# Patent

## Title
WLAN having load balancing based on access point loading

## Abstract

A network having distribution of access point loading includes access points to which mobile stations can associate themselves based upon access point beacon signal levels and loading levels for the various access points. A mobile station receives beacon signals from various access points and determines a signal strength for the received beacon signals. The mobile station also receives access point loading information from the access points. The mobile station associates with an access point based upon the access point beacon signal strengths and the access point loading information.

## Background
CROSS REFERENCE TO RELATED APPLICATIONS
The present application is a continuation of prior U.S. patent application Ser. No. 12/157,464 filed on Jun. 11, 2008, which is a continuation of prior U.S. patent application Ser. No. 10/292,862 filed on Nov. 12, 2002, and issued as U.S. Pat. No. 7,400,901 on Jul. 15, 2008, which claims the benefit of U.S. Provisional Patent Application No. 60/332,957, filed on Nov. 19, 2001, each of which is incorporated herein by reference.


STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH
Not Applicable.
FIELD OF THE INVENTION
The present invention relates generally to communication networks and, more particularly, to wireless communication networks.
BACKGROUND OF THE INVENTION
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.
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.
It would, therefore, be desirable to adjust the loading of network access points to reduce network congestion.
SUMMARY OF THE INVENTION
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.
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.



BRIEF DESCRIPTION OF THE DRAWINGS
The invention will be more fully understood from the following detailed description taken in conjunction with the accompanying drawings, in which:
FIG. 1 is a schematic depiction of a wireless network having access point selection by mobile stations in accordance with the present invention;
FIG. 2 is a schematic block diagram of an exemplary access point that can form a part of the network of FIG. 1;
FIG. 3 is a pictorial representation of an exemplary capability field that can be contained in a message exchanged by an access point and a mobile station in accordance with the present invention; and
FIG. 4 is a schematic depiction of a mobile station that can form a part of the network of FIG. 1.



DETAILED DESCRIPTION OF THE INVENTION
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 APa-N having associated coverage areas CAa-P serving the various mobile stations MSa-M. Access point load balancing is achieved by each of the mobile stations MSa-M determining to which one of the access points APa-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.
In general, each mobile station MS receives a beacon signal from access points Apa-N having a coverage area CAa-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 Msa-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.
For example, as illustrated in FIG. 1, a first mobile station MSc may be located within the coverage areas CAa,b of associated first and second access points APa,b. The first mobile station MSc receives a beacon signal of acceptable power level, e.g., above a predetermined threshold, from the first and second access points APa,b. In an exemplary embodiment, the first and second access points APa,b also broadcast their loading levels in a Beacon Frame that is received by the first mobile station MSc. Since the first access point APa supports significantly more mobile stations, e.g., MSd-i, than the second access point APb (supports MSa,b), the first mobile station MSc associates itself with the second access point APb.
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.
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.
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.
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.
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.







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









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.







TABLE 2







Probe Request frame body








Order
Information





1
SSID


2
Supported rates
















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









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.







TABLE 4







Association/Reassociation Request frame body








Order
Information





1
Capability information


2
Listen interval


3
SSID


4
Supported rates
















TABLE 5







Association Response frame body








Order
Information





1
Capability information


2
Status code


3
Association ID (AID)


4
Supported rates









In accordance with the present invention, the mobile station selects an access point based upon beacon signal strength and access point loading levels.
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.
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.
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.
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.
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.
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.
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.
For example, assume that the beacons of access points AP1, AP2, . . . APN 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 Vi for the access point APi. A cost function f(Vi) can be defined to calculate the cost if the mobile station associates with a given access point APi. 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.
In one particular embodiment, the association module uses the number of stations associated with the APs as the sole parameter of the cost function. Let n1, n2, . . . nN be the number of stations associated with AP1, AP2, . . . APN, and let f(Vi)=ni. 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={APj|nj=min{n1n2, . . . nN}}, which is known as the Join-the-shortest-queue (JSQ) algorithm.
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.
In another embodiment, it is assumed that there is an acceptable level of beacon signal power level Pa. When a mobile station receives beacon signals from multiple access points, each of which has a power level equal or greater than Pa, the access point with the lowest traffic loading among such plurality of access points is selected for association with the mobile station.
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.
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.

## Claims

1. A method for establishing an association between a mobile station and an access point in a wireless network comprising:
determining values for a plurality of parameters comprising load level information of the access point, an access point priority, mobile station priority weighting, and an overall network loading;
transmitting the values for a plurality of parameters to the mobile station; and
associating the mobile station with the access point based on the values for a plurality of parameters.

2. The method of claim 1 further comprising:
storing the values for a plurality of parameters.

3. The method of claim 1 wherein the load level information further comprises a transmission queue length and a number of transmitted error packets.

4. The method of claim 1 further comprising:
transmitting a beacon signal from the access point to the mobile station.

5. The method of claim 1 wherein transmitting the values for a plurality of parameters comprises:
transmitting the values for a plurality of parameters in a beacon frame.

6. The method of claim 1 further comprising:
receiving a request for the values for a plurality of parameters from the mobile station, wherein the transmitting the values for a plurality of parameters to the mobile station is in response to the request.

7. The method of claim 1 further comprising:
storing link quality information.

8. The method of claim 1 further comprising:
determining a cost function based on the values for a plurality of parameters.

9. The method of claim 8 further comprising:
transmitting the cost function to the mobile station.

10. The method of claim 8, wherein determining the cost function further comprises:
determining the cost function based on a traffic intensity of the access point.

11. The method of claim 8, wherein determining the cost function further comprises:
determining the cost function based on a signal strength of the access point.

12. An access point comprising:
a processor;
a module coupled to the processor, the module configured to determine values for a plurality of parameters comprising load level information of the access point, an access point priority, mobile station priority weighting, and overall network loading;
an interface configured to transmit the values for a plurality of parameters to a mobile station; and
a memory coupled to the processor, the memory configured to store the values for the plurality of parameters and a number of currently associated mobile stations.

13. The access point according to claim 12 wherein the load level information comprises a transmission queue length and a number of transmitted error packets.

14. The access point according to claim 12 wherein the memory is further configured to store link quality information.

15. A mobile station comprising:
a processor; and
a module coupled to the processor, the module configured to:
select to associate the mobile station with one of a plurality of access points based on values for a plurality of parameters comprising load level information of an access point, an access point priority, a mobile station priority weighting, and an overall network loading; and
associate the mobile station with one of the plurality of access points based on the values for a plurality of parameters.

16. The mobile station according to claim 15 further comprising:
a memory coupled to the processor, the memory configured to store the values for a plurality of parameters.

17. The mobile station according to claim 16 wherein the memory is further configured to store link quality information.

18. The mobile station according to claim 15 wherein the load level information comprises a transmission queue length and a number of transmitted error packets.