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LectA. - 04/06/05

CDA 6505 Network Architecture and Client/Server Computing. Wireless LANs by Zornitza Genova Prodanoff. LectA..ppt - 04/06/05. Wireless LAN Applications. LAN Extension Cross-building interconnect Nomadic Access Ad hoc networking. 0 2. LAN Extension.

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LectA. - 04/06/05

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  1. CDA 6505 Network Architecture and Client/Server Computing Wireless LANs by Zornitza Genova Prodanoff LectA..ppt - 04/06/05

  2. Wireless LAN Applications • LAN Extension • Cross-building interconnect • Nomadic Access • Ad hoc networking 02

  3. 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

  4. Multiple-cell Wireless LAN

  5. 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 05

  6. 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 06

  7. 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 07

  8. Wireless LAN Requirements • Throughput • Number of nodes • Connection to backbone LAN • Service area • Battery power consumption • Transmission robustness and security • Collocated network operation • License-free operation • Handoff/roaming • Dynamic configuration 08

  9. Wireless LAN Categories • Infrared (IR) LANs • Spread spectrum LANs • Narrowband microwave 09

  10. 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 010

  11. 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 011

  12. IR Data Transmission Techniques • Directed Beam Infrared • Ominidirectional • Diffused 012

  13. Directed Beam Infrared • Used to create point-to-point links • Range depends on emitted power and degree of focusing • Focused IR data link can have range of kilometers • Cross-building interconnect between bridges or routers 013

  14. Ominidirectional • Single base station within line of sight of all other stations on LAN • Station typically mounted on ceiling • Base station acts as a multiport repeater • Ceiling transmitter broadcasts signal received by IR transceivers • IR transceivers transmit with directional beam aimed at ceiling base unit 014

  15. Diffused • All IR transmitters focused and aimed at a point on diffusely reflecting ceiling • IR radiation strikes ceiling • Reradiated omnidirectionally • Picked up by all receivers 015

  16. Spread Spectrum LAN Configuration • Multiple-cell arrangement (Figure 13.2) • Within a cell, either peer-to-peer or hub • Peer-to-peer topology • No hub • Access controlled with MAC algorithm • CSMA • Appropriate for ad hoc LANs 016

  17. Spread Spectrum LAN Configuration • Hub topology • Mounted on the ceiling and connected to backbone • May control access • May act as multiport repeater • Automatic handoff of mobile stations • Stations in cell either: • Transmit to / receive from hub only • Broadcast using omnidirectional antenna 017

  18. Narrowband Microwave LANs • Use of a microwave radio frequency band for signal transmission • Relatively narrow bandwidth • Licensed • Unlicensed 018

  19. Licensed Narrowband RF • Licensed within specific geographic areas to avoid potential interference • Motorola - 600 licenses in 18-GHz range • Covers all metropolitan areas • Can assure that independent LANs in nearby locations don’t interfere • Encrypted transmissions prevent eavesdropping 019

  20. Unlicensed Narrowband RF • RadioLAN introduced narrowband wireless LAN in 1995 • Uses unlicensed ISM spectrum • Used at low power (0.5 watts or less) • Operates at 10 Mbps in the 5.8-GHz band • Range = 50 m to 100 m 020

  21. IEEE 802 Protocol Layers

  22. IEEE 802.11 Wireless LANProtocol Architecture • Functions of physical layer: • Encoding/decoding of signals • Preamble generation/removal (for synchronization) • Bit transmission/reception • Includes specification of the transmission medium 022

  23. IEEE 802.11 Wireless LANProtocol 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 023

  24. 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 024

  25. 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 025

  26. 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 026

  27. 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 027

  28. Differences between LLC and HDLC • LLC uses asynchronous balanced mode of operation of HDLC (type 2 operation) • LLC supports unacknowledged connectionless service (type 1 operation) • LLC supports acknowledged connectionless service (type 3 operation) • LLC permits multiplexing by the use of LLC service access points (LSAPs) 028

  29. 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 029

  30. IEEE 802.11 Services

  31. Distribution of Messages Within a 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 031

  32. 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 032

  33. 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 033

  34. 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 034

  35. IEEE 802.11 Medium Access Control • MAC layer covers three functional areas: • Reliable data delivery • Access control • Security 035

  36. 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 036

  37. Access Control

  38. Medium Access Control Logic

  39. 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 039

  40. 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 040

  41. MAC Frame Format

  42. 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 042

  43. 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 043

  44. 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 044

  45. 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 045

  46. 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 046

  47. Management Frame Subtypes • Association request • Association response • Reassociation request • Reassociation response • Probe request • Probe response • Beacon 047

  48. Management Frame Subtypes • Announcement traffic indication message • Dissociation • Authentication • Deauthentication 048

  49. Wired Equivalent Privacy

  50. Authentication • Open system authentication • Exchange of identities, no security benefits • Shared Key authentication • Shared Key assures authentication 050

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