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WLAN (Wireless Local Area Networks)

WLAN (Wireless Local Area Networks). By: Muhammad Hanif. Wireless LANs. Chapter 17 – Wireless LAN s. Eighth Edition by William Stallings Lecture slides by Lawrie Brown. Quotes of the day. GREED AND FAITH CAN NEVER CO-EXIST IN THE HUMAN HEART.

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WLAN (Wireless Local Area Networks)

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  1. WLAN (Wireless Local Area Networks) By: Muhammad Hanif

  2. Wireless LANs Chapter 17 – Wireless LANs Eighth Edition by William Stallings Lecture slides by Lawrie Brown

  3. Quotes of the day • GREED AND FAITH CAN NEVER CO-EXIST IN THE HUMAN HEART. • Hadith of An-Nasa’i on the authority of Abu Hurayrah

  4. Agenda WLAN Categories of Wireless Networks Wi-Fi Wireless Physical Layer Two Additional Problems CSMA / CA

  5. Wireless Local Area Networks A wireless local area network (WLAN) links two or more devices using some wireless distribution method. When two or more than two devices connect through air medium is called Wireless LAN. The laptop computers and other mobile devices (PDAs and cell phones) created high level demand for wireless local area networking. Companies jumped in, quickly developing wireless products.

  6. Categories of Wireless Networks Infrastructure Wireless :: base station network is connected to the wired Internet. Ad hoc Wireless :: wireless nodes communicate directly with one another. MANETs (Mobile Ad Hoc Networks) :: ad hoc nodes are mobile.

  7. Wireless LANs Figure(a) Wireless networking with a base station. (b) Ad hoc networking.

  8. Wi-Fi (wireless Fidelity) • Wi-Fi is a technology that allows an electronic device to exchange data or connect to the internet wirelessly using 2.4 GHz and 5.5 GHz radio waves.

  9. Wireless Physical Layer • Physical layer of WLANs include • 1997: 802.11 infrared • 1999: 802.11a OFDM and 802.11b HR-DSSS • 2001: 802.11g OFDM • And more …….. • 802.11 Infrared • Two capacities 1 Mbps or 2 Mbps. • Range is 10 to 20 meters and cannot pass through walls. • 802.11a OFDM (Orthogonal Frequency Divisional Multiplexing) • Data rate of 54Mbps and using 5.5 GHz band. • More difficulty penetrating walls. • Range is 35m (115 ft) indoor and 100m (330 ft) outdoor.

  10. Wireless Physical Layer • 802.11b HR-DSSS (High Rate Direct Sequence Spread Spectrum) • 11b approved and hit the market before 11a. • Up to 11 Mbps in 2.4 GHz band. • Range is 38m (125 ft) indoor and 125m (412 ft) outdoor. • 11b and 11a are incompatible!! • 802.11g OFDM(Orthogonal Frequency Division Multiplexing) • An attempt to combine the best of both 802.11a and 802.11b. • Supports bandwidths up to 54 Mbps. • Using 2.4 GHz frequency. • Is compatible with 802.11b. • Range is 38m (125 ft) indoor and 125m (412 ft) outdoor.

  11. Two Additional Problems • Using CSMA/CD Technology the following two problems occur during the transmission. • Two additional problems: • Hidden Terminal Problem • Exposed Terminal Problem

  12. Hidden Terminal Problem For three devices (A, B, C) Device A and C are in the range of device B. Device B is in the range of A and C. Device C is not in the range of A. Device A is not in the range of C. Device A want to communicate with device B, and at the same time Device C also want to communicate with Device B. Both start Transmission and at the same time data reach to the Device B, where there collision occur. Because using CSMA/CD only one device can transmit data at a time but in device B range two devices are transmitting i.e. A, C so collision is occurring.

  13. Hidden Terminal Problem Figure (a)The Hidden Terminal Problem.

  14. The Exposed Terminal Problem • For four devices (A, B, C, D). • In C range all the other three devices are Present i.e. A, B, D • While in A range device B and C are present. • Device C want to communicate with Device D. • Device C will sense the medium no other device is currently transmitting so both C and D will start transmission. • After that device A want to communicate with device B. • Using CSMA/CD device A will sense the medium and the medium is already busy by device C and D. so device A and B will wait until they got medium free. • As both devices A and B are free but can not start communication due to some other node communicating in their range. • This is called expose terminal problem.

  15. Expose Terminal Problem

  16. CSMA / CA • Uses CSMA / CA (CSMA with Collision Avoidance). solved hidden, exposed terminal: • Send Ready-to-Send (RTS) and Clear-to-Send (CTS) first • Source send RTS before transmission start and receiver when ready respond with CTS. • RTS, CTS helps determine who else is in range or busy (Collision avoidance).

  17. CSMA / CA (a) A sending an RTS to B.(b) B responding with a CTS to A.

  18. Quiz • Next Week First Lecture in all LAN

  19. Summary WLAN Categories of Wireless Networks Wi-Fi Wireless Physical Layer Two Additional Problems CSMA / CA

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