Fast switching method, system and client in wireless communication system

Technical Field

The present invention relates to the field of wireless communi - cations, and particularly, to a fast switching method, a fast switching system, a client and an access point (AP) in a wire- less communication system of a rail transit system.

Background Art At present, with the development of urbanization, rail transit systems are becoming increasingly popular. The current rail transit systems use wireless communication technology in communication between trains and roads. In a rail transit system, typical application scenarios of communications between trains and roads based on wireless local area network (WLAN) is as shown in Fig. 1. As shown in Fig. 1, a plurality of access points (APs) are set on the road (for clarity, Fig. 1 shows only three APs : AP0, API and AP2) , with every two adjacent APs being separated by an interval of a specific distance to ensure that no coverage dead zone exists. A client is located at the train side and can communicate with an AP which covers it. By means of the communication connection, a client on the train can communicate with a server on the road side through the access point, and the AP is referred to as the current AP of the client.

When the train moves, the client will depart from the coverage area of the current AP and interrupt the communication with the current AP, and at this moment, the client needs to switch to another AP (referred to as the target AP) in order to ensure normal communication. Therefore, the switching mechanism is critical to guarantee reliable and uninterrupted communication between the client and the AP.

In order to establish a communication connection with the tar- get AP, multiple bidirectional information interactions are required between the client and the target AP, as shown in Fig. 2. The client sends a probe request to the target AP and the target AP returns a probe response to the client; and the client sends an association request to the target AP and the tar- get AP returns an association response to the client. For application scenarios where the requirements on security are relatively high, it is also required that the client sends an authentication request to the target AP, and the target AP returns an authentication response to the client; and if the cli- ent passes the authentication, then the client establishes a communication connection with the target AP, and the target AP becomes the current AP of the client.

In the example shown in Fig. 1, both the APs and the client are using directional antennas, the application of which is very common in the rail transit system. However, the use of directional antennas usually causes the strength of the received signal to suddenly weaken when the client passes by the current AP.

In the example shown in Fig. 1, the current AP of the client is API, and when the client moves from right to left to point B, the strength of the signal emitted by API suddenly weakens. When the communication with API is completely interrupted, the client must try to switch to another AP (i.e. the target AP; in Fig. 1, the target AP is AP2) in order to maintain communication with the server. In order to ensure communication reliability, the time required for the client to switch from the current AP to the target AP is as short as possible. In the prior art, the switching shown in Fig. 1 needs to spend a time of more than 100 ms ; that is to say, data transmission is interrupted in the time of 100 ms . In the rail transit system, an interruption as long as 100 ms is unbearable whether it is passenger information or train control information. Therefore, a fast switching mechanism is required to reduce the switching time .

The process of a client switching from one AP to another AP can usually be divided into two stages: the switching-deciding stage and the connection-establishment stage. The existing fast switching mechanisms are generally achieved by reducing the switching-deciding time or reducing the connection- establishment time.

A fast switching method in the prior art is as shown in Fig. 3, where API is the current AP of the client, and the client starts switching when the client moves to point A which is close to API. In this case, when the strength of the signal of API received by the client remains strong and is greater than a switching threshold, the switching starts. This avoids the situation where the strength of the signal emitted by API suddenly weakens when the client moves to point B. Therefore, using this method can reduce a sudden interruption of communication between the client and its current AP . Moreover, combining the method with the background scanning method in the prior art (the client in the idle state continuously scans all the chan- nels to collect information about the candidate APs) can help reduce the switching-deciding time.

However, since in this method, the client starts switching when the strength of the signal emitted by the current AP is still strong, more overlapping coverage areas are required between the current AP and the target AP, so to ensure that the switching point can correctly receive signals from the target AP, which will increase the number of APs. If the original distance between the APs remains, then the strength of the signal from the target AP may be relatively weak and this will seriously affect the switching process. In the worst case, a probe request, an association request and an authentication request transmitted by the client are all lost on the target AP side, and therefore multiple retransmissions are required, which will increase the connection-establishment time, thereby offsetting the advantages obtained by the switching-deciding stage. Moreo- ver, after the connection with the current AP is disconnected, this method cannot reduce the time of connection establishment with the target AP .

In the process of the client establishing a connection with the target AP, the authentication will take a lot of time. If an authentication server is used, the time spent will be more than 100 ms . Another fast switching method in the prior art is the pre-authentication method: in the IEEE802.11x standard, the client can perform a pre-authentication operation with the au- thentication server before the switching starts, thereby reducing the connection-establishment time. However, this pre- authentication method can only reduce the authentication operation time with the authentication server, but can neither reduce the time of other operations during connection establish- ment nor reduce the switching-deciding time.

Contents of the Invention

In view of this, the main object of the present invention is to provide a fast switching method, a fast switching system, a client and an AP in a wireless communication system, thereby effectively reducing the switching time of the client from the current AP to the target AP . To achieve the abovementioned object, one embodiment of a fast switching method in a wireless communication system according to the present invention comprises: when a client decides to switch, said client establishes, via its current access point (AP) , virtual links with candidate

APs, said virtual links including a communication link between said client and said current AP and communication links between said current AP and said candidate APs; said client sends, via said virtual links, a probe request to said candidate APs and receives probe responses of said candidate APs ; said client selects, according to the probe responses of said candidate APs, a target AP from said candidate APs; said client sends, via said virtual link, an association request to said target AP and receives an association response of said target AP; and when said client starts switching, said client terminates the communication with said current AP and switches to said target AP.

Another embodiment of a fast switching method in a wireless communication system according to the present invention comprises : when a client decides to switch, a current access point (AP) of said client is used to establish virtual links between said client and candidate APs, said virtual links including a communication link between said client and said current AP and communication links between said current AP and said candidate APs; said current AP receives a probe request sent, via said virtual links, by said client to said candidate APs and forwards the probe request to said candidate APs, and receives probe responses sent, via said virtual links, by said candidate APs to said client and forwards the probe responses to said client; said current AP receives an association request sent, via said virtual link, by said client to said target AP and forwards the association request to said target AP, and receives an association response sent, via said virtual link, by said target AP to said client and forwards the association response to said client ; and when said client starts switching, said current AP terminates the communication with said client.

Yet another embodiment of a fast switching method in a wireless communication system according to the present invention comprises : when a client decides to switch, candidate access points (APs) establish, via a current AP of said client, virtual links with said client, said virtual links including communication links between said candidate APs and said current AP and a communication link between said current AP and said client; said candidate APs receive, via said virtual links, a probe re- quest of said client and send probe responses to said client; and said candidate APs receive, via said virtual links, an association request of said client and send association responses to said client.

One embodiment of a fast switching system in a wireless communication system according to the present invention comprises a client and a plurality of wireless access points (APs) , said client is used for establishing, via its current access point (AP) , virtual links with candidate APs when it decides to switch, said virtual links including a communication link between said client and said current AP and communication links between said current AP and said candidate APs; sending, via said virtual links, a probe request to said candidate APs and receiving probe responses of said candidate APs; selecting, ac- cording to the probe responses of said candidate APs, a target AP from said candidate APs; sending, via said virtual link, an association request to said target AP and receiving an association response of said target AP; and terminating the communica- tion with said current AP and switching to said target AP when switching is started; said current AP is used for establishing virtual links between said client and candidate APs when said client decides to switch; receiving a probe request sent, via said virtual links, by said client to said candidate APs and forwarding the probe request to said candidate APs, and receiving probe responses sent, via said virtual links, by said candidate APs to said client and forwarding the probe responses to said client; re- ceiving an association request sent, via said virtual link, by said client to said target AP and forwarding the association request to said target AP, and receiving an association response sent, via said virtual link, by said target AP to said client and forwarding the association response to said client; and terminating the communication with said client when said client starts switching; and said candidate APs are used for establishing, via a current AP of said client, virtual links with said client when said client decides to switch; receiving, via said virtual links, a probe request of said client and sending probe responses to said client; and receiving, via said virtual links, an association request of said client and sending association responses to said client .

One embodiment of a fast switching client in a wireless communication system according to the present invention comprises: a first virtual link establishment module for establishing, via a current AP of said client, virtual links between said client and candidate APs when said client decides to switch, said virtual links including a communication link between said client and said current AP and communication links between said current AP and said candidate APs; a first probe module for sending, via said virtual links, a probe request to said candidate APs and receiving probe responses of said candidate APs; a selection module for selecting, according to the probe responses of said candidate APs, a target AP from said candidate APs; a first association module for sending, via said virtual link, an association request to said target AP and receiving an association response of said target AP; and a first switching module for terminating the communication with said current AP and switching to said target AP when said client starts switching. One embodiment of an access point (AP) in a wireless communication system according to the present invention comprises: a second virtual link establishment module for establishing virtual links between a client and candidate APs when said cli- ent decides to switch, said virtual links including a communication link between said client and said AP and communication links between said AP and said candidate APs;

a second probe module for receiving a probe request sent, via said virtual links, by said client to said candidate APs and forwarding the probe request to said candidate APs, and receiving probe responses sent, via said virtual links, by said candidate APs to said client and forwarding the probe responses to said client; a second association module for receiving an association request sent, via said virtual link, by said client to said target AP and forwarding the association request to said target AP, and receiving an association response sent, via said virtual link, by said target AP to said client and forwarding the association response to said client; and a second switching module for terminating the communication with said client when said client starts switching.

Another embodiment of the access point (AP) in a wireless communication system according to the present invention comprises: a third virtual link establishment module for establishing, via a current AP of a client, a virtual link between said AP and said client when said client decides to switch, said virtual link including a communication link between said AP and said current AP and a communication link between said current AP and said client; a third probe module for receiving, via said virtual link, a probe request of said client and sending a probe response to said client; and a third association module for receiving, via said virtual link, an association request of said client and sending an association response to said client.

Description of the accompanying drawings

Fig. 1 shows a schematic diagram of WLAN-based communication in a rail transit system in the prior art;

Fig. 2 shows a schematic diagram of information interaction used for establishing a communication connection between a client and a candidate AP in the prior art; Fig. 3 shows a schematic diagram of a fast switching method in the prior art; Fig. 4 shows a flow chart of a fast switching method in a wireless communication system of an embodiment of the present invention; Fig. 5 shows a schematic diagram of a fast switching method in a wireless communication system of an embodiment of the present invention;

Fig. 6 shows a flow chart of a fast switching method in a rail transit wireless communication system of an embodiment of the present invention;

Fig. 7 shows a structural schematic diagram of a fast switching client in a wireless communication system of an embodiment of the present invention;

Fig. 8 shows a structural schematic diagram of a current AP in a wireless communication system of an embodiment of the present invention; and

Fig. 9 shows a structural schematic diagram of a candidate AP in a wireless communication system of an embodiment of the present invention. Particular Embodiments

In order to make the object, technical solutions and advantages of the present invention more apparent and clear, the present invention will be further described in detail below with refer- ence to the accompanying drawings and by way of embodiments.

The embodiments of the present invention propose a fast switching method in a wireless communication system as shown in Fig. 4, the method comprising the following steps: step 401, when a client decides to switch, the client establishes, via its current AP, virtual links with candidate APs, the virtual link including a communication link between the client and the current AP and communication links between the current AP and the candidate APs; step 402, the client sends, via the virtual links, a probe request to the candidate APs and receives probe responses of the candidate APs; step 403, the client selects, ac- cording to the probe responses of the candidate APs, a target AP from the candidate APs; step 404, the client sends, via the virtual link, an association request to the target AP and receives an association response of the target AP; and step 405, when the client starts switching, the client terminates the communication with the current AP and switches to the target AP.

Taking a wireless communication scenario in a rail transit system for example, said virtual link includes a wireless link be- tween the client and the current AP and wired links between the current AP and the candidate APs. Fig. 5 shows a schematic diagram of a fast switching method in a wireless communication system of an embodiment of the present invention. As shown in Fig. 5, the current AP of the client is API, and the client de- cides to switch when moving to point A which is close to API, and at this moment pre- switching starts.

The pre- switching mentioned in the embodiments of the present invention refers to the preparations made before the client switches from the current AP to another AP (i.e. the target

AP) , such as operations like probe, association and authentication, and so on. In this embodiment, the timing for deciding to switch can be determined using a variety of methods, one of which is to determine the timing of switching according to the strength of the signal emitted by the current AP (i.e. API) .

Due to the fact that at least one of the client and the AP uses directional antennas in the rail transit system, in Fig. 5, when the client moves from right to left, the signal strength of API will first become increasingly strong, and when the cli- ent moves to point A, the signal strength reaches a threshold, and then the train will pass by API quickly, and when the train moves to point B where API resides, the signal strength will suddenly weaken. Therefore, in the embodiments of the present invention, when the client is informed that the signal strength of API exceeds a certain preset signal threshold which can be set in advance based on empirical values, the client starts to perform pre- switching . Another method is to determine the timing of switching according to the location of the client. In this method, if all the APs are fixed to a certain location on the road, the train route is also fixed, and the client has stored location information about all the APs in advance, when the train moves, the client is capable of calculating its distance from API, and when the distance is less than a certain preset distance threshold, it indicates that the train will pass by API quickly, and hence pre- switching is required. Of course, a person skilled in the art should understand that oth- er methods can also be used to determine the timing of switching according to the needs of the practical application, as long as pre- switching is completed when the client moves from point A to point B where API resides, and all these methods should be within the scope of protection of the present inven- tion.

The particular implementations of the present invention are described in detail below in conjunction with the flow chart of the fast switching method in a rail transit wireless communica- tion system as shown in Fig. 6.

Step 601, when a client decides to switch, virtual links are established between the client and candidate APs, the virtual links including a wireless link between the client and the cur- rent AP and wired links between the current AP and the candidate APs. In this step, in conjunction with the example shown in Fig. 5, the current AP is API, and the client decides to switch when moving to a location (i.e. point A) close to API. In the embodiments of the present invention, a variety of methods can be used to determine a candidate AP. One of the relatively common methods is the following: all the APs store loca- tion information about their own and information about neighbor APs. When deciding to switch, the client sends a pre- switching request to API; and after receiving the pre- switching request, API contains the information about the neighbor APs in a pre- switching response and sends same to the client, wherein the information about the neighbor APs contains the location information about the neighbor APs. After receiving the information about the neighbor APs sent by API, according to the running direction of the train, the client selects as a candidate AP a neighbor AP with the same running direction as the train from the neighbor APs sent by API, wherein the candidate AP may be the next one or next two APs adjacent to API along the running direction of the train. Another method to determine a candidate AP is the following: the client stores a list of APs which rec- ords the information about the APs in accordance with the sequence of how the APs are set on the road. Since the train route is fixed in the rail transit system, the client can determine which AP is to pass by next according to the list, that is, the client can determinate a candidate AP according to the current AP and the information about the APs recorded in the list. Yet another method to determine a candidate AP is the following: information about all the APs set on the road are stored in a server, the client can then access the server via the current AP to obtain information about the neighbor APs of the current AP, and then according to the running direction of the train, the client selects as a candidate AP a neighbor AP with the same running direction as the train from the neighbor APs obtained. Additionally, the client can also use a method in the prior art to continuously scan, in the idle state, all the channels to collect information about the candidate AP . Of course, other methods can also be used to determine a candidate AP, on which the embodiments of the present invention do not impose restrictions. In the embodiments of the present invention, a variety of methods can be used to establish a virtual link between the client and the candidate AP. One method allows doing no more changes to the client, but the current AP modifies the MAC address filter thereof so as to be able to receive a message sent by the client to the candidate AP and forward the message to the candidate AP via its wired link with the candidate AP . For exam- pie, in conjunction with the example shown in Fig. 5, the current AP (i.e. API) can add the MAC address of a candidate AP (e.g. AP2) in its MAC address filter, so that API can receive a message sent by the client to AP2 via a wireless link with API, and forward the message to AP2 via its Ethernet link with AP2. Another method allows doing no more changes to the current AP, but the client sends, via its wireless link with the current AP, to the current AP the message which is sent to the candidate AP, and then the current AP forwards the message to the candidate AP via its wired link with the candidate AP; and con- versely, the candidate AP sends, via its wired link with the current AP, to the current AP the message which is sent to the client, and then the current AP forwards the message to the client via its wireless link with the client. In the second method, if the client and the candidate AP are in the same sub- net, the client can use a distribution flag DS specified in the IEEE802.il standard in a message which is sent to the current AP to indicate to the current AP to forward a message the destination MAC address of which is the candidate AP, thus achieving the MAC layer communication with the candidate AP; and if the client and the candidate AP are in different subnets, the client can, in the message which is sent to the current AP, indicate to the current AP to forward, via an IP route, a message the destination IP address of which is the candidate AP, thus achieving the IP layer (layer 3) communication with the candi- date AP . Using the first method, the current AP needs to re- encapsulate the message sent by the client to the candidate AP, but the client does not need to change the header of the message. However, for the second method, the client needs to encapsulate the message which is forwarded by the current AP and the destination address of which is the candidate AP, but the current AP does not need to re-encapsulate the message, just simply forward the message instead. The embodiments of the pre- sent invention do not impose restrictions on the method for establishing a virtual link, and other methods that can be used should also be within the scope of protection of the present invention .

Step 602, the client sends a probe request to the candidate AP via the virtual link established between the client and the candidate AP. In conjunction with the example shown in Fig. 5, description is made by taking AP2 and AP3 as the candidate APs for example. In the embodiments of the present invention, when the first method for establishing a virtual link is used, the client sends a probe request to AP2 and AP3 over a channel where it currently resides, API changes the MAC address filter thereof to receive the probe request sent by the client to AP2 and AP3 , and then API re-encapsulates the received probe request and forwards same to AP2 and AP3 respectively via an Ethernet link. When the abovementioned second method for establishing a virtual link is used, the client can use a distributed flag to send to API, via a channel where it currently resides, a probe request that needs to be sent to AP2 and AP3 , and API will forward the probe request to AP2 and AP3 respectively via an Ethernet link. The channel where the client currently resides refers to the chan- nel over which the client communicates with API, i.e. the channel of API .

Step 603, the candidate AP receives, via the virtual link, the probe request sent by the client and returns a probe response to the client.

After receiving the probe request, AP2 and AP3 can transfer onto the channel indicated in the probe request to measure the strength of the received signal so as to generate a received signal strength indicator (RSSI) , and contain the RSSI value in the probe response and return same to the client via the established virtual link. The channel indicated in the probe request is the channel where the client currently resides, i.e. the channel over which the client communicates with API. Since at this moment the client still maintains communication with API, AP2 and AP3 can continuously monitor the message sent by the client, thus quickly obtaining the RSSI value so as to facilitate the reduction of switching-decision time of the client. In an application scenario where the client has the capability of performing multi -channel processing, the channel indicated in the probe request can also be the channel of AP2 or AP3 , or other channels that can be used in probe operations. For example, if the channel indicated in the probe request is the chan- nel of AP2 or AP3 , the client sends a probe message over the channels of AP2 and AP3 respectively, and AP2 and AP3 then, respectively, measure the strength of the received signal over their channels, generate the received signal strength indicator, and contain the RSSI value in the probe response and re- turn same to the client via the established virtual link.

After receiving the probe request forwarded by API, AP2 and AP3 can also transfer onto the channel of API according to the channel information stored therein or obtained from a server, to measure the strength of the received signal, and contain the measured strength information about the received signal in the probe response. Since the channel of API is the channel where the client currently resides, AP2 and AP3 can continue monitoring the message sent by the client so as to quickly obtain the strength information about the received signal.

AP2 and AP3 can send, via a wired link with API, a probe response which contains the RSSI value to API, and after receiving the probe response from AP2 and AP3 , API forwards the re- ceived probe response to the client via a wireless link with the client. Step 604, after receiving all the probe responses returned by the candidate AP, the client compares the RSSI values contained in the probe responses, and selects an AP with the maximum RSSI value as the target AP.

For example, assuming that the RSSI value returned by AP2 is greater than the RSSI value returned by AP3 , the client selects AP2 as the target AP . Step 605, association and authentication are performed between the client and the target AP via the virtual link.

Before the client switches to the target AP, information interaction between the client and the target AP is performed via a virtual link. In this step, when the client selects AP2 as the target AP according to the RSSI value returned by the candidate AP, the client sends an association request to AP2 via a virtual link with AP2 and receives an association response returned by AP2 ; in addition, in the application scenario where the re- quirements regarding security are relatively high, the client sends an authentication request to AP2 via the virtual link and receives an authentication response returned by AP2. Taking the abovementioned second method for establishing a virtual link for example, the client can send to API, via its wireless link with API, an association request that needs to be sent to AP2 , and after receiving the association request, API forwards the association request to AP2 via its wired link with AP2 ; after receiving the association request, AP2 sends to API, via its wired link with API, an association response that needs to be sent to the client, and after receiving the association response, API forwards the association response to the client via its wireless link with the client. Likewise, the client can complete the authentication operation with AP2 by way of relaying API, which will not be described here.

Since the operation performed by the client in steps 601 to 605 is pre-switching, and the client still maintains communication with the current AP, the information interaction process between the client and target AP does not take up the switching time . Furthermore, the association response returned by the target AP can also include channel information about the target AP .

Step 606, after receiving, via the virtual link, the authentication successful response returned by the target AP, the cli- ent can generate a virtual connection table for maintaining the switching information required for establishing a connection between the client and the target AP. In this embodiment, the switching information may comprise: the link status information (such as RSSI value) , the beacon interval information, and the MAC address, SSID and channel of the target AP, and other information .

Step 607, when the client starts switching, the client terminates the communication with the current AP and switches to the channel of the target AP, thus completing the switching to the target AP.

In addition, the client can update its system parameters according to the switching information maintained in the virtual connection table and send a switching complete signal to the target AP.

In this step, in conjunction with the example shown in Fig. 5, when the client moves to point B where API resides, the signal emitted by API suddenly weakens, while the signal emitted by AP2 strengthens, and at this moment, the client starts switching. The client terminates the communication with API and switches to the channel of AP2 , thus completing the switching operation .

Step 608, after receiving the switching complete signal sent by the client, the target AP updates its system parameters and completes switching. After switching, the target AP becomes the current AP of the client.

So far, the switching process ends.

Compared with the fast switching mechanism in the prior art, the switching method proposed in the embodiments of the present invention has the following advantages: In the fast switching method proposed in the embodiments of the present invention, a virtual link is established using a communication link between the client and the current AP and communication links between the current AP and the candidate APs . Via this virtual link, the client can complete the information interaction with the target AP before switching to the target AP without interrupting the communication with the current AP, for example, the client obtains the RSSI values emitted by the candidate APs, selects the target AP and performs association and authentication with the target AP, and all the operations before switching can be completed at the pre- switching stage through the virtual link, whereas when the signal strength emitted by the current AP weakens and the real switching starts, the only thing the client needs to do is to update its system parameters based on the switching information maintained by itself. Therefore, the switching method can not only reduce the switching-deciding time, but also reduce the connection- establishment time, so that the time for the client to switch from one AP to another AP will be greatly reduced. In the switching method proposed in the embodiments of the present invention, all the information interaction between the client and the candidate AP is conducted through the virtual link, which can guarantee the quality of communications between the client and the candidate APs, thus avoiding an increase in the number of APs. Moreover, since the client decides to switch before the signal strength of the current AP weakens, and at this moment, its communication with the current AP has not been interrupted, the quality of communication between the client and the current AP can be guaranteed.

In the switching method proposed in the embodiments of the pre- sent invention, using the method of the candidate AP

transfering onto the channel where the client currently resides to monitor the message sent by the client, the client can frequently transfer onto a channel where another AP resides and to send a message without interrupting the communication with the current AP, thereby simplifying the operation of the client. Moreover, since the client still maintains communication with the current AP, the candidate AP can continuously monitor the message sent by the client, thus quickly obtaining the RSSI value so as to facilitate the reduction of the switching- deciding time of the client.

In the switching method proposed in the embodiments of the present invention, all the information used for switching can be interacted through the virtual link before the client switches to the target AP, and can be implemented by means of a distributed flag or an IP route under the IEEE802.il standard framework; therefore, no changes need to be made to the relevant part in the IEE802.11 standard, but simply modifying the software of the client and the AP instead.

The fast switching method proposed in the embodiments of the present invention is particularly suitable for the wireless communication scenario where at least one of the AP and the client is using directional antennas, such as a wireless commu- nication scenario in a rail transit system.

Of course, the switching method proposed in the embodiments of the present invention is also suitable for the wireless communication scenario where the AP and the client use omnidirec- tional antennas. In the application scenario where omnidirectional antennas are used, there is no such a case as that the signal strength suddenly weakens when the client is closest to the current AP, instead, the signal strength of the current AP increasingly weakens as the client is further away from the current AP . Therefore, the method for the client to decide the switching timing can be slightly modified, for example, when the client is informed that the signal strength of the current AP is below a preset threshold, the client decides to switch.

Another embodiment of the present invention also proposes a fast switching method in a wireless communication system, the method comprising the steps of: when a client decides to switch, its current AP is used to establish virtual links between the client and candidate APs, the virtual links including a communication link between the client and the current AP and communication links between the current AP and the candidate APs; the current AP receives a probe request sent, via the virtual links, by the client to the candidate APs and forwards the probe request to the candidate APs, and receives probe responses sent, via the virtual links, by the candidate APs to the client and forwards the probe responses to the client; the current AP receives an association request sent, via the virtual link, by the client to a target AP and forwards the association request to the target AP, and receives an association response sent, via the virtual link, by the target AP to the client and forwards the association response to the client; and when the client starts switching, the current AP terminates the communication with the client.

In this embodiment, when the abovementioned first method for establishing a virtual link is used, the current AP can estab- lish virtual links between the client and the candidate APs by modifying its MAC address filter. In application scenarios where the requirements on security are relatively high, before the client switches to the target AP, the method may further comprise: the current AP receives an authentication request sent, via the virtual link, by the client to the target AP and forwards the authentication request to the target AP, and receives an authentication response sent, via the virtual link, by the target AP to the client and forwards the authentication response to the client. Another embodiment of the present invention also proposes a fast switching method in a wireless communication system, the method comprising the steps of: when a client decides to switch, candidate access points (APs) establish, via a current AP of the client, virtual links with the client, the virtual links including communication links between the candidate APs and the current AP and a communication link between the current AP and the client; the candidate APs receive, via the virtual links, a probe request of the client and send probe responses to the client; and the candidate APs receive, via the virtual links, an association request of the client and send association responses to the client.

Preferably, the probe request can contain information about a channel where the client currently resides. Then in this embodiment, the method further comprises: the candidate APs transfer onto the channel to measure the strength of a received signal, and contain the measured strength information about the received signal in the probe response.

Furthermore, in this embodiment, the method may further com- prise: the candidate APs receive, via the virtual links, an authentication request of the client and send authentication responses to the client, to meet the high security requirements. The abovementioned steps performed by the current AP and candidate APs have been described in detail in the foregoing embodiments, and will not be described here.

According to the switching method proposed in the abovementioned embodiments, the embodiments of the present invention also propose a fast switching system in a wireless communication system, the system comprising a client and a plurality of APs.

The client is used for establishing, via its current AP, virtual links with candidate APs when it decides to switch, the virtual links including a communication link between the client and the current AP and communication links between the current AP and the candidate APs; sending a probe request, via the virtual links, to said candidate APs and receiving probe responses of said candidate APs; selecting, according to the probe responses of said candidate APs, a target AP from the candidate APs; sending an association request, via said virtual link, to said target AP and receiving an association response of said target AP; and terminating the communication with the current AP and switching to the target AP when switching is started; the current AP is used for establishing virtual links between the client and the candidate APs when the client decides to switch; receiving a probe request sent, via the virtual link, by the client to the candidate APs and forwarding the probe request to the candidate APs, and receiving probe responses sent, via the virtual links, by the candidate APs to the client and forwarding the probe responses to the client; receiving an association request sent, via the virtual link, by the client to the target AP and forwarding the association request to the target AP, and receiving an association response sent, via the virtual link, by the target AP to the client and forwarding the association response to the client; and terminating the communication with the client when the client starts switching; and the candidate APs are used for establishing, via the current AP of the client, virtual links with the client when the client decides to switch; receiving, via the virtual links, a probe request of the client and sending probe responses to the client; and receiving, via the virtual link, an association request of the client and sending association responses to the client . In this system, for the detailed functions of the client and the AP and the processing done thereby, please refer to the relative description in the abovementioned method embodiment, which will not described here. According to the switching method proposed in the abovementioned embodiments, one embodiment of the present invention also proposes a structural schematic diagram of a fast switching client in a wireless communication system. As shown in Fig. 7, the client comprises: a first virtual link establishment module 701 for establishing, via a current AP of the client, virtual links between the client and candidate APs when the client decides to switch, the virtual links including a communication link between the client and the current AP and communication links between the current AP and the candidate APs; a first probe module 702 for sending a probe request, via said virtual link, to said candidate APs and receiving probe re- sponses of said candidate APs; a selection module 703 for selecting, according to the probe responses of the candidate APs, a target AP from the candidate APs;

a first association module 704 for sending an association request, via the virtual link, to said target AP and receiving an association response of said target AP; and a first switching module 705 for terminating the communication with the current AP and switching to the target AP when the client starts switching.

In this embodiment, the first virtual link establishment module

701 is used for establishing, via the current AP, virtual links with the candidate APs using a distributed flag when the client and the candidate APs are in the same subnet. If the client and the candidate APs are in different subnets, then the first virtual link establishment module 701 can use an IP route to establish, via the current AP, virtual links with the candidate APs . In this embodiment, the probe request can contain information about a channel where the client currently resides; and the probe response can contain strength information about received signals measured by the candidate APs over the channel. In this embodiment, when the requirements regarding system security are relatively high, the client can further comprise a first authentication module for sending, via the virtual link, an authentication request to the target AP and receiving an authentication response of the target AP.

In this embodiment, the association response can contain information about a channel of the target AP; and when the client starts switching, the first switching module 705 is used for switching to the channel of the target AP .

In this embodiment, the client can further comprise: a switching deciding module for deciding to switch when the signal strength of the current AP received by the first probe module

702 exceeds a preset signal threshold; or for deciding to switch when the distance between the client and the current AP is smaller than a preset distance threshold. As shown in Fig. 8, one embodiment of the present invention also provides an AP in a wireless communication system. The AP is a current AP of a client, and can comprise: a second virtual link establishment module 801 for establishing virtual links between the client and candidate APs when the client decides to switch, the virtual links including a communication link between the client and the AP and communication links between the AP and the candidate APs; a second probe module 802 for receiving a probe request sent, via the virtual links, by the client to the candidate APs and forwarding the probe request to the candidate APs, and receiving probe responses sent, via the virtual links, by the candi- date APs to the client and forwarding the probe responses to the client; a second association module 804 for receiving an association request sent, via the virtual link, by the client to the target AP and forwarding the association request to the target AP, and receiving an association response sent, via the virtual link, by the target AP to the client and forwarding the association response to the client; and a second switching module 805 for terminating the communication with the client when the client starts switching.

In this embodiment, optionally, the second virtual link establishment module 801 can be used for establishing virtual links between the client and the candidate APs by modifying a MAC address filter of the AP .

In addition, in this embodiment, said AP can further comprise a second authentication module 806 for receiving an authentica- tion request sent, via the virtual link, by the client to the target AP and forwarding the authentication request to the target AP, and receiving an authentication response sent, via the virtual link, by the target AP to the client and forwarding the authentication response to the client.

As shown in Fig. 9, one embodiment of the present invention al- so provides an AP in a wireless communication system. The AP is a candidate AP, and can comprise: a third virtual link establishment module 901 for establishing, via a current AP of a client, a virtual link with the client when the client decides to switch, the virtual link including a communication link between the AP and the current AP and a communication link between the current AP and the client; a third probe module 902 for receiving, via the virtual link, a probe request of the client and sending a probe response to the client; and a third association module 904 for receiving, via the virtual link, an association request of the client and sending an asso- ciation response to the client.

In this embodiment, the probe request can preferably contain information about a channel where the client currently resides; and the third probe module 902 is then used for measuring the strength of a received signal over the current channel of the client, and containing the measured strength information about the received signal in the probe response.

In addition, in this embodiment, the candidate AP can further comprise a third authentication module 906 for receiving, via said virtual links, an authentication request of said client and sending an authentication response to said client.

For a detailed description of the fast switching system, client and AP which are provided in the embodiments of the present invention, please refer to the relevant description in the above- mentioned method embodiment, which will not be described here. Using the switching system, client and AP according to the embodiments of the present invention can also obtain the technical effects that can be obtained by the abovementioned fast switching method according to the embodiments of the present invention, which will not be described in detail here.

Although a description has been made above with regard to the particular embodiments of the present invention by taking the rail transit wireless communication system as an example, a person skilled in the art should understand that the present invention is not limited to this. In other wireless communication systems that involve switching issues, such as mesh systems, etc., the fast switching solutions according to the embodiments of the present invention are all applicable without departing from the technical concept of the present invention.

It is to be noted that not all of the steps and modules in each of the abovementioned flow diagrams and structural schematic diagrams are necessary, and it is possible to ignore certain steps or modules according to actual needs. The execution sequence of each step is not fixed and can be adjusted as needed. The system structure as described in each of the abovementioned embodiments may be a physical structure or a logical structure, i.e., some modules may be implemented by one and the same phys- ical entity, or some modules may be implemented respectively by a plurality of physical entities, or may be implemented by certain components in a plurality of independent devices together.

In each of the above embodiments, the hardware unit may be im- plemented either mechanically or electrically. For example, a hardware unit may include a permanently dedicated circuit or logic (such as a dedicated processor, FPGA, or ASIC) to perform a corresponding operation. The hardware unit may further include a programmable logic or circuit (such as a general pur- pose processor, or other programmable processors) that can perform a corresponding operation by way of temporary software settings. Particular implementations (a mechanical means, or a dedicated permanent circuit or a temporarily set circuit) may be determined based on considerations of cost and time.

The present invention also provides a machine-readable medium which stores an instruction that enables a machine to execute the fast switching method according to the embodiments of the present invention. Particularly, a system or a device with a storage medium can be provided, on which storage medium there are stored software program codes for implementing the func- tions of any of the abovementioned embodiments, and a computer (or CPU or MPU) of the system or device is enabled to read and execute the program codes stored in the storage medium.

In this case, the functions of any of the abovementioned embod- iments can be implemented by the program codes per se read from the storage medium, therefore the program codes and the storage medium storing the program codes constitute a part of the present invention. The embodiments of the storage medium for providing program codes include a floppy disk, a hard disk, a magnetic optical disk, an optical disk (such as CD-ROM, CD-R, CD-RW, DVD-ROM, DVD-RAM, DVD-RW, and DVD+RW) , a magnetic tape, a non-volatility storage card, and a ROM. Selectively, the program codes can be downloaded from a server computer via a communication network.

In addition, it should be clarified that the operating system running in the computer can be enabled to complete part of or all of the practical operations, not only can be by way of exe- cuting the program codes read out by the computer, but also can be by way of instructions based on the program codes, thereby realizing the function of any one of the above-mentioned embodiments . In addition, it can be understood that writing the program codes read from the storage medium into the memory provided in the extension board inserted inside the computer or into the memory provided in the extension unit connected to the computer, subsequently the instructions based on the program codes cause the CPU mounted on the extension board or the extension unit to execute part of or all of the practical operations, thereby realizing the function of any one of the abovementioned embodiments .

What are mentioned above are merely the preferred embodiments of the present invention, and they are not intended to limit the protection scope of the present invention. All modifications, equivalent replacements and improvements within the spirit and principle of the present invention should be covered within the scope of protection of the present invention.