PCT/CN2004/000784 English Translation A METHOD FOR DETECTING FREQUENT HANDOVER IN THE WIRELESS LOCAL AREA NETWORK Technical Field The present invention relates to a wireless local area network (LAN) system in communication field, especially relates to a method for detecting frequent handover in wireless LAN. Technical Background Wireless Local Area Network (LAN), such as the wireless network according with IEEE802. 1 a/b/g, means interconnecting the computer devices through wireless communication technique to construct a network system capable of mutual communication and resource sharing. In wireless LAN, a typical network topology structure is called an extended service set (ESS). The ESS system is composed of one or several basic service sets (BSS) and a distribution system which is for connecting the BSSs. Each BSS includes a wireless access point (WAP) device. In the ESS structure in wireless LAN as shown in figure 1, there are two basic service sets 1 and 2 which are connected by the distribution system. The ESS is distinguished by ESS identifier, and WAP is distinguished by BSS identifier, i.e. access point identifier. By scanning the channel, the station acquires the information of the ESS and BSS in the wireless LAN where the station locates, and chooses a suitable BSS to join in. When a station needs to join a wireless LAN, the normal communication can only be started by connecting the access point. The station can establish connection with only one access point at one moment. When the station needs to change certain connection attribute, it may start a reconnection process. After establishing connection with one access point, the station checks whether there is an 2 PCT/CN2004/000784 English Translation access point with better signal quality by scanning the channel. If such access point exists, the station will establish connection with the new access point through the reconnection process and cut off the connection with the old access point. The above process is called handover. The current wireless LAN protocols do not prescribe the condition for the station to perform handover. Usually, it is realized by the method of handing over windows, i.e., only when the station finds out that there is a new access point having a better signal quality than that of the old access point with a predetermined handover window value, the station will perform the handover. However, the violent changes of the wireless environment and unsuitably set handover window value will result in the frequent handover of the station between two access points which have similar signal qualities. Summary of the Invention The purpose of the invention is to advance a method for detecting the frequent handover in wireless LAN, wherein, the wireless terminal records the historical information of the connected access points, based on which it determines whether multiple handovers have happened between two access points during a short period of time, thereby providing the basis for avoiding frequent handover of the station in wireless LAN, and further avoiding the dropping of the access efficiency resulting from frequent handover. In order to realize the above purpose, the invention raises a method for detecting frequent handover in wireless LAN, characterized in that it includes the following steps of: Step 1, creating handover information recording which is created and maintained by a station; Step 2, determining handover condition, after the station accessing to an access point, searching for another access point that satisfies the handover condition by scanning the channel, if such an access point existing, proceeding with the next step, otherwise continuing to scan the handover state; 3 PCT/CN2004/000784 English Translation Step 3, determining the frequent handover state, the station determining whether itself being in the frequent handover state according to the valid record in the current handover information recording, and handing over when not being in frequent handover state; Step 4, recording the handover information, the station updating the handover information record after completing the handover, and incrementing a new recording item; and Step 5, starting time counter mechanism for the newly incremented recording item, maintaining and processing the handover information, at the same time, the state continuing to access or scan. Said method for detecting frequent handover in wireless LAN, in said step 1, said handover information recording is created locally by the station, and each recording item needs to store at least one identifier field of the current access point and one identifier field of a new access point that can be handed over. Said method for detecting frequent handover in wireless LAN, in said step 5, a time counter timer is started to maintain and process the handover information recording, each recording item in the handover information recording is valid during a fixed period of time, when the time counter timer corresponding to the recording item expires, the corresponding handover information recording item will be deleted, and the corresponding time counter timer will be cancelled. Said method for detecting frequent handover in wireless LAN, in which said step 2 further includes the following steps of: the station creating connection with an access point; the station being in non-handover state; determining whether the recording item in the handover information recording of the station is invalid, if yes, deleting the corresponding handover recording, and canceling the corresponding time counter timer; and scanning the channel to search for another access point that satisfies the handover 4 PCT/CN2004/000784 English Translation condition. Said method for detecting frequent handover in wireless LAN, in which said step 3 further includes the following steps of: the station finding another access point that satisfies the handover condition; the station being in pre-handover state; the station counting up the recording items in the current handover information recording that are valid and match the identifier numbers of the current access point and the new access point; if the number of the suited recording items being greater than a predetermined frequent handover threshold, then the station being in the frequent handover state, cutting off the handover, and continuing to scan the handover state; and if the number of the suited recording items is no greater than the predetermined frequent handover threshold, then setting the station being in the state of handover available, and handing over the station to another access point. Brief Discription of the Drawings Figure 1 is the schematic view of the extended service set (ESS) in wireless LAN according to this invention. Figure 2 is the schematic view of one embodiment according to the method of this invention. Figure 3 shows the specific implementation process of determining station handover according to this invention; Figure 4 shows the state switch of the terminal station according to this invention. Preferred Embodiment of the Invention 5 PCT/CN2004/000784 English Translation The method of the invention is adaptive to the wireless LAN network structure illustrated in figure 1. In the wireless LAN applying the method of this invention, there are at least two access points API and AP2 which belong to a same ESS, and a terminal station STA. The content relating to the method of the invention includes the following steps of: (1) creating the handover information recording: the station STA constructing the handover information recording locally and maintaining the same, each recording item needs to store at least one identifier field of a new access point and one identifier field of a current access point; (2) determining the handover condition: after connecting the access point AP1, the station STA searching whether there is another access point AP2 which satisfies the existing handover condition by scanning aerial wireless signal, if finding such access point AP2 that satisfies the handover condition, performing the next step, otherwise, continuing to scan the handover state; (3) determining the frequent handover state: the station STA counting up the recording items in the current handover information recording that are valid and match the identifier numbers of the current access point API and/or the new access point AP2, if the number counted up exceeding a fixed threshold value, the station STA being in the frequent handover state; if the number counted up does not exceed the fixed threshold value, the station STA being not in the frequent handover state; (4) recording the handover information: if the station STA having performed the handover between the access points API and AP2, and the station STA having established connection with the access points AP2, then in the new handover information recording, the station STA filling the identifier number of the access point AP2 into the identifier field of the new access point and filling the identifier number of the access point API into the identifier field of the current access point; and (5) starting a time counter mechanism for the incremented record: the station STA being valid for each recording item in the handover information recording table during a fixed period 6 PCT/CN2004/000784 English Translation of time, when the recorded existing time exceeding the above fixed time period, deleting the recording item, the station STA immediately starting a time counter timer for the above newly incremented item of the table, if the time counter timer corresponding to the item of the table expiring, deleting the above item of the table from the handover information recording table, and also canceling the corresponding time counter timer. In the following, the invention will be further described in detail in combination with the drawings and the embodiment. The application environment according to one embodiment of the invention is shown as figure 2. Device 100 is a wireless LAN card (WLAN card) according with wireless LAN standard IEEE 802.11/a/b/g. Devices 101 and 102 are wireless access point devices according with wireless LAN standard IEEE 802.1 1/a/b/g. Device 100 detects devices 101 and 102 by scanning the channel, and selects device 101 to establish connection. A frequent handover detection process of device 100 according to the description of the invention is shown as figure 3, it is described as follows (for the conversion of the state of device 100, please refer to figure 4): Step 1, device 100 establishing a connection with device 101; Step 2, setting device 100 to be in the non-handover state; Step 3, deciding whether the handover record in the handover information recording table of device 100 is invalid, if there being invalid record, turning to step 4, otherwise, turning to step 5; Step 4, deleting the invalid record in the handover information recording table, and canceling the corresponding timer; Step 5, device 100 searching whether there being a device 102 which has better signal 7 PCT/CN2004/000784 English Translation quality and satisfies the current handover condition, if such a device being found, turning to step 6; otherwise turning to step 2; Step 6, device 100 setting itself to be in the pre-handover state; Step 7, device 100 counting up the matched recording items in the handover information recording table according to the identifier number of device 101 and the identifier number of device 102; Step 8, deciding whether the number of the matched recording items is greater than the frequent handover threshold, If yes, turning to step 9; otherwise, turning to step 11; Step 9, device 100 setting itself to be in the frequent handover state; Step 10, device 100 cutting off the handover, and turning to step 2; Step 11, device 100 setting itself to be in the handover available state; Step 12, handing over device 100 from device 101 to device 102 according to the current handover flow; Step 13, device 100 being in the handover available state, after completing the current handover process, device 100 updating the handover information table, incrementing a recording item in the handover information recording table, said incremented item at least recording the identifier numbers of device 101 and device 102; and Step 14, starting time sliding window timer for the incremented recording item in the handover information recording table, timing the life period of said recording item, after the sliding window finishing timing, canceling the recording item and its corresponding sliding window timer, and turning to step 2. Surely, this invention can also have various other embodiments. People skilled in the art are able to make various corresponding variations and modifications based on this invention without departing from the spirit and essential of this invention. Therefore, all those corresponding variations and modifications should fall into the protection scope of the appended claims. 8 PCT/CN2004/000784 English Translation Industrial Applicability This invention decides whether the station is in frequent handover state by analyzing the handover information recorded in the latest period of time, provides the basis for avoiding the frequent handover, and improves the access efficiency during the station handover in wireless LAN. 9