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Ethernet Networks: Design, Implementation, Operation, Management 4th phần 8

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một số bổ sung các tiêu chuẩn IEEE. Một Ngoài ra là các đặc điểm kỹ thuật IEEE 802.11b, mở rộng tỷ lệ hoạt động của DSSS đến 5,5 Mbps và 11 Mbps và đại diện cho loại phổ biến nhất của mạng LAN không dây khi sửa đổi cuốn sách này xảy ra. | 408 chapter eight ISO OSI Reference Model IEEE 802.11 layers Data link layer Logical link control Media access control CSMA CA Physical layer FHSS DSSS IR Legend FHSS Frequency Hopping Spread Spectrum DSSS Direct Sequence Spread Spectrum IR Infrared Figure 8.1 IEEE 802.11 architecture. several additions to the IEEE standard. One addition was the IEEE 802.11b specification which extended the operating rate of DSSS to 5.5 Mbps and 11 Mbps and which represented the most popular type of wireless LAN when this book revision occurred. Both the basic 802.11 and the 802.11b specifications operate in the 2.4 GHz unlicensed Industrial Scientific and Medical ISM band. While the Federal Communications Commission FCC in the U.S. regulates the maximum power and transmission method the fact that the ISM band is unlicensed means that a user does not have to obtain a license to use equipment in that frequency band. A second addendum to the IEEE 802.11 standard is the 802.11a specification. This specification defines the use of a multi-carrier frequency transmission method in the 5 GHz ISM band. The multi-carrier frequency method is referred to as orthogonal frequency division multiplexing OFDM which results in a large number of carriers being used each of which operates at a low data rate but cumulatively they support a high data rate up to 54 Mbps. Because higher frequencies attenuate more rapidly than lower frequencies the range of 802.11a-compatible devices is significantly less than that of 802.11b devices. This results in a requirement to install additional access points to obtain the same area of wireless LAN coverage and increases the cost of a very high speed wireless LAN. Because many network operators require more speed than that wireless ethernet 409 provided by the 802.11b specification but a higher range than that supported by the 802.11a specification the IEEE has been working on a new standard referred to as 802.11g which doubles the data rate of 802.11b networks to