1、1 1Unit 7 WiFi and Bluetooth117Text AHow WiFi Works?125Text BBluetooth133参考译文参考译文WiFi是如何工作的？是如何工作的？2 2If youve been at an airport,in a coffee shop,a library or a hotel recently,chances are youve been right in the middle of a wireless network.Many people also use wireless networking,also called WiFi
2、or 802.11 networking,to connect their computers at home,and some cities are trying to use the technology to provide free or low-cost Internet access to residents.How WiFi Works？3 3In the near future,wireless networking may become so widespread that you can access the Internet just about anywhere at
3、any time,without using wires.(See Figure 7.1)4 4Wireless networks make it easy to connect to the Internet.Figure 7.1 WiFi works without using wires5 5WiFi has a lot of advantages.Wireless networks are easy to set up and inexpensive.Theyre also unobtrusiveunless youre on the lookout for a place to us
4、e your laptop,you may not even notice when youre in a hotspot.In this article,well look at the technology that allows information to travel over the air.Well also review what it takes to create a wireless network in your home.6 61.What Is WiFi?A wireless network uses radio waves,just like cell phone
5、s,televisions and radios do.In fact,communication across a wireless network is a lot like two-way radio communication.Heres what happens:A computers wireless adapter translates data into a radio signal and transmits it using an antenna.A wireless router receives the signal and decodes it.The router
6、sends the information to the Internet using a physical,wired Ethernet connection.7 7The process also works in reverse,with the router receiving information from the Internet,translating it into a radio signal and sending it to the computers wireless adapter.The radios used for WiFi communication are
7、 very similar to the radios used for walkie talkies,cell phones and other devices.They can transmit and receive radio waves,and they can convert 1s and 0s into radio waves and convert the radio waves back into 1s and 0s.But WiFi radios have a few notable differences from other radios:8 8They transmi
8、t at frequencies of 2.4 GHz or 5 GHz.This frequency is considerably higher than the frequencies used for cell phones,walkie-talkies and televisions.The higher frequency allows the signal to carry more data.They use 802.11 networking standards,which come in several flavors:802.11a transmits at 5 GHz
9、and can move up to 54 megabits of data per second.It also uses Orthogonal Frequency-Division Multiplexing(OFDM),a more efficient coding technique that splits that radio signal into several sub-signals before they reach a receiver.9 9This greatly reduces interference.802.11b is the slowest and least
10、expensive standard.For a while,its cost made it popular,but now its becoming less common as faster standards become less expensive.802.11b transmits in the 2.4 GHz frequency band of the radio spectrum.It can handle up to 11 megabits of data per second,and it uses Complementary Code Keying(CCK)modula
11、tion to improve speeds.802.11g transmits at 2.4 GHz like 802.11b,but its a lot fasterit can handle up to 54 megabits of data per second.802.11g is faster because it uses the same OFDM coding as 802.11a.802.11n is the newest standard that is widely available.10 10This standard significantly improves
12、speed and range.For instance,although 802.11g theoretically moves 54 megabits of data per second,it only achieves real-world speeds of about 24 megabits of data per second because of network congestion.802.11n,however,reportedly can achieve speeds as high as 140 megabits per second.Other 802.11 stan
13、dards focus on specific applications of wireless networks,like Wide Area Networks(WANs)inside vehicles or technology that lets you move from one wireless network to another seamlessly.11 11WiFi radios can transmit on any of three frequency bands.Or,they can frequency hop rapidly between the differen
14、t bands.Frequency hopping helps reduce interference and lets multiple devices use the same wireless connection simultaneously.As long as they all have wireless adapters,several devices can use one router to connect to the Internet.This connection is convenient,virtually invisible and fairly reliable
15、;however,if the router fails or if too many people try to use high-bandwidth applications at the same time,users can experience interference or lose their connections.12 122.WiFi HotspotsIf you want to take advantage of public WiFi hotspots or start a wireless network in your home,the first thing yo
16、ull need to do is make sure your computer has the right gear.Most new laptops and many new desktop computers come with built-in wireless transmitters.If your laptop doesnt,you can buy a wireless adapter that plugs into the PC card slot or USB port(See Figure 7.2).Desktop computers can use USB adapte
17、rs,or you can buy an adapter that plugs into the PCI slot inside the computers case.Many of these adapters can use more than one 802.11 standard.13 13Wireless adapters can plug into a computers PC card slot or USB port.Figure 7.2 Wireless a dapters14 14Once youve installed your wireless adapter and
18、the drivers that allow it to operate,your computer should be able to automatically discover existing networks.This means that when you turn your computer on in a WiFi hotspot,the computer will inform you that the network exists and ask whether you want to connect to it.If you have an older computer,
19、you may need to use a software program to detect and connect to a wireless network.Being able to connect to the Internet in public hotspots is extremely convenient.Wireless home networks are convenient as well.They allow you to easily connect multiple computers and to move them from place to place w
20、ithout disconnecting and reconnecting wires.15 153.Building a Wireless NetworkIf you already have several computers networked in your home,you can create a wireless network with a wireless access point.If you have several computers that are not networked,or if you want to replace your Ethernet netwo
21、rk,youll need a wireless router.This is a single unit that contains:A port to connect to your cable or DSL modem;A router;An Ethernet hub;A firewall;A wireless access point.16 16A wireless router allows you to use wireless signals or Ethernet cables to connect your computers to one another,to a prin
22、ter and to the Internet.Most routers provide coverage for about 100 feet(30.5 meters)in all directions,although walls and doors can block the signal.If your home is very large,you can buy inexpensive range extenders or repeaters to increase your routers range.As with wireless adapters,many routers c
23、an use more than one 802.11 standard.802.11b routers are slightly less expensive,but because the standard is older,theyre slower than 802.11a,802.11g and 802.11n routers.Most people select the 802.11g option for its speed and reliability.17 17Once you plug in your router,it should start working at i
24、ts default settings(See Figure 7.3).Most routers let you use a Web interface to change your settings.You can select:The name of the network,known as its Service Set IDentifier(SSID)The default setting is usually the manufacturers name.The channel that the router usesMost routers use channel 6 by def
25、ault.If you live in an apartment and your neighbors are also using channel 6,you may experience interference.Switching to a different channel should eliminate the problem.18 18A wireless router uses an antenna to send signals to wireless devices and a wire to send signals to the Internet.Figure 7.3
26、Wireless router19 19Your routers security optionsMany routers use a standard,publicly available sign-on,so its a good idea to set your own username and password.Security is an important part of a home wireless network,as well as public WiFi hotspots.If you set your router to create an open hotspot,a
27、nyone who has a wireless card will be able to use your signal.Most people would rather keep strangers out of their network,though.Doing so requires you to take a few security precautions.2020Its also important to make sure your security precautions are current.The Wired Equivalency Privacy(WEP)secur
28、ity measure was once the standard for WAN security.The idea behind WEP was to create a wireless security platform that would make any wireless network as secure as a traditional wired network.But hackers discovered vulnerabilities in the WEP approach,and today its easy to find applications and progr
29、ams that can compromise a WAN running WEP security.To keep your network private,you can use one of the following methods:21 21WiFi Protected Access(WPA)is a step up from WEP and is now part of the 802.11i wireless network security protocol.It uses Temporal Key Integrity Protocol(TKIP)encryption.As w
30、ith WEP,WPA security involves signing on with a password.Most public hotspots are either open or use WPA or 128-bit WEP technology,though some still use the vulnerable WEP approach.Media Access Control(MAC)address filtering is a little different from WEP or WPA.It 2222doesnt use a password to authen
31、ticate usersit uses a computers physical hardware.Each computer has its own unique MAC address.MAC address filtering allows only machines with specific MAC addresses to access the network.You must specify which addresses are allowed when you set up your router.This method is very secure,but if you b
32、uy a new computer or if visitors to your home want to use your network,youll need to add the new machines MAC addresses to the list of approved addresses.The system isnt foolproof.2323A clever hacker can spoof a MAC addressthat is,copy a known MAC address to fool the network that the computer he or
33、she is using belongs on the network.Wireless networks are easy and inexpensive to set up,and most routers Web interfaces are virtually self-explanatory.2424Bluetooth is a proprietary open wireless technology standard for exchanging data over short distances(using short wavelength radio transmissions
34、 in the ISM band from 2400-2480 MHz)from fixed and mobile devices,creating Personal Area Networks(PANs)with high levels of security.Created by telecoms vendor Ericsson in 1994,it was originally conceived as a wireless Bluetooth2525alternative to RS-232 data cables.It can connect several devices,over
35、coming problems of synchronization.(See figure 7.4)2626Figure 7.4 Bluetooth Logo27271.ImplementationBluetooth uses a radio technology called frequency-hopping spread spectrum,which chops up the data being sent and transmits chunks of it on up to 79 bands(1 MHz each;centered from 2402 to 2480 MHz)in
36、the range 2,400-2,483.5 MHz(allowing for guard bands).This range is in the globally unlicensed Industrial,Scientific and Medical(ISM)2.4 GHz short-range radio frequency band.Originally Gaussian Frequency-Shift Keying(GFSK)modulation was the only modulation scheme available;2828subsequently,since the
37、 introduction of Bluetooth 2.0+EDR,/4-DQPSK and 8DPSK modulation may also be used between compatible devices.Devices functioning with GFSK are said to be operating in Basic Rate(BR)mode where an instantaneous data rate of 1 Mbit/s is possible.The term Enhanced Data Rate(EDR)is used to describe/4-DPS
38、K and 8DPSK schemes,each giving 2 and 3 Mbit/s respectively.The combination of these(BR and EDR)modes in Bluetooth radio technology is classified as a BR/EDR radio.2929Bluetooth is a packet-based protocol with a master-slave structure.One master may communicate with up to 7 slaves in a piconet;all d
39、evices share the masters clock.Packet exchange is based on the basic clock,defined by the master,which ticks at 312.5 s intervals.Two clock ticks make up a slot of 625 s;two slots make up a slot pair of 1250 s.In the simple case of single-slot packets the master transmits in even slots and receives
40、in odd slots;the slave,conversely,receives in even slots and transmits in odd slots.Packets may be 1,3 or 5 slots long but in all cases the master transmit will begin in even slots and the slave transmit in odd slots.3030Bluetooth provides a secure way to connect and exchange information between dev
41、ices such as faxes,mobile phones,telephones,laptops,personal computers,printers,Global Positioning System(GPS)receivers,digital cameras,and video game consoles.A master Bluetooth device can communicate with a maximum of seven devices in a piconet(an ad-hoc computer network using Bluetooth technology
42、),though not all devices support this limit.The devices can switch roles,by agreement,and the slave can become the master(for example,31 31a headset initiating a connection to a phone will necessarily begin as master,as initiator of the connection;but may subsequently prefer to be slave).The Bluetoo
43、th Core Specification provides for the connection of two or more piconets to form a scatternet,in which certain devices simultaneously play the master role in one piconet and the slave role in another.3232At any given time,data can be transferred between the master and one other device(except for th
44、e little-used broadcast mode).The master chooses which slave device to address;typically,it switches rapidly from one device to another in a round-robin fashion.Since it is the master that chooses which slave to address,whereas a slave is(in theory)supposed to listen in each receive slot,being a mas
45、ter is a lighter burden than being a slave.Being a master of seven slaves is possible;being a slave of more than one master is difficult.The specification is vague as to required behaviour in scatternets.33332.UsesBluetooth is a standard wire-replacement communications protocol primarily designed fo
46、r low power consumption,with a short range(power-class-dependent,but effective ranges vary in practice;see table below)based on low-cost transceiver microchips in each device.Because the devices use a radio(broadcast)communications system,they do not have to be in visual line of sight of each other,
47、however a quasi optical wireless path must be viable.34343535 The effective range varies due to propagation conditions,material coverage,production sample variations,antenna configurations and battery conditions.In most cases the effective range of class 2 devices is extended if they connect to a cl
48、ass 1 transceiver,compared to a pure class 2 network.This is accomplished by the higher sensitivity and transmission power of Class 1 devices.36363737 While the Bluetooth Core Specification does mandate minimums for range,the range of the technology is application specific and is not limited.Manufac
49、turers may tune their implementations to the range needed to support individual use cases.38382.1 Bluetooth profilesTo use Bluetooth wireless technology,a device has to be able to interpret certain Bluetooth profiles,which are definitions of possible applications and specify general behaviors that B
50、luetooth enabled devices use to communicate with other Bluetooth devices.These profiles include settings to parameterize and to control the communication from start.Adherence to profiles saves the time for transmitting the parameters anew before the bi-directional link becomes effective.There are a
