1- 無線網(wǎng)絡(luò)概念簡(jiǎn)介

1、Introduction to WirelessNetworking Concepts,1- 無線網(wǎng)絡(luò)概念簡(jiǎn)介,Wireless Local-Area Networks(無線局域網(wǎng)),Sir William Herschel (1738 to 1822) made a discovery that infrared light(紅外線) existed The discovery of infrared light led the

2、way to the electromagnetic wave（電磁波） theory electromagnetic waves travel at the speed of light and that electricity can be carried on these waves,how they relate to wireless local-area networks (WLANs)？ In standard L

3、ANs, data is propagated over wires such as an Ethernet cable（以太網(wǎng)線路）, in the form of electrical signals（電信號(hào)）. The discovery opens the airways to transfer the same data, as electrical signals, without wires. This is acco

4、mplished by way of Radio Frequencies (RF)（射頻）. With RF, the goal is to send as much data as far as possible and as fast as possible.,The problem is the numerous influences on radio frequencies that need to be either ove

5、rcome or dealt with. One of these problems is interference（干擾）,How Bandwidth Is Achieved from RF Signals,To achieve bandwidth（帶寬） from RF signals, you need to send data as electrical signals using some type of emission m

6、ethod（發(fā)送方式）. One such emission method is known as Spread Spectrum（擴(kuò)展頻譜）.In 1986, the FCC agreed to allow the use of spread spectrum in the commercial market using what is known as the industry, scientific, and medical

7、(ISM) frequency bands.,To place data on the RF signals, you use a modulation technique（調(diào)制技術(shù)）. Modulation is the addition of data to a carrier signal. You are probably familiar with this already. To send music, news, or

8、 speech over the airwaves, you use frequency modulation (FM)（調(diào)頻） or amplitude modulation (AM)（調(diào)幅）.,Bandwidth（帶寬）,In wireless networking, the word bandwidth can mean two different things. In one sense of the word, it can

9、refer to data rates（數(shù)據(jù)率）. In another sense of the word, it can refer to the width of an RF channel（RF頻道寬度）.This book uses the term bandwidth to refer to the width of the RF channel and not to data rates.,,the standard

10、unit of measure is the Hertz (Hz)（赫茲）例如：FM radio is generally a higher quality, using about 175 kHz of bandwidth. Compare that to a television signal, which sends both voice and video over the air. The TV signal you re

11、ceive uses almost 4500 kHz of bandwidth.,As Figure 1-1 illustrates, the electromagnetic spectrum spans from Extremely Low Frequency (ELF)（極低頻） to Extremely High Frequency (EHF)（極高頻）. In fact, the data you send using WLAN

12、s is either in the 900-MHz, 2.4-GHz, or 5-GHz frequency ranges.,,,900 MHz： cordless phone（對(duì)講機(jī)） operate in this range. 2.4 GHz： is probably the most widely used frequency range in WLANs. It is used by the 802.11, 802.11

13、b, 802.11g, and 802.11n IEEE standards. Some channels overlap（信道重疊） with others and cause interference. each channel is 22-MHz wide。 For this reason, channels 1, 6, and 11 are most commonly used because they do no

14、t overlap.,,,,The 2.4-GHz range uses direct sequence spread spectrum (DSSS) （直接序列擴(kuò)展頻譜）modulation. Data rates of 1 Mbps, 2 Mbps, 5.5 Mbps, and 11 Mbps are defined for this range.,,5 GHz： The 5-GHz range is used by the 80

15、2.11a standard and the new 802.11n draft standard. In the 802.11a standard, data rates can range from 6 Mbps to 54 Mbps. The 5-GHz ranges use Orthogonal Frequency Division Multiplexing (OFDM) （正交頻分復(fù)用）. Data rates of 6,

16、 9, 12, 18, 24, 36, 48, and 54 Mbps are defined.,Modulation Techniques（調(diào)制技術(shù)） and How They Work,In short, the process of modulation is the varying in a signal or a tone called a carrier signal（載體）. Data is then added to t

17、his carrier signal in a process known as encoding（編碼）.A modulated waveform consists of three parts:Amplitude（振幅）: The volume of the signalPhase（相位）: The timing of the signal between peaksFrequency（頻率）: The pitch of t

18、he signal,,Wireless networks use a few different modulation techniques, including these:DSSSOFDMMultiple-Input Multiple-Output (MIMO),DSSS （直接序列擴(kuò)展頻譜）,To encode data using DSSS, you use a chip sequence（碼序列）. A chip and

19、 a bit are essentially the same thing, but a bit represents the data, and a chip is used for the carrier encoding.To understand how data is encoded in a wireless network and then modulated, you must first understand ch

20、ipping codes(編碼).,Chipping Codes,Because of the possible noise interference（噪聲干擾） with a wireless transmission, DSSS uses a sequence of chips. it sends a single data bit as a string of chips or a chip stream. In the fi

21、gure, the chipping code for the bit value of 1 is expanded to the chip sequence of 00110011011,,,Barker Code（巴克碼）,To achieve rates of 1 Mbps and 2 Mbps, 802.11 uses a Barker code. This code defines the use of 11 chips wh

22、en encoding the data. The 11-chip Barker code used in 802.11 is 10110111000. In the end, and for the exam, each bit of data sent is encoded into an 11-bit Barker code and then modulated with DSSS.,Complementary Code Key

23、ing-CCK（補(bǔ)碼）,When you are using DSSS, the Barker code works well for lower data rates such as 1-Mbps, 2-Mbps,5.5-Mbps,and11-Mbps.DSSS uses a different method（CCK） for higher data rates, which allows The 802.11 standard t

24、o achieve rates of 5.5 and 11 Mbps.,DSSS Modulation Techniques and Encoding,Now that the data has been encoded using Barker code or CCK, it needs to be transmitted or modulated out of the radio antennas. You can think of

25、 it this way:■ Encoding is how the changes in RF signal translate to the 1s and 0s.■ Modulation is the characteristic of the RF signal that is manipulated.For example, amplitude modulation（AM）, frequency modulation(FM

26、), and phase-shift keying are modulations.,,802.11b uses phase to modulate the data. Specifically, in 802.11b, BPSK and QPSK are used.,BPSK (Binary Phase Shift Keying)-二進(jìn)制相移鍵控,,,BPSK is the simplest method of PSK. I

27、n BPSK, two phases are used that are separated by 180 degrees. BPSK can modulate 1 bit per symbol. To simplify this, a phase shift of 180 degrees is a 1, and a phase shift of 0 degrees is a 0, as illustrated in Figure 1-

28、5.,QPSK-正交相移鍵控（Quadrature Phase Shift Keyin）,In BPSK, 1 bit per symbol is encoded. This is okay for lower data rates. QPSK has the capability to encode 2 bits per symbol. This doubles the data rates available in BPSK whi

29、le staying within the same bandwidth.,OFDM (Orthogonal Frequency Division Multiplexing)正交頻分復(fù)用,OFDM is not considered a spread spectrum technology, but it is used for modulation in wireless networks. Using OFDM, you can

30、achieve the highest data rates with the maximum resistance to corruption of the data caused by interference.OFDM defines a number of channels in a frequency range. These channels are further divided into a larger number

31、 of small-bandwidth subcarriers.OFDM is not used in 802.11b because 802.11b devices use DSSS. 802.11g and 802.11a both used OFDM.,MIMO (Multiple-Input Multiple-Output),MIMO is a technology that is used in the new 802.

32、11n specification.A device that uses MIMO technology uses multiple antennas(天線) for receiving signals (usually two or three) in addition to multiple antennas for sending signals.MIMO technology can offer data rates hig

33、her than 100 Mbps by multiplexing（復(fù)用） data streams simultaneously in one channel.With the use of MIMO technology, an access point (AP) can talk to non-MIMO-capable devices and still offer about a 30 percent increase in

34、performance of standard 802.11a/b/g networks.,Dynamic Rate Shifting,the farther away you get from the access point, the lower the data rates are that you can achieve. This is true regardless of the technology.though you

35、 can achieve higher data rates with different standards, you still have this to deal withAll Cisco wireless products can perform a function called dynamic rate shifting (DRS). In 802.11 networks, operating in the 2.4-GH

36、z range, the devices can rate-shift from 11 Mbps to 5.5 Mbps, and further to 2 and 1 Mbps depending on the circumstances.,Sending Data Using CSMA/CA,Wireless networks have to deal with the possibility of collisions（沖突）.

37、If you are in a wired network, a jam signal is heard by listening（監(jiān)聽） to the wire.To avoid collisions on a wireless network, carrier sense multiple access collision avoidance(CSMA/CA)（載波偵聽多路訪問／沖突避免） is used.,collision a

38、voidance（CA 沖突避免） means that when a device wishes to send, it must listen first. If the channel is considered idle, the device sends a signal informing others that it is going to send data and that they should not send.

39、Another way to supplement this is using request to send (RTS)(發(fā)送請(qǐng)求) and clear to send (CTS)（清空發(fā)送） packets. With the RTS/CTS method, the sending device uses an RTS packet, and the intended receiver uses a CTS packet. Th