1 / 36

Lecture 8

Lecture 8. Wireless Local Area Network. Why Wireless. Mobility : Wireless LAN systems can provide LAN users with access to real-time information anywhere in their organization. This mobility supports productivity and service opportunities not possible with wired networks.

sidney
Download Presentation

Lecture 8

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Lecture 8 Wireless Local Area Network

  2. Why Wireless • Mobility: Wireless LAN systems can provide LAN users with access to real-time information anywhere in their organization. This mobility supports productivity and service opportunities not possible with wired networks. • Installation Speed and Simplicity: Installing a wireless LAN system can be fast and easy and can eliminate the need to pull cable through walls and ceilings. • Installation Flexibility: Wireless technology allows the network to go where wire cannot go. • Reduced Cost-of-Ownership: While the initial investment required for wireless LAN hardware can be higher than the cost of wired LAN hardware, overall installation expenses and life-cycle costs can be significantly lower. Long-term cost benefits are greatest in dynamic environments requiring frequent moves and changes. • Scalability: Wireless LAN systems can be configured in a variety of topologies to meet the needs of specific applications and installations. Configurations are easily changed and range from peer-to-peer networks suitable for a small number of users to full infrastructure networks of thousands of users that enable roaming over a broad area.

  3. Wireless LAN Technologies (I) • Considerations for choosing IR technology • Advantages • No government regulations controlling use • Immunity to electro-magnetic (EMI) and RF interference • Disadvantages • Generally a short-range technology (30 – 50 ft. radius under ideal conditions • Signal cannot penetrate solid objects • Signal affected by ice, snow, light, fog • Dirt can interfere with infrared

  4. Wireless LAN Technologies (II) • Considerations for choosing Narrowband (UHF) technology • Advantages • Longest range • Low cost solution for large sites with low medium data throughput requirements • Disadvantages • Low throughput • No multi-vendor interoperability • Interference potential • RF site licence required for protected bands • Large radios and antennas increase wireless client size

  5. Wireless LAN Technologies (III) Wireless range for UHF compared to other RF technologies in a typical warehouse environment and outdoors in open area

  6. Wireless LAN Technologies (IV) Wireless Technology Data Rates 400 MHz UHF 4.8 - 19.2 Kbps 900 MHz Spread Spectrum 100 - 400 Kbps 2.4 GHz Spread Spectrum 1 - 2 Mbps 2.4 GHz More than10 Mbps 5.7 GHz Future More than 20 Mbps

  7. Wireless LAN Technologies (V) Considerations for choosing 900 MHz technology Considerations for choosing 2.4 GHz technology

  8. Wireless LAN Considerations (I) Business factors for selecting a WLAN vendor

  9. Wireless LAN Considerations (II) Technical factors for selecting a WLAN solution

  10. Multiplexing • Capacity of transmission medium usually exceeds capacity required for transmission of a single signal • Multiplexing - carrying multiple signals on a single medium • More efficient use of transmission medium

  11. Multiplexing

  12. Reasons for Widespread Use of Multiplexing • Cost per kbps of transmission facility declines with an increase in the data rate • Cost of transmission and receiving equipment declines with increased data rate • Most individual data communicating devices require relatively modest data rate support

  13. Multiple Access • Three basic methods for multiplexing data from mobile devices into a given frequency spectrum • Frequency Division Multiple Access (FDMA) • Time Division Multiple Access (TDMA) • Code Division Multiple Access (CDMA)

  14. Frequency Division Multiple Access (FDMA) • Takes advantage of the fact that the useful bandwidth of the medium exceeds the required bandwidth of a given signal • Each source is given its own frequency band and can use it permanently. • Multiple sources send on different frequency bands at the same time.

  15. Time Division Multiple Access (TDMA) (I) • Takes advantage of the fact that the achievable bit rate of the medium exceeds the required data rate of a digital signal • Each source is given certain time intervals during which it can use all the bandwidth. • Multiple sources send at different points in time on the same frequency bandwidth.

  16. Time Division Multiple Access (TDMA) (II) • Synchronous TDMA Fixed bandwidth allocation. Good for constant bit-rate streams. Inefficient for variable bit-rate, bursty streams. Simple, cost effective. • Asynchronous TDMA On demand bandwidth allocation. Much more efficient for variable bit-rate, bursty streams. Various solutions.

  17. Code Division Multiple Access (CDMA) • Multiple sources send with different data encoding, at the same time, on the same bandwidth. Each source encodes its data using a different code, from a set of orthogonal codes. • Data divided into small packets and distributed into a predetermined pattern across the frequency spectrum. • Each pattern designed by a code known as Pseudo-random Noise (PN) code. • PN corresponds to a time slot number in TDMA or a carrier frequency in FDMA.

  18. IEEE 802.11 Wireless LAN Standard

  19. Protocol Architecture • Functions of physical layer: • Encoding/decoding of signals • Preamble generation/removal (for synchronization) • Bit transmission/reception • Includes specification of the transmission medium

  20. Protocol Architecture • Functions of medium access control (MAC) layer: • On transmission, assemble data into a frame with address and error detection fields • On reception, disassemble frame and perform address recognition and error detection • Govern access to the LAN transmission medium • Functions of logical link control (LLC) Layer: • Provide an interface to higher layers and perform flow and error control

  21. Separation of LLC and MAC • The logic required to manage access to a shared-access medium not found in traditional layer 2 data link control • For the same LLC, several MAC options may be provided

  22. MAC & LLC Frames Format • Characteristics of LLC not shared by other control protocols: • Must support multi access, shared-medium nature of the link • Relieved of some details of link access by MAC layer • MAC control • Contains Mac protocol information • Destination MAC address • Destination physical attachment point • Source MAC address • Source physical attachment point • CRC • Cyclic redundancy check

  23. LLC Services • Unacknowledged connectionless service • No flow- and error-control mechanisms • Data delivery not guaranteed • Connection-mode service • Logical connection set up between two users • Flow- and error-control provided • Acknowledged connectionless service • Cross between previous two • Datagrams acknowledged • No prior logical setup

  24. IEEE 802.11 Services

  25. IEEE 802.11 Architecture • Distribution system (DS) • Access point (AP) • Basic service set (BSS) • Stations competing for access to shared wireless medium • Isolated or connected to backbone DS through AP • Extended service set (ESS) • Two or more basic service sets interconnected by DS

  26. Distribution of Messages Within a Distribution Service (DS) • Distribution service • Used to exchange MAC frames from station in one BSS to station in another BSS • Integration service • Transfer of data between station on IEEE 802.11 LAN and station on integrated IEEE 802.x LAN

  27. Transition Types Based On Mobility • No transition • Stationary or moves only within BSS • Basic Service Set (BSS) transition • Station moving from one BSS to another BSS in same ESS • Extended Service Set (ESS) transition • Station moving from BSS in one ESS to BSS within another ESS

  28. IEEE 802.11 Medium Access Control • MAC layer covers three functional areas: • Reliable data delivery • Access control • Security – WEP (Wired Equivalent Privacy), WPA (Wi-Fi Protected Access), WPA-PSK (Wi-Fi Protected Access with the PreShared Key)

  29. Reliable Data Delivery • More efficient to deal with errors at the MAC level than higher layer (such as TCP) • Frame exchange protocol • Source station transmits data • Destination responds with acknowledgment (ACK) • If source doesn’t receive ACK, it retransmits frame • Four frame exchange • Source issues request to send (RTS) • Destination responds with clear to send (CTS) • Source transmits data • Destination responds with ACK

  30. Access Control

  31. MAC Frame Format & Fields • Frame Control – frame type, control information • Duration/connection ID – channel allocation time • Addresses – context dependant, types include source and destination • Sequence control – numbering and reassembly • Frame body – MAC Service Data Unit (MSDU) or fragment of MSDU • Frame check sequence – 32-bit Cyclic Redundancy Check (CRC)

  32. Frame Control Fields • Power management – 1 if transmitting station is in sleep mode • More data – Indicates that station has more data to send • WEP (Wired Equivalent Privacy) – 1 if wired equivalent protocol is implemented • Order – 1 if any data frame is sent using the Strictly Ordered service • Protocol version – 802.11 version • Type – control, management, or data • Subtype – identifies function of frame • To DS – 1 if destined for DS • From DS – 1 if leaving DS • More fragments – 1 if fragments follow • Retry – 1 if retransmission of previous frame

  33. Physical Media Defined by Original 802.11 Standard • Direct-sequence spread spectrum • Operating in 2.4 GHz ISM band • Data rates of 1 and 2 Mbps • Frequency-hopping spread spectrum • Operating in 2.4 GHz ISM band • Data rates of 1 and 2 Mbps • Infrared • 1 and 2 Mbps • Wavelength between 850 and 950 nm

  34. IEEE 802.11a and IEEE 802.11b • IEEE 802.11a • Makes use of 5-GHz band • Provides rates of 6, 9 , 12, 18, 24, 36, 48, 54 Mbps • Uses orthogonal frequency division multiplexing (OFDM) • Subcarrier modulated using BPSK, QPSK, 16-QAM or 64-QAM • IEEE 802.11b • Provides data rates of 5.5 and 11 Mbps • Complementary code keying (CCK) modulation scheme

  35. Sources • From: • William Stallings - Wireless communications and networks /Second Edition, Prentice Hall 2005 • Chapter 2 – 2.5 • Chapter 14 • Whitepapers: • Wireless LAN Concepts – available at: http://www.utdallas.edu/ir/wlans/whitepapers/wlanconcepts.pdf • Guide to Wireless LAN Technologies – available at: http://www.utdallas.edu/ir/wlans/whitepapers/wlan_wp.pdf • Wireless Networking – available at: http://www.utdallas.edu/ir/wlans/whitepapers/WLANA.pdf • Wireless LAN Evaluation Guide – available at: http://www.utdallas.edu/ir/wlans/whitepapers/wlangde.pdf • What is a Wireless LAN – available at: http://www.utdallas.edu/ir/wlans/whitepapers/whatwlan.pdf • In-building Wireless LANs – available at: http://www.utdallas.edu/ir/wlans/whitepapers/1862.pdf

More Related