外文翻譯---無線局域網(wǎng)

1、<p><b>　　無線局域網(wǎng)</b></p><p>　　概覽無線局域網(wǎng)（ WLAN ）通常是一個(gè)有線局域網(wǎng)的延伸。無線局域網(wǎng)組件將數(shù)據(jù)包轉(zhuǎn)換成無線電波或紅外線（ IR ）光脈沖，并傳送到其他無線裝置，或到一個(gè)作為通往有線局域網(wǎng)服務(wù)的接入點(diǎn)?，F(xiàn)今大多數(shù)無線局域網(wǎng)是基于IEEE 802.11和802.11b標(biāo)準(zhǔn)之間的無線通信設(shè)備和網(wǎng)絡(luò)。這些標(biāo)準(zhǔn)允許數(shù)據(jù)傳輸分別在1到2 Mbps

2、或5至11 Mbps。并指定一個(gè)共同的架構(gòu)，傳輸方法，及其他方面的無線數(shù)據(jù)傳輸，以改善互操作性的產(chǎn)品。</p><p>　　技術(shù)在設(shè)計(jì)一個(gè)無線局域網(wǎng)解決方案時(shí)，制成品無線局域網(wǎng)有多種技術(shù)選擇。每種技術(shù)來都有自身的優(yōu)點(diǎn)和局限性。</p><p>　　窄帶技術(shù)窄帶無線電系統(tǒng)在一個(gè)特定的無線電頻率傳輸和接收用戶信息。窄帶無線電頻率保持其頻帶為可能剛剛通過信息的大小。在不同的頻道頻率仔細(xì)協(xié)

3、調(diào)不同用戶可避免通訊渠道之間產(chǎn)生不良的串音。</p><p>　　私人電話線很像一個(gè)電臺頻率。當(dāng)每個(gè)家庭在一個(gè)居委會有自己的私人電話線，人們在一個(gè)主頁不能聽其他家的電話。在無線電系統(tǒng)，隱私和不干涉是完成所要求作出其他的家園。無線電接收器過濾掉所有的無線電信號，除了那些指定的頻率。</p><p>　　從客戶角度來看，有一個(gè)缺點(diǎn)，窄帶技術(shù)是最終用戶，針對每個(gè)雇用網(wǎng)站的地方，必須取得FCC的許

4、可。</p><p>　　擴(kuò)頻技術(shù)大多數(shù)無線局域網(wǎng)系統(tǒng)使用擴(kuò)頻技術(shù)，軍事開發(fā)的寬帶射頻技術(shù)使用在可靠，安全，關(guān)鍵任務(wù)通信系統(tǒng)。擴(kuò)展頻譜是旨在提供高的帶寬利用率，可靠性，完整性和安全性。換言之，在窄帶傳輸中更多的帶寬被消耗，但權(quán)衡所產(chǎn)生的信號，在效果，聲音方面，更容易偵測，只要接收機(jī)知悉對擴(kuò)頻信號播出的參數(shù)。如果接收機(jī)沒有調(diào)整在正確的頻率，擴(kuò)頻信號看起來像背景噪音。有兩種類型的無線擴(kuò)頻：跳頻和直接序列。<

5、/p><p>　　跳頻擴(kuò)頻技術(shù)跳頻擴(kuò)譜（ fhss ）無論發(fā)射機(jī)和接收機(jī)，在一個(gè)眾所周知的模式使用窄頻載波變化的頻率。妥善同步，凈效果是要維持一個(gè)單一的邏輯通道。對于一個(gè)意想不到的接收器， fhss似乎是短時(shí)脈沖噪聲。</p><p>　　直接序列擴(kuò)頻技術(shù)直接序列擴(kuò)展頻譜（擴(kuò)頻）為每個(gè)位元生成一個(gè)多余的比特模式，以轉(zhuǎn)交。這位模式是所謂的子碼（或刨切代碼）。子碼越長，可以回收的概率原始數(shù)

6、據(jù)更大（當(dāng)然，要求更多的帶寬） 。如果該子碼一個(gè)或多個(gè)比特在傳輸過程中損壞。一個(gè)意想不到的接收器，直接序列擴(kuò)頻顯示為低功耗寬帶噪聲，并被窄帶接收機(jī)拒絕（忽略不計(jì)）。</p><p>　　紅外技術(shù)第三個(gè)技術(shù)，很少用在商業(yè)無線局域網(wǎng)，是紅外。紅外（ IR ）系統(tǒng)使用的頻率非常高，在電磁波譜中僅低于可見光，進(jìn)行數(shù)據(jù)。像光線一樣，紅外不能穿透不透明的對象;這是不是針對（瞄準(zhǔn)線）或彌漫性的技術(shù)。廉價(jià)的系統(tǒng)提供非常有限的

7、范圍內(nèi)，而且通常是用于個(gè)人區(qū)域網(wǎng)絡(luò)，但偶爾會用在具體的無線局域網(wǎng)的應(yīng)用。高性能定向紅外是不切實(shí)際的移動用戶，因而是只能用來實(shí)施固定子網(wǎng)絡(luò)。彌漫性（或反射）紅外無線局域網(wǎng)系統(tǒng)不需要瞄準(zhǔn)線，但細(xì)胞對個(gè)別房間是有限的。</p><p>　　無線局域網(wǎng)如何工作無線局域網(wǎng)使用電磁電波（無線電或紅外線）從一個(gè)點(diǎn)到另一個(gè)點(diǎn)通信不依靠任何物理連接。無線電波通常被稱為無線電運(yùn)營商，因?yàn)樗麄冎皇菆?zhí)行功能，為一個(gè)遙控接收器提供能源

8、。數(shù)據(jù)傳輸是疊加在廣播運(yùn)營商，以便它能夠在接收端準(zhǔn)確地提取。這是一般稱為調(diào)制承載者的信息傳播。一旦數(shù)據(jù)疊加（調(diào)制）在無線電載波上，無線電信號占用一個(gè)以上的頻率，由于頻率或比特率的調(diào)制信息添加到承運(yùn)人。</p><p>　　多個(gè)電臺運(yùn)營商們可以在同一個(gè)空間在同一時(shí)間存在，如果無線電波傳送不同的無線電頻率，不干擾對方，。提取數(shù)據(jù)時(shí)，無線電接收機(jī)接受在一個(gè)無線電頻率的頻率，而拒絕所有其他的頻率。</p>

9、<p>　　在一個(gè)典型的無線局域網(wǎng)配置中，一個(gè)發(fā)送器/接收器（收發(fā)器）裝置，稱為接入點(diǎn)，從一個(gè)固定的位置使用標(biāo)準(zhǔn)的布線連接到有線網(wǎng)絡(luò)。在最低限度，接入點(diǎn)在無線局域網(wǎng)和有線網(wǎng)絡(luò)的基礎(chǔ)設(shè)施之間接收，緩沖器，并傳送數(shù)據(jù)。一個(gè)單一的接入點(diǎn)可以支持一小群用戶其功能范圍為少于一百至幾百英尺。接入點(diǎn)（或天線接入點(diǎn)）通常是采取高，但可能采取的任何地方，基本上是可行的，只要預(yù)期的無線電覆蓋獲得。</p><p>　　Wi

10、reless LAN</p><p><b>　　OVERVIEW</b></p><p>　　A wireless LAN (WLAN) is typically an extension of a wired LAN. WLAN components convert data packets into radio waves or infrared (IR) li

11、ght pulses and send them to other wireless devices or to a access point that serves as a gateway to the wired LAN. Most WLANs today are based on the IEEE 802.11 and 802.11b standards for wireless communication between de

12、vices and a LAN. These standards permit data transmissions at 1 to 2 Mbps or 5 to 11 Mbps. Respectively, and specify a common architecture, tra</p><p>　　TECHNOLOGY</p><p>　　Manufactures of wirel

13、ess LANs have a range of technologies to choose from when designing a wireless LAN solution. Each technology comes with its own set of advantages and limitations.</p><p>　　Narrowband Technology</p>&l

14、t;p>　　A narrowband radio system transmits and receives user information on a specific radio frequency. Narrowband radio keeps the radio signal frequency as narrow as possible just to pass the information. Undesirable

15、crosstalk between communications channels is avoided by carefully coordinating different users on different channel frequencies.</p><p>　　A private telephone line is much like a radio frequency. When each ho

16、me in a neighborhood has its own private telephone line, people in one home cannot listen to calls made to other homes. In a radio system, privacy and noninterference are accomplished by the calls made to other homes. Th

17、e radio receiver filters out all radio signals except the ones on its designated frequency.</p><p>　　From a customer standpoint, one drawback of narrowband technology is that the end-user must obtain an FCC

18、license for each site where it is employed.</p><p>　　Spread Spectrum Technology</p><p>　　Most wireless LAN systems use spread-spectrum technology, a wideband radio frequency technique developed

19、by the military for use in reliable, secure , mission-critical communications systems. Spread-spectrum is designed to trade off bandwidth efficiency for reliability, integrity, and security. In other words, more bandwidt

20、h is consumed than in the case of narrowband transmission, but the tradeoff produces a signal that is, in effect, louder and thus easier to detect, provided that the receiver </p><p>　　Frequency-Hopping Spre

21、ad Spectrum Technology</p><p>　　Frequency-hopping spread-spectrum (FHSS) uses a narrowband carrier that changes frequency in a pattern known to both transmitter and receiver. Properly synchronized, the net e

22、ffect is to maintain a single logical channel. To an unintended receiver, FHSS appears to be short-duration impulse noise.</p><p>　　Direct-Sequence Spread Spectrum Technology</p><p>　　Direct-seq

23、uence spread-spectrum (DSSS) generates a redundant bit pattern for each bit to be transmitted. This bit pattern is called a chip (or chipping code). The longer the chip, the greater the probability that the original data

24、 can be recovered (and , of course, the more bandwidth required). Event if one or more bits in the chip are damaged during transmission. To an unintended receiver, DSSS appears as low-power wideband noise and is rejected

25、 (ignored) by most narrowband receivers.</p><p>　　Infrared Technology</p><p>　　A third technology, little used in commercial wireless LANs, is infrared. Infrared (IR) systems use very high frequ

26、encies, just below visible light in the electromagnetic spectrum, to carry data. Like light, IR cannot penetrate opaque object; it is either directed (line-of-sight) or diffuse technology. Inexpensive directed systems pr

27、ovide very limited range and typically are used for personal area networks but occasionally are used in specific wireless LAN applications. High performance directed</p><p>　　How Wireless LANs Work</p>

28、<p>　　Wireless LANs use electromagnetic airwaves (radio or infrared) to communication from one point to another without relying on any physical connection. Radio waves are often referred to as radio carriers becau

29、se they simply perform the function of delivering energy to a remote receiver. The data being transmitted is superimposed on the radio carrier so that it can be accurately extracted at the receiving end. This is generall

30、y referred to as modulation of the carrier by the information being trans</p><p>　　Multiple radio carriers can exits in the same space at the same time without interfering with each other if the radio waves

31、are transmitted on different radio frequencies. To extract data, a radio receiver tunes in one radio frequency while rejecting all other frequencies.</p><p>　　In a typical wireless LAN configuration, a trans

32、mitter/receiver (transceiver) device, called an access point, connects to the wired network from a fixed location using standard cabling. At a minimum, the access point receives, buffers, and transmits data between the w

33、ireless LAN and the wired network infrastructure. A single access point can support a small group of user and can function within a range of less than one hundred to several hundred feet. The access point (or the antenna

34、 to the acce</p><p>　　Real-time network protection required for wireless networking</p><p>　　Wireless LANs have experienced tremendous growth since the introduction of the 802.11b wireless netwo

35、rking standard spurred the deployment of “Wi-Fi” solutions developed by network equipment vendors. Flexibility, ease of deployment and low component costs constitute three major drives for the popularity of WLANs. Howeve

36、r, the same flexibility and mobility provided by wireless networking also introduces new security vulnerabilities in addition to those that threaten conventional LANs. For real-ti</p><p>　　WLAN security thre

37、ats inhibit build-out</p><p>　　Because WLANs use publicly available radio spectrum as the medium to carry data, unauthorized access and eavesdropping are key concerns. Major security threats to WLANs include

38、 the following:</p><p>　　WLAN access points can be probed by anyone within reach of the network’s radio signal. Thus constituting physically unbounded entry points from which to launch intrusions, viruses an

39、d all other types of attacks that threaten landline networks.</p><p>　　WLAN access points are often deployed inside corporate networks behind conventional firewalls, making these access points even more attr

40、active as points for launching attacks.</p><p>　　WLANs are extremely vulnerable to denial-of-service attack and interruption. Any malicious hacker with a laptop and a wireless Network Interface Card can tran

41、smit wireless signal interrupters in close proximity to company sites where WLANs are deployed and effectively jam a Wi-Fi signal.</p><p>　　Internal employees can set up their WLAN interface standards were a

42、ware of these vulnerabilities and communicate directly with people outside of the company.</p><p>　　Naturally, the framers of the 802.11b wireless standards were aware of these include the following:</p&g

43、t;<p>　　The use of Service Set Identifier (SSID):</p><p>　　The SSID is a shared secret (typically an ASCII string) that has to configured by network administrators into all access points and wireless

44、terminals (e.g.,PCs) that share a common WLAN. The weakness of the SSID is that it’s a relatively simple password, common to all devices on the WLAN, and once the SSID is compromised, any device with the SSID can gain un

45、restricted access. Furthermore, the default setting of SSID is often not changed in WLAN deployments, and access points are typically confi</p><p>　　Media Access Control (MAC) address filtering:</p>&

46、lt;p>　　Since every WLAN terminal’s network card has a unique MAC address, it’s possible to manually maintain a set of allowed MAC address lists for physical address filtering. Using a MAC address list, the systems adm

47、inistrator needs to update the list constantly to accommodate changes, including when users get new or replacement WLAN interface cards. In addition, MAC address filtering merely verifies the identity of the WLAN interfa

48、ce card and not the identity of the PC into which it’s inserted or the</p><p>　　Wired Equivalent Privacy (WEP):</p><p>　　Using WEP, communications between mobile terminals and access points are

49、scrambled using a symmetrical encryption technique called RC4 on the data link layer. This prevents eavesdropping and prevents unauthorized access by users that haven’t been configured with the necessary encryption key.

50、WEP offers both 40-bit and 128-bit encryption strengths; however, WEP suffers from a number of drawbacks. For example, as with the SSID, all users within a service area have the same encryption key; if one u</p>&

51、lt;p>　　In addition to these Wi-Fi-specific security mechanisms, other techniques can be applied to WLANs to make them robust against attacks. One approach for enhancing the network-level security in wireless LANs is t

52、o use IPSec virtual private network (VPN) technology in conjunction with Wi-Fi security methods. VPN technology provides for data privacy via strong encryption to prevent eavesdropping and also provides for authenticatio

53、n of wireless terminals and their users using a variety of means rangi</p><p>　　Another important area of concern for wireless LANs is protection against content-based attacks. Wireless LAN users who are bro

54、wsing the Internet can be exposed to viruses and worms in Web (HTTP) downloads and applications that aren’t scanned by conventional-firewall, e-mail-based antivirus software. To prevent these attacks, real-time antivirus