Patent Grant
7400901
Not Applicable.
The present invention relates generally to communication networks and, more particularly, to wireless communication networks.
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.
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.
The invention will be more fully understood from the following detailed description taken in conjunction with the accompanying drawings, in which:
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
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.
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.
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.
In accordance with the present invention, the mobile station selects an access point based upon beacon signal strength and access point loading levels.
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.
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 V1 for the access point APi. A cost function f(V1) 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(V1)=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{n1,n2, . . . 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 (
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.
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.
| Number | Name | Date | Kind |
|---|---|---|---|
| 5305308 | English et al. | Apr 1994 | A |
| 5513210 | Vook et al. | Apr 1996 | A |
| 5594731 | Reissner | Jan 1997 | A |
| 5654959 | Baker et al. | Aug 1997 | A |
| 5812531 | Cheung et al. | Sep 1998 | A |
| 5815811 | Pinard et al. | Sep 1998 | A |
| 5844900 | Hong et al. | Dec 1998 | A |
| 5901362 | Cheung et al. | May 1999 | A |
| 5987018 | Freeburg et al. | Nov 1999 | A |
| 6002918 | Heiman et al. | Dec 1999 | A |
| 6067297 | Beach | May 2000 | A |
| 6157668 | Gilhousen et al. | Dec 2000 | A |
| 6374311 | Mahany et al. | Apr 2002 | B1 |
| 6522881 | Feder et al. | Feb 2003 | B1 |
| 6580700 | Pinard et al. | Jun 2003 | B1 |
| 6643521 | Bourgoin et al. | Nov 2003 | B1 |
| 6842616 | Takano et al. | Jan 2005 | B2 |
| 6859654 | Reynolds et al. | Feb 2005 | B1 |
| 6879579 | Myles et al. | Apr 2005 | B1 |
| 6895258 | Scherzer et al. | May 2005 | B1 |
| 7177649 | Nielsen | Feb 2007 | B1 |
| 20020045424 | Lee | Apr 2002 | A1 |
| 20020085498 | Nakamichi et al. | Jul 2002 | A1 |
| 20020110105 | Awater et al. | Aug 2002 | A1 |
| 20030163579 | Knauerhase et al. | Aug 2003 | A1 |
| 20040039817 | Lee et al. | Feb 2004 | A1 |
| 20040053624 | Frank et al. | Mar 2004 | A1 |
| 20040068668 | Lor et al. | Apr 2004 | A1 |
| 20040110524 | Takano et al. | Jun 2004 | A1 |
| 20060007951 | Meier | Jan 2006 | A1 |
| 20060234754 | Takano et al. | Oct 2006 | A1 |
| Number | Date | Country |
|---|---|---|
| 1 133 208 | Sep 2001 | EP |
| 1 156 623 | Nov 2001 | EP |
| 1 178 630 | Feb 2002 | EP |
| 1 206 070 | May 2002 | EP |
| 1 207 452 | May 2002 | EP |
| WO 9835453 | Aug 1998 | WO |
| WO 0213429 | Feb 2002 | WO |
| Number | Date | Country | |
|---|---|---|---|
| 20030139197 A1 | Jul 2003 | US |
| Number | Date | Country | |
|---|---|---|---|
| 60332957 | Nov 2001 | US |
