| 单词 | 802.11b |
| 释义 | 802.11b Also known as Wi-Fi ( Wi reless Fi delity). The IEEE standard for an 802.11 wireless local area network (WLAN) operating in the 2.4-GHz band. 802.11b transmission options include infrared (IR), which is rarely used, and radio frequency (RF). 802.11b uses direct sequence spread spectrum (DSSS) modulation. DSSS involves the transmission of a bit stream that is modulated with the Barker code chip-ping sequence. Each bit is encoded into a redundant 11-bit Barker code (e.g., 10110111000), with each resulting data object forming a chip. The chip is put on a carrier frequency in the 2.4 GHz range (2.4–2.483 GHz) and the waveform is modulated using one of several modulation schemes, depending on link quality.The highest link quality employs the most complex technique, which yields the greatest signaling rate. As the link quality deteriorates, the modulation technique ratchets down to the least capable. In order of complexity and yield, those techniques are as follows: • Binary phase-shift keying (BPSK) at 1 Mbps. • Quadrature phase-shift keying (QPSK) at 2 Mbps. • Complementary code keying (CCK) at 5.5 and 11 Mbps. Radio link quality is always uncertain, especially when using unlicensed frequencies due to the potential for mutual interference with other systems. At the lowest rate, however, 802.11b link quality generally is acceptable at distances of up to 100 meters or so. 802.11b divides the available spectrum into 14 channels, each of which has a width of 25 MHz. In the United States, the FCC allows the use of 11 channels. Four channels are available in France, 13 in the rest of Europe, and only 1 in Japan. There also is overlap between adjacent channels as each has a width of 25 MHz and all share a band that is only 83 MHz (2.4–2.483 GHz) wide. See also 802.11, Barker code, BPSK, carrier, CCK, DSSS, frequency, IR, modulation, QPSK, RF, and WLAN. |
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