Abstract:
Disclosed is a method for performing handoff in a communication system of access points and a mobile station, where the mobile station is associated with a first access point. The first access point receives a first transmission at a first signal quality from the mobile station and receives a second transmission that indicates a second signal quality at which a second access point is receiving transmissions from the mobile station. The first access point compares the first signal quality with the second signal quality and when the second signal quality is greater than the first signal quality, the first access point signals the second access point to initiate a handoff with the mobile station. The second access point responds by sending a disassociation message to the mobile station to disassociate with the first access point, wherein the disassociation message is addressed as if it originated from the first access point.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to a method for performing a seamless handoff in a communication system. 
     BACKGROUND OF THE INVENTION 
     Handoff mechanisms in current communication systems, such as 802.11 wireless local area network (“WLAN”), are simple, but slow. Slow handoffs may result in large numbers of transmissions being delayed and/or lost during handoff from one access point (“AP”) to another. This problem can become very severe if a mobile station (“MS”) is traveling from one AP coverage area to another at vehicular speeds. 
     The standards-based solution to this problem consists of the MS determining that the signal quality from the currently associated AP has dipped below some threshold value, or worse yet, that the MS tries to transmit a transmission to an AP, and all retries are exhausted. The decision process in the MS can take several hundred milliseconds (e.g., 600+). At that point, the MS begins searching for additional APs that it can associate with. Once a different AP is found, the MS associates with it and the handoff is complete. 
     A major disadvantage to this standards-based approach is that the MS hangs on to the old AP for quite a while before seeking a new AP with better signal quality. Thus, while the standards-based approach may be simple, it is not conducive to seamless handoffs. 
     Thus, there exists a need for a method for performing a seamless handoff. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       A preferred embodiment of the invention is now described, by way of example only, with reference to the accompanying figures in which: 
         FIG. 1  illustrates a system topology in accordance with the present invention; 
         FIG. 2  illustrates a message sequence flow diagram in accordance with the preferred embodiment of the present invention; 
         FIG. 3  illustrates a flowchart depicting the operation of the first access point in accordance with the preferred embodiment of the present invention; and 
         FIG. 4  illustrates a flowchart depicting the operation of the second access point in accordance with the preferred embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     It will be appreciated that for simplicity and clarity of illustration, elements shown in the figures have not necessarily been drawn to scale. For example, the dimensions of some of the elements are exaggerated relative to each other. Further, where considered appropriate, reference numerals have been repeated among the figures to indicate identical elements. 
     The present invention reduces handoff delays from one AP to another by exploiting the fact that adjacent/neighboring APs are likely to “hear”/receive a transmission from a MS well before the MS reaches a threshold where it decides the signal quality from its currently associated AP is poor enough to trigger the handoff procedure. Thus, the present invention adds additional AP processing and inter-AP messaging to the handoff procedure that provides for a more seamless (less delay, fewer dropped transmissions, fewer out of order transmissions, or the like) handoff for subscriber units. It should be noted, however, that in a first embodiment of the present invention, modifications are not required to be made to the MS for the purposes of improving the handoff decision process (i.e., the present invention is transparent to the MS); however, in an alternative embodiment of the present invention, modifications are required to be made to the MS for the purposes of improving the handoff decision process. 
       FIG. 1  illustrates a communication network having a first AP (“AP 1 ”)  100 , a second AP (“AP 2 ”)  102 , and the MS  104 . The coverage area of AP 1   100  is represented by circle  106 , and the coverage area for AP 2   102  is represented by circle  108 . The present invention assumes that the AP 1   100 , AP 2   102 , and the MS  104  are functioning properly, and the MS  104  is currently registered and associated with AP 1   100 . 
       FIG. 2  illustrates an example of a message sequence flow diagram of the operation of the communication network in accordance with the preferred embodiment of the present invention. As the MS  104  starts to move out of the coverage area  106  of AP 1   100  and into the coverage area  108  of AP 2   102 , AP 2   102  will begin to receive transmissions from the MS  104  ( 200 ). It should be noted that the MS  104  may or may not still be within the coverage area  106  of AP 1   100 . When AP 2   102  starts to receive transmissions from the MS  104 , AP 2   102  compares the signal quality of the transmission to a threshold in accordance with the present invention. If the signal quality of the received transmission exceeds the threshold, AP 2   102 , or some other entity, signals AP 1   100  that AP 2   102  is receiving transmissions from the MS  104  currently associated with AP 1   100  at the given signal quality ( 202 ). AP 1   100  compares the signal quality of transmissions received by AP 2   102  to its own, and if AP 1   100  determines that the signal quality of transmissions received by AP 2   102  is better than its own, AP 1   100  transmits a “handoff trigger” to AP 2   102  that signals AP 2  to initiate a handoff with the MS  104  ( 204 ). It is important to note that the signal quality of the transmissions received by a device may be based on, but is not limited to, signal strength, slicer error, a Viterbi path metric, cyclic redundant check errors, or the like. 
     AP 2   102  processes the “handoff trigger” received from AP 1   100  and commits to associating with the MS  104  by transmitting a disassociation message to the MS  104 , with the message addressed as if the disassociation message originated from AP 1   100  ( 206 ); such a technique is known to those individuals ordinarily skilled in the art as “spoofing”, which is implemented in the present invention so that a standard (off-the-shelf) MS  104  will accept the disassociation message from AP 2   102 . The MS  104  transmits a disassociation acknowledgement back to what it believes is AP 1   100  ( 208 ); if AP 1   100  receives the disassociation acknowledgement, AP 1   100  ignores the acknowledgement. 
     AP 2   102  receives and processes the disassociation acknowledgement from the MS  104  even though it is addressed to AP 1   100 . Upon receipt of the disassociation acknowledgement from the MS  104 , AP 2   102  transmits a beacon message ( 210 ), which in turn triggers the MS  104  to re-associate with AP 2   102 . 
     Upon receipt of the beacon message from AP 2   102 , the MS  104  transmits a re-associate request to AP 2   102  ( 212 ). AP 2   102  responds with a re-associate response to the MS  104  ( 214 ). Once the response is received, the MS  104  begins directing its outgoing transmissions to AP 2   102 . AP 2   102  immediately signals AP 1   100  to forward to AP 2   102  any buffered packets destined to the MS  104  ( 216 ), and AP 1   100  forwards to AP 2   102  any buffered packets destined to the MS  104  ( 218 ). It should be noted that AP 1   100  could forward to AP 2   102  any buffered packets destined to the MS  104  at any time after AP 2   102  commits to associate with the MS  104 . 
     Further optimizations can be added to the present invention to reduce the signaling overhead between AP 1   100  and AP 2   102  if the MS  104  happens to be within overlapping coverage areas for an extended period of time. For example, if AP 1   100  does not signal AP 2   102  to initiate a handoff with the MS  104  within a predetermined time period after being notified of the signal quality at which AP 2   102  is receiving transmissions from the MS  104 , AP 2   102  can interpret the non-response as meaning that the signal quality of the transmissions received by AP 2   102  from the MS  104  was not strong enough to warrant a handoff. As a result, when AP 2   102  receives additional transmissions from the MS  104 , AP 2   102  can hold off signaling AP 1   100  of the presence of the MS  104  unless the signal quality has improved, preferably for some fixed amount of time, or by some percentage or threshold, over previously received transmissions. 
     In an alternative embodiment, before AP 1   100  signals AP 2   102  to initiate a handoff with the MS  104 , AP 1   100  takes into consideration the signal quality of transmissions received by the MS  104  from AP 2   102  ( 220 ). In this alternative embodiment, in addition to AP 2   102  indicating to AP 1   100  the signal quality of transmissions received by AP 2   102  from the MS  104 , the MS  104  also indicates to AP 1   100  the signal quality of transmissions received by the MS  104  from AP 2   102 . Taking into consideration the perspective of the MS  104 , particularly the signal quality of transmissions the MS  104  receives from neighboring APs, prior to instructing a particular AP to initiate a handoff with the MS  104 , improves the handoff decision process because the inbound (i.e., from the MS to the AP) and outbound (i.e., from the AP to the MS) radio frequency communication paths are not necessarily symmetrical; in other words, just because AP 2   102  is receiving transmissions transmitted by the MS  104  at a given signal quality does not mean that the MS  104  is receiving transmission transmitted by AP 2   102  at the same or similar signal quality. Thus, AP 1  will instruct AP 2   102  to initiate a handoff with the MS  104  when the signal quality of the transmissions received by the MS  104  from AP 2  is greater than the signal quality of the transmissions received by the MS  104  from AP 1   100 , and the signal quality of the transmissions received by AP 2   102  from the MS  104  is greater than a threshold in accordance with the alternative embodiment of the present invention; alternatively, AP 1   100  will instruct AP 2   102  to initiate a handoff with the MS  104  when the signal quality of the transmissions received by AP 2   102  from the MS  104  is greater than the signal quality of the transmissions received by AP 1   100  from the MS  104 , and the signal quality of the transmissions received by the MS  104  from AP 2   102  is greater than a threshold in accordance with the alternative embodiment of the present invention; in yet a further alternative, AP 1   100  will instruct AP 2   102  to initiate a handoff with the MS  104  when the signal quality of the transmissions received by AP 2   102  from the MS is greater than the signal quality of the transmissions received by AP 1   100  from the MS  104 , and the signal quality of the transmissions received by the MS  104  from AP 2   102  is greater than the signal quality of the transmissions received by the MS  104  from AP 1   100  in accordance with the alternative embodiment of the present invention. It will be appreciated by those individuals skilled in the art that other combinations of signal quality may be used to determine whether AP 1   100  will instruct AP 2   102  to initiate a handoff with the MS  104 , other than those listed above, and still remain within the spirit and scope of the present invention. 
     It should be noted that, in accordance with the present invention, the devices apply hysteresis as known to those individuals skilled in the art, particularly when computing signal quality values and/or comparing signal quality values to each other and/or a threshold. Further, it should be noted that the present invention is backwards compatible in that APs implementing the present invention can be mixed with, for example, standard 802.11 APs within the same system. For example, if an enhanced AP (i.e., an AP implementing the present invention) attempts to signal a standard AP (i.e., an AP not implementing the present invention) that a handoff is possible, the standard AP will ignore the transmission since it is unrecognizable. The lack of a response will indicate to the enhanced AP that the particular AP does not support this feature, and thus, will not transmit any subsequent “handoff advertisements” to this particular AP. This does not, however, preclude the completion of the handoff. For example, if the signal quality at AP 2   102  exceeds a certain threshold (or other metric that indicates that a handover should occur), then AP 2   102  may determine it does not need to receive a handoff trigger from AP 1   100 , and may proceed with the “spoofed” disassociation message to the MS  104 . 
     While the invention has been described in conjunction with specific embodiments thereof, additional advantages and modifications will readily occur to those skilled in the art. The invention, in its broader aspects, is therefore not limited to the specific details, representative apparatus, and illustrative examples shown and described. Various alterations, modifications and variations will be apparent to those skilled in the art in light of the foregoing description. Thus, it should be understood that the invention is not limited by the foregoing description, but embraces all such alterations, modifications and variations in accordance with the spirit and scope of the appended claims.