Abstract:
An access point (AP) ( 120   a ) manages and coordinates the switching of mobile devices ( 110 ) to other access points ( 120   b, c, d ). The access point ( 120   a ) monitors the quality of its communications link with the mobile device ( 110 ). When the access point ( 120   a ) determines that the quality is degrading, or when the access point ( 120   a ) determines that its traffic is excessive, or when another switch-triggering event ( 202 ) occurs, the access point ( 120   a ) sends a transfer-request to other access points ( 120   b, c, d ) in its vicinity. If another access point can accept the mobile device ( 110 ), it notifies the requesting access point ( 120   a ) that it is available. The requesting access point ( 120   a ) selects from among the available access points ( 120   b, c, d ), and notifies the mobile device ( 110 ) to switch to the selected access point. The notification to the mobile device ( 110 ) includes the appropriate attributes of the selected access point, so that the mobile device ( 110 ) can effect the switch to this selected access point with minimum overhead or delay. The requesting access point ( 120   a ) maintains a message buffer during this transfer process, so that any data lost during the transfer can be easily recovered.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of U.S. provisional application Ser. No. 60/492,199 filed Aug. 1, 2003, which is incorporated herein whole by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to the field of communications, and in particular to a method and system for managing a wireless network wherein wireless devices are free to roam among multiple Basic Service Sets (BSSs) in an Extended Service Set (ESS). 
     Mobile devices, such as laptop computers, personal data assistants (PDAs), headsets, voice over IP (VOIP) phones, and so on, are becoming increasingly popular, and standards and protocols, such as IEEE 802.11 and Bluetooth have been established to facilitate the networking of such devices to form wireless local area networks (WLANs). 
     Conventionally, the mobile device establishes a connection via an access point (AP) on the WLAN and monitors the quality of the connection. When the quality of the connection becomes marginal, such as occurs when the mobile device travels away from the access point, the mobile device searches for another access point in the network having potentially better quality. Upon detection of another access point with better quality, the mobile device establishes communication with this other access point and cancels the connection with the prior access point. The other access point may be in the same sub-network, or Basic Service Set (BSS), as the original access point, or in a different sub-network that is coupled to the original sub-network, forming an Extended Service Set (ESS). 
     The continued monitoring of signal quality, and searching for alternative access points by the mobile device consumes power that could otherwise be used for the communication of information over the network. U.S. patent application “METHOD AND SYSTEM FOR ENABLING CENTRALIZED CONTROL OF WIRELESS LOCAL AREA NETWORKS”, publication number 2002/0085719, filed 22 Oct. 2001 for David B. Crosbie, incorporated by reference herein, discloses the use of a roaming server that monitors the quality of the links between access points and wireless devices, and effects a substantially seamless transfer of the wireless devices to alternative access points as required. The transfer of a wireless device to an alternative access point may also be triggered based on the congestion of particular links, the location of the wireless device, or an explicit request from either the access point or the mobile device. The seamless transfer is achieved by “address-spoofing”. The roaming server transfers the “session data” from the original access point to the new access point, so that the new access point adopts the address of the original access point, along with other communications parameters, such as the frequency offset, hop sequence, encryption key, and so on. To assure continuous communication, the clocks are synchronized by having the wireless device assume a ‘master’ role with the original access point, then assume a ‘slave’ role with the new access point. Although this scheme provides a seamless transfer of the wireless device from the original access point to the new access point, this session-data spoofing and master-then-slave clock synchronization is contrary to the underlying autonomous principles of conventional networking protocols. 
     SUMMARY OF THE INVENTION 
     It is an object of this invention to provide a management method and system for a wireless network that minimizes the tasks that need to be performed by the wireless device. It is a further object of this invention to provide a network management method and system that is substantially consistent with existing standards, such as IEEE 802.11 a/b/g. 
     These objects, and other, are achieved by a switching scheme wherein an access point (AP) manages and coordinates the switching of mobile devices to other access points. The access point monitors the quality of its communications link with the mobile device. When the access point determines that the quality is degrading, or when the access point determines that its traffic is excessive, or when another switch-triggering event occurs, the access point sends a transfer-request to other access points in its vicinity. If another access point can accept the mobile device, it notifies the requesting access point that it is available. The requesting access point selects from among the available access points, and notifies the mobile device to switch to the selected access point. The notification to the mobile device includes the appropriate attributes of the selected access point, so that the mobile device can effect the switch to this selected access point with minimum overhead or delay. The requesting access point maintains a message buffer during this transfer process, so that any data lost during the transfer can be easily recovered. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates an example block diagram of a wireless network in accordance with this invention. 
         FIG. 2  illustrates an example flow diagram of a wireless network switching process in accordance with this invention. 
         FIG. 3  illustrates an example block diagram of an access point device in accordance with this invention. 
     
    
    
     DETAILED DESCRIPTION 
     Throughout the drawings, the same reference numeral refers to the same element, or an element that performs substantially the same function. 
     In the following description, the IEEE 802.11 a/b/g wireless device communications protocol is used as the paradigm of a wireless communications system, although one of ordinary skill in the art will recognize that the principles of this invention are not limited to the particular examples provided. 
       FIG. 1  illustrates an example block diagram of a wireless network  100  in accordance with this invention. The example network  100  includes three access points AP  120 , with which mobile devices M  110  communicate to transfer data across the network, and/or to other networks. Also included in the example network  100  are optional switch-assist devices SA  130 , whose function is explained below. The mobile devices M communicate with the access points AP via wireless communications, whereas the AP and SA devices may communicate with each other by wireless or wired communications, depending upon the particular network environment. 
     Each mobile device M is associated with one access point, and sends and receives each of its messages via this access point. When a mobile device M first encounters the network  100 , it sends an access request to a detected access point AP. If the access point AP can accept this mobile device M, the access point AP provides the mobile device a “description set” that identifies the communications and protocol parameters required for joining the network via this access point AP. An access point AP may support a plurality of mobile devices M contemporaneously. 
     In accordance with this invention, an access point AP monitors the quality of the, communication channel of each of its associated mobile devices M, and attempts to locate a more suitable access point AP for a mobile device M when the quality of the channel deteriorates. If a more suitable AP is found, the original AP obtains the required description set for communicating with the new access point, and forwards it to the mobile device M with the command to switch to the new access point. In a preferred embodiment of this invention, this new description set conforms to the description set that would have been provided to the mobile device M after a conventional access negotiation between the mobile device and the new access point, thereby maintaining substantial conformity with the conventional protocol specification. With this new description set, the mobile device M is able to switch to the new access point without incurring the overhead associated with the conventional process of searching for and negotiating access with the new access point. 
       FIG. 2  illustrates an example flow diagram of a wireless network switching process in accordance with this invention. The mobile device M  110  communicates messages  201  with its associated access point AP  120   a . The access point AP  120   a  continuously assesses the quality of the communications link with the mobile device M  110 . At some point, a “trigger event” T  202  occurs that causes the access point AP  120   a  to initiate a search for a more suitable access point for servicing the mobile device M  110 . This trigger event T  202  may be a decrease in signal strength from the mobile device M  110 , a decrease in throughput capability for the device M  110  due to increased traffic load from other devices through this access point AP  120   a , a reported movement of the mobile device M  110  away from the access point AP  120   a , and so on. Any of a number of techniques may be used to assess the quality of the communications link to and from the mobile device M. For example, a measure of the received signal strength, such as the common RSSI signal, and Signal quality signal, provides an assessment of the quality of communications from the mobile device M, and the number of requested retransmissions from the mobile device M provides an assessment of the quality of communications to the mobile device M. In a preferred embodiment of this invention, the mobile device M is configured to periodically send a message to the access point  120   a  during periods of inactivity, to facilitate continuous quality assessments and/or location determinations. 
     The optional switch-assist device SA  130  in  FIG. 1  is configured to facilitate a determination of this trigger event T  202 . In a preferred embodiment of this invention, the switch-assist device SA monitors communications from mobile devices M, and reports measures related to the these communications to one or more of the access points AP  120 . From these reports, and knowledge of the location of each switch-assist device SA, the access point  120   a  of  FIG. 2  is able to estimate the approximate location of the mobile device M  110 , and thereby determine the direction of movement, if any, of the mobile device M. In this way, a detected movement of the mobile device M toward the periphery of the range of the access point  120   a  will trigger an event T  202 . 
     Upon the occurrence of a trigger event T  202 , the access point AP  120   a  initiates a search for a more suitable access point for servicing the mobile device M. Access point AP  120   a  send a “switch-assessment” request SAReq  221  to each of the other access points in the network. Optionally, if the approximate location and/or traveling direction of the mobile device M is known, or the identification of immediately adjacent access points are known, the switch-assessment request SAReq  221  may be sent to only a select few of the other access points in the network. In a preferred embodiment of this invention, the switch-assessment request SAReq  221  includes the communications parameters and/or traffic requirements associated with the mobile device M, so that each of the other access points can provide an assessment of its suitability for specifically servicing the device M. 
     Each access point  120   b - d  that receives the request  221  evaluates the request, illustrated in  FIG. 2  as E  222 ,  223 ,  224 , respectively. Each access point  120   b - c  that can accommodate the mobile device M responds with a switch-assessment response  231 ,  232 . Optionally, access points  120   d  that cannot accommodate the mobile device M respond with an explicit negative response. 
     The original access point  120   a  evaluates the responses  231 ,  232  to determine the most suitable access point  120   b ,  120   c  for servicing the mobile device M, at  241  in  FIG. 2 . This evaluation is preferably based on the compatibility of the mobile device M with each of the access points  120   b ,  120   c , the available capacity of access points  120   b ,  120   c  in view of the traffic requirements of the mobile device M, and other factors. In a preferred embodiment of this invention, the user of the mobile device M, or the provider of the access points AP, are provided the option of explicitly declaring factors that affect the determination of a suitable pairing of a mobile device to a particular access point. For example, a user or network administrator may provide an access classification for each access point relative to each mobile device, the classification being, for example, preferred, acceptable, forbidden, and so on. In a commercial environment, this classification may be based on a subscription level, a quality-of-service parameter, and so on. 
     Also in a preferred embodiment of this invention, the evaluation of the responses  231 ,  232  may also include consideration of the path of motion of the mobile device M, to minimize subsequent AP-switches. For example, if the predicted path of the mobile device M crosses the periphery of the field of a first AP switch, then into the core of the field of a second AP switch, and if the second AP switch is currently suitable for accepting the mobile device M, the original access point preferably selects the second AP switch, even if the first AP switch might be considered more favorable for other factors. In this manner, an interim switch to the first AP switch is avoided. In like manner, the original access point AP  120   a  may delay switching until a particular access point in the predicted path of the mobile device M is able to support the mobile device M. 
     Optionally, the access point AP  120   a  also notifies the mobile device M of a potential switch of access points, via a “switch-imminent” request SIReq  211 . The mobile device M acknowledges receipt of this request  211  with a switch-imminent response SIRes  212 . In this optional configuration, the access point AP  120   a  may be configured to postpone some or all of the above described process of searching and selecting an alternative access point for the mobile device M until the response SIRes  212  is received. The mobile device M may be configured to refuse to switch to a new access point for security purposes, or it may be configured to prompt the user for approval of a switch of access point, or it may be configured to only allow a switch upon completion of each message session, and so on. 
     Having selected the most suitable access point for servicing the mobile device M, which in the example of  FIG. 2  is access point  120   c , the original access point  120   a  directs the mobile device M to switch to the new access point  120   c  via a switch-start command  251 . In a preferred embodiment of this invention, the aforementioned switch-assessment responses SARes  231 ,  232  include the description set required to establish a communications channel with each access point AP  120   b ,  120   c . Optionally, this information can be requested from the selected access point AP  120   c  after the original access point  120   a  completes the evaluation process  241 . This description set is communicated to the mobile device M as part of the switch-start command  251 . In response to this command  251 , the mobile device M configures itself according to the aforementioned description set to join the network via the new access point  120   c , at  261 , and sends a message to associate itself to the new access point  120   c , at  271 . In the IEEE 802.11 protocol, this association is established via a reassociation request, which, if Inter Access Point protocol 802.11f is implement, also initiates a set of authentication messages between the original access point  120   a  and the new access point  120   c . Thereafter, messages  291  are communicated between the mobile device M and the network via the new access point  120   c.    
     In a preferred embodiment of this invention, the original access point AC  120   a  is configured to store data communications to and from the mobile device M during the transition of the mobile device M to the new access point AP  120   c , to facilitate recovery in the event that the transition is unsuccessful or problematic. 
       FIG. 3  illustrates an example block diagram of an access point device  120  in accordance with this invention. The access point device  120  includes a transceiver  310  for wireless communication with mobile devices, a transceiver  330  for wired or wireless communication with other access points, and, optionally, a transceiver  360  for wired or wireless communication with switch-assist devices. During routine operation, the access point device  120  communicates messages between the mobile device and the network via a data handler  340 . 
     The device  120  includes a trigger control module  350  that is configured to initiate a search for an alternative access point for servicing a mobile device. The module  350  receives signal quality measures and other measures from the transceiver  310 , message error rate statistics, traffic flow statistics, and other measures from the data handler  340 , and, optionally, signal quality measures and other measures from the transceiver  360 . Based on this received information, the trigger controller  350  determines whether to initiate an access-point-switch procedure, as discussed above. 
     The trigger control module  350  communicates the initiation signal to a switch control module  320 . The switch control module  320  sends the aforementioned switch-assessment request to one or more other access points, and receives corresponding switch-assessment replies from one or more of the other access points, via the transceiver  330 . Optionally, the switch control module  320  also communicates the aforementioned switch-imminent request and response signals to and from the mobile device via the transceiver  310 . Upon receipt of the switch-assessment replies from the other access points, the switch control module  320  determines the most suitable new access point, if any, for the mobile device, as discussed above. If a more suitable access point is determined, the switch control module  320  issues a switch command to the mobile device to effect the new association of the mobile device to the selected access point. As noted above, in a preferred embodiment of this invention, the command to switch access points includes all of the information required to effect this new association, thereby freeing the mobile device of the conventional overhead of searching for and negotiating with the other access devices as it travels through the network, thereby minimizing the disruption of traffic flow. 
     The foregoing merely illustrates the principles of the invention. It will thus be appreciated that those skilled in the art will be able to devise various arrangements which, although not explicitly described or shown herein, embody the principles of the invention and are thus within its spirit and scope. For example, separate transceivers  310 ,  330 , and  360  are illustrated to provide communications with the mobile devices, other access points, and switch-assist modules, respectively, although fewer transceivers could be employed if the mobile device, access points, and/or switch-assist modules share a common communication technology. These and other system configuration and optimization features will be evident to one of ordinary skill in the art in view of this disclosure, and are included within the scope of the following claims.