tailieunhanh - 802.11® Wireless Networks: The Definitive Guide phần 2

Sau khi khung truyền đã hoàn thành và DIFS đã trôi qua, trạm có thể cố gắng để truyền tải các dữ liệu dựa trên tắc nghẽn. Một giai đoạn được gọi là cửa sổ tranh hoặc cửa sổ backoff theo DIFS. Cửa sổ này được chia thành các khe cắm. Khe cắm chiều dài là mediumdependent; | After frame transmission has completed and the DIFS has elapsed stations may attempt to transmit congestion-based data. A period called the contention window or backoff window follows the DIFS. This window is divided into slots. Slot length is mediumdependent higher-speed physical layers use shorter slot times. Stations pick a random slot and wait for that slot before attempting to access the medium all slots are equally likely selections. When several stations are attempting to transmit the station that picks the first slot the station with the lowest random number wins. As in Ethernet the backoff time is selected from a larger range each time a transmission fails. Figure 3-7 illustrates the growth of the contention window as the number of transmissions increases using the numbers from the direct-sequence spread-spectrum DSSS physical layer. Other physical layers use different sizes but the principle is identical. Contention window sizes are always 1 less than a power of 2 . 31 63 127 255 . Each time the retry counter increases the contention window moves to the next greatest power of two. The size of the contention window is limited by the physical layer. For example the DS physical layer limits the contention window to 1023 transmission slots. Figure 3-7. DSSS contention window size When the contention window reaches its maximum size it remains there until it can be reset. Allowing long contention windows when several competing stations are attempting to gain access to the medium keeps the MAC algorithms stable even under maximum load. The contention window is reset to its minimum size when frames are transmitted successfully or the associated retry counter is reached and the frame is discarded. Fragmentation and Reassembly Higher-level packets and some large management frames may need to be broken into smaller pieces to fit through the wireless channel. Fragmentation may also help improve reliability in the presence of interference. The primary sources .