HK1105064A - Neighbor scanning in wireless local area networks - Google Patents

Description

Proximity scanning in wireless local area networks

Technical Field

The present invention relates generally to Wireless Local Area Networks (WLANs), and more particularly to a method for scanning for neighboring Access Points (APs).

Background

WLANs are becoming increasingly popular because of their convenience and flexibility, and their popularity is expected to increase significantly as new applications for such networks are developed. One promising area is the use of Voice over Internet Protocol (VoIP), and when the user is a mobile subscriber, the need for seamless service continuity support (i.e., handover) in adjacent WLAN deployment areas is increasing.

In the 802.11 standard, Stations (STAs) can use two different modes to identify APs: an active scanning mode and a passive scanning mode. The STA uses either an active scanning mode or a passive scanning mode, usually determined by its configuration, in practice both modes are used. In the active scanning mode, the STA selects a frequency channel and transmits a Probe Request frame (Probe Request frame), and then waits for a certain time to receive a response in the form of the Probe Request frame. The probe request frame is typically sent by the AP when the Basic Service Set (BSS) is operating in infrastructure mode. In this case, if the STA does not receive the probe request frame after a certain time, it adjusts to a new frequency and repeats the procedure.

In passive scanning mode, the STA tries to find out the presence of a BSS on a particular frequency channel by adjusting the frequency and listening for a certain amount of time in order to capture beacon frame broadcasts by the AP in a fixed time interval. In this case, if the STA does not receive the beacon frame broadcast after a certain time, it adjusts to a new frequency and repeats the procedure.

When using passive mode, the STA may know which frequency channel it is likely to find candidate APs, but it cannot know exactly when a neighboring AP will send a beacon frame. Typically, beacon frames are sent at predetermined fixed time intervals, such as: every 100ms, in the worst case, the STA tunes to the target frequency and must wait at least 100ms until a beacon frame occurs. In the case where the STA has only one receiver, its service on the old frequency is interrupted while the STA performs passive scanning on the target frequency.

Performing an efficient delivery over a WLAN must include several requirements, such as: identification and measurement of appropriate candidate APs for delivery, establishment of STA authentication and security content in the target AP, re-association with the target AP, and communication of the data to the target AP.

While WLAN has not traditionally been developed with the goal of providing completely seamless mobility support, a problem with current WLAN systems is that the identification and measurement of appropriate candidate APs by the STA is a lengthy procedure, and may last for hundreds of milliseconds. In addition, the STA behavior is not well characterized and the duration of the measurement procedure can vary greatly depending on the different implementation chosen by the factory.

To avoid interruption of important services by the user, for example, the delivery procedure needs to be performed quickly during the execution of a VoIP call (service interruption time should typically not exceed tens to hundreds of milliseconds). In addition, the procedure by which the STA measures and identifies the neighboring candidate APs should not impact the performance of the ongoing service in any significant way.

Therefore, there is a need to improve the performance of the passive scanning mode, so that the use of the passive mode can ensure the continuity of the service and seamless delivery, especially in VoIP.

Disclosure of Invention

The invention includes methods, signaling mechanisms, and timing information regarding transmission intervals, and scheduling of neighbor candidate APs. The AP sends timing information about neighboring candidate APs to the STA, which can then use the timing information to schedule its adjustments to the target frequency and perform the identification and measurement of the target AP in a minimum amount of time.

Timing information for the re-adjacent candidate APs may be provided to STAs using broadcast/multicast type frames (e.g., included in beacon frames) or single broadcast type Medium Access Control (MAC) frames. In addition, Information Elements (IEs) containing timing information may be sent in MAC management frames or piggybacked on MAC control or data frames.

A method for proximity scanning in a WLAN having a STA, a first AP associated with the STA, and a second AP. Beginning with generating timing information about a beacon signal transmitted by the second AP, the timing information being reported by the first AP to the STA, the STA scheduling a time based on the timing information to listen for beacon signals transmitted by the second AP.

A system for proximity scanning in a WLAN includes a STA, a first AP associated with the STA, and a second AP. The STA includes a first timing information device, a scheduling device configured to receive timing information from the first timing information device, and a receiver for receiving communication signals and controllable by the scheduling device. The first AP includes a second timing information device that sends timing information to the first timing information device in the STA. The second AP includes a signal transmission device for transmitting a beacon signal, wherein the timing information relates to the beacon signal.

Drawings

The invention will be understood in more detail from the following description of a preferred embodiment, given as an example, and with reference to the accompanying drawings, in which:

FIG. 1 is a flowchart of a method for communicating timing information between a STA, an AP associated with the STA, and a candidate AP;

FIG. 2 is a timing diagram illustrating a scan of a candidate AP;

FIG. 3 is a timing diagram illustrating scanning of N channels; and

fig. 4 is a diagram of a system for communicating timing information between a STA, an AP associated with the STA, and a candidate AP.

Detailed Description

Hereinafter, the term "Station (STA)" includes, but is not limited to, a wireless transmit/receive unit, a user equipment, a mobile station, a fixed or mobile subscriber unit, a pager, or any other type of device capable of operating in a wireless environment. When referred to hereafter, the terminology "Access Point (AP)" includes but is not limited to a base station, a node B, a site controller, or any type of interfacing device in a wireless environment.

The present invention includes timing information about the transmission interval of neighboring candidate APs, typically the beacon frame transmission time, from where to transmit to a STA to improve the efficiency of the passive scanning mode.

The AP sends timing information about the neighboring candidate APs to the STA, which can then use the timing information to schedule it to adjust to the target frequency and identify and measure the target AP in a minimum time.

Fig. 1 is a flow diagram of a method 100 for communicating timing information between a STA 102, an AP (AP1)104 in communication with the STA 102, and a candidate AP (AP2) 106. In an optional first step, the STA 102 requests timing information of the candidate AP2106 from the AP 1104 (step 110). The AP 1104 then requests timing information from the AP2106 (step 112). The AP2106 reports its timing information to the AP1 (step 114), which is only needed if the AP1 has not previously obtained the timing information of the AP 2; the AP1 may have additional means to obtain the timing information (discussed below). The AP1 reports the timing information of the AP2 to the STA 102 (step 116). The STA 102 then schedules to adjust to the frequency of the AP2 in order to listen for the beacon of the AP2 (step 118).

Timing information for neighboring APs may include, for example: beacon interval (periodicity of beacon frame occurrence), a target signal frame transmission time, or a contention-free and contention-based period. Timing information regarding neighboring candidate APs may be communicated to the STA in absolute time reference form (e.g., a timestamp like "neighboring beacon frames will occur at time xyz"), or relative time difference to a known reference point (e.g., indicating a difference from the frame to which the AP1 sent the timing information to the STA, or from the previous or current beacon frame of AP 1).

Because the timing of the transmission of the next beacon frame is unknown to an accuracy in excess of a few millimeters due to the need for the device to wait for the end of any ongoing transmission/reception before transmitting the beacon, the AP sends a predicted reception interval (or equivalently, a target time plus an uncertainty reserve amplitude) to the STA.

Timing information provided to STAs can always be supplemented by the uncertainty period or a specific rule allows the STA to obtain the timing information and/or the uncertainty period. In general, the active AP will inform the STA that the candidate AP's beacon frame will occur N time units earlier than the active AP's beacon frame, and will also inform the STA that the candidate AP's beacon frame will occur M time units before and L time units after the indicated time or time interval due to the unforeseen factors. Another possibility is that the uncertainty period is not described every time the AP provides timing information, but rather is sent separately (e.g., via beacons), or sent as a specified fixed value, both of which save signaling bandwidth.

Timing information for neighboring candidate APs may be sent to STAs using solicitation and/or non-solicitation broadcast/multicast type frames (e.g., included in a beacon frame), or solicitation and/or non-solicitation unicast type MAC frames (e.g., in Association Response frames, Reassociation Response frames, or probe Response frames). Information Elements (IEs) containing timing information can be sent in (or partially in) MAC management frames or piggybacked on MAC control or data frames. Communicating timing information to the STAs may also include using inter-layer service origination (e.g., MAC-physical layer (PHY) -STA Management Entity (SME)) to initiate, acknowledge, and report on activities including sending MAC signaling frames, measuring activity, and the like.

Timing information for neighbor candidate APs may be generated at a particular AP in several ways, including: the AP uses the network to send signals to retrieve timing information of neighboring APs, the AP uses its own neighboring APs measurements, the AP uses reports from STA measurements, or the AP uses general timing equipment on the network.

In terms of network-side signaling, APs exchange transmission time information about their beacons through a decentralized system that connects APs together. There are several possible network side signaling implementations, such as: an AP broadcasts information about its beacon transmission timing to all APs on the distributed system, or the AP requests beacon timing information like another AP, and the other AP responds through the distributed system. Alternatively, the AP may query a network timing database, preferably implemented as part of a central control or local network management entity, for example, to obtain current timing information about its neighboring APs.

When the AP uses its own neighboring APs for measurement, the measuring AP listens for beacons of other APs and measures the transmission time of the beacon, from which beacon transmission interval the measuring AP can infer an approximate future transmission time. This approach is useful when neighboring APs use the same frequency channel as the measuring AP, or it would otherwise require the measuring AP to adjust to other frequency channels from time to time in order for it to listen for beacons, a less attractive solution.

In terms of the AP using reports from the STA measurements, STAs report the number of times a peer AP listens for beacons from neighboring APs(s), including the beacon transmission interval, the identification of neighboring APs, and the timestamp of the neighboring AP. The peer AP may use this absolute and relative time reference combination to obtain timing information, which the peer AP stores in memory and infers about future transmission times of beacons of these APs.

When a STA enters a BSS, it can set up a flag in the association request frame, the reassociation request frame, or the probe request frame. The flag, which indicates that the STA wants to receive a neighbor report element in the corresponding association request frame, re-association request frame or probe request frame, may be implemented in several ways, for example, as a simple bit flag or an IE containing values indicating the form of information that the STA wants to retrieve from the AP. The neighbor report element may include a Timing Synchronization Function (TSF) information field including a TSF offset value and a beacon interval value of a neighboring AP. The TSF offset value is expressed in Timing Units (TUs), which is, for example and without loss of generality, one micron in length, and is the timing offset between the peer AP and the neighboring AP represented in TUs with respect to the peer AP. The beacon interval value, which may be referred to in a preferred embodiment and without loss of generality as a Target Beacon Transmission Time (TBTT), has a typical default value of 100 ms.

Timing information regarding neighbor candidate APs may be stored, accessed or configured in an AP Management Information Base (MIB). The MIB may be a MAC layer MIB or a PHY layer MIB.

Fig. 2 and 3 show two embodiments. In fig. 2, when the STA only knows the approximate arrival time of the beacon frame of the neighboring candidate AP, the scan period during which the STA needs to stay on a given frequency to listen to a particular AP, if timing information is known, is typically on the order of milliseconds, and may fall to a full beacon interval (typically 100ms) if timing information is not known.

The gain of the proposed method when scanning several APs at different channels is shown in fig. 3. Typically, when timing information is used, the STA may establish a schedule based on the beacon frame transmissions and measure all beacon frames in a single or multiple beacon intervals. Conversely, if timing information is not used, several beacon intervals must be requested. The "uncertainty interval" shown in fig. 3 refers to the exact time for the beacon transmission due to the need to distinguish from other transmissions.

The above-described method is applicable to IEEE 802.11-based WLANs, and in particular to WLANs based on 802.11r (fast BSS transmission), 802.11s (extended service set (ESS) network), 802.11k (radio resource measurement), and 802.11n (high throughput WLAN). These methods are also applicable to other wireless network forms.

Fig. 4 illustrates a system 400 for communicating timing information between a STA 402, an AP (AP1)404 in communication with the STA 402, and a candidate AP (AP2) 406. The system 400 may be used when timing information is transmitted from the AP2 to the AP1 via a distributed network. The STA 402 includes a timing information device 410, a scheduling device 412, a receiver 414, and an antenna 416. The AP1404 includes a timing information device 420. The AP 2406 includes a timing information device 430, a beacon transmission device 432, and an antenna 434.

The system 400 operates as follows. In an optional first step, the STA 402 sends a request through the timing information device 410 to the timing information device 420 on the AP1404 requesting timing information for the candidate AP 2406. The AP 1104 then receives timing information about the AP 2406 through the timing information device 420 and the timing information device 430, respectively. As described above with reference to fig. 1, the AP1404 can receive timing information about the AP 2406 in different manners.

In AP 2406, the beacon transmitting device 432 transmits its beacon via antenna 434 and communicates the timing information to transmit the beacon to the timing information device 430. The timing information is sent from the timing information device 430 to the timing information device 420 in the AP 1404. AP1404 sends the timing information of AP 2406 from the timing information device 420 to the timing information device 410 in the STA 402.

Once the STA 402 receives the timing information for the AP 2406, which is passed from the timing information device 410 to the scheduling device 412, the scheduling device 412 determines when the STA 402 will adjust its receiver 414 to scan for and receive beacon transmissions from the AP 2406.

Although the features and elements of the present invention are described in the embodiments in particular combinations, each feature or element can be used alone (without the other features and elements of the preferred embodiments) or in various combinations with or without other features and elements of the present invention.

Claims (36)

1. A method for neighbor scanning in a wireless local area network having a station, a first Access Point (AP) associated with the station, and a second AP, comprising:

generating timing information regarding a beacon signal transmitted by the second AP;

reporting the timing information from the first AP to the station; and

scheduling, by the station, a time to listen for the beacon signal transmitted by the second AP, the scheduling based on the timing information.

2. The method of claim 1 wherein the timing information includes an absolute time reference for transmitting the beacon signal.

3. The method of claim 1 wherein the timing information includes a time difference relative to a known reference time for transmitting the beacon signal.

4. The method of claim 3, wherein the time difference is a deviation in time units.

5. The method of claim 3 wherein the reference time is a target beacon transmission time of the second AP.

6. The method of claim 3 wherein the timing information of the second AP is referenced to the beacon period of the first AP.

7. The method of claim 3 wherein the timing information of the second AP is referenced to the beacon transmission time interval of the first AP.

8. The method of claim 1 wherein the timing information includes a time window for transmitting the beacon signal.

9. The method of claim 1 wherein the generating step includes sending the timing information from the second AP to the first AP by signaling through a network.

10. The method according to claim 1, wherein the generating step includes making, by the first AP, measurements of beacon signals transmitted by the second AP.

11. The method according to claim 1, wherein the generating step includes receiving at the first AP measurements from the station associated with the first AP, the measurements relating to beacon signals transmitted by the second AP.

12. The method of claim 1 wherein the reporting step includes using a beacon frame to send the timing information.

13. The method of claim 1 wherein the reporting step includes using a probe request frame to send the timing information.

14. The method of claim 1 wherein the reporting step includes using a link response frame to send the timing information.

15. The method of claim 1 wherein the reporting step includes using a reassociation response frame to send the timing information.

16. The method of claim 1 wherein the reporting step includes using an information element in a Medium Access Control (MAC) frame to send the timing information.

17. The method of claim 16 wherein the MAC frame is a management sub-form.

18. The method of claim 1, further comprising the steps of: requesting timing information for the second AP from the station, wherein the station sends a request to the first AP.

19. The method according to claim 18, wherein the request sent by the station is part of a probe request.

20. The method according to claim 18, wherein the request sent by the station is part of a connection request.

21. The method according to claim 18, wherein the request sent by the station is part of a request for reconnection.

22. The method of claim 1, further comprising the steps of: requesting the timing information from the second AP by the first AP.

23. The method of claim 1, further comprising the steps of: requesting the timing information from the connecting station by the first AP.

24. A system for proximity scanning in a wireless local area network, the system comprising:

a station, comprising:

a first timing information device;

a scheduling means for receiving timing information from the first timing information means; and

a receiver for receiving communication signals, the receiver being controllable by the scheduling device;

a first Access Point (AP) in communication with said station, said first AP including a second timing information device, said second timing information device sending timing information to said first timing information device in said station; and

a second AP comprising a beacon transmitting device for transmitting a beacon signal, wherein the timing information is associated with the beacon signal.

25. The system of claim 24 wherein the timing information includes an absolute time reference for the beacon signal transmission.

26. The system of claim 24 wherein the timing information includes a time difference relative to a known reference time of transmission of the beacon signal.

27. The system of claim 24 wherein the timing information includes a time window for transmission of the beacon signal.

28. The system according to claim 24, further comprising a third timing information device in said second AP, said third timing information device receiving timing information from said beacon transmitting device and signaling said timing information to said second timing information device in said first AP via a network.

29. The system according to claim 24, wherein the timing information is generated by the first AP performing measurements on beacon signals transmitted by the second AP.

30. The system according to claim 24, wherein said timing information is generated by said first AP receiving measurements from stations connected to said first AP, said measurements relating to beacon signals transmitted by said second AP.

31. The system of claim 24 wherein the scheduling means receives timing information regarding beacon signals from the second AP and schedules the receiver a time to receive beacon signals from the second AP.

32. The system according to claim 24, wherein said station sends a request for timing information about said second AP from said first timing information device to said second timing information device in said first AP.

33. The system according to claim 24, wherein said first AP sends a request for timing information about said second AP from said second timing information device to said third timing information device in said second AP.

34. The system according to claim 24, wherein said second timing information device in said first AP sends a beacon frame including said timing information to said first timing information device in said station.

35. The system of claim 24 wherein the second timing information device in the first AP sends an information element in a Medium Access Control (MAC) frame to the first timing information device in the station, the information element including the timing information.

36. The system of claim 35 wherein the MAC frame is in a management sub-form.