潘书阳-文献翻译

天津职业技术师范大学 Tianjin University of Technology and Education 文 献 翻 译 专    业：       网络工程        班级学号：     网络1001 – 21      学生姓名：        潘书阳        指导教师：    刘建卿  副教授     二〇一四年六月 目    录 1 英文原文    1 2 中文译文    8 1 英文原文 CCNA Wireless Official Exam Certification Guide Brandon James Carroll CHAPTER 6  Overview of the 802.11 WLAN Protocols The wireless space consists of numerous protocols. Specifically in the WLAN area, the Institute of Electrical and Electronic Engineers (IEEE) has created several protocols within the 802.11 category to facilitate the networking process. These protocols define the data rates, the modulation techniques, and more. An understanding of these protocols is essential for any administrator of wireless networks. In this chapter, you will learn about the 802.11 family of protocols, including 802.11, 802.11a, b, and g. In addition, you will gain an introduction to the 802.11n draft standard. The 802.11 Protocol Family Overview The IEEE helps to standardize wireless protocols. Those that you must be familiar with for the CCNA Wireless Exam are the 802.11 a/b/g and n protocols. These four IEEE standards define the wireless family that is used in almost all wireless LANS today. The standardization of wireless networking started with the original 802.11 protocol in 1997, and each protocol thereafter has simply added to the benefit of wireless technologies. This chapter looks at the 802.11 protocol families, their history, and how they operate. The 802.11 protocols encompass the 2.4-GHz and 5-GHz range. The Original 802.11 Protocol The original 802.11 protocol was where wireless LANs find there beginnings. It is rare to find this original protocol in new hardware today, probably because it only operates at 1 and 2 Mbps. The 802.11 standard describes frequency-hopping spread spectrum (FHSS), which operates only at 1 and 2 Mbps. The standard also describes direct sequence spread spectrum (DSSS), which operates only at 1 and 2 Mbps. If a client operates at any other data rate, it is considered non-802.11 compliant, even if it can use the 1- and 2-Mbps rates. Table 6-2 highlights the characteristics of the original 802.11 protocol. Table 6-2 the 802.11 Protocol Ratified 1997 RF Technology FHSS and DSSS Frequency Spectrum 2.4-GHz     The original 802.11 protocol falls within the industry, scientific, and medical (ISM) bands and operates only in the 2.4-GHz range. The 2.4-GHz range has up to 14 channels depending on the country you are in. In the United States, the FCC allows channels 1 through 11 to be used. This gives you 3 nonoverlapping channels: 1, 6, and 11. This is important because you do not want to have APs and clients operating on the same channel placed near each other for interference reasons. The 802.11b Protocol 802.11b is a supplement to the 802.11 protocol. To get a better feel for how the 802.11 protocols progressed, understand that technology moves faster than the standards do. 802.11 was quickly outgrown because wired networks offered 10 Mbps versus the 1 and 2 Mbps of 802.11. Vendors developed methods of achieving higher data rates. The danger in vendor-designed protocols, of course, is interoperability. The job of the IEEE was simply to define a standard that all vendors could follow based on the proprietary implementations that they were using. 802.11b offers higher data rates—up to 11 Mbps—with backward compatibility at 1 and 2 Mbps. At 1 and 2 Mbps, the same coding and modulation as 802.11 is used. When operating at the new speeds—5.5 Mbps and 11 Mbps—a different modulation and coding is used. 802.11 uses Barker 11 coding, as covered in Chapter 1, “Introduction to Wireless Networking Concepts,” and 802.11b uses complementary code keying (CCK) for coding. For modulation, 802.11 uses differential binary phase-shift keying (DBPSK), whereas 802.11b uses differential quadrature phase-shift keying (DQPSK). The result is more data sent in the same period. 802.11b was ratified in September 1999. The United States has 11 channels, the same as 802.11. In Europe, the ETSI defines 13 channels, and Japan has 14. 802.11b allows dynamic rate shifting (DRS) to enable clients to shift rates to lower rates as they travel farther away from an AP and higher rates as they get closer to an AP. Today, 802.11b is the most popular and most widely deployed wireless standard.    Table 6-3 gives some basic in-formation on the 802.11b standard. Table 6-3 the 802.11b Protocol Ratified 1999 RF Technology DSSS Frequency Spectrum 2.4-GHz Coding Barker 11 and CCK Modulation DBPSK and DQPSK Data Rates 1, 2, 5.5, 11 Mbps Nonoverlapping Channels 1, 6, 11     The 802.11g Protocol The IEEE ratified 802.11g in June 2003. In addition to the four data rates of 802.11b, it added eight more. The maximum data rate of 54 Mbps places 802.11g in the same speed range as 802.11a; however, it remains in the 2.4-Ghz frequency range. On the lower end, 802.11g is still compatible with 802.11b, using the same modulation and coding as 802.11b for the 1-, 2-, 5.5-, and 11-Mbps rates. To achieve the higher data rates, 802.11g uses orthogonal frequency division multiplexing (OFDM) for modulation. OFDM is the same modulation that 802.11a uses. There are still only three nonoverlapping channels. With OFDM, you must be careful about power outputs; the power needs to be reduced to handle the peaks in the modulation technique and still fall within governmental regulations. Table 6-4 shows some details about 802.11g. 继续阅读