Patent Publication Number: US-2011051693-A1

Title: Method and apparatus for roaming seamlessly

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a communication system, and more particularly, to a method and apparatus for roaming seamlessly. 
     2. Description of the Related Art 
     Wireless local area network (WLAN) technology is now widely used in various applications. Numerous organizations devote extensive resources to research seeking improvements in WLAN data communication quality. In a WLAN, a wireless transmit/receive unit (WTRU) may be located within communication range of several access points (APs). When the WTRU roams among various APs, the AP with which the WTRU associates may be switched in accordance with a location of the WTRU and the signal quality. 
     In order to ensure service quality for WLAN applications, some requirements for transmission of packets are defined. For example, for voice over IP (VoIP) services, acceptable network transmission environments with good packet data processing performance ensure that the packet delay is less than 150 ms. Slower packet transmission rates cause echoes and tremolos caused by delays of packets, and thus longer delays will cause poor sound quality for users. According to the IEEE 802.11r standard, the switching time for a roaming WTRU from one AP to another AP is required to be less than 50 ms to maintain the quality of VOIP services in a wireless network. Therefore, finding ways to complete related procedures for roaming in advance to decrease the delay caused by an AP switching operation is an important issue for the market. 
     SUMMARY OF THE INVENTION 
     A method and apparatus for roaming seamlessly determine whether an authentication procedure and a handshaking procedure are performed in advance to obtain a key which is needed for data transmissions in accordance with received signal strength indication (RSSI) values of the AP with which the apparatus associates. During the processes of performing the authentication procedure and the handshaking procedure for the next AP, the apparatus continues to transceive data to/from the AP with which the apparatus currently associates. The conditions for switching are set by a user. For example, when an RSSI value of the AP with which the apparatus associates is less than a threshold value set by a user, the apparatus is switched to associate with another AP. 
     One embodiment of the present invention discloses a method for roaming seamlessly, comprising the steps of: associating with a first access point; selecting a second access point if a first RSSI value of the first access point is less than a first threshold value; performing an authentication procedure and a handshaking procedure with the second access point; and associating with the second access point if a second RSSI value of the first access point is less than a second threshold value. 
     Another embodiment of the present invention discloses a method for roaming seamlessly, comprising the steps of: associating with a first access point; performing a channel scanning procedure if a first RSSI value of the first access point is less than a first threshold value; generating a candidate list in accordance with the channel scanning procedure; selecting a second access point in accordance with the candidate list; performing an authentication procedure and a handshaking procedure with the second access point; and associating with the second access point if a second RSSI value of the first access point is less than a second threshold value. 
     Another embodiment of the present invention discloses an apparatus for roaming seamlessly comprising a forwarding unit, a receiving unit, a determining unit, a selecting unit, an authenticating/handshaking unit and a switching unit. The forwarding unit is utilized to forward a plurality of packets to a first access point or a second access point. The receiving unit is utilized to receive a plurality of packets forwarded from the first access point or the second access point. The determining unit is utilized to determine whether at least one RSSI value of the first access point is less than a first threshold value or less than a second threshold value. The selecting unit is utilized to select the second access point in accordance with at least one candidate list and a first determining result of the determining unit. The authenticating/handshaking unit is utilized to perform an authentication procedure and a handshaking procedure with the second access point. The switching unit is utilized to switch a connection to the second access point in accordance with a second determining result of the determining unit. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be described according to the appended drawings in which: 
         FIG. 1  illustrates a circumstance in which a station roams from the communication range of one AP to the communication range of another AP; 
         FIG. 2  shows a flowchart of a method for roaming seamlessly in accordance with an exemplary embodiment of the present invention; 
         FIG. 3  shows a flowchart of a method for roaming seamlessly in accordance with another exemplary embodiment of the present invention; and 
         FIG. 4  illustrates a block diagram of an apparatus for roaming seamlessly in accordance with another exemplary embodiment of the present invention. 
     
    
    
     PREFERRED EMBODIMENT OF THE PRESENT INVENTION 
       FIG. 1  illustrates a circumstance in which a station  13  roams from the communication range of an AP  10  to the communication ranges of an AP  11  and an AP  12 .  FIG. 2  shows a flowchart of a method for roaming seamlessly in accordance with an exemplary embodiment of the present invention. In order to enable those skilled in the art to practice the present invention in accordance with the exemplary embodiment,  FIG. 1  and  FIG. 2  are utilized to describe a procedure of the method for roaming seamlessly. The above-mentioned AP  10 , AP  11 , AP  12  and station  13  all comply with IEEE 802.11 standards. 
     In step S 201 , the procedure of the exemplary embodiment is activated. In step S 202 , the station  13  is associated with the AP  10 . Step S 203  determines whether a candidate list has been generated. If NO, a channel scanning procedure is performed in step S 204 . According to the IEEE 802.11 standard, the scanning phase scans all channels in order to determine which channels can be utilized by a station. The two currently available modes for scanning are active scanning and passive scanning. For active scanning, a station selects a channel and sends a broadcast probe request frame and then waits for a predetermined period of time. If no probe response frame is transmitted by the AP(s) in response to the probe request frame within the predetermined period of time, the station selects a next channel and repeats the procedure. For passive scanning, a station simply goes to the channel of interest and passively listens for any periodic beacon frames sent out by AP(s). If no periodic beacon frame is transmitted by the AP(s) within a predetermined period of time, the station selects a next channel and repeats the passive scanning operation. In step S 205 , a candidate list is generated in accordance with the channel scanning procedure. Subsequently, a specified time is set by a user and a time counting procedure is performed. Step S 206  determines whether the elapsed time of the time counting procedure reaches the specified time. The specified time can be set to 30 seconds, for example. If the specified time is reached, the process is returned to step S 204 . Steps S 204 -S 206  are utilized to perform a channel scanning procedure periodically. If a candidate list has been generated, step S 207  determines whether a received signal strength indication (RSSI) value of the AP  10  associated with the station  13  is less than a first threshold value. In accordance with the exemplary embodiment, the candidate list includes the AP  10 , the AP  11  and the AP  12 . In accordance with the exemplary embodiment, service set identifiers (SSID) of the AP  10 , the AP  11  and the AP  12  are the same. If step S 207  returns a result of YES, an AP is selected in accordance with the candidate list in step S 208 . In accordance with the exemplary embodiment, the AP with the highest RSSI value is selected. The selected AP is the AP  11 , for example. Thus, the AP  11  is selected in step S 208 . In step S 209 , an authentication procedure is performed. The authentication procedure is an open system authentication procedure, an extensible authentication protocol procedure or a shared-key authentication protocol procedure. In step S 210 , a handshaking procedure is performed to obtain a key which is needed for data transmissions, wherein the handshaking procedure is a four-way handshaking procedure or a two-way handshaking procedure. Step S 211  determines whether an RSSI value of the AP  10  associated with the station  13  is less than a second threshold value. If YES, the station  13  is switched to associate with the AP  11  in step S 211 . Step S 213  determines whether the procedure is to be repeated. If YES, the process is returned to step S 203 . If NO, the procedure is ended in step S 214 . 
       FIG. 3  is a flowchart of a method for roaming seamlessly in accordance with another exemplary embodiment of the present invention. In order to enable those skilled in the art to practice the present invention in accordance with the exemplary embodiment,  FIG. 1  and  FIG. 3  are utilized to describe the procedure of a method for roaming seamlessly. In step S 301 , the procedure of the exemplary embodiment is activated. In step S 302 , a station  13  is associated with an AP  10 . Step S 303  determines whether an RSSI value of the AP  10  associated with the station  13  is less than a first threshold value. If YES, a channel scanning procedure is performed in step S 304 . The mode of the channel scanning procedure can be an active scanning mode or a passive scanning mode. In step S 305 , a candidate list is generated in accordance with the channel scanning procedure. In accordance with the exemplary embodiment, the candidate list includes the AP  10 , an AP  11  and an AP  12 . In accordance with the exemplary embodiment, SSIDs of the AP  10 , the AP  11  and the AP  12  are the same. In step S 306 , an AP is selected in accordance with the candidate list. In accordance with the exemplary embodiment, the AP with the highest RSSI value is selected. The AP with the highest RSSI value is the AP  11 , for example. Thus, the AP  11  is selected in step S 306 . In step S 307 , an authentication procedure is performed. The authentication procedure is an open system authentication procedure, an extensible authentication protocol procedure or a shared-key authentication protocol procedure. In step S 308 , a handshaking procedure is performed to obtain a key which is needed for transmissions, wherein the handshaking procedure is a four-way handshaking procedure or a two-way handshaking procedure. Step S 309  determines whether an RSSI value of the AP  10  associated with the station  13  is less than a second threshold value. If YES, the station  13  is switched to associate with the AP  11  in step S 310 . Step S 311  determines whether the procedure is to be repeated. If YES, the process is returned to step S 303 . If NO, the procedure is ended in step S 312 . 
       FIG. 4  illustrates a block diagram of an apparatus for roaming seamlessly in accordance with another exemplary embodiment of the present invention.  FIG. 1  and  FIG. 4  are utilized to describe the embodiment. The apparatus  400  for roaming seamlessly comprises a forwarding unit  401 , a receiving unit  402 , a determining unit  403 , a selecting unit  404 , an authenticating/handshaking unit  405 , a switching unit  406 , a channel scanning unit  407  and a generating unit  408 , wherein a station  13  is equipped with the apparatus  400  for roaming seamlessly. Assume that the station  13  has been associating with an AP  10 . The forwarding unit  401  is utilized to forward a plurality of packets to the AP  10  while the receiving unit  402  is utilized to receive a plurality of packets forwarded from the AP  10 . The apparatus  400  roams from the communication range of an AP  10  to the communication ranges of an AP  11  and an AP  12 . The determining unit  403  is utilized to determine whether RSSI values of the AP  10  at various time points are less than a first threshold value or less than a second threshold value. The channel scanning unit  407  is utilized to perform a channel scanning procedure periodically or perform the channel scanning procedure in accordance with a determining result of the determining unit  403 . The generating unit  408  is utilized to generate a candidate list in accordance with at least one scanning result of the channel scanning unit  407 . In accordance with the exemplary embodiment, the candidate list selectively includes the AP  10 , the AP  11  and the AP  12 . In accordance with the exemplary embodiment, SSIDs of the AP  10 , the AP  11  and the AP  12  are the same. The selecting unit  404  is utilized to select an AP in accordance with another determining result of the determining unit  403 . In accordance with the exemplary embodiment, the AP with the highest RSSI value is selected. The AP with the highest RSSI value is the AP  11 , for example. The authenticating/handshaking unit  405  is utilized to perform an authentication procedure and a handshaking procedure with the AP  11 . The authentication procedure is an open system authentication procedure, an extensible authentication protocol procedure or a shared-key authentication protocol procedure. The handshaking procedure is a four-way handshaking procedure or a two-way handshaking procedure. The authenticating/handshaking unit  405  receives packets from the AP  11  and forwards packets to the AP  11  through the forwarding unit  401  and the receiving unit  402 , respectively, to complete the authentication procedure and the handshaking procedure. In the meantime, the forwarding unit  401  forwards packets to the AP  10  while the receiving unit  402  receives packets from the AP  10 . In other words, while forwarding and receiving normal communication packets to and from the AP  10 , the forwarding unit  401  and the receiving unit  402  simultaneously forwards and receives, respectively, authentication and handshaking packets to and from the AP  11 , to complete the authentication procedure and the handshaking procedure. In accordance with an exemplary embodiment of the present invention, the functions of the above-mentioned forwarding unit  401  and the receiving unit  402  can be achieved by providing two instance layers. The switching unit is utilized to switch the apparatus  400  to associate with the AP  11  in accordance with another determining result of the determining unit  403 . The above-mentioned apparatus  400  for roaming seamlessly can be implemented with software, firmware, hardware, or a platform with a single processor or with multiple processors. 
     In accordance with the present invention, a method and apparatus for roaming seamlessly determine whether an authentication procedure and a handshaking procedure are performed in advance to obtain a key which is needed for data transmissions in accordance with RSSI values of the AP with which the apparatus associates. During the processes of performing the authentication procedure and the handshaking procedure for the next AP, the apparatus continues to transceive data to/from the AP with which the apparatus currently associates. The conditions for switching are set by a user. For example, when an RSSI value of the AP with which the apparatus associates is less than a threshold value set by a user, the apparatus is switched to associate with another AP. 
     The above-described embodiments of the present invention are intended to be illustrative only. Numerous alternative embodiments may be devised by persons skilled in the art without departing from the scope of the following claims.