the WLAN according to the present invention. The illustrated signal flow is for vertical handoff of an MSS  810  from an 802.16 BS  850  to an 802.11 AP  890 . 
     Referring to  FIG. 8 , the MSS  810  establishes a connection with the 802.16 BS  850  by initialization in steps  811  and  813 . During a normal operation, the MSS  810  acquires a signal from the 802.11 AP  890  by scanning in step  815  and decides on a vertical handoff to the 802.11 AP  890  in step  817 . The scanning complies with the 802.11f draft. 
     In step  819 , the MSS  810  transmits an 802.16 MSSHO_REQ message to the BS  850 . A target BS is set to the AP  890 . The BS  850  transmits an HO-notification message to a WS  870  in step  821 , and the WS  870  awaits reception of an IAPP MOVE-notify message from the AP  890  in step  823 . 
     After transmitting the MSSHO_REQ message, the MSS  810  transmits a reassociation request message to the AP  890  and awaits reception of a reassociation response message from the AP  890  in step  825 . When the MSS  810  associated with the AP  890 , the AP  890  transmits an IAPP MOVE-notify message to the WS  870  in step  827 .  
     Then the WS  870  converts the IAPP MOVE-notify message to an 802.16 HO-notification-response message in step  829  and transmits the HO-notification-response message to the BS  850  in step  831 . In step  833 , the BS  850  transmits an HO-notification-confirm message to the WS  870 . The BS  850  also transmits a BSHO_RSP message for the MSSHO_REQ message to the MSS  810  in step  835 . The MSS  810  transmits an HO_IND message to the BS  850  in step  837 . Upon reception of the HO_IND message, the BS  850  releases existing resources and connection from the MSS  810  in step  839 . 
     Meanwhile, upon receipt of the HO-notification-confirm message, the WS  870  converts the HO-notification-confirm message to an 802.11 IAPP MOVE-notify message in step  841  and transmits the IAPP MOVE-notify message to the AP  890  in step  843 . Then the WS  870  updates information about the MSS  810  (e.g., position and forwarding information) in step  845 . 
     The case where the 802.11 AP is connected to the 802.16 NS by a wired link has been described so far. It can be further contemplated as another embodiment of the present invention that the 802.11 AP is connected to the 802.16 NS by a wireless link. Then the 802.16 BS recognizes the 802.11 AP as an SS. 
       FIG. 9  illustrates a system model according to the present invention. 
     This system model is characterized in that the 802.16 BS serves as a last mile. In other words, the 802.11 AP is one MSS for the 802.16 BS. Therefore, the 802.11 AP should be provided with both 802.11 and 802.16 radio interfaces. The system model is viable because the 802.16 standards define a fixed 802.16 SS. It advantageously enables free deployment or elimination of an 802.11 AP.  
     Referring to  FIG. 9 , a consideration for the system model is which one between the 802.16 BS and the 802.11 AP is to manage an MSS connected to the 802.11 AP. Another consideration is how to transmit an inter-802.11 AP handoff message and a vertical handoff message. The two considerations will be addressed below. 
     Management of MSS Associated with 802.11 AP 
     Management of an MSS involves all of registration, IP address allocation, and authorization. 
     In the case where the 802.11 AP manages the SS, the 802.16 BS is not involved in managing an MSS associated with the AP. When the 802.11 AP is connected, the 802.16 BS allocates a plurality of IP addresses, for example, an IP prefix to the AP. The 802.11 AP allocates one of the IP addresses to the SS, as illustrated in  FIG. 10 . 
     Referring to  FIG. 10 , 802.11 APs  1040  and  1050  (AP# 1  and AP# 2 ) associate with an 802.16 BS  1070  and are allocated a plurality of IP addresses each in step  1011 . For example, they are allocated IP prefixes. Meanwhile, the 802.16 BS  1070  has an ARP table in which IP addresses allocated to SSs are mapped to their MAC addresses. When a new MSS  1010  attempts to associate with the AP  1040 , the AP  1040  allocates one of the IP addresses allocated by the 802.16 BS  1070  to the MSS  101  in step  1013 . 
     In the case where the 802.16 BS also manages an MSS associated with the 802.11 AP, information about the MSS is also registered in the 802.16 BS. The 802.16 BS allocates an IP address and the allocated IP address is notified to the MSS via the 802.11 AP. This MSS management technique advantageously obviates the need for allocating a new IP address at a handoff between 802.11 APs or at a 802.11-802.16 vertical handoff. An example is presented in  FIG. 11 .  
     Referring to  FIG. 11 , 802.11 APs  1140  and  1150  (AP# 1  and AP# 2 ) associate with an 802.16 BS  1170  in step  1111 . Later, a new MSS  1110  attempts to associate with the AP  1140  in step  1113 . The AP  1140  transmits association information of the MSS  1110  to the 802.16 BS  1170 . The 802.16 BS  1170  allocates a new IP address for the MSS  1110 , while simultaneously updating the ARP table, and then informs the AP  1140  of the allocated IP address in step  1115 . The AP  1140  transmits the allocated IP address to the MSS  1110  in step  1117 . 
     The following description is made of management of an MSS associated with an 802.11 AP by an 802.16 BS, taking an example. 
     In the present invention, handoff between 802.11 APs complies with the 802.11f draft. One thing to note is that every message is converted to an 802.16 message and the 802.16 BS functions to relay handoff messages between the 802.11 APs. The handoff messages are sensitive to time and thus, they are signaled by Unsolicited Grant Service (USG). 
       FIG. 12  is a diagram illustrating a signal flow for a handoff procedure from the WLAN to the broadband wireless communication network according to the second embodiment of the present invention. The call flow is for a vertical handoff of an MSS  1210  from an 802.11 AP  1250  to an 802.16 BS  1290 . 
     Referring to  FIG. 12 , the 802.11 AP  1250  associates with the 802.16 BS  1270  by initial ranging in step  1211  and performs authorization, registration, and IP connection in step  1213 . In step  1215 , the MSS  110  associates with the AP  1250  by exchanging association request and response messages. The AP  1350  acquires an IP address to be allocated to the MSS  1210  by authorization,  registration, and IP connection to the BS  1270  in step  1217  and transmits the IP address to the MSS  1210  in step  1219 . 
     In step  1221 , the MSS  1210  acquires a signal from the BS  1270  by scanning and decides on a vertical handoff to the BS  1270 . The MSS  1210  then transmits a VHO request message (refer to Table 16) to the AP  1250  in step  1223 . The AP  1250  converts the VHO request message to an 802.16 HO-notification message in step  1225  and transmits the HO-notification message to the BS  1270  in step  1227 . 
     The BS  1270  responds to the HO-notification message with an HO-notification-response message to the AP  1250  in step  1229  and the AP  1250  transmits an HO-notification-confirm message to the BS  1270  in step  1231 . In step  1232 , the AP  1250  transmits a VHO response message for the VHO request message to the MSS  1210 . 
     The AP  1250  then dissociates from the MSS  1210  in step  1233 . Hence, the MSS  1210  associates with the BS  1270  by initial ranging in step  1235  and performs initialization through authorization, registration and IP connection in step  1237 . The authorization or IP connection may not be performed because the 802.16 BS  1270  already has knowledge of the MSS  1210 . 
       FIG. 13  is a diagram illustrating a signal flow for a handoff procedure from the broadband wireless communication network to the WLAN according to the second embodiment of the present invention. The call flow is for a vertical handoff of an MSS  1310  from an 802.16 BS  1350  to an 802.11 AP  1370 . The following description is made under the assumption that the 802.11 AP  1370  has been registered as an MSS to the 802.16 BS  1350 .  
     Referring to  FIG. 13 , the MSS  1310  performs initial ranging in step  1311  and performs initialization through authorization, registration and IP connection in step  1313 . Thus, the MSS  1310  associates with the 802.16 BS  1350 . In step  1315 , the MSS  1310  acquires a signal from the 802.11 AP  1370  by downlink scanning and decides on a vertical handoff to the AP  1370 . 
     The MSS  1310  transmits an 802.11 MSSHO_REQ message to the BS  1350  in step  1317 . The AP  1370  is set as a target BS. Upon reception of the MSSHO_REQ message, the BS  1350  determines the handoff target and if the target is an 802.11 AP, converts the MSSHO_REQ message to an HO-notification message in step  1319 . In step  1321 , the BS  1350  transmits the HO-notification message to the AP  1370 . 
     The AP  1370  responds to the BS  1350  with an HO-notification-response message in step  1323  and the BS  1350  transmits an HO-notification-confirm message to the AP  1370  in step  1325 . 
     Meanwhile, the AP  1370  transmits a BSHO_RSP message for the MSSHO_REQ message to the MSS  1310  in step  1327 . The MSS  1310  transmits an HO_IND message to the BS  1350  in step  1329 . In step  1331 , the BS  1350  releases an existing connection form the MSS  1310 . Later, the MSS  1310  associates with the AP  1370  by exchanging association request and response messages in step  1333 . Since, the BS  1350  provides information about authorization, registration, and IP connection to the AR  1370 , there is no need for re-setting the information. 
     As described above, an MSS of the present invention is a dual-mode MSS that can associate with both the WLAN and the broadband wireless communication network. The operation of the dual-mode MSS will be described below.  
       FIG. 14  is a flowchart illustrating a vertical handoff procedure from an 802.11 AP to an 802.16 BS in a dual-mode MSS according to the present invention. 
     In the case of a vertical handoff from an 802.11 AP to an 802.16 BS, an MSS associated with the 802.11 AP acquires a signal from a neighbor 802.16 BS by scanning in step  1411  and decides on a vertical handoff by a vertical handoff determination algorithm in step  1413 . In generation, the MSS decides on the vertical handoff by comparing the received signal strength of a serving BS with those of neighboring BSs. If the vertical handoff is not available, the MSS returns to step  1411 . 
     If the vertical handoff is determined, the MSS generates a VHO request message (refer to Table 16) and transmits it to the 802.11 AP in step  1415  and monitors reception of a VHO response message (refer to Table 17) in step  1417 . 
     Upon receipt of the VHO response message, the MSS determines whether the VHO response message acknowledges the VHO request in step  1419 . If the VHO request is not acknowledged, the MSS returns to step  1411 . If the VHO request is acknowledged, the MSS releases an existing connection form the 802.11 AP and associates with the 802.16 BS in step  1421 . 
       FIG. 15  is a flowchart illustrating a vertical handoff procedure form the 802.16 BS to the 802.11 AP in the dual-mode MSS according to the present invention. 
     In the case of a vertical handoff from an 802.16 BS to an 802.11 AP, an MSS associated with the 802.16 BS acquires a signal from a neighbor 802.11 AP by scanning in step  1511  and decides on a vertical handoff by a vertical handoff  determination algorithm in step  1513 . If the vertical handoff is not available, the MSS returns to step  1511 . 
     If the vertical handoff is decided, the MSS generates a 802.16 HO_REQ message and transmits it to the 802.16 BS in step  1515  and monitors reception of an HO_RSP message in step  1517 . 
     Upon reception of the HO_RSP message, the MSS determines whether the HO_RSP message acknowledges the vertical handoff request in step  1419 . If the vertical handoff request is not acknowledged, the MSS returns to step  1511 . If the vertical handoff request is acknowledged, the MSS releases an existing connection form the 802.16 BS and associates with the 802.11 AP in step  1521 . 
     As described above, the present invention provides a vertical handoff technique between an 802.11 WLAN and an 802.16 broadband wireless network. Since, the present invention supports the 802.11-802.16 vertical handoff, handoff between heterogeneous networks efficiently applies to a network with 802.11 and 802.16 networks being coexistent. 
     While the invention has been shown and described with reference to certain preferred embodiments thereof, they are merely exemplary applications. For example while the embodiments of the present invention have been described focusing on handoff between an 802.11 WLAN and an 802.16 broadband wireless network, the present invention is easily applicable to handoff between the broadband wireless network and any other network interworkable with the broadband wireless network. Therefore, 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 spirit and scope of the invention as defined by the appended claims.