CN101827409A - Quick handover method based on 802.11 network - Google Patents
Quick handover method based on 802.11 network Download PDFInfo
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Abstract
The invention provides a quick handover method based on an 802.11 network, which comprises the following steps of: (1) comparing a current latest signal to noise ratio (SNR) of an AP (Access Point) with an old SNR received from the AP last time by a mobile node (MN): if a current SNR value is larger than a neighbor chart (NC) threshold and larger than an old SNR value, and carrying out no channel scanning by the MN; if the current SNR value is smaller than the old SNR value and smaller than the NC threshold, starting the channel scanning to generate a NC; (2) when the MN detects that the SNR value of the current AP is smaller than a cell search threshold, starting a selective channel scanning algorithm for scanning; and (3) when the MN detects that the SNR value of an optical AP in an AP list is larger than the cell search threshold and after the difference between the SNR value of the optimal AP and the SNR value of the current AP exceeds a handover differential (delta SNR), disconnecting the connection with the current AP by the MN and carrying out authentication and reconnection with a new AP. The invention solves the technical problem of overlong delay time taken by a wireless mobile node in the process of handover in the 802.11 network.
Description
Technical field
The present invention relates to the quick handoff technique of wireless IP, more specifically, the present invention relates to a kind of fast switch over method based on 802.11 networks.Use quick handoff technique described in the invention, can reduce the MAC layer handover delay of mobile node in 802.11 networks.
Background technology
In 802.11 networks, mobile node is by the access of access point AP realization Internet, and AP is equivalent to the base station in the mobile communication system, and different is, in an AP service area, a plurality of mobile nodes insert by the CSMA/CA mode of competition rather than the TDMA mode of preassignment channel.In 802.11 WLAN (wireless local area network), BSS is an element.It is made up of a plurality of mobile nodes of obeying the common coordination system.In 802.11 networks of centralized distribution, BSS also can regard the overlay area of an AP as.A plurality of such area of coverage integrated local local area network (LAN)s are formed an expansion service district ESS.A plurality of AP overlay area is connected to Internet by a couple in router (AR).Couple in router AR is in the edge of Access Network, connects for mobile node provides wireless IP.Mobile node between different AP when mobile, the switching that all can produce link layer at every turn.
According to the IEEE802.11 protocol specification, link layer switches and mainly to be divided into three phases: find AP (Probe) stage, authentication (Authenticate) stage and reconnect (Re-Associate) stage.
Finding the AP stage, in order to find a new potential AP, and connecting with it that website must be earlier by scanning the relevant information that (scan) mechanism obtain available AP.Scan mechanism is divided into active scan and drive sweep.At sweep phase, website sends beacon frame (Beacon) signal that AP sends around claim frame (Probe) or the passive search by active scan.If receive Probe acknowledgement frame or beacon frame that a plurality of AP send simultaneously, selecting wherein, the best AP of radio link quality carries out next stage.
In authentication phase, interactive authentication information between website and the AP has only the website by authentication just can use Internet resources.
In the stage of reconnecting, website sends the request of connecting to AP, receives connection request as if AP, will make connection response to website, communicate so website just can have been set up the AP that connects by this, connects to set up to finish, and link layer switching is so far finished.
The state transition graph detailed to this three phases is referring to Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications[S] .IEEE Standerd 802.11,1999.
Because the handover delay value of MN between AP can't satisfy the demand of existing real time business than higher in 802.11 agreements, so a lot of correlative study has all proposed improvement project to the handoff procedure of agreement.
At document " Fast Inter-AP Handoff using Predictive-Authentication Scheme in Public Wireless LAN ", Networks 2002, Aug 2002., (Pre-Authenticate) that people such as Sangheon Pack have proposed a kind of authentication in advance fast switching model shortens authenticated time when switching, and then the reduction handover delay, but this method is not very general; Document " Fast and Scalable WirelessHandoff in Support of Mobile Internet Audio[J] .Mobile Networks and Applications ", 1998,3:351-363., propose a kind of handoff algorithms of data forwarding, reduced the packet loss when switching.But because information exchange and processing between the new and old AP need certain hour, switching delay does not reduce; At document " Fast Handoff Scheme in Wireless LAN for real-time System[C] ", the ThirdIEEE WLAN Workshop.2001., Choyi, people such as V Kumar have proposed a kind of based on XCAST switch mode [3], further reduce switching delay and packet loss, but this method must have an XCAST router, and lacks a kind of safe authentification of user mechanism.
At document " An Empirical Analysis of the IEEE 802.11 MAC Layer Handoff Process.ACM SIGCOMMComputer Communication Review; 33 (2): 93-102; among the Apirl 2003.; people such as A.Mishra have provided each several part and postponed big or small experimental result, and the experiment confirm delayed sweep accounts for the MAC layer and switches more than 90% of total delay.The present invention is based on 802.11 networks fast switch over method this be to have used this conclusion.Switch more than 90% of total delay because delayed sweep accounts for the MAC layer, the delay meeting that therefore changes this part significantly changes the performance of whole handoff procedure.
Summary of the invention
The purpose of this invention is to provide a kind of fast switch over method based on the 802.1l network, it provides a solution that brand-new mobile node switches fast in WLAN.
According to a kind of fast switch over method of the present invention based on 802.11 networks, wherein, described 802.11 networks are based on the network configuration of center (Centroid is a wireless aps), and the overlay area between the AP is overlapped, and it is characterized in that: described fast switch over method comprises the steps:
1) mobile site MN is according to comparing from the old_SNR that this AP receives the up-to-date signal to noise ratio (snr) of current AP and last time:: if current SNR value greater than neighbor picture threshold value (NC Threshold) and greater than the old_SNR value, MN does not carry out scan channel.If current SNR value just starts scan channel less than old_SNR and less than the neighbor picture threshold value, generate neighbor picture (NC);
When 2) the SNR value that detects current AP as MN was found thresholding (Cell Search Threshold) value less than the sub-district, startup selective channel scanning algorithm scanned;
3) the MN SNR value that detects the AP of the optimum in the AP tabulation is found threshold value greater than the sub-district, and the difference of the SNR value of the SNR value of this AP and current AP above switching difference (Delta SNR) after, the MN disconnection is connected with current AP's, authenticates with new AP, reconnects.
According to above-mentioned based on 802.11 fast switch over method, it is characterized in that: switchover policy is as follows in the described step 1): mobile site MN compared from the old_SNR that this AP receives with last time according to the up-to-date signal to noise ratio (snr) of current AP:: if current SNR value is greater than neighbor picture threshold value (NC Threshold), and greater than the old_SNR value, show that MN is connected normally with current AP, and MN moves to the direction near current AP, does not carry out the scan channel operation this moment.If current SNR value, shows then that MN moves to the direction away from current AP less than old_SNR, MN switches possibly.If the SNR value of this moment is in case less than the neighbor picture threshold value, MN just starts scan channel, generates neighbor picture NC.
According to above-mentioned based on 802.11 fast switch over method, it is characterized in that: the maintenance of neighbor figure that the MN needs are extra, when MN normally is connected with current AP, if the SNR value of current AP is during less than the neighbor picture threshold value, MN can carry out scan channel the free time, close on the beacon frame (Beacon) that APs sends by reception, obtain close on APs the BSSID channel at place extremely, generate neighbor picture NC={C
I1, C
I2..., C
Im.
Based on 802.11 fast switch over method, it is characterized in that: the step of selective channel scanning algorithm is as follows described step 2) according to above-mentioned:
1.1) the SNR value of current AP is in case less than sub-district handoff threshold value, MN inquiry neighbor picture NC{C
I1, C
I2..., C
Im, if NC is not empty, execution in step 1.2), otherwise the AP tabulation is set for empty, startup standard 802.11 switched scan algorithms.
1.2) the AP tabulation is set for empty, execution in step 1.3).
1.3) Initial Channel Assignment C is set
InitBe C
k(C
kBe the current channel that connects of MN).
1.4) judge in NC, whether to exist not have a scanned channel C
Ij, and | C
Ij-C
Init| 〉=5, if exist, then execution in step 1.5); As there not being the channel that satisfies condition, see then whether the AP tabulation is empty.If the AP tabulation is sky, execution in step 1.9); If the AP tabulation is not empty, finish the selective channel scanning process.
1.5) judge | C
Ij-C
k| whether 〉=5 set up, if set up then execution in step 1.6); Otherwise, with channel C
IjChannel status is set to scan, and execution in step 1.4 then).
1.6) scanning channel C
Ij, execution in step 1.7).
1.7) if at channel C
IjOn find the SNR of the SNR of AP value greater than current AP arranged, then with channel C
Ij, C
IjOn the SNR of the AP that scans and this AP correspondence record in the AP tabulation execution in step 1.8).Otherwise execution in step 1.4).
1.8) C is set
InitBe C
Ij, execution in step 1.4).
1.9) all not scanned channels among the scan N C successively, if have the SNR value of the SNR of AP greater than current AP, then the channel number itemize at the AP that all are satisfied condition, the pairing SNR of each AP and place thereof records in the AP tabulation, finishes scanning process then; If there be not the SNR of the SNR value of AP, then withdraw from the selective channel scanning process, operative norm 802.11 switched scan algorithms greater than current AP.
According to above-mentioned based on 802.11 fast switch over method, it is characterized in that: switchover policy is as follows in the described step 3): MN can send Probe Request by the regular AP to optimum, the Probe Response that AP returns can judge MN to close on the state of signal-to-noise of optimum AP, its signal to noise ratio constantly compares with the signal to noise ratio of the current AP of connection, after the difference of the signal to noise ratio of these two AP surpasses switching difference (Delta SNR), carry out verification process (Authenticate), after authentication, MN and new AP reconnect (Re-associate) process.
The key parameter explanation:
● thresholding (Cell Search Threshold: we are 10dB with its value among the present invention) is found in the sub-district
SNR in the beacon frame of the AP that the MN detection is connected, if SNR dies down when being lower than a certain thresholding system, just begin to carry out the selective channel scanning process and advance switching between AP, we claim that this handoff threshold value is that thresholding (Cell SearchThreshold) 10dB is found in the sub-district.MN began to carry out the selective channel scanning process when signal to noise ratio snr of current AP was lower than this value, and after scanning process finished, MN had just write down the parameter of relevant neighbor AP in the AP tabulation of his self maintained, and selected the AP of optimums to switch according to these parameters.
● switch difference (Delta SNR: we are 6dB with its value among the present invention)
MN can send Probe Request by the regular AP to optimum, the Probe Response that AP returns can judge MN to close on the state of signal-to-noise of optimum AP, MN constantly compares this signal to noise ratio with the signal to noise ratio of the current AP of connection, when the difference of the signal to noise ratio of two AP surpass switch difference (Delta SNR) 6dB after, the MN disconnection is connected with current AP's, begin to initiate and new AP between verification process (Authenticate), after authentication, reconnecting Re-associate will be issued.The change procedure of SNR when Fig. 1 has shown switching judging.
● neighbor picture thresholding (NC Threshold: we are 12dB with its value among the present invention)
In our invention, MN needs extra maintenance of neighbor figure, when MN normally is connected with current AP, if the SNR value of current AP is less than neighbor picture threshold value (NC Threshold) 12dB, MN can carry out scan channel the free time, just write down its BSSID and the channel at place thereof if detect the existence of adjacent AP, generate neighbor picture NC.
● shift out scope threshold value (Out of Range Threshold: we are 2dB with its value among the present invention)
If the SNR value that the SNR value of all new AP that MN scans all can't satisfy with current AP differs this condition of Delta SNR (6dB), but the SNR value rapid deterioration of current AP is when shifting out scope threshold value (Out of Range Threshold) 2dB, MN will interrupt and being connected of current AP, and choosing on every side, the maximum AP of SNR value authenticates and reconnects.
Description of drawings
Fig. 1 judges the switching schematic diagram according to a kind of being changed by SNR of fast switch over method based on 802.11 networks of the present invention
Fig. 2 is the neighbor picture NC data structure diagram according to a kind of fast switch over method based on 802.11 networks of the present invention
Fig. 3 is the selective channel scanning algorithm flow chart according to a kind of fast switch over method based on 802.11 networks of the present invention
Fig. 4 is the AP list data structure figure according to a kind of fast switch over method based on 802.11 networks of the present invention
Embodiment
Because the limited coverage area of access point AP in 802.11 networks, so mobile node is between different AP when mobile, the switching that all can produce link layer at every turn.Designing a kind of rapid and reliable handoff algorithms is a key factor that guarantees seamless switching communication traffic in 802.11 WLAN (wireless local area network).Algorithm of the present invention is by obtaining neighbor picture NC and adopt the selective channel scanning algorithm, reduce the quantity of scanning channel, greatly reducing maximum MAC layer scanning delays consuming time in the handoff procedure, thereby significantly reduced handover delay.
Below just will be described in detail various embodiment of the present invention in conjunction with the accompanying drawings.
Fig. 1 is that the SNR according to a kind of fast switch over method based on 802.11 networks of the present invention changes and judges and switch schematic diagram.In the IEEE802.11 network, the core parameter that decision mobile site (MN) switches to another AP from an AP is the signal to noise ratio (snr) that the AP of the current association of measurement can provide, and the value of SNR has been represented the available signal quality and the noise magnitude of physical channel simultaneously.MN obtains the value of SNR from the AP per second sends 10 times beacon (beacon) frame.
Mobile site MN compared from the old_SNR that this AP receives with last time according to the up-to-date signal to noise ratio (snr) of current AP: if current SNR value is greater than neighbor picture threshold value (NC Threshold) (we are set to 12dB example of the present invention), and greater than the old_SNR value, show that MN is connected normally with current AP, and MN moves to the direction near current AP, does not carry out the scan channel operation this moment.If current SNR value, shows then that MN moves to the direction away from current AP less than old_SNR, MN switches possibly.If the SNR value of this moment is in case less than neighbor picture threshold value (12dB), MN just starts scan channel, generates neighbor picture NC.
SNR in the beacon frame of the AP that the MN detection is connected, if SNR dies down when being lower than a certain thresholding system, just begin to carry out the selective channel scanning process and advance switching between AP, we claim that this handoff threshold value is that thresholding (Cell Search Threshold) (we are set to 10dB in example of the present invention) is found in the sub-district.MN began to carry out the selective channel scanning process when signal to noise ratio snr of current AP was lower than this value, and after scanning process finished, MN had just write down the parameter of relevant neighbor AP in the AP tabulation of his self maintained, and selected the AP of optimums to switch according to these parameters.
MN can send Probe Request by the regular AP to optimum, the Probe Response that AP returns can judge MN to close on the state of signal-to-noise of optimum AP, MN constantly compares this signal to noise ratio with the signal to noise ratio of the current AP of connection, when surpassing, the difference of the signal to noise ratio of two AP switches difference (Delta SNR) (we are set to 6dB in example of the present invention), the MN disconnection is connected with current AP's, begin to initiate and new AP between verification process (Authenticate), after authentication,, MN and new AP reconnect (Re-associate) process.
If the SNR value that the SNR value of all new AP that MN scans all can't satisfy with current AP differs this condition of Delta SNR (6dB), but the SNR value rapid deterioration of current AP is less than shifting out scope threshold value (Out of Range Threshold) (we are taken as 2dB with it in example of the present invention), MN will interrupt and being connected of current AP, and choosing on every side, the maximum AP of SNR value authenticates and reconnects.
Fig. 2 is the neighbor picture NC data structure diagram according to a kind of fast switch over method based on 802.11 networks of the present invention.According to a kind of fast switch over method of the present invention based on 802.11 networks, MN needs extra maintenance of neighbor figure, when MN normally is connected with current AP, if the SNR value of current AP is during less than the neighbor picture threshold value, MN can carry out scan channel the free time, close on the beacon frame (Beacon) that APs sends by reception, obtain close on APs the BSSID channel at place extremely, generate neighbor picture NC={C
I1, C
I2..., C
Im, 1≤m≤14 wherein.
Fig. 3 is according to a kind of selective channel scanning algorithm flow chart of the present invention.The core concept of selective channel scanning algorithm: when MN switches, can follow numbering priority scan and current channel complete nonoverlapping channel (these channels channel number difference to each other is more than or equal to 5) on frequency domain, for example 1,6 according to current AP place channel, 11 or 2,7,12, perhaps 3,8,13 or the like the channels of non-overlapping copies like this when the value of the SNR of the AP on these channels does not satisfy condition, scan other channel again.
Step 101,102, the SNR value of current AP is in case less than sub-district handoff threshold value, and MN inquires about neighbor picture NC{C
I1, C
I2..., C
Im, if NC is not empty, execution in step 104, otherwise execution in step 103.
Step 104,105, the AP tabulation is set for empty, Initial Channel Assignment C is set
InitBe C
k
Step 111,114, all not scanned channels among the scan N C judge whether the SNR value of the SNR of AP greater than current AP successively, if having, execution in step 116; Otherwise, execution in step 117 (operative norm 802.11 switched scan algorithms).
Step 115,118, this AP that satisfies condition and SNR value record thereof in the AP tabulation, are write down channel C simultaneously
Ij, C is set then
InitBe C
Ij, turn to execution in step 106.
Step 116,119, the channel number at this AP, the pairing SNR of this AP and place thereof is recorded in the AP tabulation, finish the selective channel scanning process then.
The selective scanning process for example
Such as be that channel is channel 7 at the beginning.(C
init＝C
k＝7)
1) AP (if channel 1 is included among the neighbor picture NC) on the first scanning channel 1, if the AP that satisfies condition is arranged on the channel 1 then it is recorded in the AP tabulation, direct execution in step 118;
2) if channel 1 is not included among the neighbor picture NC, perhaps the AP that does not satisfy condition in the channel 1 turns to execution in step 106 this moment; See whether channel 2 is included among the neighbor picture NC.If this moment, channel 2 was in neighbor picture NC, the AP on the scanning channel 2 then, if the AP that satisfies condition is arranged on the channel 2 then it is recorded in the AP tabulation, execution in step 118 then.
3) then MN is that its numbering both differed 5 with the channel (channel 1 or channel 2) that the 1st step or the scanning of the 2nd step obtain with the channel of scanning, again and initial channel (channel 7) differ 5 channel.The channel that satisfies the numbering minimum of this condition is channel 12 (supposing that channel 12 is in NC), if the AP that satisfies condition is arranged on the channel 12 then it is recorded in the AP tabulation, execution in step 118,106,108,112 then, at this moment the selective channel end of scan.
4) if channel 12 is not included among the neighbor picture NC, the perhaps AP that does not satisfy condition in the channel 12, just continue scanning channel 13 (supposing that channel 13 is in NC), if the AP that satisfies condition is arranged on the channel 13 then it is recorded in the AP tabulation, execution in step 118,106 then, and 108,112, the selective channel end of scan at this moment
5) if channel 13 is not included among the neighbor picture NC, the perhaps AP that does not satisfy condition in the channel 13, just continue scanning channel 14 (supposing that channel 14 is in NC), if the AP that satisfies condition is arranged on the channel 14 then it is recorded in the AP tabulation, execution in step 118,106 then, and 108,112, the selective channel end of scan at this moment
Selective channel scan method of the present invention preferential scanning and current channel complete nonoverlapping channel (these channels channel number difference to each other is more than or equal to 5) on frequency domain earlier, if find the AP that satisfies condition in these channels, the selective channel scanning process finishes soon; If do not satisfy the AP of this condition, remaining channel among the ability scan N C.If also do not find the AP that satisfies condition, ability operative norm 802.11 switched scan algorithms.
This selective channel scan method can significantly reduce the time of scanning, algorithm selects corresponding channel to scan according to the difference of initial channel, priority scan and current channel be complete nonoverlapping channel (difference of these channels channel number to each other is more than or equal to 5) on frequency domain, makes this scanning algorithm that very big flexibility arranged.Adopt this inventive method, can significantly reduce the quantity of scanning channel, reduced maximum MAC layer scanning delays consuming time in the handoff procedure, thereby significantly reduced handover delay.
Fig. 4 is the AP list data structure figure according to a kind of fast switch over method based on 802.11 networks of the present invention.
The AP tabulation is the available AP tabulation that is higher than the SNR value of current AP in the SNR value that the selective scanning process finishes to write down the back.Not only write down the MAC Address of new AP, the SNR value has also write down the channel number at its place, wherein 1≤m≤14.
Claims (6)
1. fast switch over method based on 802.11 networks, wherein, described 802.11 networks are based on the network configuration of center (Centroid is a wireless aps), and the overlay area between the AP is overlapped.It is characterized in that: described fast switch over method comprises the steps:
1) mobile site MN compared from the old_SNR that this AP receives with last time according to the up-to-date signal to noise ratio (snr) of current AP; If current SNR value is greater than neighbor picture threshold value (NC Threshold) and greater than the old_SNR value, MN does not carry out scan channel.If current SNR value just starts scan channel less than old_SNR and less than the neighbor picture threshold value, generate neighbor picture (NC:Neighbor Chart);
When 2) the SNR value that detects current AP as MN was found thresholding (Cell Search Threshold) value less than the sub-district, startup selective channel scanning algorithm scanned;
3) the MN SNR value that detects the AP of the optimum in the AP tabulation is found threshold value greater than the sub-district, and the difference of the SNR value of the SNR value of this AP and current AP above switching difference (Delta SNR) after, the MN disconnection is connected with current AP's, authenticates with new AP, reconnects.
According to claim 1 based on 802.11 fast switch over method, it is characterized in that: the signal to noise ratio snr that AP provides in the described step 1) is the core parameter that decision MN switches to another AP from an AP, and the SNR value has been represented the available signal quality and the noise magnitude of physical channel simultaneously.The value of SNR is present in the beacon frame (Beacon) that the AP per second sends 10 times.
According to claim 1 based on 802.11 fast switch over method, it is characterized in that: switchover policy is as follows in the described step 1): MN sends the SNR value that comprises in beacon (Beacon) frame according to current AP at every turn and compares with the old_SNR that received last time: if current SNR value is greater than neighbor picture threshold value (NC Threshold), and greater than the old_SNR value, show that MN is connected normally with current AP, and MN moves to the direction near current AP, does not carry out the scan channel operation this moment.If current SNR value, shows then that MN moves to the direction away from current AP less than old_SNR, MN switches possibly.If the SNR value of this moment is in case less than the neighbor picture threshold value, MN just starts scan channel, generates neighbor picture NC.
According to claim 1 based on 802.11 fast switch over method, it is characterized in that: the maintenance of neighbor figure that the MN needs are extra, when MN normally is connected with current AP, if the SNR value of current AP is during less than the neighbor picture threshold value, MN can carry out scan channel the free time, close on the beacon frame (Beacon) that APs sends by reception, obtain the BSSID that closes on APs and the channel at place thereof, generate neighbor picture NC={C
I1, C
I2..., C
Im.
According to claim 1 based on 802.11 fast switch over method, it is characterized in that: the step of selective channel scanning algorithm is as follows described step 2):
1.1) the SNR value of current AP is in case less than sub-district handoff threshold value, MN inquiry neighbor picture NC={C
I1, C
I2..., C
Im, if NC is not empty, execution in step 1.2); Otherwise the AP tabulation is set for empty, startup standard 802.11 switched scan algorithms.
1.2) the AP tabulation is set for empty, execution in step 1.3).
1.3) Initial Channel Assignment C is set
InitBe C
k(C
kBe the current channel that connects of MN).
1.4) judge in NC, whether to exist not have a scanned channel C
Ij, and | C
Ij-C
Init| 〉=5, if exist, then execution in step 1.5).As there not being the channel that satisfies condition, see then whether the AP tabulation is empty.If the AP tabulation is sky, execution in step 1.9); If the AP tabulation is not empty, then finish the selective channel scanning process.
1.5) judge | C
Ij-C
k| whether 〉=5 set up, if set up, then execution in step 1.6); Otherwise, with channel C
IjChannel status is set to scan, and execution in step 1.4 then).
1.6) scanning channel C
Ij, execution in step 1.7).
1.7) if at channel C
IjOn find the SNR of the SNR of AP value greater than current AP arranged, then with channel C
Ij, C
IjOn the SNR of the AP that scans and this AP correspondence record in the AP tabulation execution in step 1.8).Otherwise execution in step 1.4).
1.8) C is set
InitBe C
Ij, execution in step 1.4).
1.9) all not scanned channels among the scan N C successively, if have the SNR value of the SNR of AP greater than current AP, then the channel number itemize at the AP that all are satisfied condition, the pairing SNR of each AP and place thereof records in the AP tabulation, finishes scanning process then; If there be not the SNR of the SNR value of AP, then withdraw from the selective channel scanning process, operative norm 802.11 switched scan algorithms greater than current AP.
According to claim 1 based on 802.11 fast switch over method, it is characterized in that: switchover policy is as follows in the described step 3): MN can send Probe Request by the regular AP to optimum, the Probe Response that AP returns can judge MN to close on the state of signal-to-noise of optimum AP, its signal to noise ratio constantly compares with the signal to noise ratio of the current AP of connection, after the difference of the signal to noise ratio of these two AP surpasses switching difference (DeltaSNR), carry out verification process (Authenticate), after authentication, MN and new AP reconnect (Re-associate) process.
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