tailieunhanh - 802.11® Wireless Networks: The Definitive Guide phần 6

các kỹ thuật để ghi nhận một kênh bận rộn. Ngưỡng sức mạnh nhận được tín hiệu xác định xem kênh được sử dụng, nhưng các hướng dẫn chính cho các thiết bị chuẩn là nó phải đáp ứng các tiêu chuẩn hiệu suất nhất định. Việc triển khai miễn phí để sử dụng trường Chiều dài gói từ tiêu đề | b The data bits per symbol is a function of the rate of the convolution code. Clear Channel Assessment The OFDM PHY specification leaves implementers a great deal of latitude in selecting techniques for noting a busy channel. Received signal strength thresholds determine whether the channel is in use but the main guideline for equipment is that it must meet certain performance standards. Implementations are free to use the Packet Length field from the PLCP header to augment clear channel assessment but this is not required. An Example of OFDM Encoding OFDM encoding as you can no doubt see by now is an intense multistep process. One of the additions that made to the original specification was Annex G an encoding of Schiller s Ode to Joy for transmission over an Shortly after was published the IEEE working group discovered several errors in the example and published a correction. If you are interested in learning about OFDM encoding in detail you can refer to this example. 2 Well an English translation anyway. Characteristics of the OFDM PHY Parameters specific to the OFDM PHY are listed in Table 11-4. Like the physical layers presented in Chapter 10 the OFDM PHY also incorporates a number of parameters to adjust for the delay in various processing stages in the electronics. As a final note the extra radio bandwidth provided by the U-NII bands offers a great deal of throughput. There are eight overlapping channels available for the OFDM PHY so it can offer up to 432 Mbps in an area where all eight channels are co-located. Table 11-4. OFDM PHY parameters Parameter Value Notes Maximum MAC frame length 4 095 bytes Slot time 9ps SIFS time 16ps The SIFS is used to derive the value of the other interframe spaces DIFS PIFS and EIFS . Contention window size 15 to 1 023 slots Preamble duration 20ps PLCP header 4ps Table 11-4. OFDM PHY parameters Parameter Value Notes duration Like the other physical layers