Abstract:
A method and an apparatus select an access point (AP) in a wireless LAN to associate or reassociate, based on considerations that take into account the quality-of-service (QoS) status of the stations (STAs) and the potential hidden terminal effect. The method utilizes advertised or requested information obtained from an AP which includes the QoS status in each basic service set (BSS) and estimates the potential hidden terminal (HT) effect based on local channel sensing by the STA. The method selects the AP in a manner that reduces the possibility of collision from equal and higher priority HTs, thus providing greater transmission throughput and improving performance.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present patent application is related to and claims priority of U.S. Provisional Patent Application (“Provisional Application”), entitled “Method and Apparatuses for Access Point Selection in Wireless LAN,” by D. Lei et al., Ser. No. 60/862,873, filed on Oct. 25, 2006. The Provisional patent application is hereby incorporated by reference in its entirety. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention is related to mobile computing. In particular, the present invention is related to methods for providing an efficient infrastructure for mobile computing. 
         [0004]    2. Discussion of the Related Art 
         [0005]    A typical prior art wireless local area network (WLAN) is configured to provide a number of access points (APs) interconnected by and to an ethernet backbone. Each AP provides network access over a service area that is referred to as a “basic service set” (ESS) in the IEEE standard IEEE 802.11 1 . The service areas of the APs in a typical WLAN are preferably overlapping to allow a seamless coverage and to provide capacity enhancement where the APs overlap. In a WLAN, a mobile user or station (STA) scans frequently for candidate channels to identify APs for association. Since the media access control (MAC) throughput performance under the IEEE 802.11 depends significantly on the BSS load (e.g. the number of associated stations), a strategy for selecting an AP in an effective manner is important.  1  IEEE Std. 802.11, “Wireless LAN Media Access Control (MAC) and Physical Layer (PHY) Specifications,” 1999. 
         [0006]    In the meantime, as the use of multimedia services (e.g., voice, video, and web browsing) in a heterogeneous environment continues to proliferate, WLANs need to provide higher transmission rates and at lower costs. As each type of service has a different specific quality-of-service (QoS) requirement, WLAN standards now provide service differentiation by assigning services to different access categories and according to user priorities (see, e.g., service categories under the IEEE standard IEEE 802.11e 2 ). Because different services may co-exist in a WLAN, an effective AP selection method should take into consideration the service type and access priority.  2  IEEE Computer Society, “Part 11: Wireless LAN Medium Access Control (MAC) and physical Layer (PHY) specifications: Medium Access Control (MAC) enhancements for Quality of Service (QoS)”, IEEE Std. 802.11e/D1.4, 2001 
         [0007]    Various studies have been made to improve AP selection. The most common and simplest approach is based on received signal strength (RSS) measurements. An STA, upon entering the service area of a WLAN or upon detecting degradation in its existing link, scans each candidate channel and estimates the RSS of the received frames from the available APs. The goal of selecting the AP with the strongest RSS is to access a higher quality channel and at a higher data rate. This method results in a larger admission capacity in a WLAN. While an RSS-based selection method appears to provide a maximum data transmission rate to each STA, such a method ignores the contention-based nature in WLAN access. When STAs gather around one AP, an RSS-based method tends to cause traffic aggregation in some BSSs and severe frame collisions, thereby resulting in throughput degradation. 
         [0008]    AP selection methods have also been studied as a possible efficient way to achieve load balancing among different BSSs. For example, in the article “Performance of an efficient method for association admission control in public wireless LAN,” by Hyun-woo Lee, Se-han Kim and Won Ryu, and published in the Vehicular Technology Conference (VTC), Fall, 2004, the authors suggest taking BSS load levels of the stations, and their traffic, into account in an AP selection method. Under that method, an AP inserts into one or more reserved fields of beacon or probe response frames values representing their BSS load or traffic, so as to allow the STA to select an AP with a light load and to avoid traffic congestion. Similarly, in the article “A Novel Load Sensitive Algorithm for AP selection in 4G Networks” by S. Misra and A. Banerjee, published in the International Conference of Computer and Devices for Communication (CODEC), Calcutta, India, 2003, the authors propose an association algorithm which further considers an STA&#39;s movement, in addition to the load level. The STA&#39;s direction of movement allows an AP with a light load along that direction to be selected. While these load-balancing methods may alleviate traffic imbalance between adjacent BSSs to some extent, neither the number of STAs nor the amount of traffic is an accurate measure of the load level. 
         [0009]    The Chinese patent application “Method and Apparatus to select AP in WLAN,” serial no., 200610110741.0, filed on Aug. 8, 2007, discloses a method for AP selection which enhances total performance by taking into account hidden terminals that may cause much severe throughput degradation. Under that method, the STA avoids collisions with hidden terminals by anticipating their effects in their respective BSSs. 
         [0010]    Other proposals present more precise load estimates to allow AP selection. For example, the article “Facilitating access point selection in IEEE 802.11 wireless networks,” by S. Vasudevan, K. Papagiannaki, C. Diot, J. Kurose, and D. Towsley, published in ACM Internet Measurement Conf., New Orleans, La., October 2005, describes a method for an STA to estimate a potential bandwidth based on delays experienced by beacon frames received from an AP. In this method, the same access priority is assumed between beacon and other data frames, so that a channel status may be predicted from an offset between the timestamp of a beacon and the target beacon transmission time (TBTT). However, using beacon delay for bandwidth estimation assumes that the beacon frames experience the same contention as other frames. In a WLAN, where the beacon frames are typically sent prior to data frames, such an assumption is typically not valid. 
         [0011]    In addition, the article “Link Quality based Association Mechanism in IEEE 802.11h compliant Wireless LANs,” by T. Korakis et al, presented in the Workshop on Resource Allocation in Wireless Networks (RAWNET), April 2005, proposes considering the physical layer channel condition (e.g., bi-directional link rate or other channel quality metrics on the uplink and downlink) in selecting the AP to associate. However, these quality metrics (e.g., the bidirectional link rates) are available only for point coordination function (PCF) operation. Since PCF is not enabled in most WLAN products, the Korakis method has limited applications. 
       SUMMARY OF THE INVENTION 
       [0012]    According to one embodiment of the present invention, a method for selecting an AP in a WLAN having overlapping basic service sets (BSSs) improves both the total WLAN throughput and load balancing among adjacent BSSs. The method uses AP selection to provide service differentiation and to further enhance the performance of high priority services. In a BSS which includes multiple types of services, different access rules govern different access categories. For example, a “best effort” service may experience a longer delay before being allowed to access the channel than the delay experienced by a voice service, since the voice service is more sensitive to the delay characteristics. Therefore, when an STA that requires voice service selects an AP to access a WLAN, among APs with the same loads in their BSSs, the STA selects the AP having the least number of STAs that use services of equal or higher priority than the voice service. In this manner, because the STA using the higher priority service preempts access opportunities of those STAs using lower priority services, the performance and throughput of the higher priority service is improved. 
         [0013]    According to another embodiment of the present invention, a method selects an AP according to current service type and distribution in BSSs. In this method, APs notify STAs of its load status, expressed in either channel utilization or STA count, for each kind of service the AP supports. A STA, upon receiving the information, scans the channel to estimate the potential hidden terminal effect arising from the STAs with an equal or higher access priority than itself. By selecting the AP with the smallest potential hidden terminal effect, the STA is least likely to be preempted an opportunity to transmit by other STAs that use high priority services, while mitigating frame collisions with the hidden terminals. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]      FIG. 1  illustrates a WLAN to which a method of AP selection according to the present invention is applicable. 
           [0015]      FIG. 2  shows a Quality-of-service Basic Service Set (QBSS) load element in a beacon, response frame or a management/Action frame, as specified under the IEEE standard, IEEE 802.11e. 
           [0016]      FIG. 3(   a ) shows proposed quality-of-service BSS (QBSS) load subelements that may be used in a beacon, a probe response or a management/action frame transmitted by an AP, in accordance with one embodiment of the present invention. 
           [0017]      FIG. 3(   b ) shows proposed load elements that may be used in a beacon, probe response or a management/action frame transmitted by an AP, in accordance with one embodiment of the present invention. 
           [0018]      FIG. 4  shows an alternative proposed QBSS load sub-element format that can be provided without lengthening the beacon or management frame, in accordance with one embodiment of the present invention. 
           [0019]      FIG. 5  ( a ) is an operation flow chart of an AP selection method, according to one embodiment of the present invention. 
           [0020]      FIG. 5  ( b ) is an operation flow chart of an AP selection method, according to another embodiment of the present invention. 
           [0021]      FIG. 6  shows implementation of the method of  FIG. 5(   a ), according to one embodiment of the present invention. 
           [0022]      FIG. 7  shows implementation of the method of  FIG. 5(   b ), according to another embodiment of the present invention. 
           [0023]      FIG. 8(   a ) shows a BSS load sub-element proposed to be included in the beacon, probe response or the management/action frame under any embodiment of  FIGS. 6 ,  7  and  9 . 
           [0024]      FIG. 8(   b ) shows another BSS load sub-element proposed to be included in the beacon, probe response or management/action frames under any embodiment of  FIGS. 6 ,  7  and  9 . 
           [0025]      FIG. 9  shows implementation of the method of  FIG. 5(   a ), according to yet another embodiment of the present invention. 
           [0026]      FIG. 10  shows a BSS load element proposed to be included in the beacon, probe response or management/action frames under any embodiment of  FIGS. 6 ,  7  and  9 . 
           [0027]      FIG. 11  shows components within an STA to support an AP selection method according to one embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0028]    The present invention provides a QoS-aware AP selection method in a WLAN. In one embodiment, the STA selects an AP which provides the least hidden terminal effect from other STAs having equal or higher priorities relative to the STA&#39;s own access category (AC). 
         [0029]      FIG. 1  illustrates a WLAN  100  to which a method of AP selection according to the present invention is applicable. As shown in  FIG. 1 , WLAN  100  includes at least access points, AP 1  and AP 2 , which provide service on different channels without co-channel interference. Coverage areas for access points AP 1  and AP 2  are overlapping, as indicated by coverage areas  101  and  102  (in solid lines). Mobile terminals MT 1 , MT 2  and MT 3 , each using different qualities of services, access WLAN  100 . For example, mobile terminal MT 1  uses a voice-over-IP (VoIP) service, and is shown associated with access point AP 1 . Mobile terminal MT 2 , for example, uses a “best effort (BE) service and is shown associated with access point AP 2 . Mobile terminal MT 3 , for example, may be another VoIP user entering overlapping area  103  between coverage areas  101  and  102 . In overlapping area  103 , mobile terminal MT 3  detects signals from both access points AP 1  and AP 2 . Mobile terminal MT 3  may therefore select one of the two access points AP 1  and AP 2  to associate, according to one embodiment of the present invention. 
         [0030]      FIG. 2  shows a Quality-of-service Basic Service Set (QBSS) load element in a beacon or response frame, as specified under the IEEE standard, IEEE 802.11e. As shown in  FIG. 2 , (i) a “station count” field indicates the number of stations currently associated with this QBSS, (ii) a “channel utilization” field indicates the percentage of time, normalized to 255, a QOS AP (QAP) senses the medium to be busy, as measured by either a physical or a virtual carrier sense mechanism. 
         [0031]      FIG. 5(   a ) is an operation flow chart  500  of an AP selection method, according to one embodiment of the present invention. As shown in  FIG. 5  ( a ), when a STA enters a WLAN or detects a link degradation in the WLAN, it requests or acquires BSS load information from a candidate AP in each channel (step  503 ), for ACs equal to or higher than its own AC. In  FIG. 5(   a ), the channels allocated to candidate APs are indexed by index k=1, . . . , N and the ACs are indexed by index i=1, . . . , m. In one embodiment, m=4, corresponding respectively to access categories defined in IEEE 802.11e: voice (VO), video (VI), best effort (BE) and background (BK), indexed in decreasing priority order (i.e., the lower priority services are assigned the larger numbers). The STA&#39;s own AC index is denoted by M For example, when M has the value 1, the STA is initiating a VoIP service. The BSS load information for the k-th channel for priority service i is denoted u ki , which may be extracted from the value of QBSS station count or channel utilization sub-element corresponding to AC i  in a beacon, a probe-response, or a management/action frame transmitted by the AP in the k-th channel. 
         [0032]    Prior to requesting or acquiring the BSS load information, the method initializes the parameters k and u ki  (step  501 ). The default value for u ki  is −1, indicating that the BSS load information corresponding to AC i  for channel k has not been obtained. Possible quantities to derive u ki  include station count and channel utilization.  FIG. 3(   a ) shows proposed quality-of-service BSS (QBSS) load sub-elements that may be used in a beacon, a probe-response, or a management/action frame transmitted by an AP, in accordance with one embodiment of the present invention. Such QBSS sub-elements may appear in beacon frames, probe response frames or the neighbor report elements included in roaming management frames. Sub-element  3 - 1  is the format for a QBSS station count sub-element, which includes (a) a station count bitmask field which indicates the ACs for which station counts are provided; and (b) a station count list, which lists the station counts. Each bit in the station count bitmask that is set to ‘1’ corresponds to an AC for which a station count is provided. The encoding for the bitmask for m=4 is shown in  3 - 3 . The station count list provides a sequence of station count corresponding respectively to station counts for ACs indicated by the station count bitmask. The station count may be provided, for example, by the number of stations belonging to each corresponding AC currently associated with the QBSS. 
         [0033]    Sub-element  3 - 2  is a format for a QBSS channel utilization sub-element, which includes (a) a channel utilization bitmask field which indicates the ACs for which channel utilization values are provided; and (b) a channel utilization list, which lists the channel utilization values. Channel utilization may be expressed, for example, by the percentage of time (e.g., normalized to 255) the AP senses the medium busy, as a result of channel utilization by STAs belonging to the corresponding AC. Channel utilization may be measured, for example, by either a physical or a virtual carrier sense mechanism. 
         [0034]      FIG. 4  shows an alternative proposed QBSS load sub-element format that can be provided without lengthening the beacon, the probe-response, or the management/action frame, in accordance with one embodiment of the present invention. Such a sub-element is particularly beneficial for a beacon frame, which is desired to have minimal overhead. As shown in  FIG. 4 , sub-element  4 - 1  includes a station count field, which consists of: (a) the station count for AC 1  (i.e., the number of VoIP STAs currently associated with the QBSS); and (b) the number of STAs other than the STAs of AC 1 . This format allows a VoIP STA to select an AP using the station count for AC 1 , while another STA may use instead the total number of STAs in the QBSS (i.e., the sum of the two parts of the station count field). Similarly, sub-element  4 - 2  illustrates the alternate format for expressing channel utilization as a channel utilization for AC 1 , and channel utilization by all other STAs associated with the QBSS. 
         [0035]    Although in  FIGS. 3 and 4  QBSS station count and QBSS channel utilization are each expressed as a “sub-element” of the QBSS load element shown in  FIG. 2 , QBSS station count and QBSS channel utilization may also be used as an independent information element, such as shown in  FIG. 3(   b ). Returning to  FIG. 5(   a ), in this embodiment, the N channels are processed sequentially. While not all channels have been processed (step  502 ), the STA scans for or requests BSS load information (step  503 ) from the k-th channel, specifying its own AC i  or another AC of interest. When the STA successfully obtains the BSS load information (step  504 ), the STA extracts the channel utilization or station count sub-element or elements according to the AC i  (i≦m) from the received frame. The STA then records or updates the values as u ki , as appropriate ( 505 ). Meanwhile, based on the RSS measurement of received frames, the STA estimates the maximum supportable data rate v k  in channel k (step  506 ). 
         [0036]    If the STA cannot obtain the BSS load information, the STA determines whether or not such information has previously been recorded for this channel k (i.e., all u ki  have the value of −1; step  511 ). If no information has been recorded for this channel k, the STA determines if sufficient time has elapsed since the beginning of processing in the channel ( 512 ). If the elapsed time has reached maximum scanning period, T max , STA sets the potential hidden terminal effect f k  for the k th  channel to a very large value (“infinity”), indicating that the channel is currently unavailable ( 513 ). Otherwise, the STA returns to step  503 , to continue checking whether or not the BSS load information is detected in channel k. 
         [0037]    After the STA estimates the supportable data rate v k  (step  506 ), or if the current BSS load information cannot be obtained, but the BSS load information has previously been recorded (i.e., after step  511 ), the STA determines whether or not the scanning period has exceeded the minimum required scanning period T min  (step  507 ). If so, the STA calculates the heard BSS load information r ki  for channel k and for each relevant AC i , i≦m (step  508 ). r ki  refers to the overheard number of STAs corresponding to the relevant AC i , or the relevant channel utilization ratio (i.e. the percentage of time that the STA senses the medium busy due to the STAs corresponding to the relevant AC i , as indicated by either the physical or virtual carrier sense mechanism). The AC type of r ki  may be obtained, for example, by extracting the AC values from the QBSS field of overheard frames. 
         [0038]    In one embodiment, to obtain the heard BSS load information, the mobile terminal senses the channel overhearing the received frames that are not destined to itself, and extracts the identities and the ACs of the stations transmitting the frames. The heard BSS load may be represented by the number of stations with different identities persisting to each AC that are overheard by the mobile terminal. Alternatively, the mobile terminal extracts the ACs of the stations transmitting the frames and accumulates the channel time occupied by these frames in each AC. The heard BSS load may be represented by these channel times. The parameters T min  and T max  controls the duration over which the mobile station scans a channel and accumulates the heard BSS load. Based on activities during the duration (i.e., for at least T min , but at most T max ), the mobile station determines the condition of the channel and whether or not the channel is available. The setting of T min  may take into account such parameters as the traffic load in the currently scanned channel, and the beacon interval. 
         [0039]    After the STA has obtained a set of (u ki , r ki , v k ), i≦m, for all N candidate channels, the STA calculates the potential hidden terminal effect f k  for each channel k, which is given by: 
         [0000]    
       
         
           
             
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         [0000]    (step  509 ), where L DATA  is the average data length the STA is going to transmit. The STA then examines all f k &#39;s and selects the AP with the least f k  to associate or reassociate (step  514 ). 
         [0040]      FIG. 5  ( b ) shows an operation flow chart  520  of an AP selection method, according to another embodiment of the present invention. In contrast with  FIG. 5  ( a ), step  521  replaces steps  506 - 509 . That is, when the STA records or updates the values as u ki , as appropriate (step  505 ), or when the BSS load information has previously been recorded for this channel k (i.e., not all u ki  have the value of −1; step  511 ), the STA calculates the load status that use services of equal or higher priority than the STA&#39;s AC, f k , for each channel k, which is given by: (step  521 ) 
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       The STA then examines all f k &#39;s and selects the AP with the least f k  to associate or reassociate (step  514 ). 
       [0041]      FIGS. 6 ,  7  and  9  illustrate the method of  FIG. 5(   a ) in different embodiments. The method of  FIG. 5(   b ) is substantially the same as that of  FIG. 5(   a ), except that step  506 - 509  in  FIG. 5(   a ) are replaced in  FIG. 5(   b ) by step  521  of  FIG. 5(   b ). In each of these embodiments, a BSS load sub-element or element is proposed to be included in the beacon, the probe-response, or the management frames. FIG.  8 -( a ) illustrates using the formats of  FIG. 3(   a ) in the embodiments of  FIGS. 6 ,  7  and  9 .  FIG. 8(   b ) illustrates using the formats as  FIG. 4  in the embodiments of  FIGS. 6 ,  7  and  9 .  FIG. 10  illustrates using the formats of  FIG. 3(   b ) to the embodiment of  FIGS. 6 ,  7  and  9 . 
         [0042]      FIG. 6  shows application of the method of  FIG. 5(   a ) for a STA operating under a passive scanning mode. The IEEE 802.11 standard defines passive and active scanning modes for an STA to acquire synchronization in a WLAN. As shown in  FIG. 6 , under the passive mode, access points AP 1  and AP 2  periodically broadcast beacon frames containing BSS-specific information at a series of target beacon transmission times (TBTTs), which are separated by a fixed beacon interval. Transmission of the beacon frame may be deferred if the channel is sensed busy at a TBTT, until the channel is sensed idle again. For this embodiment, any one of the BSS load sub-elements or elements proposed to be included in the beacon frames shown in  FIG. 8(   a ),  FIG. 8(   a ) and  FIG. 10  may be used. 
         [0043]    As shown in  FIG. 8(   a ), both the beacon and the probe-response frames may contain a “QBSS load” field, which includes “station count” and “channel utilization” elements, similar to those specified in the IEEE standard, IEEE 802.11e, shown in  FIG. 2 . In  FIG. 8(   a ), however, “station count” values, “channel utilization” values, or both, are given for each AC, with the appropriate format adopted from the formats shown in  FIG. 3 . Depending on the implementation, it may not be necessary to provide both “station count” and “channel utilization” values by ACs; nor, for overhead reasons, is it necessary to include the station count or channel utilization data for all ACs. For example, only higher priority ACs (e.g., VO and VI) need be included in the beacon or probe response frames. The corresponding bits in the bitmask indicate whether the value for any specified AC is included. 
         [0044]    Alternatively, the QBSS load sub-element of  FIG. 8(   b ), which does not lengthen the beacon frame (relative to the existing standard) may be used. The QBSS load element of  FIG. 8(   b ) takes the format explained above, with respect to  FIG. 4 . In this manner, the overhead associated with the beacon frame is not increased. This format is preferable under the passive scanning mode. As shown in  FIG. 8(   b ), a first part in the previous QBSS load element provides the station count or channel utilization for a specified AC (e.g., AC 1 , or VoIP), while the other part provides for same statistic for other ACs. Based on this beacon format, VoIP STAs can be differentiated by calculating the potential hidden terminal effect from VoIP STAs in this QBSS. 
         [0045]    As shown in  FIG. 6 , at time T 1 , a VoIP STA enters the WLAN or detects a link degradation, a method for AP selection is triggered. The STA selects a channel (e.g. CH 1 ) from its list of candidate channels to listen to a beacon frame. At time T 2 , the STA successfully detects a beacon frame in CH 1 , and extracts the QBSS load information (i.e., either the station count or channel utilization sub-element). In this example, the beacon provides values only for higher priority ACs (e.g., u 11  and u 12 , for VO and VI respectively). Meanwhile, the STA measures the signal strength of the received beacon and estimates the maximum supported data rate on this channel as v 1 . 
         [0046]    After receiving the beacon frame at time T 2 , the STA determines whether or not the elapsed scanning time in CH 1  exceeds the minimum scanning time T min . As the elapsed scanning time exceeds the minimum scanning time T min , a heard BSS load r ki  is estimated for CH 1 . As only the sub-elements corresponding to the ACs equal or higher than the STA&#39;s AC are considered, r ki  includes only the number of VoIP STAs or the channel utilization due to VoIP transmission. The same procedure is then repeated in candidate channel CH 2 . 
         [0047]    At time T 3 , the STA successfully detects a beacon frame in channel CH 2 , and thereby obtains a set of BSS load information U 2 , and u 22  and signal strength-based data rate estimate v 2 . As time T 3  is within the minimum required scanning period T min , scanning continues in the channel until T min  is reached (i.e., until time T 4 ). At time T 4 , the heard BSS load r 21 , corresponding to the ACs equal or higher than that of itself, is estimated. 
         [0048]    As the STA has by time T 4  scanned both candidate channels CH 1  and CH 2 , the STA calculates the potential hidden terminal effect f k  for each channel using the information it has gathered for that channel during channel scanning. Since the STA uses the VoIP service, corresponding to the highest AC, only VoIP STAs are considered in estimating the potential hidden terminals effect (i.e. f k =max(0, L DATA /v k ·(u ki −r ki )). In this example, f 2  is less than f 1 , access point AP 2  is selected. The STA hence associates or reassociates with access point AP 2 . 
         [0049]      FIG. 7  illustrates application of the method of  FIG. 5(   a ) to a STA operating under active scanning mode. In active scanning mode, unlike the passive mode, the STA sends out probe messages to the candidate channels. As seen from  FIG. 7 , a STA begins scanning channel CH 1  (one of two candidate channels in its candidate channel list) at time T 1  by sending a “probe request” frame (P_Req) which addresses access point AP 1  of channel CH 1 . Access point AP 1 , upon receiving the P_Req matching its identity at time T 2 , replies with a probe response frame (P_Res) enclosing BSS load information u 11  and u 12 , using a frame format shown in any one of  FIGS. 8(   a ),  8 ( b ) and  10  (described above with respect to the beacon frame of  FIG. 6) . Meanwhile, the maximum supported data rate v k  is estimated from the measured RSS of the P_Res frame. At time T 3 , the STA replies with an acknowledgement (ACK) frame to confirm the successful reception of P_Res. 
         [0050]    At time T 4 , the STA determines that the scanning period has exceeded minimum scanning time T min , a heard BSS load r 11  is estimated, corresponding to ACs equal or higher priority than the STA&#39;s own AC (i.e., VoIP, in this example). The same procedure is then repeated for channel CH 2 . In channel CH 2 , at time T 6 , as the scanning period does not exceed T min , the STA continues the scanning until time T 7 , when a heard BSS load r 21  may be estimated, having scanned for the minimum period T min . In the same manner as described above for  FIG. 6 , the hidden terminal effect f k  is estimated for each channel (i.e., f k =max(0, L DATA /v k  ·(u ki −r ki )), taking into consideration only the VoIP service, in this example). The AP with the lesser f k  is selected, according to the method of  FIG. 5  ( a ). 
         [0051]      FIG. 9  shows implementation of the method of  FIG. 5(   a ), in which action frames are used to convey the BSS load sub-elements or elements used in an AP selection, according to yet another embodiment of the present invention. Under this method, the STA and the AP exchange a series of frames enabling the roaming capability: “roaming management query” frame (R_M_Query), “roaming management request” frame (R_M_Req) and “roaming management response” frame (R_M_Res). When a STA in a WLAN using a VoIP service detects a link degradation, the STA reselects an AP so as to maintain its access to the WLAN. The roaming management frames, defined in IEEE standard, IEEE 802.11v, draft 3 , may be used for obtaining the BSS load information from an AP. As shown in  FIG. 9 , at time T 1 , the STA sends to access point AP 1  of channel CH 1  a “roaming management query” frame to request information. Access point AP 1 , upon receiving the STA&#39;s R_M_Query, replies at time T 2  by a R_M_Req frame. In this R_M_Req frame is included a BSS load element, such as shown in  FIG. 10 .  FIG. 10  shows a roaming management request frame that, other than including the proposed BSS load element, conforms substantially with the format specified in the IEEE 802.11v draft for this action frame, which supports a roaming function. The roaming management request frame includes a “roaming candidate list entries” field, which contains one of more “neighbor report elements.” The neighbor report element includes the proposed BSS load sub-element to support an AP selection method according to the present invention.  3  P802.11v-D0.03 
         [0052]    The BSS load element in the neighbor report element may take a format, for example, described in conjunction with  FIG. 3  above. Upon receiving the roaming management request frame, the STA extracts, for example, the BSS load information u 11  and u 12 . At time T 3 , the STA sends a R_M_Res frame to acknowledge receipt of the R_M_Req frame. The frame sequence is similar to the probe-response-acknowledgement sequence described above with respect to  FIG. 7 . Steps  506 - 510  of  FIG. 5(   a ) are followed substantially in the same manner discussed with respect to  FIGS. 6-7  above. The same procedure is followed to collect BSS information in channel CH 2  from access point AP 2 . Access point selection based on selecting the access point at which the hidden terminal effect is least is then followed, in substantially the same manner as described above with respect to  FIGS. 6-7 . 
         [0053]      FIG. 11  shows components within STA  1100  provided to support an AP selection method according to one embodiment of the present invention. As shown in  FIG. 11 , judgment or determination means  1101  comprises primarily a state machine to implement the steps of the method  FIG. 5(   a ) or  FIG. 5(   b ). BSS load request means  1102  perform channel scanning, in any of the passive or active scanning modes, and action frames to obtain the required BSS load sub-elements or elements (See, e.g., the implementations of  FIGS. 6 ,  7 ,  9 ), and also to obtain the heard BSS load based on the traffic in each monitored channel. Calculation and storage means  1103  records the required information for AP selection (e.g., u ki , v k , r ki ) and calculates the corresponding f k &#39;s (i.e. 
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         [0000]    AP selection means  1104  select an AP based on the f k  for all the candidate i=1 channels, according to methods discussed above. Transmitting and receiving means  1105  transmit and receive frames from the radio interface. 
         [0054]    The detailed description above is provided to illustrate the specific embodiments of the present invention and is not intended to be limiting. Numerous variations and modifications within the scope of the present invention are possible. The present invention is set forth in the following claims.