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802.11® Wireless Networks: The Definitive Guide phần 4

trạm đó chọn các khe đầu tiên (trạm với số ngẫu nhiên thấp nhất) thắng. Như trong Ethernet, thời gian backoff được chọn từ một phạm vi lớn hơn mỗi lần truyền dữ liệu không thành công. Hình 3-7 minh họa sự phát triển của cửa sổ tranh như số lượng tăng lên truyền, | frame with WEP. The header identifying the frame as an authentication frame is preserved but the information elements are hidden by WEP. After receiving the third frame the access point attempts to decrypt it and verify the WEP integrity check. If the frame decrypts to the Challenge Text and the integrity check is verified the access point will respond with a status code of successful. Successful decryption of the challenge text proves that the mobile station has been configured with the WEP key for the network and should be granted access. If any problems occur the access point returns an unsuccessful status code. 7.3.3 Preauthentication Stations must authenticate with an access point before associating with it but nothing in 802.11 requires that authentication take place immediately before association. Stations can authenticate with several access points during the scanning process so that when association is required the station is already authenticated. This is called preauthentication. As a result of preauthentication stations can reassociate with access points immediately upon moving into their coverage area rather than having to wait for the authentication exchange. In both parts of Figure 7-6 there is an extended service set composed of two access points. Only one mobile station is shown for simplicity. Assume the mobile station starts off associated with AP1 at the left side of the diagram because it was powered on in AP1 s coverage area. As the mobile station moves towards the right it must eventually associate with AP2 as it leaves AP1 s coverage area. U No imautliữriitđĩon Cr ST . J h API APi . . Ềíỉĩ ẼÍỈỈ ìỌ Figure 7-6. Time savings of preauthentication Preauthentication is not used in the most literal interpretation of 802.11 shown in Figure 7-6a. As the mobile station moves to the right the signal from AP1 weakens. The station continues monitoring Beacon frames corresponding to its ESS and will eventually note the existence of AP2. At some point the