Enhanced passive scanning

#This invention relates generally to methods of operating wireless communication systems and wireless local area networks. More specifically, the invention relates to scanning methods for establishing communications between a mobile station and an access point.

#Wireless local area networks (WLANs) typically transmit via radio or infrared frequencies to connect data devices. In a WLAN, the wireless communication devices are often mobile, moving around more or less freely within the networked area. WLANs combine with infrastructure networks systems that can be connected to the Internet, thereby providing communication over long distances.

#WLANs link portable and wireless computer devices, also called mobile stations or terminals, to a wired LAN via a plurality of fixed access points (APs), also called base stations. Allowing WLAN devices to communicate with the infrastructure network, access points provide for wireless communications within respective cells and are typically spaced throughout a designated networked area. The access points facilitate communications between a networked set of 802.11-compliant devices called a basic service set (BSS), as well as communications with other BSSs and wired devices in or connected to wired infrastructure network systems.

#Various embodiments of the present invention are illustrated by the accompanying figures, wherein:

#FIG. 1 illustrates an enhanced passive scanning system for a wireless local area network, in accordance with one embodiment of the current invention;

#FIG. 2 illustrates an enhanced passive scanning system for a wireless local area network, in accordance with another embodiment of the current invention;

#FIG. 1 shows an enhanced passive scanning system for a wireless local area network (WLAN), in accordance with one embodiment of the present invention at 100. WLAN 100 includes one or more wireless communication devices referred to herein as mobile stations 110, 112, 114 and 116, and one or more access points 120 and 124. Access points 120 and 124 may be connected to an infrastructure network 130, which in turn may be connected to other wired and wireless networks (not shown). Mobile stations 110, 112, 114 and 116 include radio transmitters and receivers for transmitting and receiving signals such as voice over IP, data packets, control frames, and network management frames. Mobile stations 110, 112, 114 and 116 can communicate wirelessly with access points 120 and 124, and can be set to communicate with each other. In one embodiment, two mobile stations 110 and 112 form an ad hoc network with the ability to establish communications when in close proximity to each other. Often, one or more mobile stations 110, 112 and 114 will connect wirelessly to an access point 120, using standardized protocols such as IEEE 802.11, commonly referred to as Wi-Fi. These industry standards allow communication channels to be established and maintained between combinations of mobile stations and access points for the transmission of digital information by using techniques such as carrier-sense multiple access protocol with collision avoidance (CSMA/CA) to help provide rapid, equitable connectivity for all stations.

#Due in part to battery power limitations, the transmission range for reliable communications between mobile stations and access points is limited. Often within a well-configured wireless LAN, a mobile station may be within range of more than one access point, and the access point to which the mobile station connects needs to be determined. For example, mobile station 110 is connected to access point 120, sometimes referred to as the serving access point. Mobile station 114, however, may be connected to either access point 120 or access point 124, with one of the access points 120, 124 acting as the serving AP while the other is what can be referred to as a neighboring AP. As mobile station 114 moves or roams, or as environmental conditions for radio transmissions change, the serving AP may be changed accordingly.

#Access point 120 has a transmission range 122 within which data transmissions can be sent to and received from one or more mobile stations 110, 112, and 114. Similarly, access point 124 has a transmission range 126 within which data transmissions can be sent to and received from one or more mobile stations 114 and 116. To begin the process of connecting with a mobile station, an AP transmits, for example, a beacon signal on a prescribed channel. The prescribed channel comprises a prescribed frequency or frequency band that can be received by any mobile station within the transmission range when the mobile station is appropriately tuned and ready to receive the transmission. The beacon signal, commonly referred to as the beacon, comprises, for example, an access-point timestamp, a beacon interval, a basic service set identifier (BSSID), and a traffic indication map (TIM). The access-point timestamp contains timer information from the serving AP such as a copy of the AP's TSF timer to be used for synchronizing time-sensitive operations. The beacon interval indicates the time between two targeted start times of a beacon signal. In one embodiment, the beacon interval is substantially 1024 microseconds. The SSID is an identifying name assigned to the local network comprising the mobile stations and the APs serving the mobile stations. A BSS is a basic service set that includes a set of stations controlled by a single coordination function. The BSSID in an infrastructure BSS is the MAC address of the serving AP. The traffic indication map, an information element present within beacon frames generated by APs, contains a DTIM count that indicates how many beacons will appear before the next DTIM, a DTIM period indicating the number of beacon intervals between successive DTIMs, a bitmap control field that provides an indication of broadcast or multicast frames buffered at the AP, and a traffic-indication virtual bitmap containing information corresponding to traffic buffered for a specific station within the BSS that the AP is prepared to deliver at the time the beacon frame is transmitted. The DTIM is a beacon signal that contains a delivery traffic information message (DTIM) after which an AP sends out buffered broadcast and multicast media access control (MAC) service data units (MSDU), followed by any unicast frames. The beacon signal may also include within the beacon frame fields containing information such as capability information, supported rates, and parameters related to frequency hopping (FH) or direct sequence spread spectrum (DSSS) physical layers (PHYs).