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Lecture 7: WLAN

Lecture 7: WLAN. Anders Västberg vastberg@kth.se 08-790 44 55. Slides are a selection from the slides from chapter 13, and 14 from: http://williamstallings.com/Wireless/Wireless2e.html. Wireless LAN. Advantages Flexibility Planning Robustness Cost Disadvantages Quality of service

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Lecture 7: WLAN

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  1. Lecture 7: WLAN Anders Västberg vastberg@kth.se 08-790 44 55 Slides are a selection from the slides from chapter 13, and 14 from: http://williamstallings.com/Wireless/Wireless2e.html

  2. Wireless LAN • Advantages • Flexibility • Planning • Robustness • Cost • Disadvantages • Quality of service • Safety and security

  3. Wireless LAN Categories • Infrared (IR) LANs • Spread spectrum LANs

  4. Strengths of Infrared Over Microwave Radio • Spectrum for infrared virtually unlimited • Possibility of high data rates • Infrared spectrum unregulated • Equipment inexpensive and simple • Reflected by light-colored objects • Ceiling reflection for entire room coverage • Doesn’t penetrate walls • More easily secured against eavesdropping • Less interference between different rooms

  5. Drawbacks of Infrared Medium • Indoor environments experience infrared background radiation • Sunlight and indoor lighting • Ambient radiation appears as noise in an infrared receiver • Transmitters of higher power required • Limited by concerns of eye safety and excessive power consumption • Limits range

  6. Spread Spectrum LAN Configuration • Advantages: • Cover large areas • Penetrate walls • Higher transmission rates • Disavantages: • Interference with other senders or electrical devices • Limited free spectrums

  7. Wireless LAN Applications • LAN Extension • Cross-building interconnect • Nomadic Access • Ad hoc networking

  8. Wireless LAN Configurations

  9. LAN Extension • Wireless LAN linked into a wired LAN on same premises • Wired LAN • Backbone • Support servers and stationary workstations • Wireless LAN • Stations in large open areas • Manufacturing plants, stock exchange trading floors, and warehouses

  10. Cross-Building Interconnect • Connect LANs in nearby buildings • Wired or wireless LANs • Point-to-point wireless link is used • Devices connected are typically bridges or routers

  11. Nomadic Access • Wireless link between LAN hub and mobile data terminal equipped with antenna • Laptop computer or notepad computer • Uses: • Transfer data from portable computer to office server • Extended environment such as campus

  12. Ad Hoc Networking • Temporary peer-to-peer network set up to meet immediate need • Example: • Group of employees with laptops convene for a meeting; employees link computers in a temporary network for duration of meeting

  13. IEEE 802.11 • Simple and robust WLAN • Time-bounded and asynchronous services • Multiple physical layers • Power management • World wide operation

  14. IEEE 802.11 Architecture • Stations (STA) • 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 • Identified by an ESSID

  15. IEEE 802 Protocol Layers

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

  17. 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

  18. 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

  19. MAC Frame Format • MAC control • Contains Mac protocol information • Destination MAC address • Destination physical attachment point • Source MAC address • Source physical attachment point • CRC • Cyclic redundancy check

  20. Logical Link Control • Characteristics of LLC not shared by other control protocols: • Must support multiaccess, shared-medium nature of the link • Relieved of some details of link access by MAC layer

  21. 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

  22. IEEE 802.11 Services

  23. Transition Types Based On Mobility • No transition • Stationary or moves only within BSS • BSS transition • Station moving from one BSS to another BSS in same ESS • ESS transition • Station moving from BSS in one ESS to BSS within another ESS

  24. Association-Related Services • Association • Establishes initial association between station and AP • Reassociation • Enables transfer of association from one AP to another, allowing station to move from one BSS to another • Disassociation • Association termination notice from station or AP

  25. Access and Privacy Services • Authentication • Establishes identity of stations to each other • Deathentication • Invoked when existing authentication is terminated • Privacy • Prevents message contents from being read by unintended recipient

  26. IEEE 802.11 Medium Access Control • MAC layer covers three functional areas: • Reliable data delivery • Access control • Security

  27. 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

  28. Access Control

  29. Access Control

  30. Medium Access Control Logic

  31. Access Control

  32. Interframe Space (IFS) Values • Short IFS (SIFS) • Shortest IFS • Used for immediate response actions • Point coordination function IFS (PIFS) • Midlength IFS • Used by centralized controller in PCF scheme when using polls • Distributed coordination function IFS (DIFS) • Longest IFS • Used as minimum delay of asynchronous frames contending for access

  33. IFS Usage • SIFS • Acknowledgment (ACK) • Clear to send (CTS) • Poll response • PIFS • Used by centralized controller in issuing polls • Takes precedence over normal contention traffic • DIFS • Used for all ordinary asynchronous traffic

  34. MAC Frame Format

  35. MAC Frame 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 – MSDU or fragment of MSDU • Frame check sequence – 32-bit CRC

  36. Frame Control Fields • 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

  37. Frame Control Fields • Power management – 1 if transmitting station is in sleep mode • More data – Indicates that station has more data to send • WEP – 1 if wired equivalent protocol is implemented • Order – 1 if any data frame is sent using the Strictly Ordered service

  38. Control Frame Subtypes • Power save – poll (PS-Poll) • Request to send (RTS) • Clear to send (CTS) • Acknowledgment • Contention-free (CF)-end • CF-end + CF-ack

  39. Data Frame Subtypes • Data-carrying frames • Data • Data + CF-Ack • Data + CF-Poll • Data + CF-Ack + CF-Poll • Other subtypes (don’t carry user data) • Null Function • CF-Ack • CF-Poll • CF-Ack + CF-Poll

  40. Management Frame Subtypes • Association request • Association response • Reassociation request • Reassociation response • Probe request • Probe response • Beacon

  41. Management Frame Subtypes • Announcement traffic indication message • Dissociation • Authentication • Deauthentication

  42. Authentication • Open system authentication • Exchange of identities, no security benefits • Shared Key authentication • Shared Key assures authentication

  43. 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

  44. 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

  45. IEEE 802.11g and IEEE 802.11n • IEEE 802.11g: Provides data rates up to 108 Mbps and is compatible with 802.11b • IEEE 802.11n: Even higher data rates, up to 600 Mbps.

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