Revised August 2013 Wireless LANs I Chapter 6

Revised August 2013 Wireless LANs I Chapter 6 Panko and Panko Business Data Networks and Security, 9 th Edition © 2013 Pearson

6. 1: Perspective Chapter 5 ◦ Ethernet wired switched LANs ◦ Switched, so Require standards at Layer 1 (physical) and Layer 2 (data link) ◦ Physical and data link layer standards are almost always OSI standards. © 2013 Pearson 2

Perspective Chapters 6 and 7 ◦ Wireless LANs (WLANs) ◦ Also require standards at Layers 1 and 2 ◦ So also are OSI standards Wired versus Wireless LANs ◦ Companies have been spending more on wireless LANs than wired LANs since 2008. © 2013 Pearson 3

6. 1: 802. 11 Wireless LAN Technology 802. 11 is the dominant wireless LAN (WLAN) Technology Standardized by the 802. 11 Working Group Large 802. 11 WLANs use multiple access points to cover large areas 802. 11 © 2013 Pearson 4

6. 1: 802. 11 Wireless LAN Technology Products are certified as compliant by the Wi -Fi Alliance ◦ An industry group separate from the IEEE ◦ Wi-Fi seal appears on products ◦ So 802. 11 is often called Wi-Fi © 2013 Pearson 5

6. 2: 802. 11 Wireless LAN (WLAN) Operation The wireless access point connects the wireless client to the wired Ethernet LAN. © 2013 Pearson 6

6. 2: 802. 11 Wireless LAN (WLAN) Operation The LAN connection is needed to give clients access to servers and Internet access routers on the wired LAN. © 2013 Pearson 7

6. 2: 802. 11 Wireless LAN (WLAN) Standards Speeds and Distances to Devices ◦ Products on the market have speeds up to about 300 Mbps, but usually 10 to 100 Mbps ◦ Distances of 30 to 100 meters © 2013 Pearson 8

6. 3: Electromagnetic Wave Optical fiber transmission is measured in terms of wavelength. Typical data LAN frequencies are 500 MHz to 10 GHz © 2013 Pearson 9

6. 4: Omnidirectional and Dish Antennas © 2013 Pearson 10

6. 4: Omnidirectional and Dish Antennas Questions: What type of antenna do mobile phones use? Why? © 2013 Pearson 11

6. 5: Wireless Propagation Problems 1. Wireless transmission has many propagation problems. © 2013 Pearson 2. Electromagnetic Interference (EMI) is unwanted power at the same frequency from other devices. 12

6. 5: Wireless Propagation Problems Shadow zones are places the signal cannot penetrate because of obstacles in its path. Shadow zones, also called dead spots, grow worse as frequency increases. © 2013 Pearson 13

6. 5: Wireless Propagation Problems The signal strength spreads out as the surface of a sphere. This means that its strength falls as (1/r 2), where r is the radius. If you double the distance, you only get ¼ the signal strength © 2013 Pearson 14

6. 5: Wireless Propagation Problems Radio signals spread out in a sphere. ◦ S = signal power, r = range (distance) or radius ◦ If the signal power at 10 meters is 9 milliwatts (m. W), how strong is it at 30 meters? S 2 = S 1 * (r 1/r 2)2 S 2 = 9 m. W * (10/30)2 S 2 = 9 m. W * (1/3)2 S 2 = 9 m. W * (1/9) S 2 = 1 m. W © 2013 Pearson 15

6. 5: Wireless Propagation Problems Your turn. ◦ If the signal strength at 5 meters is 48 m. W, how strong is it at 20 meters? © 2013 Pearson 16

6. 5: Wireless Propagation Problems Signal is absorbed by the air and water. Note that there are two types of attenuation: Inverse square law and absorptive attenuation. Note that this is different than shadow zones. © 2013 Pearson 17

6. 5: Wireless Propagation Problems Direct and reflected signals may interfere. Most serious propagation problem at WLAN frequencies. © 2013 Pearson 18

6. 6: Multipath Interference If the two waves are out of phase, they will negate each other, giving no signal. © 2013 Pearson 19

6. 5: Wireless Propagation Problems Recap © 2013 Pearson 20

6. 7: The Frequency Spectrum, Service Bands, and Channels © 2013 Pearson 21

6. 7: The Frequency Spectrum, Service Bands, and Channels © 2013 Pearson 22

6. 7: The Frequency Spectrum, Service Bands, and Channels © 2013 Pearson 23

6. 9: Channel Bandwidth and Transmission Speed Signal Bandwidth ◦ Figure 6 -2 shows a wave operating at a single frequency. ◦ However, most signals are spread over a range of frequencies (Figure 6 -9). © 2013 Pearson 24

6. 8: Channel Bandwidth and Transmission Speed Channel Bandwidth ◦ Channel bandwidth is the highest frequency in a channel minus the lowest frequency. ◦ An 88. 0 MHz to 88. 2 MHz channel has a bandwidth of 0. 2 MHz (200 k. Hz). ◦ Higher-speed signals need wider channel bandwidths. © 2013 Pearson 25

6. 8: Channel Bandwidth and Transmission Speed Shannon Equation C = B [Log 2 (1+S/N)] ◦ C = Maximum possible speed in the channel in bits per second Not the actual speed, although the actual speed may be close ◦ B = Bandwidth in Hz ◦ S/N = Signal-to-Noise Ratio (SNR)—the signal power divided by the average noise power Better S/N ratios produce fewer errors. © 2013 Pearson 26

6. 8: Channel Bandwidth and Transmission Speed Shannon Equation C = B [Log 2 (1+S/N)] ◦ Note that doubling the bandwidth doubles the maximum possible transmission speed. ◦ Multiplying the bandwidth by X multiplies the maximum possible speed by X. ◦ Wide bandwidth is the key to fast transmission. © 2013 Pearson 27

6. 8: Channel Bandwidth and Transmission Speed Shannon Equation C = B [Log 2 (1+S/N)] ◦ Increasing S/N helps slightly, but usually cannot be done to any significant extent Ratio 100: 1 1, 000: 1 10, 000: 1 © 2013 Pearson Approximate Log 2 7 10 (40% improvement) 13 (30% improvement) 28

6. 8: Channel Bandwidth and Transmission Speed Broadband Narrowband Channels ◦ Broadband means wide channel bandwidth and therefore high speed. ◦ Narrowband means narrow channel bandwidth and therefore low speed. ◦ Traditionally, narrowband is below 200 kbps; broadband is above 200 kbps. © 2013 Pearson 29

6. 8: Channel Bandwidth and Transmission Speed The Golden Zone ◦ Most organizational radio technologies operate in the golden zone in the 500 MHz to 10 GHz range. ◦ At higher frequencies, there is more available bandwidth. Golden zone frequencies are high enough for there to be large total bandwidth. ◦ At lower frequencies, signals propagate better. Golden zone frequencies are low enough to allow fairly good propagation characteristics. © 2013 Pearson 30

6. 10: Line-of-Sight © 2013 Pearson 31

6. 11: Licensed and Unlicensed Service Bands Licensed Service Bands ◦ If two nearby radio hosts transmit in the same channel, their signals will interfere. ◦ Most service bands are licensed bands, in which hosts need a license to transmit. ◦ The government limits licenses to reduce interference. ◦ Television bands, AM radio bands, and so on are licensed. ◦ In cellular telephone bands, which are licensed, only the central transceivers are licensed, not the mobile phones. © 2013 Pearson 32

6. 11: Licensed and Unlicensed Service Bands ◦ Some service bands are set aside as unlicensed bands. ◦ Hosts do not need to be licensed to be turned on or moved. ◦ 802. 11 operates in unlicensed radio bands. ◦ This allows access points and hosts to be moved freely. © 2013 Pearson 33

6. 11: Licensed and Unlicensed Service Bands ◦ However, there is no way to stop interference from other nearby users. ◦ Your only recourse is to negotiate with others. ◦ At the same time, you may not cause unreasonable interference—for instance, by transmitting at excessive power. © 2013 Pearson 34

6. 12: 802. 11 in the 2. 4 GHz and 5 GHz Unlicensed Service Bands The 2. 4 GHz Unlicensed Service Band ◦ Defined the same in almost all countries (2. 400 GHz to 2. 485 GHz) Commonality reduces radio costs ◦ Propagation characteristics are good © 2013 Pearson 35

6. 12: 802. 11 in the 2. 4 GHz and 5 GHz Unlicensed Bands The 2. 4 GHz Unlicensed Band ◦ Potential interference from microwave ovens, cordless telephones, and so on ◦ For 20 MHz 802. 11 channels, only three nonoverlapping channels are possible Channels 1, 6, and 11 This creates mutual channel interference between nearby access points transmitting in the same 20 MHz channel © 2013 Pearson 36

6. 13: Co-Channel Interference in 2. 4 GHz The 2. 4 GHz Unlicensed Band ◦ Difficult or impossible to put nearby access points on different channels © 2013 Pearson 37

6. 12: 802. 11 in the 2. 4 GHz and 5 GHz Unlicensed Service Bands The 5 GHz Unlicensed Service Band ◦ 5 GHz radios are more expensive than 2. 4 GHz radios because somewhat different frequency ranges are used in different countries. ◦ Shorter propagation distance than in the 2. 4 GHz band because of higher frequencies. ◦ Deader shadow zones than in the 2. 4 GHz band because of higher frequencies. © 2013 Pearson 38

6. 12: 802. 11 in the 2. 4 GHz and 5 GHz Unlicensed Service Bands The 5 GHz Unlicensed Service Band ◦ More total bandwidth than 2. 4 GHz, so between 11 and 24 non-overlapping 20 MHz channels. ◦ Allows nearby access points to operate on nonoverlapping channels. ◦ Some access points can operate on two channels to provide faster service. © 2013 Pearson 39

6. 12: 802. 11 in the 2. 4 GHz and 5 GHz Unlicensed Bands What is the main advantage of 2. 4 GHz operation? What is the main advantage of 5 GHz operation? © 2013 Pearson 40

6. 14: Spread Spectrum Transmission ◦ You are required by law to use spread spectrum transmission in unlicensed bands. ◦ Spread spectrum transmission reduces propagation problems. Especially multipath interference ◦ Spread spectrum transmission is NOT used for security in WLANs. © 2013 Pearson 41

6. 15: Normal vs Spread Spectrum Transmission Why do you not want to make radio channel bandwidth greater than the signal requires? © 2013 Pearson 42

6. 15: Normal vs Spread Spectrum Transmission Wasteful of bandwidth but necessary to avoid propagation problems. © 2013 Pearson 43

6. 16: Orthogonal Frequency Division Multiplexing (OFDM) © 2013 Pearson 44

6. 17: WLAN Frames and Packets Sender puts a packet for the destination host into an 802. 11 frame, then sends the frame wirelessly to the access point. © 2013 Pearson 45

6. 17: WLAN Frames and Packets The 802. 11 frame has the wrong frame format to travel over an 802. 3 Ethernet network. The switches and destination host would not know what to do with it. The access point removes the packet from the 802. 11 frame and discards the frame. © 2013 Pearson 46

6. 17: WLAN Frames and Packets The access point encapsulates the packet in an 802. 3 frame and sends this frame on to the destination host via Ethernet switches. © 2013 Pearson 47

6. 17: WLAN Frames and Packets The Wired Ethernet Network is called the Distribution System The server removes the packet from the 802. 3 frame. © 2013 Pearson 48

6. 17: WLAN Frames and Packets Does the IP packet travel all the way to the destination host? Does the 802. 11 frame travel all the way to the destination host? Why or why not? © 2013 Pearson 49

6 -18: Basic Service Set (BSS) An access point and its wireless hosts Service Set ID (SSID) is the name of the network (abc) © 2013 Pearson 50

6 -18: Extended Service Set (ESS) Collection of access points that all have the same SSID © 2013 Pearson 51

6 -18: Roaming/Handoff All access points must have the same SSID © 2013 Pearson 52

6. 19: Transmitting in a Single Channel © 2013 Pearson 53

6. 20: CSMA/CA+ACK Box CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) ◦ Sender listens for traffic ◦ 1. If there is traffic, waits © 2013 Pearson 54

6. 20: CSMA/CA+ACK Box CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) ◦ 2. If there is no traffic: 2 a. If there has been no traffic for less than the critical time value, waits a random amount of time, then returns to Step 1. 2 b. If there has been no traffic for more than the critical value for time, sends without waiting. This avoids collision that would result if hosts could transmit as soon as one host finishes transmitting. © 2013 Pearson 55

6. 20: CSMA/CA+ACK (Acknowledgement) Box ◦ Receiver immediately sends back an acknowledgement. If original sender does not receive the acknowledgement, retransmits using CSMA. ◦ CSMA/CA plus ACK is a reliable protocol. CSMA/CA+ACK is reliable because wireless transmission has high error rates. Ethernet has lower error rates and so can be unreliable. © 2013 Pearson 56

6. 21: RTS-CTS © 2013 Pearson Box 57

6. 21: RTS-CTS Box 0 © 2013 Pearson 58

Comparison Box CSMA/CA is Mandatory ◦ It is the default MAC method. ◦ It is more efficient than RTS/CTS ◦ Is usually optional. ◦ Is good if two or more client stations cannot hear each other. ◦ It will prevent them from transmitting at the same time. © 2013 Pearson 59

6 -22: 802. 11 Standards The 802. 11 Working Group has produced several transmission standards. Existing Standards ◦ 802. 11 g ◦ 802. 11 n In Development ◦ 802. 11 ac (still in development but pre-standard ac products are now available and popular) ◦ 802. 11 ad © 2013 Pearson 60

6. 22: 802. 11 g and 802. 11 n Characteristic Remarks 802. 11 g Today’s dominant 802. 11 standard in terms of installed base. 802. 11 n Today’s fastestgrowing 802. 11 standard. Unlicensed band 2. 4 GHz. And 5 GHz if dual-band © 2013 Pearson 61

6. 22: 802. 11 g and 802. 11 n Characteristic 802. 11 g 802. 11 n Channel Bandwidth 20 MHz 40 MHz but may drop back if there is interference Spread spectrum OFDM method © 2013 Pearson OFDM 62

6. 22: 802. 11 g and 802. 11 n Characteristic 802. 11 g 802. 11 n Number of overlapping channels (varies by country) 3 @ 20 MHz In the U. S. 2. 4 GHz: 3 @ 20 MHz 1 @ 40 MHz 5 GHz: 12 @ 40 MHz © 2013 Pearson 63

6. 22: 802. 11 g and 802. 11 n Characteristic 802. 11 g 802. 11 n Rated Speed 54 Mbps 100 Mbps to 600 Mbps Actual throughput, 3 m 25 Mbps Closer to the rated speed Actual throughput, 30 m 20 Mbps Closer to the rated speed © 2013 Pearson 64

6. 22: 802. 11 g and 802. 11 n Characteristic Typical Maximum Distance © 2013 Pearson 802. 11 g 30 m (100 ft) 802. 11 n 70 m (230 ft) 65

6. 22: 802. 11 g and 802. 11 n Characteristic MIMO? 802. 11 g No 802. 11 n Yes MIMO is multiple input/multiple output Allows a sender to transmit two or more signals in the same channel simultaneously Uses multipath transmission as a benefit instead of a problem Also in 802. 11 ac © 2013 Pearson 66

6. 23: MIMO Access point transmits two signals in the same channel—one from Antenna A and one from Antenna B. These are called spatial streams. © 2013 Pearson 67

6. 23: MIMO The two signals arrive at different times at the two receiving antennas. Time differences allow them to be separated and understood. © 2013 Pearson 68

6. 23: MIMO (Multiple Input/Multiple Output) MIMO Benefits ◦ MIMO brings higher speeds because it can send more information in a channel. ◦ MIMO also brings longer propagation distances for technical reasons we will not discuss. © 2013 Pearson 69

802. 11 ac Standard is under development Products based on the draft standard are beginning to come to the market Uses OFDM in the 5 GHz band Channel bandwidth is 80 MHz or 160 MHz 6 channels at 80 MHz in the United States 3 channels at 160 MHz in the United States © 2013 Pearson 70

802. 11 ac Maximum Number of Spatial Streams ◦ 802. 11 n: 4 ◦ 802. 11 ac: 8 ◦ However, most products contain fewer antennas and so fewer spatial streams © 2013 Pearson 71

802. 11 ac Rated Speeds ◦ 433 Mbps to 6. 9 Gbps, depending on channel bandwidth and the number of spatial streams ◦ 867 Mbps and 1. 3 Gbps are common initially ◦ So called Gigabit 802. 11 © 2013 Pearson 72

802. 11 ad Gigabit speed but for very short distances ◦ Operates in the 60 GHz band (not 2. 4 or 5 GHz) ◦ Channel bandwidth is 2. 1 GHz! ◦ 3 possible channels in the United States, 4 in Europe ◦ Uses MIMO, beamforming and multiuser MIMO (later) ◦ 7 Gbps ◦ Replaces in-room cables for video ◦ Probably not able to work between rooms © 2013 Pearson 73

6. 24: Beam Forming Beamforming allows an access point to focus its transmissions and reception © 2013 Pearson 74

6. 24: Beam Forming Multiuser MIMO allows two wireless hosts to transmit at the same time. © 2013 Pearson 75

Multiuser MIMO Defined in 802. 11 n, but a single method was not defined Defined in 802. 11 ac, and there is a single standard, so adoption is more likely However, initial pre-standard 802. 11 ac products typically do not offer it. © 2013 Pearson 76

6. 25: Speed, Throughput, and Distance Throughput is aggregate throughput shared by all wireless hosts using an access point ◦ But only by the hosts that are actively trying to send and receive at the moment ◦ Example Access point has a rated speed of 125 Mbps Actual aggregate throughput is 100 Mbps There are 20 people served by the access point Five are transmitting at a moment What is their individual throughput? © 2013 Pearson 77

6. 25: Speed, Throughput, and Distance Throughput versus distance ◦ As distance increases, signals get weaker ◦ Wireless hosts must use slower but more reliable transmission processes ◦ A distant host will therefore take longer to transmit its message ◦ This will reduce everyone else’s individual throughput ◦ So try not to have distant users. Add more access points © 2013 Pearson 78

6. 25: Speed, Throughput, and Distance Speed Killers ◦ An 802. 11 b device connecting to an access point hurts all hosts ◦ It will transmit very slowly because 802. 11 b is limited to one or two megabits per second Now a rare problem © 2013 Pearson 79

6. 26: Trends White Space Operation ◦ One way to increase available bandwidth ◦ In the United States, broadcasters were required to vacate the UHF spectrum ◦ Some UHF channels have been auctioned off ◦ Unused channels in various bands (called white space) will be made available for unlicensed use ◦ May be used for WLAN operation © 2013 Pearson 80

6. 26: Trends Impending Spectrum Scarcity ◦ Traffic has been growing explosively ◦ Governments have made many more service bands available ◦ However, traffic may outstrip capacity ◦ This spectrum scarcity will increase prices and may ultimately limit growth © 2013 Pearson 81

6. 27: 802. 11 Wi-Fi Direct Wireless hosts communicate directly, without using an access point. Standard created by the Wi-Fi Alliance, not by the 802. 11 WG © 2013 Pearson 82

6. 28: Wireless Mesh Network There is no Ethernet network (distribution system) Frames are forwarded by access points and wireless hosts © 2013 Pearson 83

6. 28: Wireless Mesh Network Mesh networks are governed by 802. 11 s It is not a mature standard © 2013 Pearson 84

Where We Are Going? Chapter 7 ◦ Wireless LANs II. ◦ More on 802. 11 networks, including security and management. ◦ Other local wireless standards, including Bluetooth and near field communication © 2013 Pearson 85

© 2013 Pearson 86