WO2013053897A1

WO2013053897A1 - Client, access point, communication system and switching method

Info

Publication number: WO2013053897A1
Application number: PCT/EP2012/070272
Authority: WO
Inventors: Mattias Lampe; Jie Zhang
Original assignee: Siemens Aktiengesellschaft
Priority date: 2011-10-14
Filing date: 2012-10-12
Publication date: 2013-04-18
Also published as: EP2754317A1; CN103052124A

Abstract

The embodiments of the present invention provide a client, access point, communication system and switching method. When the client is moving in a direction in which it first approaches the first access point and then approaches a second access point, and is within the extent of common coverage of the first access point and the second access point, a signal receiving module is used for receiving signals from the first access point and the second access point via omnidirectional antenna; a switching condition judgment module is used for judging whether a switching condition is satisfied on the basis of the signal strength of a signal received from the first access point or the signal strength of signals received from the first access point and the second access point, and if it is, sending a switching execution instruction to a switching execution module; the switching execution module is used for initiating a switching program on the basis of the switching execution instruction to switch the present client from the first access point to the second access point. The client, access point, communication system and switching method of the embodiments of the present invention ensure the reliability of switching.

Description

Client, access point, communication system and switching method

Technical field

The present invention relates to the field of wireless communication, and in particular to a client, access point, communication system and switching method .

Background art

Owing to the numerous advantages of subway control and passenger information systems based on wireless communication, such as continuous bidirectional connection, a relatively high data transmission rate and relatively low costs, such systems are receiving more and more attention at present. Taking a wireless subway communication system based on a wireless local area network (WLA ) as an example, a number of access points (AP) are positioned along a railway track, while a client is provided at the train end, so the client on the train is able to exchange information with trackside equipment via a wireless area network.

Fig. 1 is a structural schematic diagram of a WLAN containing a switching mechanism in the prior art. As Fig. 1 shows, access points APi_i, APi and APi₊i are disposed in succession along a subway track from left to right, while a client is provided on a train moving from left to right. Two directional antennas are provided on each access point, one facing in the direction of the train's motion and the other facing in a direction opposite to that of the train's motion. One directional antenna facing in the direction of the train's motion is provided on the client. A directional antenna facing in the direction of the train's motion is referred to as a forward antenna, and a directional antenna facing in a direction opposite to that of the train's motion is referred to as a backward antenna. Signals emitted by the forward antennas and backward antennas disposed on the access points cover the entire length of the subway line, and as the train moves from left to right along the track, the train-mounted client switches from one access point to another.

Specifically, when the client is in the range of coverage of APi, the forward antenna of the client receives signals emitted from the backward antenna of APi. Once the client has passed point A and reached the region to the right of APi, the signal strength of signals emitted by the backward antenna of APi in this region falls sharply, so that before the client switches from APi to APi₊i and establishes a connection with AP_i+i, the signal strength of signals received by the client from APi falls to a very low level or even to zero; thus it cannot be guaranteed that the client will be able to switch smoothly from APi to APi₊i, and a break in communication is likely to occur. However, the requirements of subway control systems with regard to reliability of communication connections are very high. In actual applications, this switching scheme is unable to meet the demands of subway control systems.

In order to solve this problem, existing technology provides a different switching solution. Fig. 2 is a structural schematic diagram of a WLAN containing a different switching mechanism in the prior art, the structure of Fig. 2 being similar to that of Fig. 1. As the client moves to point B to the left of APi, the signal strength of signals received by the client from APi is gradually increasing, and the client begins to switch at point B. In other words, when the signal strength of signals received by the client from APi is greater than or equal to a predetermined received signal strength indicator (RSSI) threshold, the client begins to switch. At this time, the signal strength of signals received by the client from APi is large enough, and there is sufficient time before the signal strength falls sharply to ensure that the client switches from APi to APi₊i . This switching scheme is capable of reducing the signal loss caused by a sharp drop in signal strength effectively, and reducing the switching decision time.

However, in particular applications, point B is the point at which switching begins, and it must be ensured that signals from APi₊i are able to cover point B in order to guarantee that the client will be able to establish a connection with APi₊i smoothly, and this will then increase the extent of the common coverage area of APi and APi₊i . In order to increase the extent of the common coverage area of APi and AP_i+i , it is necessary to provide more access points beside the subway track so as to decrease the distance between two adjacent access points, and this will increase the cost of the system. Furthermore, it is necessary to choose an appropriate point for switching to begin and an appropriate extent of common coverage area beforehand, in order to ensure that on the one hand the signal strength of signals received by the client from APi₊i is sufficiently large for the client to be able to establish a communication connection with APi₊i , and on the other that the extent of the common coverage area is as small as possible. The determination of the point at which switching begins is dependent upon the determination of the RSSI threshold, but the process of propagation of wireless signals, i.e. electromagnetic waves, is affected by various environmental factors from the surroundings, so in reality it is very difficult to determine a precise RSSI threshold for each access point.

It can be seen from the above that in the embodiment shown in Fig. 2, given that the signal strength of signals received by the forward antenna of the client from the backward antenna of APi will drop sharply, the RSSI threshold must be determined precisely so as to accurately determine the point at which switching shall begin if it is to be ensured that the client will have sufficient time to switch from APi to APi₊i ; however, the RSSI threshold is very difficult to determine precisely. If the determined RSSI threshold is inappropriate, the client will be unable to switch from APi to APi₊i , and the reliability of switching cannot be guaranteed. Furthermore, the quality of switching is not high, and the system has a relatively high cost .

Content of the invention

In view of the above, the embodiments of the present invention provide a client, access point, communication system and switching method, for improving the reliability of switching between access points .

The embodiments of the present invention provide a client, the client comprising a forward antenna and a signal receiving module, the signal receiving module being used to receive a signal from a first access point via the forward antenna; the client further comprises: an omnidirectional antenna, a switching condition judgment module and a switching execution module ; wherein when the client is moving in a direction in which it first approaches the first access point and then approaches a second access point, and is within the extent of common coverage of the first access point and the second access point, the signal receiving module is further used for receiving signals from the first access point and second access point via the omnidirectional antenna; the switching condition judgment module is used for judging whether a switching condition is satisfied on the basis of the signal strength of a signal received from the first access point or the signal strength of signals received from the first access point and the second access point, and if it is, sending a switching execution instruction to the switching execution module, wherein the signal received from the first access point comprises: a signal received from the first access point using the omnidirectional antenna or using the omnidirectional antenna and the forward antenna; the signals received from the first access point and the second access point comprise: signals received from the first access point and the second access point using the omnidirectional antenna or using the omnidirectional antenna and the forward antenna; the switching execution module is used for initiating a switching program on the basis of the switching execution instruction to switch the present client from the first access point to the second access point.

Preferably, the switching condition judgment module is specifically used for: judging whether the signal strength of a signal received from the first access point is less than or equal to a first RSSI threshold, and if it is, sending the switching execution instruction to the switching execution module; or judging whether the difference between the signal strength of a signal received from the first access point and the signal strength of a signal received from the second access point is less than or equal to a set value, and if it is, sending the switching execution instruction to the switching execution module .

Preferably, the client further comprises: a signal strength judgment module and an omnidirectional antenna opening module; the signal strength judgment module is used for judging whether the signal strength of a signal received from the first access point by the signal receiving module via the forward antenna is greater than or equal to a second RSSI threshold, and if it is, sending an omnidirectional antenna opening signal to the omnidirectional antenna opening module; the omnidirectional antenna opening module is used for opening the omnidirectional antenna on the basis of the omnidirectional antenna opening signal .

Preferably, the client further comprises: a switching completion judgment module and an omnidirectional antenna closing module; the switching completion judgment module is used for receiving an instruction from the switching execution module, and on the basis of this instruction judging whether the present client has completed the switching operation, and if it has, sending an omnidirectional antenna closing signal to the omnidirectional antenna closing module; the omnidirectional antenna closing module is used for closing the omnidirectional antenna on the basis of the omnidirectional antenna closing signal .

The embodiments of the present invention provide an access point, the access point comprising a backward antenna and a signal sending module, the signal sending module being used for sending a signal to the client via the backward antenna; the access point further comprises an omnidirectional antenna, and during switching of a client from the present access point to the next access point, the signal sending module is further used for sending a signal to the client at least via the omnidirectional antenna.

Preferably, the access point further comprises: a signal receiving module, a signal strength judgment module and an omnidirectional antenna opening module, wherein the signal receiving module receives a signal sent by the client via the backward antenna; the signal strength judgment module is used for judging whether the signal strength of a signal received by the signal receiving module from the client is greater than or equal to a first RSSI threshold, and if it is, sending an omnidirectional antenna opening signal to the omnidirectional antenna opening module ; the omnidirectional antenna opening module is used for opening the omnidirectional antenna on the basis of the omnidirectional antenna opening signal .

Preferably, the access point further comprises: a switching completion judgment module and an omnidirectional antenna closing module; the switching completion judgment module is used for judging whether all clients associated with the access point have completed the switching operation, and if they have, sending an omnidirectional antenna closing signal to the omnidirectional antenna closing module; the omnidirectional antenna closing module is used for closing the omnidirectional antenna on the basis of the omnidirectional antenna closing signal .

The embodiments of the present invention provide a communication system, the system comprising: a first access point, a second access point and any client described above, the client moving in a direction in which it first approaches the first access point and then approaches the second access point .

Preferably, the first access point comprises an omnidirectional antenna, and is capable of communicating with the client via the omnidirectional antenna at least during switching of the client from the first access point to the second access point. Preferably, the second access point comprises an omnidirectional antenna, and is capable of communicating with the client via the omnidirectional antenna at least during switching of the client from the first access point to the second access point.

The embodiments of the present invention provide another communication system, the system comprising: a client, a first access point and a second access point; at least one of the first access point and the second access point is any access point described above, and the client moves in a direction in which it first approaches the first access point and then approaches the second access point.

The embodiments of the present invention provide a switching method, wherein any client described above moves in a direction in which it first approaches a first access point and then approaches a second access point, the method comprising: the client moving into the area of coverage of the first access point, establishing a connection with the first access point, and communicating with the first access point via the forward antenna of the client; the client moving into the common coverage area of the first access point and the second access point, receiving signals from the first access point and the second access point at least via the omnidirectional antenna of the client, and judging whether a switching condition is satisfied on the basis of the signal strength of a signal received from the first access point or the signal strength of signals received from the first access point and the second access point, and if it is, initiating a switching program to switch from the first access point to the second access point; wherein the signal received from the first access point comprises: a signal received by the client from the first access point using the omnidirectional antenna or using the omnidirectional antenna and the forward antenna, and the signals received from the first access point and the second access point comprise: signals received by the client from the first access point and the second access point using the omnidirectional antenna or using the omnidirectional antenna and the forward antenna.

Preferably, the step of judging whether a switching condition is satisfied on the basis of the signal strength of a signal received from the first access point comprises: judging whether the signal strength of a signal received from the first access point is less than or equal to a first RSSI threshold; the step of judging whether a switching condition is satisfied on the basis of the signal strength of signals received from the first access point and the second access point comprises: judging whether the difference between the signal strength of a signal received from the first access point and the signal strength of a signal received from the second access point is less than or equal to a set value.

Preferably, the method further comprises: the client judging whether the signal strength of a signal received from the first access point using the forward antenna is greater than or equal to a second RSSI threshold, and if it is, opening the omnidirectional antenna; the client judging whether it has switched to the second access point, and if it has, closing the omnidirectional antenna.

The embodiments of the present invention provide another switching method, wherein a client moves in a direction in which it first approaches a first access point and then approaches a second access point, at least one of the first access point and the second access point being any access point described above, the method comprising: the client moving into the area of coverage of the first access point, establishing a connection with the first access point, and communicating with the first access point via the backward antenna of the first access point; the client moving into the common coverage area of the first access point and the second access point, receiving signals sent at least via omnidirectional antenna by the first access point and second access point, judging whether a switching condition is satisfied on the basis of the signal strength of a signal received from the first access point or the signal strength of signals received from the first access point and the second access point, and if it is, initiating a switching program to switch from the first access point to the second access point; wherein the signal received from the first access point comprises: a signal sent by the first access point using the omnidirectional antenna, or the omnidirectional antenna and backward antenna, and received by the client; the signals received from the first access point and the second access point comprise: signals sent by the first access point and second access point using the omnidirectional antennas, or the omnidirectional antennas and backward antennas, and received by the client.

Preferably, the method further comprises: the first access point judging whether the signal strength of a signal received from the client using the backward antenna is greater than or equal to a second RSSI threshold, and if it is, opening the omnidirectional antenna; the first access point judging whether all clients associated therewith have successfully switched to the second access point, and if they have, closing the omnidirectional antenna.

The embodiments of the present invention provide another switching method, wherein any client described above moves in a direction in which it first approaches a first access point and then approaches a second access point, at least one of the first access point and the second access point being any access point described above, the method comprising: the client moving into the area of coverage of the first access point, establishing a connection with the first access point, and communicating with the backward antenna of the first access point via the forward antenna of the client; the client moving into the common coverage area of the first access point and the second access point, receiving at least via the omnidirectional antenna of the client signals sent at least via omnidirectional antenna by the first access point and the second access point, and judging whether a switching condition is satisfied on the basis of the signal strength of a signal received from the first access point or the signal strength of signals received from the first access point and the second access point, and if it is, initiating a switching program to switch from the first access point to the second access point; wherein the signal received from the first access point comprises: a signal sent by the first access point using the omnidirectional antenna thereof or the backward antenna and omnidirectional antenna thereof and received by the client using the omnidirectional antenna thereof or the omnidirectional antenna and forward antenna thereof, and the signals received from the first access point and the second access point comprise: signals sent by the first access point and the second access point using the omnidirectional antennas thereof or the omnidirectional antennas and backward antennas thereof and received by the client using the omnidirectional antenna thereof or the omnidirectional antenna and forward antenna thereof .

By way of the technical solution of the present invention, a forward antenna and omnidirectional antenna are installed on the client, and when the client is moving in a direction in which it first approaches a first access point and then approaches a second access point, and is within the extent of common coverage of the first access point and the second access point, the client receives signals from the first access point and second access point at least via the omnidirectional antenna, and judges whether a switching condition is satisfied using the signals received by the omnidirectional antenna; if it is, the client switches from the first access point to the second access point. Since the client receives signals from the first access point and second access point using the omnidirectional antenna when it is within the extent of common coverage of the first access point and the second access point, a sharp drop in the signal strength of the signal received from the first access point can be avoided, thus it can be ensured that all clients can maintain good communication with the first access point before switching to the second access point. In such circumstances, there is no need to precisely determine the received signal strength indicator (RSSI) threshold, therefore interruption in communication caused by the determined RSSI threshold being unsuitable is avoided, ensuring the reliability of switching.

Description of the accompanying drawings

Fig. 1 is a structural schematic diagram of a WLA containing a switching mechanism in the prior art;

Fig. 2 is a structural schematic diagram of a WLAN containing a different switching mechanism in the prior art;

Fig. 3 is a schematic diagram of a first preferred structure of a client in the embodiments of the present invention;

Fig. 3a is a schematic diagram of a second preferred structure of a client in the embodiments of the present invention;

Fig. 4 is a schematic diagram of a first preferred structure of an access point in the embodiments of the present invention;

Fig. 4a is a schematic diagram of a second preferred structure of an access point in the embodiments of the present invention; Fig. 5a is a schematic diagram of a first preferred structure of the communication system in the embodiments of the present invention;

Fig. 5b is a schematic diagram of a second preferred structure of the communication system in the embodiments of the present invention;

Fig. 6 is a first preferred schematic flow chart of the switching method in the embodiments of the present invention;

Fig. 6a is a schematic diagram of a first preferred embodiment of the client, first access point and second access point in the switching method shown in Fig. 6;

Fig. 6b is a schematic diagram of a second preferred embodiment of the client, first access point and second access point in the switching method shown in Fig. 6 ;

Fig. 7 is a schematic flow chart of a second preferred embodiment of the switching method in the embodiments of the present invention;

Fig. 7a is a schematic diagram of a first preferred embodiment of the client, first access point and second access point in the switching method shown in Fig. 7;

Fig. 7b is a schematic diagram of a second preferred embodiment of the client, first access point and second access point in the switching method shown in Fig. 7;

Fig. 8 is a schematic flow chart of a third preferred embodiment of the switching method in the embodiments of the present invention; Fig. 8a is a schematic diagram of the client, first access point and second access point in the switching method shown in Fig. 8.

In the drawing :

301: forward antenna 302: omnidirectional antenna

303: signal receiving module 304: switching condition judgment module 305: switching execution module

301a: forward antenna 302a: omnidirectional antenna

303a: signal receiving module 304a: switching condition judgment module 305a: switching execution module

306a: signal strength judgment module 307a: omnidirectional antenna opening module 308a: switching completion judgment module 309a: omnidirectional antenna closing module

401: backward antenna 402: omnidirectional antenna

403 : signal sending module

401a: backward antenna 402a: omnidirectional antenna

403a: signal sending module 404a: signal receiving module 405a: signal strength judgment module 406a:

omnidirectional antenna opening module 407a: switching completion judgment module 408a: omnidirectional antenna closing module

501a: first access point 502a: second access point

503a: client

501b: first access point 502b: second access point

503b: client

601: client communicating with first access point using forward antenna 602 : judging whether a switching condition is satisfied on the basis of the signal strength of a signal received by the client at least via omnidirectional antenna, and if it is, performing switching

701: client communicating with first access point via backward antenna of first access point

702: judging whether a switching condition is satisfied on the basis of the signal strength of received signals sent at least via omnidirectional antenna by the first access point or by the first access point and the second access point, and if it is, performing switching

801: client using forward antenna thereof to communicate with backward antenna of first access point

802 : judging whether a switching condition is satisfied on the basis of the signal strength of signals sent by the first access point or by the first access point and second access point at least via omnidirectional antenna and received by the client at least via omnidirectional antenna, and if it is, performing switching.

Particular embodiments

In order to clarify the object, technical solution and advantages of the present invention, the present invention will be described in further detail below with reference to the accompanying drawings, by way of examples.

Fig. 3 is a schematic diagram of a first preferred structure of a client in the embodiments of the present invention. As the figure shows, the client comprises: a forward antenna 301, an omnidirectional antenna 302, a signal receiving module 303, a switching condition judgment module 304 and a switching execution module 305. The signal receiving module 303 is used to receive a signal from a first access point via the forward antenna 301. When the client is moving in a direction in which it first approaches the first access point and then approaches a second access point, and is within the extent of common coverage of the first access point and the second access point, the signal receiving module 303 is further used for receiving signals from the first access point and second access point via the omnidirectional antenna 302.

The switching condition judgment module 304 is used for judging whether a switching condition is satisfied on the basis of the signal strength of a signal received by the signal receiving module 303 from the first access point or the signal strength of signals received from the first access point and the second access point, and if it is, sending a switching execution instruction to the switching execution module 305. The signal received from the first access point comprises: a signal received from the first access point using the omnidirectional antenna 302 or using the omnidirectional antenna 302 and the forward antenna 301; the signals received from the first access point and the second access point comprise: signals received from the first access point and the second access point using the omnidirectional antenna 302 or using the omnidirectional antenna 302 and the forward antenna 301.

The switching execution module 305 is used for initiating a switching program on the basis of the switching execution instruction sent by the switching condition judgment module 304, to switch the present client from the first access point to the second access point.

In the embodiments of the present invention, a forward antenna and omnidirectional antenna are installed on the client, and when the client is moving in a direction in which it first approaches a first access point and then approaches a second access point, and is within the extent of common coverage of the first access point and the second access point, the client is able to receive signals from the first access point and second access point using the omnidirectional antenna, and judges whether a switching condition is satisfied using the signals received by the omnidirectional antenna; if it is, the client switches from the first access point to the second access point. Since the client receives signals from the first access point and second access point using the omnidirectional antenna when it is within the extent of common coverage of the first access point and the second access point, a sharp drop in the signal strength of the signal received from the first access point can be avoided, thus it can be ensured that all clients can maintain good communication with the first access point before switching to the second access point. In such circumstances, there is no need to precisely determine the received signal strength indicator (RSSI) threshold, therefore interruption in communication caused by the determined RSSI threshold being unsuitable is avoided, ensuring the reliability of switching.

In a particular application, the switching condition judgment module 304 judges whether the signal strength of a signal received from the first access point is less than or equal to a first RSSI threshold, and if it is, sends the switching execution instruction to the switching execution module 305; or judges whether the difference between the signal strength of a signal received from the first access point and the signal strength of a signal received from the second access point is less than or equal to a set value, and if it is, sends the switching execution instruction to the switching execution module 305. The first RSSI threshold and set value are set in advance according to actual requirements .

The embodiments of the present invention show judging whether a switching condition is satisfied on the basis of the strength of a signal received from the first access point or on the basis of the signal strength of signals received from the first access point and second access point, but it must be pointed out that actual applications are not limited to these two ways, and conditions for triggering switching may be set according to actual requirements; unnecessary details are not given here.

Fig. 3a is a schematic diagram of a second preferred structure of a client in the embodiments of the present invention. Compared to the embodiment shown in Fig. 3, in addition to containing a forward antenna 301a, an omnidirectional antenna 302a, a signal receiving module 303a, a switching condition judgment module 304a and a switching execution module 305a, the client further comprises: a signal strength judgment module 306a, an omnidirectional antenna opening module 307a, a switching completion judgment module 308a and an omnidirectional antenna closing module 309a.

As the client gradually approaches the first access point, the signal strength of signals received by the client from the first access point gradually increases. The signal strength judgment module 306a is used for receiving from the signal receiving module 303a a signal received from the first access point by the signal receiving module 303a using the forward antenna 301a, and judging whether the signal strength of the signal from the first access point is greater than or equal to a second RSSI threshold, and if it is, sending an omnidirectional antenna opening signal to the omnidirectional antenna opening module 307a. The second RSSI threshold is set in advance according to actual requirements .

The omnidirectional antenna opening module 307a is used for opening the omnidirectional antenna 302a on the basis of the omnidirectional antenna opening signal .

The switching completion judgment module 308a is used for receiving an instruction from the switching execution module 305a, and on the basis of this instruction judging whether the present client has completed the switching operation, and if it has, sending an omnidirectional antenna closing signal to the omnidirectional antenna closing module 309a.

The omnidirectional antenna closing module 309a is used for closing the omnidirectional antenna 302a on the basis of the omnidirectional antenna closing signal .

In particular operations, in addition to containing the forward antenna 301a, omnidirectional antenna 302a, signal receiving module 303a, switching condition judgment module 304a and switching execution module 305a, signal strength judgment module 306a, omnidirectional antenna opening module 307a, switching completion judgment module 308a and omnidirectional antenna closing module 309a shown in Fig. 3a, the client may also comprise only the forward antenna 301a, omnidirectional antenna 302a, signal receiving module 303a, switching condition judgment module 304a and switching execution module 305a, signal strength judgment module 306a and omnidirectional antenna opening module 307a described above, or the forward antenna 301a, omnidirectional antenna 302a, signal receiving module 303a, switching condition judgment module 304a and switching execution module 305a, switching completion judgment module 308a and omnidirectional antenna closing module 309a described above .

By way of the technical solution shown in Fig. 3a, when it is determined that the signal strength of a signal received from the first access point is greater than or equal to the second RSSI threshold, the omnidirectional antenna is opened; the omnidirectional antenna is closed after switching is complete. In this way, the omnidirectional antenna is only opened before switching, remains in an open state during switching, and is closed once switching is complete, so that signal interference caused by the omnidirectional antenna remaining continuously in an open state is avoided, improving the quality of switching. Fig. 4 is a schematic diagram of a first preferred structure of an access point in the embodiments of the present invention. As the figure shows, the access point comprises: a backward antenna 401, an omnidirectional antenna 402 and a signal sending module 403.

The signal sending module 403 is used for sending a signal to the client via the backward antenna 401, and for sending a signal to the client at least via the omnidirectional antenna 402 during switching of the client from the present access point to the next access point.

Fig. 4a is a schematic diagram of a second preferred structure of an access point in the embodiments of the present invention. As the figure shows, compared with the embodiment shown in Fig. 4, in addition to a backward antenna 401a, omnidirectional antenna 402a and signal sending module 403a, the access point also comprises: a signal receiving module 404a, a signal strength judgment module 405a, an omnidirectional antenna opening module 406a, a switching completion judgment module 407a and an omnidirectional antenna closing module 408a.

The signal receiving module 404a is used for receiving a signal sent by the client via the backward antenna 401a.

As the client gradually approaches the access point, the signal strength of signals received by the access point from the client gradually increases. The signal strength judgment module 405a is used for judging whether the signal strength of a signal received by the signal receiving module 404a from the client is greater than or equal to a first RSSI threshold, and if it is, sending an omnidirectional antenna opening signal to the omnidirectional antenna opening module 406a. The first RSSI threshold is set in advance according to actual requirements. The omnidirectional antenna opening module 406a is used for opening the omnidirectional antenna 402a on the basis of the omnidirectional antenna opening signal .

The switching completion judgment module 407a is used for judging whether all clients associated with the access point have completed the switching operation, and if they have, sending an omnidirectional antenna closing signal to the omnidirectional antenna closing module 408a.

The omnidirectional antenna closing module 408a is used for closing the omnidirectional antenna 402a on the basis of the omnidirectional antenna closing signal .

In particular operations, in addition to containing the backward antenna 401a, omnidirectional antenna 402a, signal sending module 403a, signal receiving module 404a, signal strength judgment module 405a, omnidirectional antenna opening module 406a, switching completion judgment module 407a and omnidirectional antenna closing module 408a shown in Fig. 4a, the access point may also comprise only the backward antenna 401a, omnidirectional antenna 402a, signal sending module 403a, signal receiving module 404a, signal strength judgment module 405a and omnidirectional antenna opening module 406a described above, or the backward antenna 401a, omnidirectional antenna 402a, signal sending module 403a, switching completion judgment module 407a and omnidirectional antenna closing module 408a described above .

By way of the embodiment shown in Fig. 4a, when it is determined that the signal strength of a signal received by the signal receiving module from the client is greater than or equal to a first RSSI threshold, the omnidirectional antenna is opened; the omnidirectional antenna is closed once it is determined that all clients associated with the access point have completed switching. In this way, the omnidirectional antenna is only opened before switching, remains in an open state during switching, and is promptly closed once switching is complete, so that signal interference caused by the omnidirectional antenna remaining continuously in an open state is avoided, improving the quality of switching.

Fig. 5a is a schematic diagram of a first preferred structure of the communication system in the embodiments of the present invention. As Fig. 5a shows, the communication system comprises: a first access point 501a, a second access point 502a, and the client 503a as shown in Fig. 3 or Fig. 3a. The client 503a moves in a direction in which it first approaches the first access point 501a, and then approaches the second access point 502a.

During actual operation, the first access point 501a comprises an omnidirectional antenna, and the first access point 501a communicates with the client 503a using the omnidirectional antenna at least during switching of the client 503a from the first access point 501a to the second access point 502a.

Furthermore, the second access point 502a comprises an omnidirectional antenna, and the second access point 502a communicates with the client 503a using the omnidirectional antenna at least during switching of the client 503a from the first access point 501a to the second access point 502a.

Fig. 5b is a schematic diagram of a second preferred structure of the communication system in the embodiments of the present invention. As Fig. 5b shows, the communication system comprises: a first access point 501b, a second access point 502b and a client 503b, wherein one of or each of the first access point 501b and the second access point 502b is an access point as shown in Fig. 4 or Fig. 4a. The client 503b moves in a direction in which it first approaches the first access point 501b and then approaches the second access point 502b. The above are preferred embodiments of the client, access point and communication system of the present invention; particular embodiments of the method of the present invention are set out in detail below.

Fig. 6 is a first preferred schematic flow chart of the switching method in the embodiments of the present invention. In this embodiment, the client shown in Fig. 3 or Fig. 3a moves in a direction in which it first approaches a first access point and then approaches a second access point, and the method specifically comprises:

Step 601: the client moving into the area of coverage of the first access point, establishing a connection with the first access point, and communicating with the first access point via the forward antenna of the client.

Step 602 : the client moving into the common coverage area of the first access point and the second access point, receiving signals from the first access point and the second access point at least via the omnidirectional antenna of the client, and judging whether a switching condition is satisfied on the basis of the signal strength of a signal received from the first access point or the signal strength of signals received from the first access point and the second access point, and if it is, initiating a switching program to switch from the first access point to the second access point. The signal received from the first access point comprises: a signal received from the first access point by the client using the omnidirectional antenna thereof or the omnidirectional antenna and forward antenna thereof; the signals received from the first access point and the second access point comprise: signals received from the first access point and the second access point by the client using the omnidirectional antenna thereof or the omnidirectional antenna and forward antenna thereof. In the switching method given in this embodiment of the invention, since the client receives signals from the first access point and second access point at least via the omnidirectional antenna when it is within the extent of common coverage of the first access point and the second access point, a sharp drop in the signal strength of the signal received from the first access point can be avoided, thus it can be ensured that all clients can maintain good communication with the first access point before switching to the second access point. In such circumstances, there is no need to precisely determine the RSSI threshold, therefore interruption in communication caused by the determined RSSI threshold being unsuitable is avoided, ensuring the reliability of switching.

In a particular application, the method of judging whether a switching condition is satisfied on the basis of the signal strength of a signal received from the first access point comprises: judging whether the signal strength of a signal received from the first access point is less than or equal to a first RSSI threshold, this first RSSI threshold being a value which is set in advance according to actual requirements. The step of judging whether a switching condition is satisfied on the basis of the signal strength of signals received from the first access point and the second access point comprises: judging whether the difference between the signal strength of a signal received from the first access point and the signal strength of a signal received from the second access point is less than or equal to a set value.

As the client gradually approaches the first access point, the signal strength of signals received by the client from the first access point increases continuously. In such a situation, the method further comprises: the client judging whether the signal strength of a signal received using the forward antenna thereof from the first access point is greater than or equal to a second RSSI threshold, and if it is, opening the omnidirectional antenna installed thereon. Once a switching program has been initiated, the method further comprises: the client judging whether it has switched to the second access point, and if it has, closing the omnidirectional antenna thereof .

Fig. 6a is a schematic diagram of a first preferred embodiment of the client, first access point and second access point in the switching method shown in Fig. 6. In this embodiment, APi denotes a first access point, and AP_i+i denotes a second access point. The client is fitted with a forward antenna and an omnidirectional antenna; APi and APi₊i are each fitted with an omnidirectional antenna.

The client moves from left to right, gradually approaching APi ; the client judges whether the signal strength of a signal received using the forward antenna thereof from APi is greater than or equal to a third RSSI threshold, and if it determines that the signal strength of a signal received from APi is greater than or equal to the third RSSI threshold when it moves to point A , the client opens the omnidirectional antenna installed thereon. When the client moves into the common coverage area of APi and AP_i+i , the client receives signals sent by the omnidirectional antennas of APi and APi₊i at least via the omnidirectional antenna of the client, judges whether a switching condition is satisfied on the basis of the signal strength of a signal received from APi or the signal strength of signals received from APi and APi₊i , and if it determines that the switching condition is satisfied at a point B covered by both APi and APi₊i , it initiates a switching program and switches from APi to APi₊i . The signal received from APi comprises: a signal received from the omnidirectional antenna of APi by the client using the omnidirectional antenna thereof or the omnidirectional antenna and forward antenna thereof; the signals received from APi and AP_i+i comprise: signals received from the omnidirectional antennas of APi and APi₊i by the client using the omnidirectional antenna thereof or the omnidirectional antenna and forward antenna thereof. Once the switching program has been initiated, the client further judges whether it has switched from APi to AP_i+i , and if it has, closes the omnidirectional antenna thereof.

Fig. 6b is a schematic diagram of a second preferred embodiment of the client, first access point and second access point in the switching method shown in Fig. 6. In this embodiment, APi denotes a first access point, and AP_i+i denotes a second access point. The client is fitted with a forward antenna and an omnidirectional antenna; APi and APi₊i are each fitted with a forward antenna and a backward antenna .

The client moves from left to right, gradually approaching APi ; the client judges whether the signal strength of a signal received using the forward antenna thereof from APi is greater than or equal to a fourth RSSI threshold, and if it determines that the signal strength of a signal received from APi is greater than or equal to the fourth RSSI threshold when it moves to point A , the client opens the omnidirectional antenna installed thereon. When the client moves into the common coverage area of APi and AP_i+i , the client receives at least via the omnidirectional antenna thereof signals sent by the forward antennas and backward antennas of APi and APi₊i , judges whether a switching condition is satisfied on the basis of the signal strength of a signal received from APi or the signal strength of signals received from APi and APi₊i , and if it determines that the switching condition is satisfied at a point B covered by both APi and APi₊i , it initiates a switching program and switches from APi to APi₊i . The signal received from APi comprises: signals received from the forward antenna and backward antenna of APi by the client using the omnidirectional antenna thereof or the omnidirectional antenna and forward antenna thereof; the signals received from APi and APi₊i comprise: signals received from the forward antennas and backward antennas of APi and APi₊i by the client using the omnidirectional antenna thereof or the omnidirectional antenna and forward antenna thereof. The client further judges whether it has switched from APi to AP_i+i, and if it has, closes the omnidirectional antenna thereof.

In the embodiments shown in Fig. 6a and Fig. 6b above, the clients are each fitted with a forward antenna and an omnidirectional antenna; the difference lies in the fact that in the embodiment shown in Fig. 6a, the first access point and second access point are each fitted with an omnidirectional antenna, whereas in the embodiment shown in Fig. 6b, the first access point and second access point are each fitted with a forward antenna and a backward antenna. The third RSSI threshold and fourth RSSI threshold may be the same or different, being set according to specific requirements.

Fig. 7 is a schematic flow chart of a second preferred embodiment of the switching method in the embodiments of the present invention. In this embodiment, a client moves in a direction in which it first approaches a first access point and then approaches a second access point; at least one of the first access point and the second access point is an access point as shown in Fig. 4 or Fig. 4a, and the method specifically comprises the following steps:

Step 701: the client moving into the area of coverage of the first access point, establishing a connection with the first access point, and communicating with the first access point via the backward antenna of the first access point.

Step 702: the client moving into the common coverage area of the first access point and the second access point, receiving signals sent at least via omnidirectional antenna by the first access point and second access point, judging whether a switching condition is satisfied on the basis of the signal strength of a signal received from the first access point or the signal strength of signals received from the first access point and the second access point, and if it is, initiating a switching program to switch from the first access point to the second access point. The signal received from the first access point comprises: a signal sent by the first access point using the omnidirectional antenna thereof or the omnidirectional antenna and backward antenna thereof, and received by the client; the signals received from the first access point and the second access point comprise: signals sent by the first access point and second access point using the omnidirectional antennas thereof or the omnidirectional antennas and backward antennas thereof, and received by the client.

In the switching method given in this embodiment of the invention, when within the extent of common coverage of the first access point and the second access point, the client receives signals sent at least via omnidirectional antenna by the first access point and second access point, so that a sharp drop in the signal strength of the signal received from the first access point can be avoided, thus it can be ensured that all clients can maintain good communication with the first access point before switching to the second access point. In such circumstances, there is no need to precisely determine the received signal strength indicator (RSSI) threshold, therefore interruption in communication caused by the determined RSSI threshold being unsuitable is avoided, ensuring the reliability of switching.

In particular applications, the step of judging whether a switching condition is satisfied on the basis of the signal strength of a signal received from the first access point comprises: judging whether the signal strength of a signal received from the first access point is less than or equal to a first RSSI threshold. The step of judging whether a switching condition is satisfied on the basis of the signal strength of signals received from the first access point and the second access point comprises: judging whether the difference between the signal strength of a signal received from the first access point and the signal strength of a signal received from the second access point is less than or equal to a set value. As the client gradually approaches the first access point, the signal strength of signals received by the first access point from the client increases continuously. In such circumstances, the method further comprises: the first access point judging whether the signal strength of a signal received using the backward antenna thereof from the client is greater than or equal to a second RSSI threshold, and if it is, opening the omnidirectional antenna thereof. The first access point judges whether all clients associated therewith have successfully switched to the second access point, and if they have, closes the omnidirectional antenna thereof.

Fig. 7a is a schematic diagram of a first preferred embodiment of the client, first access point and second access point in the switching method shown in Fig. 7. In this embodiment, APi denotes a first access point, and AP_i+i denotes a second access point. The client is fitted with an omnidirectional antenna; APi and APi₊i are each fitted with a backward antenna and an omnidirectional antenna. In this embodiment of the invention, the case where APi and APi₊i are each fitted with a backward antenna and an omnidirectional antenna is taken as an example, but a design in which one of APi and APi₊i has a backward antenna and an omnidirectional antenna is of course also possible, depending on actual requirements.

The client moves from left to right, gradually approaching APi; APi judges whether the signal strength of a signal received using the backward antenna thereof from the client is greater than or equal to a third RSSI threshold, and if it determines that the signal strength of a signal received from the client when the client moves to point A is greater than or equal to the third RSSI threshold, APi opens the omnidirectional antenna installed thereon. When the client moves into the common coverage area of APi and AP_i+i, the client receives signals sent by APi and APi₊i at least via the omnidirectional antennas thereof, judges whether a switching condition is satisfied on the basis of the signal strength of a signal received from APi or the signal strength of signals received from APi and APi₊i , and if it determines that the switching condition is satisfied at a point B covered by both APi and AP_i+i , it initiates a switching program and switches from APi to APi₊i . The signal received from APi comprises: a signal sent by APi using the omnidirectional antenna thereof or the omnidirectional antenna and backward antenna thereof and received from APi by the client using the omnidirectional antenna thereof; the signals received from APi and APi₊i comprise: signals sent by APi and APi₊i using the omnidirectional antennas thereof or the omnidirectional antennas and backward antennas thereof and received from APi and APi₊i by the client using the omnidirectional antenna thereof. The APi further judges whether the clients associated therewith have all switched from APi to APi₊i , and if they have, closes the omnidirectional antenna thereof .

Fig. 7b is a schematic diagram of a second preferred embodiment of the client, first access point and second access point in the switching method shown in Fig. 7. In this embodiment, APi denotes a first access point, and AP_i+i denotes a second access point. The client is fitted with a forward antenna and a backward antenna; APi and APi₊i are each fitted with a backward antenna and an omnidirectional antenna. In this embodiment of the invention, the case where APi and APi₊i are each fitted with a backward antenna and an omnidirectional antenna is taken as an example, but a design in which one of APi and APi₊i has a backward antenna and an omnidirectional antenna is of course also possible, depending on actual requirements.

The client moves from left to right, gradually approaching APi ; APi judges whether the signal strength of a signal received using the backward antenna thereof from the client is greater than or equal to a fourth RSSI threshold, and if it determines that the signal strength of a signal received from the client when the client moves to point A is greater than or equal to the fourth RSSI threshold, APi opens the omnidirectional antenna installed thereon. When the client moves into the common coverage area of APi and APi₊i , the client receives signals sent by APi and APi₊i at least via the omnidirectional antennas thereof, judges whether a switching condition is satisfied on the basis of the signal strength of a signal received from APi or the signal strength of signals received from APi and APi₊i , and if it determines that the switching condition is satisfied at a point B covered by both APi and APi₊i , it initiates a switching program and switches from APi to APi₊i . The signal received from APi comprises: a signal sent by APi using the omnidirectional antenna thereof or the omnidirectional antenna and backward antenna thereof and received from APi by the client using the forward antenna and backward antenna thereof; the signals received from APi and APi₊i comprise: signals sent by APi and AP_i+i using the omnidirectional antennas thereof or the omnidirectional antennas and backward antennas thereof and received from APi and APi₊i by the client using the forward antenna and backward antenna thereof. Once the switching program has been initiated, the APi further judges whether the clients associated therewith have all switched from APi to AP_i+i , and if they have, closes the omnidirectional antenna thereof.

In the embodiments shown in Fig. 7a and Fig. 7b above, the first access point and second access point are each fitted with a backward antenna and an omnidirectional antenna; the difference lies in the fact that in the embodiment shown in Fig. 7a, the client is fitted with an omnidirectional antenna, whereas in the embodiment shown in Fig. 7b, the client is fitted with a forward antenna and a backward antenna. The third RSSI threshold and fourth RSSI threshold may be the same or different, being set according to specific requirements.

Fig. 8 is a schematic flow chart of a third preferred embodiment of the switching method in the embodiments of the present invention. In this embodiment, the client as shown in Fig. 3 or Fig. 3a moves in a direction in which it first approaches a first access point and then approaches a second access point, at least one of the first access point and the second access point being the access point shown in Fig. 4 or Fig. 4a, and the method specifically comprises the following steps :

Step 801: the client moving into the area of coverage of the first access point, establishing a connection with the first access point, and communicating with the backward antenna of the first access point via the forward antenna of the client.

Step 802 : the client moving into the common coverage area of the first access point and the second access point, receiving at least via the omnidirectional antenna of the client signals sent by the first access point and the second access point at least via the omnidirectional antennas thereof, and judging whether a switching condition is satisfied on the basis of the signal strength of a signal received from the first access point or the signal strength of signals received from the first access point and the second access point, and if it is, initiating a switching program to switch from the first access point to the second access point. The signal received from the first access point comprises: a signal sent by the first access point using the omnidirectional antenna thereof or the backward antenna and omnidirectional antenna thereof and received by the client using the omnidirectional antenna thereof or the omnidirectional antenna and forward antenna thereof, and the signals received from the first access point and the second access point comprise: signals sent by the first access point and the second access point using the omnidirectional antennas thereof or the omnidirectional antennas and backward antennas thereof and received by the client using the omnidirectional antenna thereof or the omnidirectional antenna and forward antenna thereof. In the switching method given in this embodiment of the invention, when within the extent of common coverage of the first access point and the second access point, the client receives at least via the omnidirectional antenna thereof signals sent via omnidirectional antenna by the first access point and second access point, so that a sharp drop in the signal strength of the signal received from the first access point can be avoided, thus it can be ensured that all clients can maintain good communication with the first access point before switching to the second access point. In such circumstances, there is no need to precisely determine the received signal strength indicator (RSSI) threshold, therefore interruption in communication caused by the determined RSSI threshold being unsuitable is avoided, ensuring the reliability of switching.

In particular applications, the step of judging whether a switching condition is satisfied on the basis of the signal strength of a signal received from the first access point comprises: judging whether the signal strength of a signal received from the first access point is less than or equal to a first RSSI threshold; the step of judging whether a switching condition is satisfied on the basis of the signal strength of signals received from the first access point and the second access point comprises: judging whether the difference between the signal strength of a signal received from the first access point and the signal strength of a signal received from the second access point is less than or equal to a set value.

Fig. 8a is a schematic diagram of the client, first access point and second access point in the switching method shown in Fig. 8. In this embodiment, APi denotes a first access point, and APi₊i denotes a second access point. The client is fitted with a forward antenna and an omnidirectional antenna; APi and APi₊i are each fitted with a backward antenna and an omnidirectional antenna. In this embodiment of the invention, the case where APi and APi₊i are each fitted with a backward antenna and an omnidirectional antenna is taken as an example, but a design in which one of APi and APi₊i has a backward antenna and an omnidirectional antenna is of course also possible, depending on actual requirements.

The client moves from left to right, gradually approaching APi ; APi judges whether the signal strength of a signal received using the backward antenna thereof from the client is greater than or equal to a second RSSI threshold, and if it determines that the signal strength of a signal received from the client when the client moves to point A is greater than or equal to the second RSSI threshold, APi opens the omnidirectional antenna installed thereon. If the client determines that the signal strength of a signal received using the forward antenna thereof from APi is greater than or equal to a third RSSI threshold at point A or a position different from point A , it opens the omnidirectional antenna thereof. In particular applications, the second RSSI threshold may be equal to or not equal to the third RSSI threshold. When the client moves into the common coverage area of APi and APi₊i , the client receives at least via the omnidirectional antenna thereof signals sent by APi and APi₊i at least via the omnidirectional antennas thereof, judges whether a switching condition is satisfied on the basis of the signal strength of a signal received from APi or the signal strength of signals received from APi and APi₊i , and if it determines that the switching condition is satisfied at a point B covered by both APi and AP_i+i , it initiates a switching program and switches from APi to APi₊i . The signal received from APi comprises: a signal sent by APi using the omnidirectional antenna thereof or the omnidirectional antenna and backward antenna thereof and received from APi by the client using the omnidirectional antenna thereof or the omnidirectional antenna and forward antenna thereof; the signals received from APi and AP_i+i comprise: signals sent by APi and APi₊i using the omnidirectional antennas thereof or the omnidirectional antennas and backward antennas thereof and received from APi and APi₊i by the client using the omnidirectional antenna thereof or the omnidirectional antenna and forward antenna thereof. The APi further judges whether the clients associated therewith have all switched from APi to APi₊i, and if they have, closes the omnidirectional antenna thereof. The client must also judge whether it has switched from APi to APi₊i, and if it has, closes the omnidirectional antenna thereof.

In the client, access point, system and method disclosed in the above embodiments, switching takes place to the right of the first access point, and there is no need for the coverage of the second access point to extend to the left of the first access point. Thus the extent of the common coverage area of the first access point and second access point is reduced, and the number of access points disposed beside the subway track is correspondingly reduced, lowering the cost of the system. As seen from the above embodiments, the present invention is based on existing wireless local area networks, with no need for additional signaling, therefore the solution of the present invention is simple and easily implemented.

The embodiments of the present invention provide a client, access point, communication system and switching method. When the client is moving in a direction in which it first approaches the first access point and then approaches a second access point, and is within the extent of common coverage of the first access point and the second access point, a signal receiving module is used for receiving signals from the first access point and the second access point via omnidirectional antenna; a switching condition judgment module is used for judging whether a switching condition is satisfied on the basis of the signal strength of a signal received from the first access point or the signal strength of signals received from the first access point and the second access point, and if it is, sending a switching execution instruction to a switching execution module; the switching execution module is used for initiating a switching program on the basis of the switching execution instruction to switch the present client from the first access point to the second access point. The client, access point, communication system and switching method of the embodiments of the present invention ensure the reliability of switching .

The present invention has been set forth and described in detail above using the accompanying drawings and preferred embodiments, but it is not limited to these disclosed embodiments; other solutions derived therefrom by those skilled in the art are also included in the scope of protection of the present invention.

Claims

1. A client, the client comprising a forward antenna (301) and a signal receiving module (303), the signal receiving module (303) being used to receive a signal from a first access point via the forward antenna (301), characterized in that the client further comprises: an omnidirectional antenna (302), a switching condition judgment module (304) and a switching execution module (305); wherein when the client is moving in a direction in which it first approaches the first access point and then approaches a second access point, and is within the extent of common coverage of the first access point and the second access point, the signal receiving module (303) is further used for receiving signals from the first access point and second access point via the omnidirectional antenna (302); the switching condition judgment module (304) is used for judging whether a switching condition is satisfied on the basis of the signal strength of a signal received from the first access point or the signal strength of signals received from the first access point and the second access point, and if it is, sending a switching execution instruction to the switching execution module (305), wherein the signal received from the first access point comprises: a signal received from the first access point using the omnidirectional antenna (302) or using the omnidirectional antenna (302) and the forward antenna (301); the signals received from the first access point and the second access point comprise: signals received from the first access point and the second access point using the omnidirectional antenna (302) or using the omnidirectional antenna (302) and the forward antenna (301); the switching execution module (305) is used for initiating a switching program on the basis of the switching execution instruction to switch the present client from the first access point to the second access point.

2. The client as claimed in claim 1, wherein the switching condition judgment module (304) is specifically used for: judging whether the signal strength of a signal received from the first access point is less than or equal to a first RSSI threshold, and if it is, sending the switching execution instruction to the switching execution module (305); or judging whether the difference between the signal strength of a signal received from the first access point and the signal strength of a signal received from the second access point is less than or equal to a set value, and if it is, sending the switching execution instruction to the switching execution module (305) .

3. The client as claimed in claim 1 or 2 , wherein the client further comprises: a signal strength judgment module (306a) and an omnidirectional antenna opening module (307a) ; the signal strength judgment module (306a) being used for judging whether the signal strength of a signal received from the first access point by the signal receiving module (303a) via the forward antenna (301a) is greater than or equal to a second RSSI threshold, and if it is, sending an omnidirectional antenna opening signal to the omnidirectional antenna opening module (307a) ; the omnidirectional antenna opening module (307a) being used for opening the omnidirectional antenna (302a) on the basis of the omnidirectional antenna opening signal .

4. The client as claimed in claim 1 or 2 , wherein the client further comprises: a switching completion judgment module (308a) and an omnidirectional antenna closing module (309a) ; the switching completion judgment module (308a) being used for receiving an instruction from the switching execution module (305a) , and on the basis of this instruction judging whether the present client has completed the switching operation, and if it has, sending an omnidirectional antenna closing signal to the omnidirectional antenna closing module (309a) ; the omnidirectional antenna closing module (309a) being used for closing the omnidirectional antenna (302a) on the basis of the omnidirectional antenna closing signal .

5. An access point, the access point comprising a backward antenna (401) and a signal sending module (403), the signal sending module (403) being used for sending a signal to the client via the backward antenna (401), characterized in that the access point further comprises an omnidirectional antenna (402), and during switching of a client from the present access point to the next access point, the signal sending module (403) is further used for sending a signal to the client at least via the omnidirectional antenna (402) .

6. The access point as claimed in claim 5, wherein the access point further comprises: a signal receiving module (404a), a signal strength judgment module (405a) and an omnidirectional antenna opening module (406a) , wherein the signal receiving module (404a) receives a signal sent by the client via the backward antenna (401a) ; the signal strength judgment module (405a) is used for judging whether the signal strength of a signal received by the signal receiving module (404a) from the client is greater than or equal to a first RSSI threshold, and if it is, sending an omnidirectional antenna opening signal to the omnidirectional antenna opening module (406a) ; the omnidirectional antenna opening module (406a) is used for opening the omnidirectional antenna (402a) on the basis of the omnidirectional antenna opening signal .

7. The access point as claimed in claim 5 or 6 , wherein the access point further comprises: a switching completion judgment module (407a) and an omnidirectional antenna closing module (408a) ; the switching completion judgment module (407a) being used for judging whether all clients associated with the access point have completed the switching operation, and if they have, sending an omnidirectional antenna closing signal to the omnidirectional antenna closing module (408a) ; the omnidirectional antenna closing module (408a) being used for closing the omnidirectional antenna (402a) on the basis of the omnidirectional antenna closing signal .

8. A communication system, wherein the system comprises: a first access point (501a), a second access point (502a) and the client (503a) as claimed in any one of claims 1 to 4, the client (503a) moving in a direction in which it first approaches the first access point (501a) and then approaches the second access point (502a) .

9. The communication system as claimed in claim 8, wherein the first access point (501a) comprises an omnidirectional antenna, and is capable of communicating with the client (503a) via the omnidirectional antenna at least during switching of the client (503a) from the first access point (501a) to the second access point (502a) .

10. The communication system as claimed in claim 8 or 9, wherein the second access point (502a) comprises an omnidirectional antenna, and is capable of communicating with the client (503a) via the omnidirectional antenna at least during switching of the client (503a) from the first access point (501a) to the second access point (502a) .

11. A communication system, wherein the system comprises: a client (503b), a first access point (501b) and a second access point (502b); at least one of the first access point (501b) and the second access point (502b) is the access point as claimed in any one of claims 5 to 7 , and the client (503b) moves in a direction in which it first approaches the first access point (501b) and then approaches the second access point (502b) .

12. A switching method, wherein the client as claimed in any one of claims 1 to 4 moves in a direction in which it first approaches a first access point and then approaches a second access point, the method comprising: the client moving into the area of coverage of the first access point, establishing a connection with the first access point, and communicating with the first access point via the forward antenna of the client (601); the client moving into the common coverage area of the first access point and the second access point, receiving signals from the first access point and the second access point at least via the omnidirectional antenna of the client, and judging whether a switching condition is satisfied on the basis of the signal strength of a signal received from the first access point or the signal strength of signals received from the first access point and the second access point, and if it is, initiating a switching program to switch from the first access point to the second access point (602); wherein the signal received from the first access point comprises: a signal received by the client from the first access point using the omnidirectional antenna or using the omnidirectional antenna and the forward antenna, and the signals received from the first access point and the second access point comprise: signals received by the client from the first access point and the second access point using the omnidirectional antenna or using the omnidirectional antenna and the forward antenna.

13. The method as claimed in claim 12, wherein the step of judging whether a switching condition is satisfied on the basis of the signal strength of a signal received from the first access point comprises: judging whether the signal strength of a signal received from the first access point is less than or equal to a first RSSI threshold; the step of judging whether a switching condition is satisfied on the basis of the signal strength of signals received from the first access point and the second access point comprises: judging whether the difference between the signal strength of a signal received from the first access point and the signal strength of a signal received from the second access point is less than or equal to a set value.

14. The method as claimed in claim 12 or 13, wherein the method further comprises: the client judging whether the signal strength of a signal received from the first access point using the forward antenna is greater than or equal to a second RSSI threshold, and if it is, opening the omnidirectional antenna; the client judging whether it has switched to the second access point, and if it has, closing the omnidirectional antenna.

15. A switching method, wherein a client moves in a direction in which it first approaches a first access point and then approaches a second access point, at least one of the first access point and the second access point being the access point as claimed in any one of claims 5 to 7 , the method comprising: the client moving into the area of coverage of the first access point, establishing a connection with the first access point, and communicating with the first access point via the backward antenna of the first access point (701); the client moving into the common coverage area of the first access point and the second access point, receiving signals sent at least via omnidirectional antenna by the first access point and second access point, judging whether a switching condition is satisfied on the basis of the signal strength of a signal received from the first access point or the signal strength of signals received from the first access point and the second access point, and if it is, initiating a switching program to switch from the first access point to the second access point (702); wherein the signal received from the first access point comprises: a signal sent by the first access point using the omnidirectional antenna, or the omnidirectional antenna and backward antenna, and received by the client; the signals received from the first access point and the second access point comprise: signals sent by the first access point and second access point using the omnidirectional antennas, or the omnidirectional antennas and backward antennas, and received by the client.

16. The method as claimed in claim 15, wherein the step of judging whether a switching condition is satisfied on the basis of the signal strength of a signal received from the first access point comprises: judging whether the signal strength of a signal received from the first access point is less than or equal to a first RSSI threshold; the step of judging whether a switching condition is satisfied on the basis of the signal strength of signals received from the first access point and the second access point comprises: judging whether the difference between the signal strength of a signal received from the first access point and the signal strength of a signal received from the second access point is less than or equal to a set value.

17. The method as claimed in claim 15 or 16, wherein the method further comprises: the first access point judging whether the signal strength of a signal received from the client using the backward antenna is greater than or equal to a second RSSI threshold, and if it is, opening the omnidirectional antenna; the first access point judging whether all clients associated therewith have successfully switched to the second access point, and if they have, closing the omnidirectional antenna.

18. A switching method, wherein the client as claimed in any one of claims 1 to 4 moves in a direction in which it first approaches a first access point and then approaches a second access point, at least one of the first access point and the second access point being the access point as claimed in any one of claims 5 to 7 , the method comprising: the client moving into the area of coverage of the first access point, establishing a connection with the first access point, and communicating with the backward antenna of the first access point via the forward antenna of the client (801); the client moving into the common coverage area of the first access point and the second access point, receiving at least via the omnidirectional antenna of the client signals sent at least via omnidirectional antenna by the first access point and the second access point, and judging whether a switching condition is satisfied on the basis of the signal strength of a signal received from the first access point or the signal strength of signals received from the first access point and the second access point, and if it is, initiating a switching program to switch from the first access point to the second access point (802); wherein the signal received from the first access point comprises: a signal sent by the first access point using the omnidirectional antenna thereof or the backward antenna and omnidirectional antenna thereof and received by the client using the omnidirectional antenna thereof or the omnidirectional antenna and forward antenna thereof, and the signals received from the first access point and the second access point comprise: signals sent by the first access point and the second access point using the omnidirectional antennas thereof or the omnidirectional antennas and backward antennas thereof and received by the client using the omnidirectional antenna thereof or the omnidirectional antenna and forward antenna thereof .

19. The method as claimed in claim 18, wherein the step of judging whether a switching condition is satisfied on the basis of the signal strength of a signal received from the first access point comprises: judging whether the signal strength of a signal received from the first access point is less than or equal to a first RSSI threshold; the step of judging whether a switching condition is satisfied on the basis of the signal strength of signals received from the first access point and the second access point comprises: judging whether the difference between the signal strength of a signal received from the first access point and the signal strength of a signal received from the second access point less than or equal to a set value.