HK1186037A - A method for neighbor scanning in a wireless local area network, a station and an access point - Google Patents

Abstract

The present invention discloses a method for neighbor scanning in a wireless local area network, a station and an access point. The wireless local area network has a station, a first access point (AP) to which the station is associated, and a second AP, and the method comprises: receiving a timing information request from the station; receiving timing information at the first AP for a beacon signal sent by the second AP, wherein the timing information includes a time difference relative to a known reference time for transmission of the beacon signal, and the time difference is an offset in timing units; and reporting the timing information to the station, whereby the neighbor scanning is achieved by the station receiving the timing information.

Description

Method, station and access point for proximity scanning in a wireless local area network

The application is a divisional application of Chinese patent application with the application number of 200580023364.0, the application date of 29/6/2005 and the invention name of 'proximity scanning in wireless area network'.

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, a Station (STA) may use two different modes to identify an AP: 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 a Basic Service Set (BSS) is operating in an 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 may find a candidate AP on, but it cannot know exactly when a neighboring AP will send a beacon frame. Typically, beacon frames are transmitted in a predetermined fixed time interval, for example: every 100ms, in the worst case, the STA tunes to the target frequency and must wait at least 100m 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 handoff on a WLAN must include several requirements, such as: identification and measurement of appropriate candidate APs for handoff, establishment of STA authentication and security content in the target AP, reassociation of the target AP, and transmission of the data link 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, during the execution of a VoIP call, the handover procedure needs to be performed quickly (the service interruption time should typically not exceed tens to hundreds of milliseconds). In addition, the process of the STA measuring and identifying 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 handover, especially in VoIP.

Disclosure of Invention

The present invention encompasses methods, signaling mechanisms, and timing information regarding transmission intervals, as well as scheduling of neighboring candidate APs. The AP sends timing information about neighboring candidate APs to the STA, which can then use the timing information to schedule it to adjust to the target frequency and perform the identification and measurement of the target AP in a minimum amount of time.

The timing information of the neighbor candidate APs may be sent to the 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 neighbor scanning (WLAN) in a WLAN having a STA, a first AP associated with the STA, and a second AP. Beginning with generating timing information regarding 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 configured to receive 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 transmitting device for transmitting a beacon signal, wherein the timing information is related 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 flow diagram 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 scanning for a candidate AP;

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

fig. 4 is a system diagram 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 station controller, or any other 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 tune 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 STA102, an AP (AP 1) 104 associated with the STA102, and a candidate AP (AP 2) 106. In an optional first step, the STA102 requests timing information of the candidate AP2106 from the AP1104 (step 110). The AP1104 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 this timing information (discussed below). The AP1 reports the timing information of the AP2 to the STA102 (step 116). The STA102 then schedules a time to adjust to the frequency of the AP2 in order to listen for the beacon of the AP2 (step 118).

The timing information of the neighboring AP may include, for example: beacon interval (periodicity of beacon frame occurrence), a target signal frame transmission time, or contention-free and contention-based periods. 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).

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

The timing information provided to the STA 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 transmitted separately (e.g., via beacons), or is transmitted at a certain fixed value, both of which save signaling bandwidth.

Timing information for the neighbor candidate APs may be sent to the STAs using solicitation and/or non-solicitation broadcast/multicast type frames (e.g., contained 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). The Information Element (IE) containing timing information can be sent in (or partially in) a MAC management frame or piggybacked on a MAC control or data frame. Communicating timing information to the STA 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, measurement activities, etc.

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

In terms of network side signaling, the APs exchange transmission time information about their beacons through a distributed system that connects the APs together. There are several possible network side signaling implementations, for example: 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 neighbor AP measurements, 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 the neighboring AP uses 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 so that it can listen for beacons, which is a less attractive solution.

In terms of the AP using reports from the STA measurements, the STA reports the number of times the peer AP listens for beacons from neighboring APs, including the beacon transmission interval, the identification of the neighboring AP, and the timestamp of the neighboring AP. The peer AP may use this absolute and relative time reference combination to obtain timing information, store this information in memory, and infer approximate future transmission times of the APs' beacons.

When a STA enters a BSS, it can set a flag in the association request frame, re-association request frame, or 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 type of information that the STA wants to retrieve from the AP. The neighbor report element may include a Timing Synchronization Function (TSF) information field that includes a TSF offset value and a beacon interval value for a neighboring AP. The TSF offset value is expressed in Timing Units (TUs), which are, 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 the TU for the peer AP. The beacon interval value, which in a preferred embodiment and without loss of generality, may be represented as a Target Beacon Transmission Time (TBTT), which has a typical default value of 100 ms.

Timing information regarding neighboring 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 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, which may fall to a full beacon interval (typically 100 ms) if timing information is not known.

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

The above-described method is applicable to IEEE802.11 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 STA402, an AP (AP 1) 404 associated with the STA402, and a candidate AP (AP 2) 406. The present system 400 may be used when timing information is communicated from the AP2 to the AP1 via a distributed network. The STA402 includes a timing information device 410, a scheduling device 412, a receiver 414, and an antenna 416. AP1404 includes a timing information device 420. The AP2406 includes a timing information device 430, a beacon transmitting device 432, and an antenna 434.

The system 400 operates as follows. In an optional first step, the STA402 sends a request to the timing information device 420 on the AP1404 through the timing information device 410 requesting timing information for the candidate AP 2406. The AP1104 then receives timing information about the AP2406 via the timing information device 420 and the timing information device 430, respectively. As described previously with reference to fig. 1, the AP1404 can receive timing information for the AP2406 in different manners.

In AP2406, 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. The AP1404 sends the timing information of the AP2406 from the timing information device 420 to the timing information device 410 in the STA 402.

Once the STA402 receives the timing information for the AP2406, which is passed from the timing information device 410 to the scheduling device 412, the scheduling device 412 determines when the STA402 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 (23)

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

receiving a timing information request from the station;

receiving, at the first AP, timing information of a beacon signal transmitted by the second AP, wherein the timing information includes a time difference relative to a known reference time for transmitting the beacon signal, and the time difference is an offset in units of timing; and

reporting the timing information to the station such that the proximity scan is completed by the station receiving the timing information.

2. The method of claim 1, wherein the reference time is a target beacon transmission time of a first beacon from the second AP.

3. The method of claim 1, wherein receiving the timing information comprises receiving timing information for the second AP through network-side signaling.

4. The method of claim 1, wherein the reporting comprises sending the timing information in any one of: beacon frames, probe response frames, association response frames, re-association response frames, and information elements in Medium Access Control (MAC) frames.

5. The method of claim 4, wherein the MAC frame is a management subtype.

6. The method of claim 1, wherein the request is part of any one of: probe requests, association requests, and reassociation requests.

7. The method of claim 1, further comprising requesting timing information from the second AP.

8. The method of claim 1, further comprising:

requesting beacon measurements from stations associated with the first AP, the beacon measurements including timing information.

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

transmitting a timing information request from the station to the first AP;

receiving a timing information report from the first AP, the timing information report including timing information of a beacon signal transmitted by the second AP, wherein the timing information includes a time difference relative to a known reference time for transmitting a beacon signal, and the time difference is an offset in timing units, whereby the proximity scan is completed by the station receiving the timing information; and

scheduling a time at which the station listens for a beacon signal transmitted by the second AP, the scheduling based on the timing information report.

10. The method of claim 9, wherein the reference time is a target beacon transmission time of a first beacon from the second AP.

11. The method of claim 9, wherein the receiving comprises receiving a timing information report in an information element in a Medium Access Control (MAC) frame.

12. A station, the station comprising:

a timing information device configured to:

transmitting a timing information request from the station to a first Access Point (AP); and

receiving a timing information report from the first AP, the timing information report including timing information of a beacon signal transmitted by a second AP, wherein the timing information includes a time difference relative to a known reference time for transmitting a beacon signal, and the time difference is timing

Deviation in units; and

scheduling means in communication with the timing information means, the scheduling means configured to schedule a time at which the station listens for a beacon signal transmitted by the second AP, the scheduling based on the timing information report.

13. The station of claim 12, wherein the timing information means is configured to receive a timing information report in an information element in a Medium Access Control (MAC) frame.

14. An access point, comprising:

a transmitter configured to transmit a signal;

a receiver configured to receive a signal; and

a timing information device configured to:

receiving a timing information request from a station associated with the access point;

receiving timing information for a beacon signal transmitted by a second access point, wherein the timing information includes a time difference relative to a known reference time for transmitting the beacon signal, and the time difference is an offset in timing; and

reporting the timing information to the station.

15. The access point of claim 14, wherein the timing information request is part of any one of: probe requests, association requests, and reassociation requests.

16. The access point of claim 14, wherein the timing information is received from the second AP through network side signaling.

17. The access point of claim 14, wherein the timing information is reported to the station in any of: beacon frames, probe response frames, association response frames, re-association response frames, and information elements in Medium Access Control (MAC) frames.

18. The access point of claim 17, wherein the MAC frame is a management subtype.

19. The access point of claim 14, wherein the timing information means is further configured to request timing information from the second AP.

20. The access point of claim 14, wherein the timing information means is further configured to request beacon measurements from other stations associated with the access point, the beacon measurements including timing information.

21. An access point, comprising:

a beacon transmission device configured to:

transmitting a beacon signal; and

generating timing information about the beacon signal, wherein the timing information includes a time difference relative to a known reference time at which the beacon signal is transmitted, and the time difference is a deviation in timing; and

a timing information device configured to:

receiving the timing information from a beacon transmitting device; and

reporting the timing information to another access point.

22. The access point of claim 21, wherein the timing information means is configured to report the timing information to another access point through network side signaling.

23. The access point of claim 21, wherein the timing information means is further configured to receive a timing information request from another access point.