CN101932054B - Switching method of wireless local area network - Google Patents
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- CN101932054B CN101932054B CN2010102480414A CN201010248041A CN101932054B CN 101932054 B CN101932054 B CN 101932054B CN 2010102480414 A CN2010102480414 A CN 2010102480414A CN 201010248041 A CN201010248041 A CN 201010248041A CN 101932054 B CN101932054 B CN 101932054B
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Abstract
The invention discloses a switching method of a wireless local area network, which comprises the following steps: sending beacon frames through an access point according to a set time interval; receiving and storing a scanning threshold of the access point and the switching thresholds of neighboring access points in the beacon frames through a mobile platform; judging if the mobile platform meets a scanning starting condition, and scanning in an active scanning mode or a passive scanning mode if the mobile platform meets the scanning starting condition; if the RSSI of any neighboring access point exceeds a switching threshold, stopping scanning and selecting a neighboring access point as a target access point; if the RSSIs of all neighboring access points are lower than the switching threshold, selecting the access point with the highest RSSI as the target access point; authenticating the mobile platform and the target access point; and sending a reassociation signal through the mobile platform to the target access point, and accomplishing the switching when the mobile platform receives a reassociation response signal and the reassociation is successful. In the method of the invention, the mobile platform can be configured with different switching parameters according to different access points, and the switching method is optimized.
Description
Technical field
The present invention relates to technical field of data transmission in the communication system, relate in particular to a kind of changing method of WLAN (wireless local area network).
Background technology
1, WLAN (wireless local area network) brief introduction
WLAN (wireless local area network) since transmission rate fast, set up simply, cheap, in routine office work, domestic. applications and some engineering projects, obtained using widely.WLAN standard is announced this standard by the definition of IEEE802.11 WLAN (wireless local area network) working group in June, 1997, it is one of first generation WLAN standard.(Media Access Control, MAC) standard allow WLAN (wireless local area network) and radio equipment manufacturer to set up mutual operation network equipment for this standard definition physical layer and Media Access Controlled.802.11 physical layer has defined the signal characteristic and the modulation of transfer of data in the standard.RF in this standard (radio frequency) transmission standard be frequency-hopping spread spectrum (Frequency-Hopping Spread Spectrum, FHSS) and direct sequence spread spectrum (DirectSequence Spread Skill DSSS), is operated in 2.4000～2.4835GHz frequency range.After this announced 802.11a and 802.11b again in succession, these two standards are that wherein the 802.11a working frequency range is 5GHz to 802.11 replenish, and message transmission rate reaches as high as 54Mbit/s.802.11b 2.4GHz high-speed physical link layer is replenished description and regulation, has improved message transmission rate and can reach 11Mbit/s.2000, IEEE set up the TGg of task force 802.11g is carried out standardization effort.Purpose is for the user obtains higher data rate service, its backward compatibility 802.11b, forward compatibility 802.11a; Adopt OFDM (OrthogonalFrequency-Division Multiplexing, OFDM) technology, extra CCK (Complementary Code Keying is provided, complementary code keying)-OFDM and PBCC (PacketBinary Convolutional Code, bag binary system convolutional encoding)-22 technology, help providing multimode WLAN the product of (Wireless Local Area Network, WLAN (wireless local area network)).This standard is implemented official approval in 2003.
In addition, 802.11e is devoted to the improvement of mac-layer protocol, to support multimedia transmission.802.11h dynamic channel is selected and the transmitting power control criterionization.802.11i be devoted to improve the fail safe of network, mainly also be that the MAC layer is made amendment.
Above standard and some other standard and content that the 802.11-2007 standard is then integrated, and done some revisions and enhancing for example carried out the improvement etc. of the fail safe aspect of data link.
More than these standards mainly formulated the physical layer of WLAN (wireless local area network) transmission and the standard of medium access control layer aspect, how between WAP (wireless access point) (wireless aps, Access Point), to switch for travelling carriage or terminal and but do not make concrete regulation.But the 802.11-2007 standard code synchronously, the process of authentication, association and series of fortified passes connection, form by following three processes according to the handoff procedure of agreement WLAN (wireless local area network):
1) scanning process
When the received signal intensity of this access point is lower than sweep gate in limited time, start scanning process.
Scanning process is divided into two kinds of drive sweep and active scan again according to the difference of scan pattern.Wherein, under the drive sweep pattern, whether terminal detects beacon frame (Beacon Frame) successively and exists on the channel of WLAN (wireless local area network), and beacon frame sends with the cycle about 100ms usually.After terminal receives the Beacon frame, according to the parameter of beacon frame finish with AP synchronously.
Under active scan mode, whether terminal has AP to exist in order to survey on certain channel, initiatively send Probe Request Frame (inquiring after claim frame), after AP receives ProbeRequest Frame, can send a Probe Response Frame (inquiring after response frame) and give this travelling carriage.This travelling carriage according to the parameter of Probe Response Frame finish with AP synchronously.
The advantage of drive sweep process is than more electricity saving, and shortcoming then is that the time delay of scanning is long.Active scan is then opposite, in the wireless lan network of reality, switches and adopts active scan process usually.
No matter adopt any scan mode, travelling carriage can begin to start scanning usually when the signal strength signal intensity of this sub-district AP is lower than a certain thresholding, and startup is switched when the signal strength signal intensity of neighbor cell AP is higher than certain thresholding.
2) authentication process
In authorization phase, travelling carriage carries out authentication with the AP of the best of finding on last stage.But 802.11 standards do not require to carry out association immediately after the authentication, do not require authentication to carry out immediately after scan channel yet.Therefore, some equipment vendors have adopted pre-authentication mechanism in its product, and for example and then scanning process and pre-authentication process carry out and based on the pre-method for authenticating of IAPP (Inter-Access Point Protocol, access point interoperability agreement).In first kind of mechanism, travelling carriage carries out authentication with new AP immediately after the scan period finishes, to accelerate to set up the speed of series of fortified passes connection.Second kind of mechanism is then faster, just begins to carry out authentication when first AP of travelling carriage and ESS (Extended Service Set, extended service set) sets up association.This moment, IAPP sent to authentication information by distributed system all AP of ESS.Therefore, when needs carry out the series of fortified passes connection,, no longer need authentication process because all AP of travelling carriage and ESS have finished authentication.Based on the pre-authentication mechanism of IAPP even can just carry out before sweep phase, therefore, it can not impact the time-delay of switching.
3) series of fortified passes connection process
Series of fortified passes connection process refers to association is passed to from an AP process of another AP.In case travelling carriage and new AP authentication are passed through, series of fortified passes connection process just can be carried out, as shown in Figure 1.Series of fortified passes connection process is made up of following five steps:
A) travelling carriage sends the reassociation requests signal to new AP;
B) new AP sends the re-association response signal to travelling carriage;
C) new AP sends switching request signal to old AP;
D) old AP answers the handoff response signal to new AP;
E) new AP sends the Frame (optional) of buffering to travelling carriage.
The WLAN (wireless local area network) handoff procedure of active scan mode as shown in Figure 2.
WLAN (wireless local area network) handoff procedure during drive sweep as shown in Figure 3.
From above description as can be seen, the handoff delay of WLAN (wireless local area network) mainly is subjected to the sweep delay (drive sweep pattern) of scanning process or inquires after time-delay (active scan mode) influence.
2, the train-ground communication of City Rail Transit System
Train control system (Communication Based Train Control at City Rail Transit System based on communication, CBTC) land mobile communication system adopts the frequency range of 2.4GHz to carry out transfer of data usually, and wherein a large portion adopts wireless local area network technology.
The land mobile communication system data packets for transmission of CBTC is drawn together the train position information that train sends earthward, and ground is to the Train Control information of train transmission.These information are data very high to security requirement, and are therefore, very high to the reliability requirement of train-ground communication.In IEEE1474.1 (2004) standard, the time-delay that the regulation train-ground communication is communicated by letter with the ground car can not surpass 0.5～2 second, otherwise train will brake hard.Simultaneously, the packet loss of train-ground communication and HO-break duration also are the important indicators that influences the train-ground communication quality.On-the-spot test is the result show, the HO-break duration of train between different access points is the most key factor that influences the CBTC train-ground communication.
The condition that common triggering onboard wireless LAN equipment switches is that its radio signal quality that receives is lower than a certain specific thresholding.
In the urban track traffic train-ground communication since circuit fix, the other equipment of rail only with track on train carry out communicating by letter of wireless data, so the coverage of the other AP of rail also only is that the zone that covers rail train place gets final product.Present CBTC land mobile communication system exists wireless free wave, leaky waveguide and three kinds of transmission meanss of leaky cable, covering completely can be adopted wherein a kind of transmission means or multiple transmission means, for example adopt wireless free wave transmission means, and adopt leaky waveguide transmission means etc. in overhead zone at tunnel area.
In wireless free wave transmission means, the antenna of the other AP of rail adopts directional antenna usually, can send or obtain stronger energy like this on certain direction, thereby enlarges the coverage of AP.The coverage mode of wireless free wave employing directional antenna as shown in Figure 4.
The installation connected mode of leaky waveguide as shown in Figure 5.Leaky waveguide is installed in the both sides of rail or usually on the top in tunnel, the electromagnetic wave of launching as ground AP is when leaky waveguide is transmitted, the electromagnetic wave that transmits in leaky waveguide is radiated surrounding space from the leaky waveguide slotted eye, sew electric field its outside generation, train therefrom obtains information energy by vehicle-mounted plate aerial (antenna and waveguide upper surface distance be 30～50 centimetres), thereby realizes and the communicating by letter of ground.Equally, the electromagnetic wave that train sends by antenna is sewed electric field outside generation of leaky waveguide, also can be coupled in the leaky waveguide, realizes communicating by letter with control centre.As can be seen from the figure, in the place of two AP overlay area handing-over, train-ground communication can interrupt, and at this regional travelling carriage two switchings between AP can take place, and this switches the communication disruption distance usually less than 1 meter.
But the signal transmission characteristics of wireless free wave and leaky waveguide has very big difference.Usually wireless free wave requires received signal intensity to be higher than the receiving sensitivity 10～15dB of the receiver of wireless device in the engineering design, and waveguide transmits the influence that needs factors such as consideration sleet covering, so the requirement of received signal intensity is for being higher than more than the receiver sensitivity 20dB.If adopt wireless free wave transmission means at tunnel area, and in overhead zone employing leaky waveguide transmission means, and the switching criterion based on unique received signal intensity is also adopted in the switching between wireless local area network AP, can the performance of handoffs of WLAN (wireless local area network) be exerted an influence.
For example the wireless local area network scan thresholding of waveguide AP requirement is-70dBm, and handoff threshold is-65dBm, and the wireless local area network scan thresholding that wireless free wave AP requires is-80dBm that handoff threshold is-75dBm.Common handoff algorithms can only adopt single gate method, the performance of handoffs requirement that single completely scanning thresholding and handoff threshold can only satisfy a certain transmission means, be not suitable for existing wireless free wave on this circuit, the handover mechanism of the urban track traffic car-ground radio local area network communication of leaky waveguide transmission means is arranged again.If adopt single handoff threshold algorithm, may cause rattling and switch and the frequent generation of handover event by mistake.Therefore, be necessary to design a kind of new WLAN (wireless local area network) handover mechanism, with the performance requirement of the handover of the wireless lan network of the AP that is fit to the different radio transmission characteristic.
Summary of the invention
(1) technical problem that will solve
The technical problem to be solved in the present invention is: avoid above-mentioned the deficiencies in the prior art, and at the handoff procedure of WLAN (wireless local area network), the method that provides a kind of WLAN (wireless local area network) to switch.
(2) technical scheme
For solving the problems of the technologies described above, the invention provides a kind of changing method of WLAN (wireless local area network), may further comprise the steps:
1, access point sends beacon frame according to the time interval of setting;
2, travelling carriage receives and stores the sweep gate limit information of this access point in the beacon frame and the handoff threshold information of neighboring access point;
3, judge whether to satisfy the condition that starts scanning according to described sweep gate limit information travelling carriage,, then adopt the mode of active scan or drive sweep to scan if satisfy;
4, travelling carriage scans the neighboring access point in the access-in point information tabulation successively; If the RSSI of neighboring access point beacon frame (Received Signal Strength Indicator, received signal intensity) surpasses thresholding, then stop scanning, and select this neighboring access point as target access;
If the RSSI of all candidate target access points is lower than thresholding, then select the strongest access point of the RSSI of beacon frame as target access;
5, travelling carriage and target access are carried out authentication;
6, travelling carriage sends series of fortified passes connection signal to target access, and when travelling carriage receives the re-association response signal, and the series of fortified passes connection is when setting up successfully, and handover success is finished.
The mode of described employing drive sweep is, the beacon frame of travelling carriage search neighbor cell on the channel of setting, and obtain SSID (Service Set Identifier in this beacon frame, service set identifier) and BSSID (Basic Service Set Identifier, BSSID), if SSID and BSSID conform to canned data, then with the candidate target access point of this access point as switching;
If the RSSI of all candidate target access points is lower than handoff threshold, then select the strongest access point of the RSSI of beacon frame as target access.
Wherein, travelling carriage is lower than sweep gate startup scanning in limited time at the RSSI of this access point beacon frame.
Wherein, in described access-in point information tabulation, described neighboring access point is arranged according to priority orders, and the access point that comes the front has high priority.
Wherein, the mode of described employing drive sweep is: the beacon frame of travelling carriage search neighbor cell on the channel of setting, and obtain service set identifier SSID and BSSID BSSID in this beacon frame, if SSID and BSSID conform to canned data, then with the candidate target access point of this access point as switching;
If the RSSI of all candidate target access point beacon frames is lower than handoff threshold, then select the strongest access point of the RSSI of beacon frame as target access.
Wherein, the mode of described employing active scan is: travelling carriage sends on the channel of setting and inquires after claim frame, and response frame is inquired after in reception in the corresponding time of regulation, and obtain this and inquire after SSID and the BSSID that response frame comprises, if SSID and BSSID conform to canned data, then with the candidate target access point of this access point as switching;
All be lower than handoff threshold if all candidate target access points are inquired after the RSSI of response frame, then select to inquire after the strongest access point of the RSSI of response frame as target access.
Wherein, on the channel of described setting, search for the information frame of neighbor cell according to neighboring access point priority order from high to low, as long as search the beacon frame of neighboring access point, and the RSSI of this neighboring access point beacon frame is in case surpass handoff threshold, just stop scanning, and select this neighboring access point as target access.
Wherein, inquire after claim frame according to the transmission on the channel of setting of neighboring access point priority order from high to low, if in the stipulated time subsequently, receive and inquire after response frame, and the RSSI that inquires after response frame of this neighboring access point is in case above handoff threshold, just select this neighboring access point as target access.
(3) beneficial effect
Advantage of the present invention is: by changing method of the present invention, travelling carriage can be according to the dissimilar different handoff parameters of access point configuration, optimize changing method, can significantly reduce the probability that incidents such as table tennis switching and mistake switching take place, improved the performance of handoffs of system.
Description of drawings
Fig. 1 is a connection process in wireless aps series of fortified passes in the prior art;
Fig. 2 is the WLAN (wireless local area network) handoff procedure during active scan in the prior art;
Fig. 3 is the WLAN (wireless local area network) handoff procedure during drive sweep in the prior art;
Fig. 4 is that wireless free wave covers schematic diagram in the prior art;
Fig. 5 is the circuit connection diagram of leaky waveguide overlay area in the prior art;
Fig. 6 is the flow chart of the wireless lan handover method of embodiment of the present invention.
Embodiment
For making purpose of the present invention, content and advantage clearer, embodiment of the present invention is described further in detail below in conjunction with accompanying drawing.
The method flow diagram of embodiment of the present invention may further comprise the steps as shown in Figure 6:
1) access point sends beacon frame according to the time interval of setting;
2) travelling carriage receives and stores the sweep gate limit information of this access point in the beacon frame and the handoff threshold information of neighboring access point;
3) judge that according to described sweep gate limit information whether travelling carriage satisfies the condition that starts scanning, if satisfy, then adopts the mode of active scan or drive sweep to scan;
4) travelling carriage scans the neighboring access point in the access-in point information tabulation successively; If the received signal intensity of neighboring access point beacon frame surpasses handoff threshold, then stop scanning, and select this neighboring access point as target access;
If the received signal intensity of all neighboring access point beacon frames all is lower than handoff threshold, then select the strongest access point of the received signal intensity of beacon frame as target access;
5) travelling carriage and target access are carried out authentication;
6) travelling carriage sends series of fortified passes connection signal to target access, and when travelling carriage receives the re-association response signal, and the series of fortified passes connection is when setting up successfully, and handover success is finished.
Wherein, travelling carriage is lower than sweep gate startup scanning in limited time in the received signal intensity of this access point beacon frame.
Wherein, in described access-in point information tabulation, described neighboring access point is arranged according to priority orders, and the access point that comes the front has high priority.
Wherein, the mode of described employing drive sweep is: the beacon frame of travelling carriage search neighboring access point on the channel of setting, and obtain service set identifier and BSSID in this beacon frame, if service set identifier and BSSID conform to canned data, then with the candidate target access point of this neighboring access point as switching;
If the received signal intensity of all candidate target access point beacon frames all is lower than handoff threshold, then select the strongest access point of the received signal intensity of beacon frame as target access.
Wherein, the mode of described employing active scan is: travelling carriage sends on the channel of setting and inquires after claim frame, and response frame is inquired after in reception in the corresponding time of regulation, and obtain this and inquire after service set identifier and the BSSID that response frame comprises, if service set identifier and BSSID conform to canned data, then with the candidate target access point of this access point as switching;
All be lower than handoff threshold if all candidate target access points are inquired after the received signal intensity of response frame, then select to inquire after the strongest access point of the received signal intensity of response frame as target access.
Wherein, on the channel of described setting, search for the beacon frame of neighbor cell according to neighboring access point priority order from high to low, as long as search the beacon frame of neighboring access point, and the received signal intensity of this neighboring access point beacon frame is in case surpass handoff threshold, just stop scanning, and select this neighboring access point as target access.
Wherein, inquire after claim frame according to the transmission on the channel of setting of neighboring access point priority order from high to low, if in the stipulated time subsequently, receive and inquire after response frame, and the received signal intensity of inquiring after response frame of this neighboring access point is in case above handoff threshold, just select this neighboring access point as target access.
Embodiment
In protocol of wireless local area network, stipulated that beacon frame approximately sent periodically with the time interval of setting, the content of beacon frame is as shown in table 1:
Table 1
As previously mentioned, in the present invention, access point comprises the handover trigger thresholding of this access point in the beacon frame that sends.Travelling carriage receives and stores the handoff threshold information in the beacon frame, and when switching condition satisfied, travelling carriage started handoff procedure.The structure that beacon frame adds after scanning thresholding and the handoff threshold information is as shown in table 2.
Table 2
| Sequence number | Information | Remarks |
| 25 | The sweep gate limit information of this access point | |
| 26 | The handoff threshold information of neighboring |
|
| ... | ?... | ... |
| 25+n | The handoff threshold information of neighboring access point n | |
| At last | Manufacturer is self-defined |
As shown in Figure 6, travelling carriage is after receiving this beacon frame, and whether the received signal intensity RSSI that judges the beacon frame of this access point and neighboring access point or inquire after response frame surpasses handoff threshold.If RSSI has surpassed, just select this neighboring access point as the target access of switching, and stop scanning process, enter next step authentication and series of fortified passes connection process; If RSSI does not surpass handoff threshold, then the next access point in the tabulation of neighboring access dot information is repeated above process, up to finding suitable switching target access point.If the RSSI of all access points does not all surpass in the tabulation of neighboring access dot information, then select the highest neighboring access point of RSSI as the target access of switching.
Above execution mode only is used to illustrate the present invention; and be not limitation of the present invention; the those of ordinary skill in relevant technologies field; under the situation that does not break away from the spirit and scope of the present invention; can also make various variations and modification; therefore all technical schemes that are equal to also belong to category of the present invention, and scope of patent protection of the present invention should be defined by the claims.
Claims (6)
1. the changing method of a WLAN (wireless local area network) is characterized in that, may further comprise the steps:
1) access point sends beacon frame according to the time interval of setting;
2) travelling carriage receives and stores the sweep gate limit information of this access point in the beacon frame and the handoff threshold information of neighboring access point;
3) judge that according to described sweep gate limit information whether travelling carriage satisfies the condition that starts scanning, if satisfy, then adopts the mode of active scan or drive sweep to scan; The condition of described startup scanning is: travelling carriage is lower than sweep gate startup scanning in limited time in the received signal intensity of this access point beacon frame;
4) travelling carriage scans the neighboring access point in the access-in point information tabulation successively; If the received signal intensity of neighboring access point beacon frame surpasses handoff threshold, then stop scanning, and select this neighboring access point as target access;
If the received signal intensity of all neighboring access point beacon frames all is lower than handoff threshold, then select the strongest access point of the received signal intensity of beacon frame as target access;
5) travelling carriage and target access are carried out authentication;
6) travelling carriage sends series of fortified passes connection signal to target access, and when travelling carriage receives the re-association response signal, and the series of fortified passes connection is when setting up successfully, and handover success is finished.
2. the changing method of WLAN (wireless local area network) as claimed in claim 1 is characterized in that: in described access-in point information tabulation, described neighboring access point is according to the priority orders arrangement, and the access point that comes the front has high priority.
3. the changing method of WLAN (wireless local area network) as claimed in claim 1, it is characterized in that: the mode of described employing drive sweep is: the beacon frame of travelling carriage search neighboring access point on the channel of setting, and obtain service set identifier and BSSID in this beacon frame, if service set identifier and BSSID conform to canned data, then with the candidate target access point of this neighboring access point as switching;
If the received signal intensity of all candidate target access point beacon frames all is lower than handoff threshold, then select the strongest access point of the received signal intensity of beacon frame as target access.
4. the changing method of WLAN (wireless local area network) as claimed in claim 1, it is characterized in that: the mode of described employing active scan is: travelling carriage sends on the channel of setting and inquires after claim frame, and response frame is inquired after in reception in the corresponding time of regulation, and obtain this and inquire after service set identifier and the BSSID that response frame comprises, if service set identifier and BSSID conform to canned data, then with the candidate target access point of this access point as switching;
All be lower than handoff threshold if all candidate target access points are inquired after the received signal intensity of response frame, then select to inquire after the strongest access point of the received signal intensity of response frame as target access.
5. the changing method of WLAN (wireless local area network) as claimed in claim 3, it is characterized in that: the beacon frame of on the channel of described setting, searching for neighbor cell according to neighboring access point priority order from high to low, as long as search the beacon frame of neighboring access point, and the received signal intensity of this neighboring access point beacon frame is in case surpass handoff threshold, just stop scanning, and select this neighboring access point as target access.
6. the changing method of WLAN (wireless local area network) as claimed in claim 4, it is characterized in that: send on the channel of setting according to neighboring access point priority order from high to low and inquire after claim frame, if in the stipulated time subsequently, receive and inquire after response frame, and the received signal intensity of inquiring after response frame of this neighboring access point is in case surpass handoff threshold, just select this neighboring access point as target access, and stop scanning process, enter next step authentication and series of fortified passes connection process.
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| WO2011110125A2 (en) * | 2011-04-22 | 2011-09-15 | 华为技术有限公司 | Method and access point for changing communication link |
| CN103517381B (en) | 2012-06-12 | 2017-04-26 | 华为技术有限公司 | Method and device for getting access to wireless local area network |
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