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Overview of Wireless LANs

Use wireless transmission medium Issues of high prices, low data rates, occupational safety concerns, & licensing requirements now addressed key application areas: LAN extension cross-building interconnect ad hoc networking. Overview of Wireless LANs.

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Overview of Wireless LANs

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  1. Use wireless transmission medium Issues of high prices, low data rates, occupational safety concerns, & licensing requirements now addressed key application areas: LAN extension cross-building interconnect ad hoc networking Overview of Wireless LANs

  2. Throughput - efficient use wireless medium No of nodes - hundreds of nodes across multiple cells Connection to backbone LAN - using control modules Service area - 100 to 300 m low power consumption - for long battery life on mobiles Transmission robustness and security license-free operation handoff/roaming Dynamic configuration - addition, deletion, and relocation of end systems without disruption to users Wireless LAN Requirements

  3. Wireless LAN at a glance Airports On-off keying Old buildings Warehouses Hospitals Retail stores Pulse-position modulation Ad hoc Direct modulation Multi-subcarrier modulation Applications Topologies Transmission schemes Carrier modulation Infrastructure Wireless LAN Single-carrier modulation Spread spectrum Medium access control Transmission media Standards Direct Sequence CDMA Radio Frequency hopping FDMA CSMA/CD TDMA CSMA/CA IEEE ETSI (Hipper LAN) Infrared CDMA: Code Division Multiple Access ETSI: European Telecom. Standards Institute CSMA/CD: CSMA with Collision Detection FDMA: Frequency Division Multiple Access CSMA/CA: CSMA with Collision Avoidance TDMA: Time Division Multiple Access

  4. IEEE 802.11 Basic service set (cell) Set of stations using same MAC protocol Competing to access shared medium May be isolated May connect to backbone via access point (bridge) Extended service set Two or more BSS connected by distributed system Appears as single logic LAN to LLC level Wireless LANs

  5. Infrastructure mode All packets go through a base station Cards associate with a BSS (basic service set) Multiple BSSs can be linked into an Extended Service Set (ESS) Handoff to new BSS in ESS is pretty quick Moving to new ESS is slower, may require re-addressing Ad Hoc mode Cards communicate directly. Perform some, but not all, of the AP functions 802.11 modes

  6. Note A BSS without an AP is called an ad hoc network; a BSS with an AP is called an infrastructure network.

  7. Basic service sets (BSSs)

  8. Wireless connections: Access Point as a Bridge

  9. Extended service sets (ESSs)

  10. No transition Stationary or moves within direct communication range of single BSS BSS transition Moves between BSS within single ESS ESS transition From a BSS in one ESS to a BSS in another ESS Types of station

  11. MAC layers in IEEE 802.11 standard

  12. Distributed coordination function (DCF): CSMA/CA No collision detection After finding channel idle, the station waits for a time period called distributed interframe space (DIFS). Then sends RTS (Request to send) & waits for time period short interframe space (SIFS) Destination sends clear to send (CTS) to source station After time period of SIFS source sends data After successful transmission of data source waits for acknowledgement Point coordination function (PCF): It can be implemented in Infrastructure network. AP performs polling for all the stations. Media Access Control

  13. CSMA/CA flowchart

  14. CSMA/CA NAV (Network allocation vector)

  15. Frame format Duration of transmission Sequence control

  16. Subfields in FC field

  17. Control frames

  18. Addresses

  19. Addressing mechanisms

  20. Hidden station problem

  21. Note The CTS frame in CSMA/CA handshake can prevent collision from a hidden station.

  22. Use of handshaking to prevent hidden station problem

  23. Exposed station problem

  24. Use of handshaking in exposed station problem

  25. WIRELESS LAN :Physical layers

  26. Industrial, scientific, and medical (ISM) band

  27. Physical layer of IEEE 802.11 FHSS

  28. Physical layer of IEEE 802.11 DSSS

  29. Physical layer of IEEE 802.11 infrared

  30. BLUETOOTH Bluetooth is a wireless LAN technology designed to connect devices of different functions such as telephones, notebooks, computers, cameras, printers, coffee makers, and so on. A Bluetooth LAN is an ad hoc network, which means that the network is formed spontaneously.

  31. Short-range, high-data-rate wireless link for personal devices Originally intended to replace cables in a range of applications e.g., Phone headsets synchronization, remote controls Operates in 2.4 GHz ISM band Same as 802.11 Frequency Hopping Spread Spectrum across ~ 80 channels Bluetooth basics

  32. Protocol Architecture

  33. One master and up to 7 slave devices in each Piconet: Master controls transmission schedule of all devices in the Piconet Time Division Multiple Access (TDMA): Only one device transmits at a time Frequency hopping used to avoid collisions with other Piconets 79 physical channels of 1 MHz each, hop between channels 1600 times a sec Piconet Architecture

  34. Combine multiple Piconets into a larger Scatternet Device may act as master in one Piconet and slave in another Each Piconet using different FH schedule to avoid interference Can extend the range of Bluetooth, can route across Piconets Scatternets

  35. Frame format types

  36. L2CAP data packet format

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