Abstract:
A method for accelerating discovery of access points (APs) in a wireless local area network begins by transmitting an advertisement packet by an AP via at least one channel other than a channel on which the AP transmits beacon signals. The advertisement packet is received by a station (STA), which then determines whether to associate to the transmitting AP. The advertisement packet can include reassociation information, such that the STA can associate to the AP based on the information in the advertisement packet, or timing information, such that the STA can tune to the AP and receive the AP&#39;s beacon without dwelling on the AP for an extended period of time.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]     This application claims the benefit of U.S. Provisional Application No. 60/608,766, filed Sep. 10, 2004, which is incorporated by reference as if fully set forth herein. 
     
    
     FIELD OF INVENTION  
       [0002]     The present invention generally relates to wireless local area networks (WLANs), and more particularly, to a method for accelerating discovery of access points (APs) by a station (STA) in passive scanning mode.  
       BACKGROUND  
       [0003]     In an AP-based wireless local area network, a STA communicates with a specific AP situated in the vicinity of the STA; the STA is said to be associated to this AP. It is sometimes necessary or desirable for a STA to change the AP to which it is associated. For example, the STA may be moving out of the area covered by the AP to which it is originally associated (i.e., roaming) and experiencing poor signal conditions. Another situation is when throughput becomes poor due to congestion arising in the basic service set (BSS) served by the original AP.  
         [0004]     In order for a STA to reassociate to a new AP, it must first discover the APs in its vicinity that are potentially available. One method for performing this search is the passive scanning mode. In passive scanning, the STA listens for beacon packets sent by neighboring APs, which are typically sent approximately every 100 ms. The STA listens on each transmission channel, and changes channels at a set interval. Once the STA has listened to a beacon and decoded the information contained in the beacon, it may initiate a procedure for reassociating to the neighboring AP.  
         [0005]     The passive scanning mode suffers from a drawback, because the neighboring APs may be operating on different channels. Thus, to listen to the beacon, the STA has to tune to a different channel than the one it is currently operating on with its associated AP. Since the STA generally does not know about the timing of beacon transmissions from other APs, it may need to dwell on each channel for a relatively long time (&gt;100 ms) to ensure that it has received the beacons from all neighboring APs. While it is listening on other channels, the STA cannot receive packets from its associated AP and its communications are thus disrupted. Such disruption may adversely and significantly impact the quality of service perceived by the user.  
       SUMMARY  
       [0006]     A method for accelerating discovery of access points (APs) in a wireless local area network begins by transmitting an advertisement packet by an AP via at least one channel other than a channel on which the AP transmits beacon signals. The advertisement packet is received by a station (STA), which then determines whether to associate to the transmitting AP. The advertisement packet can include reassociation information, such that the STA can associate to the AP based on the information in the advertisement packet, or timing information, such that the STA can tune to the AP and receive the AP&#39;s beacon without dwelling on the AP for an extended period of time.  
         [0007]     In a wireless local area network including an AP having a transmitter and a receiver and at least one STA having a receiver and a transmitter, a system for accelerating discovery of APs includes an advertisement packet and determining means at each STA. The advertisement packet is transmitted by an AP on at least one channel other than a channel on which the AP transmits beacon signals and is received by at least one STA. The determining means is used to determine whether to associate a STA to the transmitting AP based on the received advertisement packet.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]     A more detailed understanding of the invention may be had from the following description of a preferred embodiment, given by way of example, and to be understood in conjunction with the accompanying drawings, wherein:  
         [0009]      FIG. 1  is a flowchart of a method for AP discovery in accordance with the present invention; and  
         [0010]      FIG. 2  is a diagram showing transmission of advertisement packets according to the method shown in  FIG. 1 . 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0011]     Hereafter, the term “station” (STA) includes, but is not limited to, a wireless transmit/receive unit, a user equipment, a fixed or mobile subscriber unit, a pager, or any other type of device capable of operating in a wireless environment. When referred to hereafter, the term “access point” (AP) includes, but is not limited to, a base station, a Node B, a site controller, or any other type of interfacing device in a wireless environment.  
         [0012]     The present invention solves the problem described above in the following manner. An AP may, under certain conditions, transmit additional packets (called “advertisement packets”) on one or more frequency channel(s) different from the one it is currently operating on. This functionality allows STAs operating on these different channels to learn about the existence of the AP without having to tune their receivers to other channels, thereby avoiding communication disruption.  
         [0013]     A method  100  for discovering APs in a WLAN in accordance with the present invention is shown in  FIG. 1 . The method  100  begins by determining whether the AP has met a predetermined condition for transmitting an advertisement packet (step  102 ). An AP may decide to advertise itself on other channels only if certain conditions are satisfied. These conditions include, for example:  
         [0014]     1) Sufficiently low load conditions in the BSS served by the AP, so that the AP can tune to the other frequency channels to transmit its advertisement packets without excessive disruption. There is no need for an AP to advertise itself if it cannot accommodate new STAs. The AP can avoid having its associated STAs transmit information to it during the time it is transmitting the advertisement packet by, for example, sending Request to Send (RTS) or Clear to Send packets (CTS) to itself or calling a contention-free period.  
         [0015]     2) Indications received from neighboring AP(s) (via inter-AP signaling) that congestion exists in their BSSs and that there is a need to off-load traffic to another channel.  
         [0016]     3) The AP could discover by itself that congestion exists in the other channels by periodically tuning to the other channels for short periods of time and listening to the traffic on each channel.  
         [0017]     4.) The AP “knows” if there are STAs in the extended service set (ESS) that require fast roaming (for example, if the STAs use delay-sensitive applications such as voice). The AP could learn this information via inter-AP signaling. If there are several delay-sensitive STAs, this would encourage the AP to send advertisement packets.  
         [0018]     It should be understood by those of skill in the art that the above conditions are merely exemplary, and that one skilled in the art can envision other conditions that an AP would need to meet prior to sending an advertisement packet.  
         [0019]     If the AP does not meet one of the conditions for transmitting an advertisement packet, the method  100  terminates (step  104 ). If the AP meets one of the conditions for transmitting an advertisement packet, then the AP transmits an advertisement packet (step  106 ). An advertisement packet contains the following information:  
         [0020]     1) The frequency channel where the AP can be found; and  
         [0021]     2) Either:  
         [0022]     a) All the information required for the STA to initiate a reassociation procedure to the AP (e.g., address of the AP, AP capabilities, etc.), a part of the information contained in the beacon, or the entire beacon; or  
         [0023]     b) The timing information for the transmission of the beacon packet by the AP on its operating channel, with respect to the time of transmission of the advertisement packet.  
         [0024]     The STA receives the advertisement packet and determines the contents of the advertisement packet (step  108 ). If the packet contains reassociation information, then if the STA desires to reassociate, it can tune to the channel where the AP is operating and immediately initiate an association procedure to the new AP (step  110 ). The method  100  then terminates (step  104 ).  
         [0025]     If the packet contains timing information (step  108 ), then if the STA desires to reassociate, it can tune to the channel where the AP is operating just before the beacon transmission time by this AP (step  112 ) and listen to the beacon, thus avoiding a long dwell time on the new channel (step  114 ). The STA then separately decides whether to reassociate to the new AP (step  116 ) and the method terminates (step  104 ).  
         [0026]      FIG. 2  shows an example of packet transmissions with three APs (AP 1 , AP 2 , and AP 3 ) operating on three channels (A, B, and C), respectively. In this example, AP 1  (which is under-loaded) transmits advertisement packets on channels B and C.  
         [0027]     An AP should typically transmit no more than one advertisement packet for each channel per beacon period. Within this period, the timing of transmission of the advertisement packet may be selected randomly. Alternatively, the AP may target a certain time offset with respect to the transmission of its own beacon. Prior to the transmission of the advertisement packet on any other channel, the AP should obey the normal medium access rules like any other STA would (i.e., waiting for a certain amount of time after the medium becomes idle on the selected channel before transmitting).  
         [0028]     The method  100  of the present invention provides following advantages.  
         [0029]     1) Reducing the time STAs utilizing the passive scanning mode spend listening to other frequency channels. The method  100  also reduces the disruption for STAs utilizing the active scanning mode, since there is no need to send probe requests on the other channels.  
         [0030]     2) Compared to an alternative approach where APs transmit beacon timing information of neighboring APs, the method  100  does not require synchronization between APs. In addition, a STA listening to an advertisement packet from a neighboring AP is also assured of being capable of hearing that AP&#39;s beacon. In the alternative approach, the STA could be far away from the neighboring AP whose beacon timing information is provided by the serving AP. The STA can also readily assess, based on the signal strength of the advertisement packet, if the neighboring AP would be suitable for transmitting data at a rate equal to or higher than its serving AP.  
         [0031]     3) The method  100  can be used as an effective mechanism for load balancing between APs. The STAs do not have to make a roaming decision before discovering the other APs that are available and accessible to them on other channels, so the STAs could more readily reassociate and balance the load more efficiently. In case a centralized architecture is adopted, the access controller can manage which APs will send advertisement packets based on load information.  
         [0032]     Although the features and elements of the present invention are described in the preferred embodiments in particular combinations, each feature or element can be used alone (without the other features and elements of the preferred embodiments) or in various combinations with or without other features and elements of the present invention. While specific embodiments of the present invention have been shown and described, many modifications and variations could be made by one skilled in the art without departing from the scope of the invention. The above description serves to illustrate and not limit the particular invention in any way.