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LECTURE 7: Wireless LAN Standards

LECTURE 7: Wireless LAN Standards. CIS 472 Wireless Communications and Networks Summer 2013 Instructor: Dr. Song Xing Department of Information Systems California State University, Los Angeles. Outline. Wireless Propagation Problems IEEE 802.11a/b/g/n 802.11a/b/g/n Capacity

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LECTURE 7: Wireless LAN Standards

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  1. LECTURE 7:Wireless LAN Standards CIS 472 Wireless Communications and Networks Summer 2013 Instructor: Dr. Song Xing Department of Information Systems California State University, Los Angeles

  2. Outline Wireless Propagation Problems IEEE 802.11a/b/g/n 802.11a/b/g/n Capacity Other WLAN Standards Wireless and Personal Communications Systems

  3. Recall Wireless Propagation Problems 1. Electromagnetic Interference (EMI) is unwanted power at the same frequency from other devices. 2. Attenuation: signal gets weaker with distance 3. Shadow Zone (Dead Spot) are places the signal cannot penetrated because of obstacles in its path. Blocking Object Wireless and Personal Communications Systems

  4. Recall Wireless Propagation Problems (Cont.) Blocking Object Direct Signal Laptop 4. Multipath Interference Reflected Signal Direct and reflected signals may interfere. Most serious propagation problem at WLAN frequencies.

  5. Higher-frequency Signal Propagation Problem • Some Problems are Frequency-Dependent. • Higher-frequency signals attenuate faster. • Absorbed more rapidly by water in the air. • Higher-frequency signals are absorbed more readily by walls and other solid objects in their path. • Higher-frequency signals are blocked more by obstacles. • At lower frequencies, signal refract (bend) around obstacles like an ocean wave hitting a buoy. • At higher frequencies, signals do not refract; leave a complete shadow behind obstacles.

  6. Outline Wireless Propagation Problems IEEE 802.11a/b/g/n • IEEE 802.11b • IEEE 802.11a • IEEE 802.11g • IEEE 802.11n 802.11a/b/g/n Capacity Other WLAN Standards Wireless and Personal Communications Systems

  7. ISM and U-NII Unlicensed Service Bands WLANs operate in unlicensed service bands. You do not need a license to have or move your stations. Two unlicensed bands are widely used: the 2.4 GHz ISM band and the 5 GHz U-NII band 5 GHz has worse propagation characteristics 2.4 GHz has fewer available channels U-NII high band: 5.725 to 5.825 GHz is sometimes referred to as U-NII / ISM due to overlap with the ISM band. 7 Wireless and Personal Communications Systems Wireless and Personal Communications Systems

  8. WLAN 802.11x Physical Layers Wireless and Personal Communications Systems

  9. IEEE 802.11 Legacy • The original WLAN standard. • Released in 1997 as 802.11. • Defines a local area network that provides cable-free data access for clients • that are either mobile or in a fixed location. • At a rate of either 1 or 2 Mbps • Using either diffused infrared or RF transmission. • Operates at 2.4 GHz band. Wireless and Personal Communications Systems

  10. Outline Wireless Propagation Problems IEEE 802.11a/b/g/n • IEEE 802.11b • IEEE 802.11a • IEEE 802.11g • IEEE 802.11n 802.11a/b/g/n Capacity Other WLAN Standards Wireless and Personal Communications Systems

  11. IEEE 802.11b • Up to 11Mbps in the 2.4GHz band. • Products that adhere to this standard are considered "Wi-Fi Certified”. • Compatible with 802.11g. • Because 802.11b and 802.11g utilize same frequencies. • The indoor range is up to 38m and outdoor range is up to 140 m. • Signal range is best and is not easily obstructed. • Lowest cost. Wireless and Personal Communications Systems

  12. Disadvantage of IEEE 802.11b • Not interoperable with 802.11a. • Because 802.11a and 802.11b utilize different frequencies. • 14 channels available in the 2.4GHz band with only three non-overlapping channels. • Supports fewer simultaneous users. • 802.11b better serves the home market. Wireless and Personal Communications Systems

  13. IEEE 802.11b Physical Layer • 802.11b standard uses the Industrial, Scientific, and Medical (ISM) band • Specifies 14 available frequencies, beginning at 2.412 GHz and incrementing by 5 MHz. • Can transmit the data at 11, 5.5, 2, or 1 Mbps. • Dynamic rate shifting (DRS) allows the transmission rate to be adjusted to a lower rate when interference is experienced. Transmitter can adjust the transmission speed down from 11 Mbps to 5.5 Mbps, 2 Mbps, or 1Mbps in an electrically noisy environment. Wireless and Personal Communications Systems

  14. IEEE 802.11b/g ISM Channels • Channels 1-11 are used in North America. Wireless and Personal Communications Systems

  15. 2.4GHz Band Channels • There are 14 channels designated in the 2.4 GHz range spaced 5 MHz apart. • However, as the protocol requires 25 MHz of channel separation, adjacent channels overlap and will interfere with each other. • For example, a radio on channel 6 will overlap over to channels 4 and 5 on the lower side and channels 7 and 8 on the upper side. • As a result, there are only 3 non-overlapping channels in the 2.4GHz used by Wireless ISPs in the Americas: Channels 1, 6, and 11. Wireless and Personal Communications Systems

  16. 3 Non-overlapping Channels for 802.11b/g • Each AP can only use one of the three non-overlapping channels. A maximum of three APs can be installed in the same area, providing a maximum of 33Mbps of bandwidth (3 x 11Mbps) for 802.11b. Wireless and Personal Communications Systems

  17. Outline Wireless Propagation Problems IEEE 802.11a/b/g/n • IEEE 802.11b • IEEE 802.11a • IEEE 802.11g • IEEE 802.11n 802.11a/b/g/n Capacity Other WLAN Standards Wireless and Personal Communications Systems

  18. IEEE 802.11a • 802.11a standard maintains the same medium access control (MAC) layer functions as 802.11b WLANs. • Differences are confined to the physical layer. • Up to 54Mbps in the 5 GHz band. • Products that adhere to this standard are considered "Wi-Fi Certified." • Less potential for RF interference than 802.11b and 802.11g. • Regulated frequencies prevent signal interference from other devices (2.4GHz cordless phone, microwave ovens, Bluetooth devices, etc). Wireless and Personal Communications Systems

  19. IEEE 802.11a (Cont.) • 802.11a achieves its increase in speed and flexibility over 802.11b through: • A higher frequency band: 5GHz band • More transmission channels: 24 non-overlapping channels • Supports more simultaneous users. • Its multiplexing technique • OFDM • A more efficient error-correction scheme • Better than 802.11b/g at supporting multimedia voice, video and large-image applications in densely populated user environments. Wireless and Personal Communications Systems

  20. Disadvantages of IEEE 802.11a • Relatively shorter range than 802.11b and 802.11g. • Higher frequency limits the range: attenuation • 802.11a signals cannot penetrate as far as those for 802.11b because they are absorbed more readily by walls and other solid objects in their path. • Approximately up to 35m indoor range. • Compared to 38m for an 802.11b or 802.11g WLAN. • Shorter range signal is more easily obstructed. Wireless and Personal Communications Systems

  21. Disadvantages of IEEE 802.11a (Cont.) • Not interoperable with 802.11b and 802.11g. • Because 802.11a and 802.11b/g utilize different frequencies • Due to its higher cost, 802.11a fits predominately in the business market. Wireless and Personal Communications Systems

  22. U-NII Frequency Band and Power Limit • IEEE 802.11a uses the Unlicensed National Information Infrastructure (U-NII) band • Intended for devices that provide short-range, high-speed wireless digital communications • U-NII spectrum is segmented into four bands • Each band has a maximum power limit

  23. 802.11a Channel Allocation • With 802.11b and 802.11g, the available frequency spectrum is divided into 11 channels in the US. • Only three 20MHz wide non-overlapping channels are available for simultaneous operation. • CH#1 (2.412 GHz), CH#6(2.437 GHz), CH#11(2.462GHz) • In 802.11a, 24 non-overlapping channels (20MHz wide each) operate simultaneously in the US. • Lower UNII band (5.15 to 5.25 GHz): 4 • Middle UNII band (5.25 to 5.35 GHz): 4 • High UNII band (5.47 to 5.725 GHz): 11 • Upper UNII band (5.725 to 5.825GHz): 4 • ISM Band (5.825GHz): 1 Wireless and Personal Communications Systems

  24. 802.11a Channel Allocation (Cont.) • As result, we can have up to 24 Access Points set to different channels in the same area without them interfering with each other. • This makes access point channel assignment much easier and significantly increases the throughput the wireless LAN can deliver within a given area. • Provide a maximum of 1.296 Gbps of aggregated bandwidth (24 x 54Mbps). • In addition, RF interference is much less likely because of the less-crowded 5 GHz band. Wireless and Personal Communications Systems

  25. 802.11a Channels in the U.S. Wireless and Personal Communications Systems

  26. Multipath Distortion Problem and the Solution • Multipath distortion • Receiving device gets the signal from several different directions at different times. • Must wait until all reflections are received • 802.11a solves this problems using OFDM. • Orthogonal Frequency Division Multiplexing (OFDM) • Splits a long high-speed digital signal into several slower signals running in parallel. • Sends the transmissions in parallel across several lower-speed sub-channels. Wireless and Personal Communications Systems

  27. Orthogonal Frequency Division Multiplexing (OFDM) • Both a modulation and multiplexing technique. • Handle transmission problems: cross-talk and multi-path propagation. • Divides a 20 MHz RF channel into 52 narrowband sub-channels and then splits a data signal into 48 separate carriers (48 of the 52 sub-channels are used for data). • Sub-channels overlap and signal bits are sent in parallel using frequency division multiplexing (FDM). Wireless and Personal Communications Systems

  28. One Non-overlapping Channel Divided into 52 Sub-channels A 20 MHz-wide non-overlapping channel supports 52 carrier signals.

  29. OFDM: One Data Transmission Using Multiple Sub-channels Wireless and Personal Communications Systems

  30. FDM and OFDM Comparison • OFDM: All the sub-channels are used by one data source transmission at a given time. • In OFDM, the sub-carrier frequencies are chosen so that the sub-carriers are orthogonal to each other, meaning that cross-talk between the sub-channels is eliminated and inter-carrier guard bands are not required. Unlike conventional FDM, a separate filter for each sub-channel is not required. • The sub-channels are overlapped in OFDM, which results in saving the available bandwidth. Wireless and Personal Communications Systems

  31. OFDM: a higher Throughput • OFDM results in a higher throughput and a faster WLAN. • The transmissions are sent in parallel, the total throughput is increased. • On the receiver, the time spent waiting for reflections to arrive is less with OFDM than with a single channel transmission. Wireless and Personal Communications Systems

  32. Outline Wireless Propagation Problems IEEE 802.11a/b/g/n • IEEE 802.11b • IEEE 802.11a • IEEE 802.11g • IEEE 802.11n 802.11a/b/g/n Capacity Other WLAN Standards Wireless and Personal Communications Systems

  33. IEEE 802.11g • Up to 54Mbps in the 2.4GHz band. • Products that adhere to this standard are considered "Wi-Fi Certified”. • Compatible with 802.11b. • Because 802.11g and 802.11b utilize same frequencies. • Improved security enhancements over 802.11. • Signal range is best and is not easily obstructed. Wireless and Personal Communications Systems

  34. IEEE 802.11g (Cont.) • 14 channels available in the 2.4GHz band with only three non-overlapping channels. • Costs more than 802.11b. • 802.11g and 802.11b NICs can operate together with an 802.11g access point. • However, the throughput of 802.11g stations falls dramatically. • 802.11g NICs will drop down to 802.11b operation when they associate with an 802.11b access point. Wireless and Personal Communications Systems

  35. 802.11g Physical Layer • Follows the same specifications for 802.11b. • Standard outlines two mandatory transmission modes along with two optional modes • Mandatory transmission modes • Same mode used by 802.11b and must support the rates of 1, 2, 5.5, and 11 Mbps • Same OFDM mode used by 802.11a but in the same frequency band used by 802.11b, and support the rates of 6, 9, 12, 18, 24, 36, 48, and 54 Mbps. • However, number of channels available with 802.11g is three • Compared with 24 channels for 802.11a Wireless and Personal Communications Systems

  36. Outline Wireless Propagation Problems IEEE 802.11a/b/g/n • IEEE 802.11b • IEEE 802.11a • IEEE 802.11g • IEEE 802.11n 802.11a/b/g/n Capacity Other WLAN Standards Wireless and Personal Communications Systems

  37. IEEE 802.11n • Significantly improve network throughput and range over previous standards. • Ratified end of 2009 • Uses multiple radios and antennas in each device • Rated speeds of 100 Mbps to 600 Mbps. • Up to 70 m indoor rang, 250 m outdoor range. • Operate in both the 2.4 GHz and 5 GHz bands. • Backward compatible with 802.11g, 802.11b and 802.11a. Wireless and Personal Communications Systems

  38. Throughout Increasing Techniques used in 802.11n • Channel bonding • MIMO (Multiple Input, Multiple Output) • Frame aggregation • Utilize dual-radio frequency band Wireless and Personal Communications Systems

  39. Multiple-Input Multiple-Output (MIMO) • A wireless device with multiple receive (Rx) antennas can improve transmissions by either: • Selecting the stronger incoming signal • Combining the individual signals at the receiver • IEEE 802.11a/b/g devices can only use a single antenna • Wireless system that uses a single antenna is called a single-input single-output (SISO) system • Has only one radio chain (radio with supporting infrastructure such as devices to amplify the signal or convert an analog signal to a digital signal)

  40. Antenna Diversity • Many 802.11a/b/g APs have two antennas. • Such APs employ Antenna diversity to improve the reception. • Antenna diversity: Ability of an access point to examine multiple copies of a received transmission and then select the best signal. • Antenna diversity can also be used in transmitting. • Transmit diversity: the AP can transmit on the antenna that most recently received the strongest incoming signal

  41. SISO Radio Chain

  42. What is MIMO? • Multiple-Input Multiple Output (MIMO): system that uses one radio chain for each antenna • that each antenna can simultaneously transmit and receive signals • IEEE made MIMO the heart of 802.11n • Estimated the MIMO alone contributes 40% to the increase in speed • 802.11n standard is sometimes called HT (MIMO) for High Throughput Multiple-Input Multiple-Output

  43. MIMO Radio Chain

  44. MIMO Signal Processing Techniques • Signaling process techniques: • Spatial diversity • Spatial multiplexing • Maximal ratio combining • Transmit beam forming • MIMO can take advantage of these signaling process techniques to create high throughput

  45. Spatial Diversity • Spatial diversity: a MIMO technique of sending the same transmission out from different antennas that will take different paths • Send redundant data signals in parallel • Can increase reliability of an RF signal • Each transmission will take different paths (called spatial paths) • It is unlikely all paths will degrade the signal in the same way

  46. Spatial Diversity (cont.) Same signals (Signal 1, Signal 2) can be sent at the same time from different antennas. The signals arrive at slightly different times at the receiving antennas. The receiver uses multipath time differences to distinguish between them.

  47. Spatial Multiplexing • Spatial diversity improves reliability and range but it does not increase speed • Instead of sending the same data out through multiple antennas, spatial multiplexing first splits up the data and sends it out over multiple antennas • Send independent data signals in parallel • Receiving antenna merges data back together • Increases speed without the need for any additional power or bandwidth

  48. Spatial Multiplexing

  49. Spatial Multiplexing (Cont.) Figure 8-13 Spatial multiplexing sending multiple (spatial) streams of data

  50. Spatial Multiplexing (Cont.) • Up to four transmitters and four receivers • Max transmission speed of 600 Mbps • Configurations such as 2x3 (2 transmitters and 3 receivers) and 3x3 (3 of each) exist Figure 8-14 802.11n MIMO devices

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