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Wireless Communications

Wireless Communications. High-Speed WLANs and WLAN Security. Objectives. Describe how IEEE 802.11a networks function and how they differ from 802.11 networks Outline how 802.11g enhances 802.11b networks Discuss new and future standards and how they improve 802.11 networks. IEEE 802.11a.

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Wireless Communications

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  1. Wireless Communications High-Speed WLANs and WLAN Security

  2. Objectives • Describe how IEEE 802.11a networks function and how they differ from 802.11 networks • Outline how 802.11g enhances 802.11b networks • Discuss new and future standards and how they improve 802.11 networks

  3. 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 • 802.11a achieves its increase in speed and flexibility over 802.11b through: • A higher frequency band • More transmission channels • Its multiplexing technique • A more efficient error-correction scheme

  4. U-NII Frequency Band • 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 • Outside the United States • 5 GHz band is allocated to users and technologies other than WLANs

  5. U-NII Frequency Band (continued)

  6. U-NII Frequency Band (continued)

  7. U-NII Frequency Band (continued) • Channel allocation • With 802.11b, the available frequency spectrum is divided into 11 channels in the United States • Only three non-overlapping channels are available for simultaneous operation • In 802.11a, eight frequency channels operate simultaneously • In the Low Band (5.15 to 5.25 GHz) and Middle Band (5.25 to 5.35 GHz) • Within each frequency channel there is a 20 MHz-wide channel that supports 52 carrier signals

  8. U-NII Frequency Band (continued)

  9. Orthogonal Frequency Division Multiplexing • 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 high-speed digital signal into several slower signals running in parallel • Sends the transmission in parallel across several lower-speed channels

  10. Orthogonal Frequency Division Multiplexing (continued) • OFDM uses 48 of the 52 subchannels for data • Modulation techniques • At 6 Mbps, phase shift keying (PSK) • At 12 Mbps, quadrature phase shift keying (QPSK) • At 24 Mbps, 16-level quadrature amplitude modulation (16-QAM) • At 54 Mbps, 64-level quadrature amplitude modulation (64-QAM) • Turbo mode or 2X mode • Few vendors have implemented higher speeds

  11. Orthogonal Frequency Division Multiplexing (continued)

  12. Orthogonal Frequency Division Multiplexing (continued)

  13. Orthogonal Frequency Division Multiplexing (continued)

  14. IEEE 802.11g • Specifies that it operates in the same frequency band as 802.11b

  15. 802.11g PHY 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 • Number of channels available with 802.11g is three • Compared with eight channels for 802.11a

  16. 802.11g PHY Layer (continued) • When both 802.11b and 802.11g devices share the same network • Standard defines how the frame header is transmitted at 1 or 2 Mbps using DSSS • The optional 22 Mbps rate is achieved by using the PBCC encoding method • Modulation is binary phase shift keying (BPSK)

  17. Other WLAN Standards • Future of WLANs will include: • Additional standards that are currently under development by the IEEE • New standards that are just beginning to appear in new equipment

  18. IEEE 802.11e • Approved for publication in November 2005 • Defines enhancements to the MAC layer of 802.11 • To expand support for LAN applications that require Quality of Service (QoS) • 802.11e allows the receiving device to acknowledge after receiving a burst of frames • Enables prioritization of frames in distributed coordinated function (DCF) mode

  19. IEEE 802.11n • Aimed at providing data rates higher than 100 Mbps using the 2.4 GHz ISM band • Bonds two 802.11 2.4 GHz ISM channels together • Uses OFDM to send two data streams at 54 Mbps • Implements multiple-in, multiple-out (MIMO)technology • Uses multiple antennas and also uses the reflected signals (multipath) • To extend the range of the WLAN • Interference with other WLANs can be a big problem

  20. IEEE 802.11r • Amount of time required by 802.11 devices to associate and disassociate • It is in the order of hundreds of milliseconds • Support voice over wireless LAN (VoWLAN) in a business environment with multiple access points • 802.11 standard needs a way to provide quicker handoffs • 802.11 MAC protocol • Does not allow a device to find out if the necessary QoS resources are available at a new AP • 802.11r is designed to resolve these issues • In addition to security concerns regarding the handoff • 802.11r is expected to enhance the convergence of wireless voice, data, and video

  21. IEEE 802.11s • Hard-to-do task • Deploy a wireless network over the entire downtown area of a medium-sized city • Provide seamless connectivity to all city employees • Ideal solution • Connect the wireless APs to each other over the wireless communications channels • 802.11s will provide the solution when it is ratified by the IEEE • Which is expected to happen in 2008

  22. Summary • Operating in the 2.4 GHz ISM frequency range, 802.11b has a maximum data rate of 11 Mbps • The 802.11a has a maximum rated speed of 54 Mbps • IEEE 802.11a networks use the Unlicensed National Information Infrastructure (U-NII) band • In 802.11a, eight frequency channels can operate simultaneously • IEEE 802.11b WLAN reception is slowed down by multipath distortion • 802.11a solves this problem using OFDM

  23. Summary (continued) • OFDM uses 48 of the 52 subchannels for data, while the remaining four are used for error correction • Number of errors in an 802.11a transmission is significantly reduced • The 802.11a standard made changes only to the physical layer (PHY layer) • Of the original 802.11 and 802.11b standard • 802.11g preserves the features of 802.11b but increases the data transfer rates to those of 802.11a • 802.11e standard adds QoS to 802.11 standards

  24. Summary (continued) • 802.11n is a proposed standard that will increase the speed of WLANs to 108 Mbps • 802.11r is a proposed standard for fast roaming • HiperLAN/2 is a high-speed WLAN specification that is similar to the IEEE 802.11a

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