WLAN having load balancing based on access point loading

[0001] The present invention claims the benefit of U.S. Provisional Patent Application No. 60/332,957, filed on Nov. 19, 2001, which is incorporated herein by reference.

[0002] Not Applicable.

[0003] The present invention relates generally to communication networks and, more particularly, to wireless communication networks.

[0004] Wireless communication networks for providing links to mobile stations are well known in the art. In one type of wireless network, a series of access points provide wireless connections to various mobile users. For example, a building can include access points located at strategic locations to serve mobile users as they move throughout the building. The mobile users migrate from access point to access point based upon the strength of beacon signals from the various access points. That is, the mobile stations use the strength of the beacon signals to select the best access point at a given point in time.

[0005] With changes in the channel environment and number of users in a Wireless Local Area Network (WLAN) system, different access points experience different traffic loading. That is, the number of users served by each of the access points varies over time. Those access points that serve a relatively high number of stations (hot spots) can become overloaded and experience reduced performance. For example, an access point can become overloaded during a meeting in a conference room proximate the access point when the attendees attempt to connect their laptops to the corporate intranet.

[0006] It would, therefore, be desirable to adjust the loading of network access points to reduce network congestion.

[0007] The present invention provides a wireless network having mobile stations that determine to which access point they will associate based upon received beacon signal power levels and access point load information sent to the mobile station. With this arrangement, overall network performance is enhanced by more efficient access point loading. While the invention is primarily shown and described in conjunction with wireless access points having beacons, it is understood that the invention is applicable to wireless networks in general in which it is desirable to distribute loading.

[0008] In one aspect of the invention, network wireless access points transmit beacon signals to mobile stations within their coverage area and the mobile stations determine the strength of each received beacon signal. Based upon the beacon signal strength, a particular mobile station can determine which access points the mobile station can associate with. The access points also broadcast, such as in a Beacon frame, loading level information for the access point. The mobile station then selects an access point based upon beacon signal strength and access point loading. By taking into consideration access point loading levels, network congestion due to overloaded access points can be reduced.

[0014] FIG. 1 shows an exemplary wireless network 100 having access point load balancing in accordance with the present invention. The network 100 includes a series of access points AP_a-N having associated coverage areas CA_a-P serving the various mobile stations MSa-M. Access point load balancing is achieved by each of the mobile stations MS_a-M determining to which one of the access points AP_a-N the mobile station should associate with based upon access point beacon signal power levels and access point loading information, as described more fully below.

[0015] In general, each mobile station MS receives a beacon signal from access points Ap_a-N having a coverage area CA_a-P in which the mobile station is located. The access points AP also broadcast, such as in a so-called Beacon Frame, loading information for the access point, as described further below. Mobile stations Ms_a-M that receive multiple access point beacon signals above a predetermined threshold level, for example, can select one of these access points based upon the load level of the access points to optimize overall access point loading. By distributing mobile station associations based on access point loading, network congestion is reduced and overall network performance is enhanced.

[0016] For example, as illustrated in FIG. 1, a first mobile station MS_c may be located within the coverage areas CA_a,b of associated first and second access points AP_a,b. The first mobile station MS_c receives a beacon signal of acceptable power level, e.g., above a predetermined threshold, from the first and second access points AP_a,b. In an exemplary embodiment, the first and second access points AP_a,b also broadcast their loading levels in a Beacon Frame that is received by the first mobile station MS_c. Since the first access point AP_a supports significantly more mobile stations, e.g., MS_d-i, than the second access point AP_b (supports MS_a,b), the first mobile station MS_c associates itself with the second access point AP_b.

[0017] It is understood that the parameters used to determine which access point a particular mobile station should associate with can vary. Exemplary parameters include loading of the access point (number of associated stations), total traffic intensity through the access point, reports from individual stations, the measurement of the received signal power from a subset of stations, mobile station priority weighting, access point priorities, and overall network loading.

[0018] It is further understood that the term “mobile station,” as used herein, should be construed broadly to include various wireless devices, such as laptops, Personal Digital Assistants (PDAs), mobile phones, and the like. Similarly, the term “access point” should be broadly construed to include transmitters/receivers in general that can provide a radio link with a mobile station.

[0019] Before describing further details of the present invention, some basic concepts are now described. In conventional wireless networks having mobile stations served by various access points, such as in a 802.11 network, there is a standard procedure by which mobile stations associate themselves with an access point. The IEEE 802.11 standard is defined in International Standard ISO/IEC 8802-111, “Information Technology—Telecommunications and Information Exchange Area Networks,” 1999 Edition, which is hereby incorporated by reference in its entirety. Before a mobile station associates with an access point, it obtains information of nearby access points by scanning the frequency channels for their beacons. The access points typically send out beacon frames periodically.

[0020] In traditional WLANs, such as 802.11 networks, beacon powers of access points are kept at a fixed level. The mobile station simply chooses the access point with the best (highest) signal strength for association. However, it is possible that one access point may be already overloaded, although it has the strongest signal strength to the mobile station.

[0021] While the following descriptions are applicable to 802.11 WLANs, it is understood that the invention applies to wireless networks in general using similar formats and mechanisms. Table 1 below shows the beacon frame body of a management frame of subtype Beacon with each of the listed components specified in the 802.11 standard.

[0022]

TABLE 1
Beacon frame body
Order	Information	Notes
1	Timestamp	Time/day/etc information
2	Beacon interval	Time between beacon intervals
3	Capability information	Resource parameters, polling parameters,
		etc.
4	SSID	Service Set ID (Wireless Network Name)
5	Supported rates	Data Rates supported by the system/device
6	FH Parameter Set	The FH Parameter Set information
		element is present within Beacon frames
		generated by STAs (Mobile STAtions)
		using frequency-hopping (FH) PHYs
		(PHYsical layer modulations)
7	DS Parameter Set	The DS Parameter Set information
		element is present within Beacon frames
		generated by STAs using direct sequence
		(DS) PHYs
8	CF Parameter Set	The CF (Contention Free) Parameter Set
		information element is only present within
		Beacon frames generated by APs (Access
		Points) supporting a PCF (Point
		Coordination Function)
9	IBSS Parameter Set	The IBSS (Independent Basic Service Set)
		Parameter Set information element is only
		present within Beacon frames generated
		by STAs in an IBSS
10	TIM	The TIM (Traffic Information Map)
		information element is only present within
		Beacon frames generated by APs

[0023] Typically, the mobile station can operate in either Passive Scanning mode or Active Scanning mode. In Passive Scanning mode, the mobile station listens to each channel scanned for no longer than a maximum duration defined by the ChannelTime parameter. The Active Scanning mode involves the generation of a Probe Request frame by the mobile stations, which is shown in Table 2 below, and the subsequent processing of a received Probe Response frame, which is shown in Table 3, by the access point.

[0024]

TABLE 2
Probe Request frame body
	Order	Information
	1	SSID
	2	Supported rates

[0025]

TABLE 3
Probe Response frame body
Order	Information	Notes
1	Timestamp	Time/day/etc. information
2	Beacon interval	Time between beacon intervals
3	Capability information	Resource parameters, polling parameters,
		etc.
4	SSID	Service Set ID (Wireless Network Name)
5	Supported rates	Data Rates supported by the system/device
6	FH Parameter Set	The FH Parameter Set information
		element is present within Beacon frames
		generated by STAs using frequency-
		hopping PHYs
7	DS Parameter Set	The DS Parameter Set information
		element is present within Beacon frames
		generated by STAs using direct sequency
		PHYs
8	CF Parameter Set	The CF Parameter Set information element
		is only present within Beacon frames
		generated by APs supporting a PCF
9	IBSS Parameter Set	The IBSS Parameter Set information
		element is only present within Beacon
		frames generated by STAs in an IBSS

[0026] Conventionally, after scanning the access point beacons, the mobile station adopts the BSSID (Basic Service Set ID: Access Point ID) and channel synchronization information in a Beacon (passive) or Probe Response (active) coming from the access point with the best signal strength. An Association/Reassociation Request is then issued by the mobile station as it attempts to associate with the selected access point. The access point then responds with an Association Response. The corresponding Association Request and Association Response frame formats are shown below in Table 4 and Table 5, respectively. It is understood that further actions, such as authentication, take place before or after the association phase.

[0027]

TABLE 4
Association/Reassociation Request frame body
Order	Information
1	Capability information
2	Listen interval
3	SSID
4	Supported rates

[0028]

TABLE 5
Association Response frame body
Order	Information
1	Capability information
2	Status code
3	Association ID (AID)
4	Supported rates

[0029] In accordance with the present invention, the mobile station selects an access point based upon beacon signal strength and access point loading levels.

[0030] FIG. 2 shows an exemplary access point 200 having a mobile station memory database 202 and a processor 204 for controlling the overall operation of the access point. The database 202 can contain various loading information including loading level, which can be provided as the number of currently associated mobile stations 202a, the link quality 202b, and the like. Further such information, such as mobile station priority, the number of access points that a given mobile station can “hear”, etc., will be readily apparent to one of ordinary skill in the art.

[0031] The access point 200 includes a module 206 for assessing the medium load condition. More particularly, the module can determine the number of mobile stations associated with the access point, their bandwidth usage, transmission queue length, number of error packets compared to the total traffic, and the like. This information can be used to determine a loading level for the access point 200, which can be used in access point selection, as described further below.

[0032] The access point 200 can also include conventional components, such as a wireless interface 250 having one or more RF transceivers, a network interface 252 for interacting with a wired network, and an I/O interface 254 for communicating with various components, such as peripheral equipment.

[0033] In an exemplary embodiment, frequency option information can be conveyed in the Association/Reassociation request frame, which is shown above in Tables 45. These two management frames contain the same Capability Information field, which is used to indicate requested or advertised capabilities. In an illustrative embodiment, the length of the Capability Information field is two octets.

[0034] FIG. 3 shows an exemplary Capability Information field 300 having sub fields of ESS 300a, IBSS 300b, CFPollable 302c, CFPoll Request 302d, and Privacy 302e, together using five of the sixteen total bits. In an exemplary embodiment, three bits form a further subfield APLI 300f indicating the loading level of the access point. It is understood that the loading level can be presented in a variety of formats including raw number of associated mobile stations, discrete values indicative of loading level, percent of rated capacity, and the like. The remaining bits 300g of the Capability Information field 300 are reserved.

[0035] FIG. 4 shows an exemplary mobile station 400 having load-based access point selection in accordance with the present invention. The mobile station 400 includes a processor 402 for controlling the overall device operation and a memory 404 for storing various information, such as configuration parameters 404a, access point IDs 404b, access point beacon signal power levels 404c, and access point loading level information 404d. The mobile station 400 can further include a conventional wireless interface 450 for transmitting and receiving radio frequency signals, and an I/O interface 452.

[0036] The mobile station 400 further includes an association module 406 for selecting an access point for association based upon the detected access point beacon signal power levels and access point loading. It is understood that the association module can select access points using a variety of functions using a range of parameters.

[0037] For example, assume that the beacons of access points AP₁, AP₂, . . . AP_N can be received by a given mobile station. For each access point AP, through its beacon signal, the mobile station can obtain various parameters, such as signal strength (RSSI), number of associated stations, traffic intensity and other related parameters. These parameters can be referred to as the signature vector V₁ for the access point AP_i. A cost function f(V₁) can be defined to calculate the cost if the mobile station associates with a given access point AP_i. Once the cost function is defined as desired, the association module of the mobile station chooses the access point AP with the minimal cost of association.

[0038] In one particular embodiment, the association module uses the number of stations associated with the AP_s as the sole parameter of the cost function. Let n₁, n₂, . . . n_N be the number of stations associated with AP₁, AP₂, . . . AP_N, and let f(V₁)=n_i. Then the association module of the mobile station chooses the access point AP with the fewest number of associated stations for association, i.e., AP_selected={AP_j|n_j=min{n₁,n₂, . . . n_N}}, which is known as the Join-the-shortest-queue (JSQ) algorithm.

[0039] This algorithm provides a mechanism for the mobile stations to choose the best server available and enables the mobile stations to explore the spatial diversity of a distributed network.

[0040] In another embodiment, it is assumed that there is an acceptable level of beacon signal power level P_a. When a mobile station receives beacon signals from multiple access points, each of which has a power level equal or greater than P_a, the access point with the lowest traffic loading among such plurality of access points is selected for association with the mobile station.

[0041] In one embodiment, the access point assesses the medium condition and inserts loading information in the Capability Information field (FIG. 3) of a Beacon/Probe Response frame. While in the scanning mode, the mobile station receives and records the access point loading information delivered in the Beacon/Probe Response frame. The mobile station can then evaluate the information and select an access point based upon beacon signal strength and access point loading levels, as described above.

[0042] One skilled in the art will appreciate further features and advantages of the invention based on the above-described embodiments. Accordingly, the invention is not to be limited by what has been particularly shown and described, except as indicated by the appended claims. All publications and references cited herein are expressly incorporated herein by reference in their entirety.