Wireless LANs (WLANs)
Local Wireless Technologies <ul><li>Physical-Layer Transmission </li></ul><ul><ul><li>Uses radio transmission </li></ul></...
Wireless availability <ul><li>43,850 locations in 2003. </li></ul><ul><li>Estimated in 2004 to grow to over 200,000 locati...
Wireless LAN (WLAN) Access Point Server Internet Router Ethernet Switch Laptop Mobile Client Wireless Access Point Large W...
Access Router with Wireless Access Point and Wireless NICs PC Card WNIC for a Notebook Computer Internal WNIC For Desktop ...
Local Wireless Technologies, Continued <ul><li>802.11 </li></ul><ul><ul><li>The dominant WLAN technology today </li></ul><...
Local Wireless Technologies, Continued <ul><li>802.11 Wireless LANs </li></ul><ul><ul><li>Speeds up to tens of megabits pe...
802.11 WLAN Standards
Specific 802.11 Wireless LAN Standards 802.11b 802.11g 802.11g if 802.11g access point serves an 802.11b station 2.4 GHz 2...
Specific 802.11 Wireless LAN Standards 802.11b 802.11g 802.11g if 802.11g access point serves an 802.11b station 11 Mbps 5...
Specific 802.11 Wireless LAN Standards 802.11g 802.11g if 802.11g access point serves an 802.11b station Aggregate through...
Specific 802.11 Wireless LAN Standards 802.11b 802.11a 802.11g 802.11g if 802.11g access point serves an 802.11b station 3...
A new Wireless LAN Standard <ul><li>A separate standard,  802.16  (or  WiMAX ), transmits at 70 Mbps and has a range of up...
Wireless LANs (WLANs) cont.
Local Wireless Technologies, Continued <ul><li>Bluetooth </li></ul><ul><ul><li>For personal area networks (PANs) </li></ul...
Local Wireless Technologies, Continued <ul><li>Other Local Wireless Technologies </li></ul><ul><ul><li>Ultrawideband: Up t...
Local Wireless Technologies, Continued <ul><li>Other Local Wireless Technologies </li></ul><ul><ul><li>Mesh networking: mu...
Radio Propagation or How wireless data gets there!
Frequency Measurement <ul><li>Frequency </li></ul><ul><ul><li>Light waves are measured in wavelengths (Ch. 3) </li></ul></...
Frequency Measurement, Continued <ul><li>Measuring Frequencies </li></ul><ul><ul><li>Frequency measures increases by facto...
Omnidirectional and Dish Antennas Omnidirectional Antenna Spread signals in all directions Rapid signal attenuation ----- ...
Wireless Propagation Problems 2. Attenuation: signal gets weaker with distance 3. Shadow Zone (Dead Spot) 1. Electromagnet...
Wireless Propagation Problems Reflected Signal Laptop Direct Signal 4. Multipath Interference Direct and reflected signals...
Inverse Square Law Attenuation <ul><li>Inverse square law attenuation </li></ul><ul><ul><li>To compare relative power at t...
Frequency-Dependent Propagation Problem <ul><li>Some problems are Frequency-Dependent </li></ul><ul><ul><li>Higher-frequen...
The Frequency Spectrum, Service Bands, and Channels Channel 5, Signal A Channel 1, Signal E Channel 2, No Signal Channel 3...
Channel Bandwidth and Transmission Speed <ul><li>Shannon Equation </li></ul><ul><ul><li>Specifies the connection between c...
Channel Bandwidth and Transmission Speed <ul><li>Shannon Equation </li></ul><ul><ul><li>C = B [ Log2 (1+S/N) ] </li></ul><...
Channel Bandwidth and Transmission Speed <ul><li>Broadband and Narrowband Channels </li></ul><ul><ul><li>Broadband means w...
Channel Bandwidth and Transmission Speed <ul><li>Channel Bandwidth and Spectrum Scarcity </li></ul><ul><ul><li>Why not mak...
Channel Bandwidth and Transmission Speed <ul><li>The Golden Zone </li></ul><ul><ul><li>Most organizational radio technolog...
Spread Spectrum Transmission <ul><li>Unlicensed Bands </li></ul><ul><ul><li>WLANs operate in unlicensed service bands </li...
Spread Spectrum Transmission, Cont. <ul><li>Spread Spectrum Transmission </li></ul><ul><ul><li>You are REQUIRED BY LAW to ...
Normal Radio Transmission and Spread Spectrum Transmission Channel Bandwidth Required for Signal Speed Normal Radio: Bandw...
Normal Radio Transmission and Spread Spectrum Transmission Channel Bandwidth Required for Signal Speed Note: Height of Box...
802.11 WLAN Operation
Typical 802.11 WLAN Operation  Server Ethernet Switch Laptop WAP Large Wired LAN Client PC UTP Radio Transmission 802.11 F...
Typical 802.11 WLAN Operation, Continued  Server Ethernet Switch Laptop WAP A Large Wired LAN Client PC WAP B UTP Handoff ...
Stations and Access Points Transmit in a Single Channel Collision if 2 Devices send Simultaneously
Media Access Control <ul><li>Only one station or the access point can transmit at a time </li></ul><ul><li>To control acce...
CSMA/CA in 802.11 Wireless LANs  <ul><li>CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) </li></ul><ul><l...
CSMA/CA in 802.11 Wireless LANs  <ul><li>CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) </li></ul><ul><u...
CSMA/CA in 802.11 Wireless LANs  <ul><li>ACK (Acknowledgement) </li></ul><ul><ul><li>Receiver  immediately  sends back an ...
Request to Send/Clear to Send (RTS/CTS)  Server Switch Laptop Access Point B Large Wired LAN Radio Link Client PC RTS 1. D...
Request to Send/Clear to Send (RTS/CTS)  Server Switch May Send Frames WAP Large Wired LAN Radio Link Client PC 2. Wireles...
Recap <ul><li>CSMA/CA+ACK is mandatory </li></ul><ul><li>RTS/CTS is optional </li></ul><ul><ul><li>However, it is mandator...
Specific 802.11 Wireless LAN Standards <ul><li>Transmission Speed and Distance </li></ul><ul><ul><li>As a station moves aw...
Specific 802.11 Wireless LAN Standards, Continued <ul><li>Transmission Speed and Distance </li></ul><ul><ul><li>When stati...
Figure 5-19: Interference Between Nearby Access Points Operating on the Same Channel Access Point Channels Should be Selec...
802.11n <ul><li>Under Development </li></ul><ul><ul><li>Rated speeds of 100 Mbps to 600 Mbps </li></ul></ul><ul><ul><li>Wi...
802.11e <ul><li>Standard for Quality of Service (QoS) </li></ul><ul><ul><li>Needed for voice and video transmission </li><...
WLAN Security
WLAN Security Threats <ul><li>Drive-By Hackers </li></ul><ul><ul><li>Sit outside the corporate premises and read network t...
WLAN Security Threats, Continued <ul><li>Rogue Access Points </li></ul><ul><ul><li>Unauthorized access points set up by de...
WLAN Security Threats, Continued <ul><li>Evil Twin Access Points </li></ul><ul><ul><li>Create a fake access point outside ...
WLAN Security Threats, Continued <ul><li>Evil Twin Access Points </li></ul><ul><ul><li>Evil twin then associates with a le...
WLAN Security Threats, Continued <ul><li>Evil Twin Access Points </li></ul><ul><ul><li>Evil twin can then read all traffic...
802.11 WLAN Management
Wireless LAN Management <ul><li>Access Points Placement in a Building </li></ul><ul><ul><li>Must be done carefully for goo...
Wireless LAN Management <ul><li>Access Points Placement in a Building </li></ul><ul><ul><li>Install access points and do s...
Wireless Access Point Management Alternatives Management intelligence can be placed in the access point or the WLAN switch
Wireless LAN Management <ul><li>Remote Access Point Management </li></ul><ul><ul><li>Desired functionality </li></ul></ul>...
Bluetooth For Personal Area Networks (PANs)
Bluetooth Personal Area Networks (PANs) <ul><li>For Personal Area Networks (PANs) </li></ul><ul><ul><li>Devices around a d...
Bluetooth Personal Area Networks (PANs) <ul><li>Cable Replacement Technology </li></ul><ul><ul><li>For example, with a Blu...
Bluetooth Personal Area Networks (PANs) <ul><li>Disadvantages Compared to 802.11 </li></ul><ul><ul><li>Short distance (10 ...
Bluetooth Personal Area Networks (PANs) <ul><li>Advantages Compared to 802.11 </li></ul><ul><ul><li>Low battery power drai...
Bluetooth Personal Area Networks (PANs) <ul><li>Bluetooth Trends </li></ul><ul><ul><li>Bluetooth Alliance is enhancing Blu...
Other Wireless Communication <ul><li>3G Cellular phones </li></ul><ul><li>VoIP on wireless </li></ul><ul><li>RFID and embe...
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Wireless networks

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  • &lt;Read the slide.&gt;
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  • 802.11 WLANs use access points. Access points are bridges that link wireless devices, which usually are clients, to the resources the wireless devices need on the wired LAN. First, servers that wireless clients will need are on the wired LAN. Second, the routers the client needs to get access to the Internet also are on the main wired LAN. In other words, wired LANs and wireless LANs today are not competitors in corporations. They work together.
  • Here is a picture of several wireless devices. In the upper-left corner, there is a residential access router with a built-in access point. &lt;This is the kind that Pat Lee uses in Chapter 1a.&gt; The other devices are wireless network interface cards (WNICs).
  • &lt;Read the slide.&gt;
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  • Now that we have discussed wireless networking in general, we will look at specific 802.11 WLAN standards.
  • [The first 802.11 standard was 802.11, which almost nobody used. It had a maximum speed of 2 Mbps.] The 802.11a standard was created to operate in the 5 GHz band, which is very uncrowded compared to the 2.4 GHz band. Unfortunately, operating in the higher 5 GHz band means that 802.11a had higher attenuation, so its signals could not travel very far. Also, 5 GHz radios were more expensive than 2.4 GHz radios, making 802.11a expensive. [Another problem is that different countries define the 5 GHz band differently. Dealing with these differences further increasing the cost of radios.] Because of these problems, 802.11a’s market acceptance was quite low. The 802.11b standard was created on the same day as the 802.11a standard. Although slower (as discussed on the next slide), 802.11b used the 2.4 GHz band, which was more crowded. However, 802.11b signals could travel farther and radios were cheaper. 802.11b took off. Today, the most widely used standard is 802.11g, which operates in the 2.4 GHz band and, as we will see on the next slide, is much faster than 802.11b.
  • Again, 802.11b only had a rated speed of 11 Mbps. Many assumed that 802.11a would eventually win because it was faster, with a rated speed of 54 Mbps. However, the 802.11g standard provided a way to bring a low-cost 54 Mbps rated speed. Note that throughput is much lower than the rated speed. Note also that throughput falls off more rapidly with distance in 802.11a, which operates in the higher 5 GHz frequency band.
  • This slide emphasizes that throughput is aggregate throughput shared by the access point and all of its stations because only one device can transmit at a time.
  • One problem with both 802.11b and 802.11g is that there are only three channels in the 2.4 GHz frequency band. These channels are 1, 6, and 11. [The band was designed to have many channels. However, it was later realized that these channels were too narrow. Channels were increased in size so that there were only three channels. These new channels were given the numbers of the old channels. In contrast, the 5 GHz band has around 24 channels in most countries (regulations vary around the world.)
  • One problem with both 802.11b and 802.11g is that there are only three channels in the 2.4 GHz frequency band. These channels are 1, 6, and 11. [The band was designed to have many channels. However, it was later realized that these channels were too narrow. Channels were increased in size so that there were only three channels. These new channels were given the numbers of the old channels. In contrast, the 5 GHz band has around 24 channels in most countries (regulations vary around the world.)
  • Bluetooth is for personal area networks (PANs)—small groups of devices (up to 8) carried by a person or on and around a desk. Bluetooth usually is limited to 10 meters. This is far too short a distance for corporate or home WLAN use. Transmission speed is 3 Mbps with a slower reverse channel. This is far too slow for a corporate WLAN. Basically, think of Bluetooth as a cable replacement technology—removing the rat’s nest of wires on a desk and allowing a person to connect the devices he or she is carrying without having to mess with wires.
  • There are three other local wireless technologies that are likely to become popular in the future. &lt;Read the slide.&gt;
  • &lt;Read the slide.&gt; [Mesh networking can also be done by wireless computers themselves, without even using access points. This is more difficult.]
  • We looked at UTP and fiber propagation in Chapter 3. Now we will look at radio propagation. A key theme is that radio propagation is much more difficult to control than UTP and optical fiber propagation because radio propagation is relatively unpredictable.
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  • Dish Antennas Focus signals in a narrow range Therefore, signals can be sent over long distances Satellite TV systems use dish antennas to talk to distant satellites Must point at sender, so good for fixed subscribers Omnidirectional Antennas Spread signals in all directions Rapid signal attenuation No need to point at receiver. Imagine if you had to carry a dish around to use your cellphone! Good for mobile subscribers
  • In UTP and fiber, propagation effects usually are highly predictable. When you send a signal into a UTP cord or optical fiber, you usually can predict what will happen quite accurately. Wireless transmission has none of this accuracy. Many factors affect radio transmission, and these factors are difficult to predict. (1) First, electromagnetic interference (EMI) often occurs in popular radio bands. Microwave ovens may cause problems. And, of course, signals from other stations will interfere. (2) Then, there is attenuation. As the wireless device gets farther from the access point, the signal gets weaker. (3) Third, if there is a thick object, such as a wall, between the sender and receiver, this will create a shadow zone, also called a dead spot. You probably are familiar with this if you have a cellular telephone.
  • (4) The fourth problem is multipath interference, which is a bit difficult to understand. In many cases, both a direct signal and a signal reflected off walls or other objects will reach the wireless client. A signal’s amplitude rises and falls constantly. If the direct signal is at its maximum when the reflected signal is at its minimum, the two will cancel out and there will be no signal that the receiving devices can read. Multipath interference usually isn’t this dramatic, it is serious problem. IN FACT, MULTIPATH INTERFERENCE USUALLY IS THE MOST SERIOUS PROPAGATION PROBLEM IN RADIO TRANSMISSION AT FREQUENCIES USED BY WIRELESS LANS!!
  • Radio attenuation is very rapid. When a radio transmits, its signal spreads out as a sphere. This produces inverse square law attenuation. &lt;Read the slide.&gt;
  • &lt;Read the slide.&gt;
  • To manage radio interference, frequency use is regulated by international agencies. (1) The range of all possible frequencies from 0 Hz to infinity is called the frequency spectrum. (2) Regulators divide the frequency spectrum into ranges of frequencies called service bands, each of which provides a single type of service, such as FM radio, AM radio, VHF television, and cellular telephony. (3) Within a service band, even smaller frequency ranges are defined. These are channels. There will be multiple channels in the service band. Each will carry a different signal. &lt;Go through the figure’s channels to show what signals are in which channel.&gt; (4) Signals in different channels do not interfere with one another.
  • How fast can you send a signal in a channel? Claude Shannon found a mathematical relationship between maximum possible speed and other factors. In Shannon’s equation, C is the maximum possible transmission speed in the channel, measured as bits per second (bps). B is the bandwidth of the channel, measured in hertz, which is abbreviated as Hz. S/N is the signal-to-noise ratio. S/N must be measured as a ratio of the signal power to the noise power. If it is expressed in terms of decibels, the equation requires decibels converted to a simple power ratio of the signal power divided by the noise power. Question: What would happen to C if you tripled B? Answer: C would triple. Question: What would happen to C if you increased the signal strength relative to the noise floor. Answer: the maximum possible signal speed would increase.
  • &lt;Read the slide.&gt; [Usually, there are government regulations on signal strength. This limits the ability to boost the signal-to-noise ratio because the noise strength generally is outside of the sender’s control.]
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  • &lt;Read the slide.&gt; [The fact that spread spectrum transmission does not provide security surprises many people because the military uses spread spectrum transmission for security. Military spread spectrum transmission provides security by hiding the spread spectrum coding pattern. This makes signals difficult to read. [In contrast, in civilian spread spectrum transmissions, the spread spectrum coding pattern must be known to all.]
  • &lt;Read the slide from top to bottom.&gt;
  • &lt;Read the slide from top to bottom.&gt; [In multipath interference, whether or not there is destructive interference depends on the frequency of the signal. By spreading the signal over many frequencies, destructive and constructive multipath interference cancel out.]
  • Having looked at radio transmission theory, we are now ready to talk about how 802.11 WLANs work.
  • Note that 802.11 uses a different frame that 802.3 Ethernet. &lt;Read the text in the lower-right side of the slide.&gt;
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  • Access point channels should be selected to minimize mutual interference. If two nearby access points operate on the same channel, they will interfere, as shown in the figure. &lt;Go through the figure.&gt;
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  • A serious problem with 802.11 WLANs is security. Security concerns have caused many firms to scale back their WLAN plans considerably.
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  • One surprise to companies who implement 802.11 WLANs is how much management work they need. In part, high management costs are a result of the immaturity of 802.11 products. However, high management cost is also do to the unpredictability of radio transmission, which requires a great deal of of on-site adjustment after installation. In addition, the number of wireless clients served by an access point changes frequently as people come and go. Sufficient capacity at one moment may not be sufficient capacity at other times.
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  • The manual labor required to manage many access points can be very large. There are two alternatives for managing all access points remotely. One alternative is to use smart access points, which can be managed directly by a central management station. Unfortunately, smart access points are expensive. The other alternative is to use WLAN switches. The management intelligence processing is in the central management station and the WLAN switch. WLAN switches can work with inexpensive dumb (normal) access points.
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  • We talked about Bluetooth briefly at the beginning of the chapter. We will now look at it in a little more detail.
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  • Wireless networks

    1. 1. Wireless LANs (WLANs)
    2. 2. Local Wireless Technologies <ul><li>Physical-Layer Transmission </li></ul><ul><ul><li>Uses radio transmission </li></ul></ul><ul><ul><li>Gives mobility </li></ul></ul>
    3. 3. Wireless availability <ul><li>43,850 locations in 2003. </li></ul><ul><li>Estimated in 2004 to grow to over 200,000 locations in 2008. (actually grew to 101,000 commercial WiFi by end of 2005) </li></ul><ul><li>25 million WiFi routers shipped in 2005 </li></ul><ul><li>January 2006, 186 US cities have Wireless municipal networks, but we’re still 14 th in the World for wireless penetration. </li></ul>
    4. 4. Wireless LAN (WLAN) Access Point Server Internet Router Ethernet Switch Laptop Mobile Client Wireless Access Point Large Wired Ethernet LAN UTP Radio Transmission Wireless access point (WAP) bridges wireless stations to resources on wired LAN—servers and routers for Internet access Communication
    5. 5. Access Router with Wireless Access Point and Wireless NICs PC Card WNIC for a Notebook Computer Internal WNIC For Desktop PC USB WNIC Access Router with Access Point
    6. 6. Local Wireless Technologies, Continued <ul><li>802.11 </li></ul><ul><ul><li>The dominant WLAN technology today </li></ul></ul><ul><ul><li>Standardized by the 802.11 Working Group </li></ul></ul>802.11
    7. 7. Local Wireless Technologies, Continued <ul><li>802.11 Wireless LANs </li></ul><ul><ul><li>Speeds up to tens of megabits per second with distances of 30 to 100 meters or more </li></ul></ul><ul><ul><ul><li>Can serve many users in a home or office </li></ul></ul></ul><ul><ul><li>Soon to be 100 Mbps to 600 Mbps with 802.11n </li></ul></ul><ul><ul><li>Organizations can provide coverage throughout a building or a university campus by installing many access points </li></ul></ul>
    8. 8. 802.11 WLAN Standards
    9. 9. Specific 802.11 Wireless LAN Standards 802.11b 802.11g 802.11g if 802.11g access point serves an 802.11b station 2.4 GHz 2.4 GHz 2.4 GHz Unlicensed Band Lower Lower Lower Attenuation Yes 802.11a 5 GHz Higher No Yes Yes Crowded Band? Lower Lower Lower Price Higher Higher Lower Market Acceptance Very Low High
    10. 10. Specific 802.11 Wireless LAN Standards 802.11b 802.11g 802.11g if 802.11g access point serves an 802.11b station 11 Mbps 54 Mbps Not Specified Rated Speed* 6 Mbps 25 Mbps 12 Mbps Throughput, 3 m 6 Mbps 802.11a 54 Mbps 25 Mbps 12 Mbps 20 Mbps 11 Mbps Throughput, 30 m Source for throughput data: Broadband.com 802.11a, operating at a higher frequency, has more attenuation Than 802.11b *Maximum rated speed. There are slower modes if propagation is poor.
    11. 11. Specific 802.11 Wireless LAN Standards 802.11g 802.11g if 802.11g access point serves an 802.11b station Aggregate throughputs; Individual throughputs are lower Are These Aggregate Or Individual Throughputs? 20 Mbps 11 Mbps Throughput, 30 m 802.11a 12 Mbps 11 Mbps 54 Mbps Not Specified Rated Speed 802.11b 6 Mbps 54 Mbps
    12. 12. Specific 802.11 Wireless LAN Standards 802.11b 802.11a 802.11g 802.11g if 802.11g access point serves an 802.11b station 3 Up to 24 3 3 Number of Non- Overlapping Channels 2.4 GHz 5 GHz 2.4 GHz 2.4 GHz Unlicensed Band 2.4 GHz non-overlapping channels are 1, 6, and 11
    13. 13. A new Wireless LAN Standard <ul><li>A separate standard, 802.16 (or WiMAX ), transmits at 70 Mbps and has a range of up to 30 miles. </li></ul><ul><li>It can operate in licensed or unlicensed bands of the spectrum from 2-6 GHz. WiMAX typically links multiple 802.11 networks or sends Internet data over long distances. </li></ul>
    14. 14. Wireless LANs (WLANs) cont.
    15. 15. Local Wireless Technologies, Continued <ul><li>Bluetooth </li></ul><ul><ul><li>For personal area networks (PANs) </li></ul></ul><ul><ul><ul><li>Multiple devices carried by a person, or </li></ul></ul></ul><ul><ul><ul><li>Multiple devices around a desk </li></ul></ul></ul><ul><ul><ul><li>Limited to about 10 meters </li></ul></ul></ul><ul><ul><ul><li>Limited to 3 Mbps with a slower reverse channel </li></ul></ul></ul><ul><ul><li>Cable replacement technology </li></ul></ul>USB Bluetooth Adapter
    16. 16. Local Wireless Technologies, Continued <ul><li>Other Local Wireless Technologies </li></ul><ul><ul><li>Ultrawideband: Up to 250 Mbps (fast) over a distance of 10 meters (short) </li></ul></ul><ul><ul><ul><ul><li>Ideal for video networking in homes </li></ul></ul></ul></ul><ul><ul><li>ZigBee for almost-always-off sensor networks at low speeds </li></ul></ul><ul><ul><ul><ul><li>Allows battery lives of months or years </li></ul></ul></ul></ul><ul><ul><li>Radio Frequency ID (RFID) tags: like UPC product tags but readable from a small distance </li></ul></ul><ul><ul><ul><ul><li>RFID reader sends probe signal that powers the RFID tag, which then responds with its information </li></ul></ul></ul></ul>
    17. 17. Local Wireless Technologies, Continued <ul><li>Other Local Wireless Technologies </li></ul><ul><ul><li>Mesh networking: multiple access points can route frames to their destination without using a wired LAN </li></ul></ul><ul><ul><li>Being standardized at 802.11s </li></ul></ul>
    18. 18. Radio Propagation or How wireless data gets there!
    19. 19. Frequency Measurement <ul><li>Frequency </li></ul><ul><ul><li>Light waves are measured in wavelengths (Ch. 3) </li></ul></ul><ul><ul><li>Radio waves are measured in terms of frequency </li></ul></ul><ul><ul><li>Measured in hertz (Hz)—the number of complete cycles per second </li></ul></ul>1 Second Two cycles in 1 second, so frequency is two Hertz (Hz).
    20. 20. Frequency Measurement, Continued <ul><li>Measuring Frequencies </li></ul><ul><ul><li>Frequency measures increases by factors of 1,000 (not 1,024) </li></ul></ul><ul><ul><li>Kilohertz (kHz) [Note the lower-case k] </li></ul></ul><ul><ul><li>Megahertz (MHz) </li></ul></ul><ul><ul><li>Gigahertz (GHz) </li></ul></ul>
    21. 21. Omnidirectional and Dish Antennas Omnidirectional Antenna Spread signals in all directions Rapid signal attenuation ----- No need to point at receiver Good for mobile subscribers Dish Antenna Focuses signals in a narrow range Signals can be sent over long distances ----- Must point at the sender Good for fixed subscribers
    22. 22. Wireless Propagation Problems 2. Attenuation: signal gets weaker with distance 3. Shadow Zone (Dead Spot) 1. Electromagnetic Interference (EMI) from Other stations, Microwave ovens, etc. Blocking Object
    23. 23. Wireless Propagation Problems Reflected Signal Laptop Direct Signal 4. Multipath Interference Direct and reflected signals may interfere Blocking Object
    24. 24. Inverse Square Law Attenuation <ul><li>Inverse square law attenuation </li></ul><ul><ul><li>To compare relative power at two distances </li></ul></ul><ul><ul><ul><li>Divide the longer distance by the shorter distance </li></ul></ul></ul><ul><ul><ul><li>Square the result; this is the relative power ratio </li></ul></ul></ul><ul><ul><li>Examples </li></ul></ul><ul><ul><ul><li>100 mW (milliwatts) at 10 meters </li></ul></ul></ul><ul><ul><ul><li>At 20 meters, 100 / (20/10) 2 = 100 mW / 4 = 25 mW </li></ul></ul></ul><ul><ul><ul><li>At 30 meters, 100 / (30/10) 2 = 100 mW / 9 = 11 mW </li></ul></ul></ul><ul><ul><li>Much faster attenuation than UTP or fiber </li></ul></ul>
    25. 25. Frequency-Dependent Propagation Problem <ul><li>Some problems are Frequency-Dependent </li></ul><ul><ul><li>Higher-frequency signals attenuate faster </li></ul></ul><ul><ul><ul><li>Absorbed more rapidly by water in the air </li></ul></ul></ul><ul><ul><li>Higher-frequency signals blocked more by obstacles </li></ul></ul><ul><ul><ul><li>At lower frequencies, signal refract (bend) around obstacles like an ocean wave hitting a buoy </li></ul></ul></ul><ul><ul><ul><li>At higher frequencies, signals do not refract; leave a complete shadow behind obstacles </li></ul></ul></ul>
    26. 26. The Frequency Spectrum, Service Bands, and Channels Channel 5, Signal A Channel 1, Signal E Channel 2, No Signal Channel 3, Signal B Channel 4, Signal D 0 Hz 2. Service Band (FM Radio, Cellular Telephony, etc.) 1. Frequency Spectrum (0 Hz to Infinity) 3. Multiple Channels within a Service Band; each Channel carries a different signal 4. Signals in different channels do not interfere with one another
    27. 27. Channel Bandwidth and Transmission Speed <ul><li>Shannon Equation </li></ul><ul><ul><li>Specifies the connection between channel bandwidth and the channel’s maximum signal transmission speed </li></ul></ul><ul><ul><li>C = B [ Log 2 (1+S/N) ] </li></ul></ul><ul><ul><ul><li>C = Maximum possible transmission speed in the channel (bps) </li></ul></ul></ul><ul><ul><ul><li>B = Bandwidth (Hz) </li></ul></ul></ul><ul><ul><ul><li>S/N = Signal-to-Noise Ratio </li></ul></ul></ul><ul><ul><ul><ul><li>Measured as a ratio </li></ul></ul></ul></ul><ul><ul><ul><ul><li>If given in dB, must convert to ratio </li></ul></ul></ul></ul>
    28. 28. Channel Bandwidth and Transmission Speed <ul><li>Shannon Equation </li></ul><ul><ul><li>C = B [ Log2 (1+S/N) ] </li></ul></ul><ul><ul><ul><li>Note that doubling the bandwidth doubles the maximum possible transmission speed </li></ul></ul></ul><ul><ul><ul><li>Increasing the bandwidth by X increases the maximum possible speed by X </li></ul></ul></ul><ul><ul><li>Wide bandwidth is the key to fast transmission </li></ul></ul><ul><ul><li>Increasing S/N helps slightly but usually cannot be done to any significant extent </li></ul></ul>
    29. 29. Channel Bandwidth and Transmission Speed <ul><li>Broadband and Narrowband Channels </li></ul><ul><ul><li>Broadband means wide channel bandwidth and therefore high speed </li></ul></ul><ul><ul><li>Narrowband means narrow channel bandwidth and therefore low speed </li></ul></ul><ul><ul><li>Narrowband is below 200 kbps </li></ul></ul><ul><ul><li>Broadband is above 200 kbps </li></ul></ul>
    30. 30. Channel Bandwidth and Transmission Speed <ul><li>Channel Bandwidth and Spectrum Scarcity </li></ul><ul><ul><li>Why not make all channels broadband? </li></ul></ul><ul><ul><li>There is only a limited amount of spectrum at desirable frequencies </li></ul></ul><ul><ul><li>Making each channel broader than needed would mean having fewer channels or widening the service band </li></ul></ul><ul><ul><li>Service band design requires tradeoffs between speed requirements, channel bandwidth, and service band size </li></ul></ul>
    31. 31. Channel Bandwidth and Transmission Speed <ul><li>The Golden Zone </li></ul><ul><ul><li>Most organizational radio technologies operate in the golden zone in the high megahertz to low gigahertz range </li></ul></ul><ul><ul><li>At higher frequencies, propagation problems are severe </li></ul></ul><ul><ul><li>At lower frequencies, there is not enough total bandwidth </li></ul></ul>Golden Zone Higher Frequency Lower Frequency
    32. 32. Spread Spectrum Transmission <ul><li>Unlicensed Bands </li></ul><ul><ul><li>WLANs operate in unlicensed service bands </li></ul></ul><ul><ul><ul><li>You do not need a license to have or move your stations </li></ul></ul></ul><ul><ul><ul><li>You must tolerate interference from other users </li></ul></ul></ul><ul><ul><ul><li>You must not cause unreasonable interference </li></ul></ul></ul><ul><ul><li>Two unlicensed bands are widely used: the 2.4 GHz band and the 5 GHz band </li></ul></ul><ul><ul><ul><li>5 GHz has worse propagation characteristics </li></ul></ul></ul><ul><ul><ul><li>2.4 GHz has fewer available channels </li></ul></ul></ul>
    33. 33. Spread Spectrum Transmission, Cont. <ul><li>Spread Spectrum Transmission </li></ul><ul><ul><li>You are REQUIRED BY LAW to use spread spectrum transmission in unlicensed bands </li></ul></ul><ul><ul><ul><li>Spread spectrum transmission is required to reduce propagation problems at high frequencies </li></ul></ul></ul><ul><ul><ul><li>Especially multipath interference </li></ul></ul></ul><ul><ul><li>Spread spectrum transmission is NOT used for security in WLANs </li></ul></ul><ul><ul><ul><li>This surprises many people </li></ul></ul></ul>
    34. 34. Normal Radio Transmission and Spread Spectrum Transmission Channel Bandwidth Required for Signal Speed Normal Radio: Bandwidth Is No Wider than Required Note: Height of Box Indicates Bandwidth of Channel To conserve spectrum channel, bandwidths usually are set to be only as wide as signals in the service band need based on their speed Normal transmission: Uses only the channel bandwidth required by your signaling speed
    35. 35. Normal Radio Transmission and Spread Spectrum Transmission Channel Bandwidth Required for Signal Speed Note: Height of Box Indicates Bandwidth of Channel Spread Spectrum Transmission: Channel Bandwidth Is Much Wider than Needed However, spread spectrum transmission uses much wider channels than are needed, which seems wasteful but improves propagation <ul><ul><li>Spread spectrum transmission: Uses channels much wider than signaling speed requires </li></ul></ul>
    36. 36. 802.11 WLAN Operation
    37. 37. Typical 802.11 WLAN Operation Server Ethernet Switch Laptop WAP Large Wired LAN Client PC UTP Radio Transmission 802.11 Frame 802.3 Frame 802.3 Frame Wireless access points (WAPs) bridge the networks (translates between the 802.11 wireless frame and the Ethernet 802.3 frame used within the LAN)
    38. 38. Typical 802.11 WLAN Operation, Continued Server Ethernet Switch Laptop WAP A Large Wired LAN Client PC WAP B UTP Handoff or Roaming (if mobile computer moves to another access point, it switches service to that access point) 802.11 Frame 802.3 Frame
    39. 39. Stations and Access Points Transmit in a Single Channel Collision if 2 Devices send Simultaneously
    40. 40. Media Access Control <ul><li>Only one station or the access point can transmit at a time </li></ul><ul><li>To control access (transmission), two methods can be used </li></ul><ul><ul><li>CSMA/CA+ACK (mandatory) </li></ul></ul><ul><ul><li>RTS/CTS (optional unless 802.11b and g stations share an 802.11g access point) </li></ul></ul>
    41. 41. CSMA/CA in 802.11 Wireless LANs <ul><li>CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) </li></ul><ul><li>CSMA </li></ul><ul><ul><li>Sender Always Listens for Traffic </li></ul></ul><ul><ul><ul><li>Carrier is the signal; sense is to listen </li></ul></ul></ul><ul><ul><li>If there is traffic, the sender waits </li></ul></ul><ul><ul><li>If there is no traffic … </li></ul></ul><ul><ul><ul><li>If the time since the last transmission is more than a critical value, the station may send immediately </li></ul></ul></ul>
    42. 42. CSMA/CA in 802.11 Wireless LANs <ul><li>CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) </li></ul><ul><ul><li>If there is no traffic </li></ul></ul><ul><ul><ul><li>If the time since the last transmission is less than a critical value, the station sets a random timer and waits </li></ul></ul></ul><ul><ul><ul><ul><li>If there is no traffic at the end of the waiting time, the station sends </li></ul></ul></ul></ul><ul><ul><ul><ul><li>If there is traffic, CSMA starts over again </li></ul></ul></ul></ul>
    43. 43. CSMA/CA in 802.11 Wireless LANs <ul><li>ACK (Acknowledgement) </li></ul><ul><ul><li>Receiver immediately sends back an acknowledgment when it receives a frame </li></ul></ul><ul><ul><ul><li>Does not wait to send an ACK </li></ul></ul></ul><ul><ul><ul><li>This avoids interference with other stations, which must wait </li></ul></ul></ul><ul><ul><li>If sender does not receive the acknowledgement, it retransmits the frame using CSMA/CA </li></ul></ul><ul><ul><li>802.11 with CSMA/CA+ACK is a reliable protocol! </li></ul></ul>
    44. 44. Request to Send/Clear to Send (RTS/CTS) Server Switch Laptop Access Point B Large Wired LAN Radio Link Client PC RTS 1. Device that wishes to transmit may send a Request-to-Send message Box
    45. 45. Request to Send/Clear to Send (RTS/CTS) Server Switch May Send Frames WAP Large Wired LAN Radio Link Client PC 2. Wireless access point broadcasts a Clear-to-Send message. Station that sent the RTS may transmit unimpeded. Other stations hearing the CTS must wait CTS Box Must Wait
    46. 46. Recap <ul><li>CSMA/CA+ACK is mandatory </li></ul><ul><li>RTS/CTS is optional </li></ul><ul><ul><li>However, it is mandatory if 802.11b and 802.11g NICs share the same 802.11g access point </li></ul></ul>
    47. 47. Specific 802.11 Wireless LAN Standards <ul><li>Transmission Speed and Distance </li></ul><ul><ul><li>As a station moves away from an access point, transmission speed falls </li></ul></ul><ul><ul><ul><li>There are several modes of operation specified in each standard </li></ul></ul></ul><ul><ul><ul><li>The fastest mode only works with a very strong signal </li></ul></ul></ul><ul><ul><ul><li>As the user moves away, the signal strength becomes too low </li></ul></ul></ul><ul><ul><ul><li>That station and the access point switch to a slower mode </li></ul></ul></ul>
    48. 48. Specific 802.11 Wireless LAN Standards, Continued <ul><li>Transmission Speed and Distance </li></ul><ul><ul><li>When stations transmit more slowly, they take longer to transmit their frames </li></ul></ul><ul><ul><ul><li>This reduces the time available for other stations to transmit </li></ul></ul></ul><ul><ul><ul><li>Consequently, throughput falls for everyone </li></ul></ul></ul><ul><ul><li>Even a few very distant stations can slow throughput for everyone substantially </li></ul></ul>
    49. 49. Figure 5-19: Interference Between Nearby Access Points Operating on the Same Channel Access Point Channels Should be Selected to Minimize Mutual Interference
    50. 50. 802.11n <ul><li>Under Development </li></ul><ul><ul><li>Rated speeds of 100 Mbps to 600 Mbps </li></ul></ul><ul><ul><li>Will operate in both the 2.4 GHz and 5 GHz bands </li></ul></ul><ul><ul><li>May use twice current bandwidth per channels (~20 MHz) to roughly double speed </li></ul></ul><ul><ul><li>Will use MIMO </li></ul></ul><ul><ul><li>Currently a draft standard </li></ul></ul>
    51. 51. 802.11e <ul><li>Standard for Quality of Service (QoS) </li></ul><ul><ul><li>Needed for voice and video transmission </li></ul></ul><ul><ul><li>Wi-Fi Alliance calls 802.11e Wi-Fi Multimedia (WMM) </li></ul></ul>
    52. 52. WLAN Security
    53. 53. WLAN Security Threats <ul><li>Drive-By Hackers </li></ul><ul><ul><li>Sit outside the corporate premises and read network traffic </li></ul></ul><ul><ul><li>Can send malicious traffic into the network </li></ul></ul><ul><ul><li>Easily done with readily available downloadable software </li></ul></ul><ul><li>War Drivers </li></ul><ul><ul><li>Merely discover unprotected access points–become drive-by hackers only if they break in </li></ul></ul>
    54. 54. WLAN Security Threats, Continued <ul><li>Rogue Access Points </li></ul><ul><ul><li>Unauthorized access points set up by department or individual </li></ul></ul><ul><ul><li>Often have very poor security, making drive-by hacking easier </li></ul></ul><ul><ul><li>Often operate at high power, attracting many clients </li></ul></ul>
    55. 55. WLAN Security Threats, Continued <ul><li>Evil Twin Access Points </li></ul><ul><ul><li>Create a fake access point outside walls of firm using a PC </li></ul></ul><ul><ul><li>Legitimate internal client associates with the evil twin access point, which operates at high power </li></ul></ul>Evil Twin AP Legitimate Client Legitimate AP Duped Association
    56. 56. WLAN Security Threats, Continued <ul><li>Evil Twin Access Points </li></ul><ul><ul><li>Evil twin then associates with a legitimate internal access point masquerading as the internal clients </li></ul></ul><ul><ul><li>This connects the evil twin to the firm’s internal network </li></ul></ul>Evil Twin AP Legitimate Client Legitimate AP 1. Associates 2. Associates As Legitimate Client
    57. 57. WLAN Security Threats, Continued <ul><li>Evil Twin Access Points </li></ul><ul><ul><li>Evil twin can then read all traffic, even if the sender and receive encrypt their messages because the evil twin steals authentication credentials passed between the clients and the legitimate access point </li></ul></ul><ul><ul><li>Also can insert traffic </li></ul></ul><ul><ul><li>Classic man-in-the-middle attack </li></ul></ul>Evil Twin AP Legitimate Client Legitimate AP
    58. 58. 802.11 WLAN Management
    59. 59. Wireless LAN Management <ul><li>Access Points Placement in a Building </li></ul><ul><ul><li>Must be done carefully for good coverage and to minimize interference between access points </li></ul></ul><ul><ul><li>Lay out 30-meter to 50-meter radius circles on blueprints </li></ul></ul><ul><ul><li>Adjust for obvious potential problems such as brick walls </li></ul></ul><ul><ul><li>In multistory buildings, must consider interference in three dimensions </li></ul></ul>
    60. 60. Wireless LAN Management <ul><li>Access Points Placement in a Building </li></ul><ul><ul><li>Install access points and do site surveys to determine signal quality </li></ul></ul><ul><ul><li>Adjust placement and signal strength accordingly </li></ul></ul><ul><ul><li>This is quite expensive </li></ul></ul>
    61. 61. Wireless Access Point Management Alternatives Management intelligence can be placed in the access point or the WLAN switch
    62. 62. Wireless LAN Management <ul><li>Remote Access Point Management </li></ul><ul><ul><li>Desired functionality </li></ul></ul><ul><ul><ul><li>Continuous transmission quality monitoring </li></ul></ul></ul><ul><ul><ul><li>Immediate notification of failures </li></ul></ul></ul><ul><ul><ul><li>Remote AP adjustment (power, channel, etc.) </li></ul></ul></ul><ul><ul><ul><li>Ability to push software updates out to all APs or WLAN switches </li></ul></ul></ul><ul><ul><ul><li>Take appropriate actions automatically whenever possible </li></ul></ul></ul>
    63. 63. Bluetooth For Personal Area Networks (PANs)
    64. 64. Bluetooth Personal Area Networks (PANs) <ul><li>For Personal Area Networks (PANs) </li></ul><ul><ul><li>Devices around a desk (computer, mouse, keyboard, printer) </li></ul></ul><ul><ul><li>Devices on a person’s body and nearby (cellphone, PDA, notebook computer, etc.) </li></ul></ul>
    65. 65. Bluetooth Personal Area Networks (PANs) <ul><li>Cable Replacement Technology </li></ul><ul><ul><li>For example, with a Bluetooth PDA, print wirelessly to a nearby Bluetooth-enabled printer </li></ul></ul><ul><ul><li>No access points are used </li></ul></ul><ul><ul><ul><li>Direct device-to-device communication </li></ul></ul></ul>Print Job
    66. 66. Bluetooth Personal Area Networks (PANs) <ul><li>Disadvantages Compared to 802.11 </li></ul><ul><ul><li>Short distance (10 meters) </li></ul></ul><ul><ul><li>Low speed (3 Mbps, with a slower reverse channel) </li></ul></ul><ul><ul><li>Insufficient for WLAN in a building </li></ul></ul>
    67. 67. Bluetooth Personal Area Networks (PANs) <ul><li>Advantages Compared to 802.11 </li></ul><ul><ul><li>Low battery power drain so long battery life between recharges </li></ul></ul><ul><ul><li>Application profiles </li></ul></ul><ul><ul><ul><li>Define how devices will work together with little or no human intervention </li></ul></ul></ul><ul><ul><ul><li>Sending print jobs to printers </li></ul></ul></ul><ul><ul><ul><li>File synchronization </li></ul></ul></ul><ul><ul><ul><li>Etc. </li></ul></ul></ul><ul><ul><ul><li>Somewhat rudimentary </li></ul></ul></ul><ul><ul><ul><li>Devices typically only automate a few access profiles </li></ul></ul></ul>
    68. 68. Bluetooth Personal Area Networks (PANs) <ul><li>Bluetooth Trends </li></ul><ul><ul><li>Bluetooth Alliance is enhancing Bluetooth </li></ul></ul><ul><ul><li>The next version of Bluetooth is likely to grow to use ultrawideband transmission </li></ul></ul><ul><ul><ul><li>This should raise speed to 100 Mbps (or more) </li></ul></ul></ul><ul><ul><ul><li>Transmission distance will remain limited to 10 meters </li></ul></ul></ul><ul><ul><ul><li>Good for distributing television within a house </li></ul></ul></ul>
    69. 69. Other Wireless Communication <ul><li>3G Cellular phones </li></ul><ul><li>VoIP on wireless </li></ul><ul><li>RFID and embedded wireless technology, e.g. credit/ID cards </li></ul><ul><li>Wireless IPODs? </li></ul>

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