tailieunhanh - 802.11® Wireless Networks: The Definitive Guide phần 5

sử dụng các số từ chuỗi trực tiếp lây lan-quang phổ (DSSS) vật lý lớp. Lớp vật lý khác sử dụng kích cỡ khác nhau, nhưng nguyên tắc là giống hệt nhau. Tranh kích thước cửa sổ luôn 1 ít hơn so với một sức mạnh của 2 Các đặc điểm kỹ thuật OFDM PHY lá thực hiện rất nhiều vĩ độ trong việc lựa chọnt. | Based on the hop sequence number the station knows the channel-hopping order. As an example say that a station has received a Beacon frame that indicates that the BSS is using the North America Europe hop sequence number 1 and is at hop index 2. By looking up the hop sequence the station can determine that the next channel is 65. Hop times are also well-defined. Each Beacon frame includes a Timestamp field and the hop occurs when the timestamp modulo dwell time included in the Beacon is 0. ISM emission rules and maximum throughput Spectrum allocation policies are the limiting factor of frequency-hopping systems. As an example consider the three major rules imposed by the FCC in the . 121 2 These rules are in rule 247 of part 15 of the FCC rules 47 CFR . 1. There must be at least 75 hopping channels in the band which is wide. 2. Hopping channels can be no wider than 1 MHz. 3. Devices must use all available channels equally. In a 30-second period no more than seconds may be spent using any one channel. Of these rules the most important is the second one. No matter what fancy encoding schemes are available only 1 MHz of bandwidth is available at any time. The frequency at which it is available shifts continuously because of the other two rules but the second rule limits the number of signal transitions that can be used to encode data. With a straightforward two-level encoding each cycle can encode one bit. At 1 bit per cycle 1 MHz yields a data rate of 1 Mbps. More sophisticated modulation and demodulation schemes can improve the data rate. Four-level coding can pack 2 bits into a cycle and 2 Mbps can be squeezed from the 1-MHz bandwidth. The European Telecommunications Standards Institute ETSI also has a set of rules for spread-spectrum devices in the ISM band published in European Telecommunications Standard ETS 300-328. The ETSI rules allow far fewer hopping channels only 20 are required. Radiated power however is controlled .