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Wireless LAN (WLAN)

Wireless LAN (WLAN)

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Wireless LAN (WLAN)

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  1. Wireless LAN (WLAN) • Wireless Ethernet • Bluetooth

  2. Introduction • Demand for wireless device connection has been growing • WLAN is generally used in campus, office as well as in cafe, petrol station etc. • WLAN can also be fixed at home for usage of Internet connection in other mobile devices

  3. IEEE 802.11 • 802.11 standard define specification that cover physical and data link layer for WLAN • Architecture • Physical layer • Media Access Control Layer (MAC) • Addressing mechanism

  4. WLAN architecture: Basic Service Set (BSS) • BSS defined as building block of WLAN • Made of mobile wireless stations and an optional central base station (access point -AP) • BSS without an AP is stand alone network and cannot send data to other BSSs. (called: ad-hoc architecture) • Stations can locate one another and agree to be part of BSS

  5. WLAN architecture : Extended Service Set (ESS)

  6. WLAN Architecture: Extended Service Set (ESS) • ESS is made of >= 2 BSSs with APs • Generally, BSSs are connected through a distribution system (wired LAN) • Distributed system connects the APs in the BSSs. • IEEE 802.11 doesn’t restrict the distribution system : It can be any IEEE LAN (such as Ethernet) • Stations in ESS: mobile & stationary • Mobile: normal stations inside a BSS • Stationary: AP stations that are part of a WLAN • When BSSs are connected, this architecture called infrastructure network, and stations in BSS no need AP to communicate one another • However communication between 2 stations from 2 difference BSSs normally occur through 2 APs

  7. Stations types • IEEE 802.11 defines 3 type of stations based on their mobility: • No-transition • Stationary or moving only inside a BSS • BSS-transition • Move from one BSS to another, but the movement is confined inside one ESS • ESS-transition • Move from one ESS to another but IEEE 802.11 doesn’t guarantee that communication is continuous during the move

  8. Physical Layer Specification • IEEE 802.11 defines specification to convert bit to signal (physical layer) • There are 5 type of specification in radio frequency domain

  9. Frequency Hopping Spread Spectrum • Generating signal in 2.4 GHz ISM band • Data transmission using 1 frequency carrier for certain period and hop to another frequency and so on. • After N hops, loop will be repeated. • If initial bandwidth signal is B, spread spectrum bandwidth is : N x B • data rate: 1 @ 2 Mbps, use FSK modulation at 1M baud/s

  10. Direct Sequence Spread Spectrum • Also in 2.4-GHz ISM band (industrial, scientific, medical) • Each bit that sent replaced with bit sequence (chip code) • To prevent the use of buffer, the time to send a chip code should = time to send original bit • If number of bit in each chip code = N, data rate to send code chip = N x original stream bit data rate • Not same as CDMA because this is physical layer implementation • bit sequence use overall band • Data rate 1 @ 2 Mbps, use PSK modulation at 1Mbaud/s

  11. MAC sub-layer in IEEE 802.11 standard • 2 MAC sub-layer • Distributed coordination function(DCF) • Point coordination function (PCF) – more complex for just infrastructure network

  12. Acess method: CSMA/CA

  13. CSMA/CA and Network Allocation Vector • NAV is CA implementation method • When RTS is sent by 1 station (including time period that needed to use channel) • Other station will start respective NAV– waiting time before checking channel status • Distributed IFS and Short IFS are waiting time between frames

  14. MAC sub-layer frame format • The wireless environment is very noisy; a corrupt frame has to be retransmitted • The division of a large frame into smaller ones. (fragmentation)

  15. Sub-field in FC field

  16. Control Frame

  17. Sub-field value in control frame

  18. Sub-field in FC field

  19. Addressing Mechanism Pengalamatan : kes 1 • Addressing in 802.11 very complex because it involve many APs • Addr 1 : next device • Addr 2 : previous device • Addr 3 : final destination if it is not defined by Addr 1 • Addr 4 : original source if it is not the same as Addr 2

  20. Figure 15.12Addressing mechanism: case 2

  21. Figure 15.13Addressing mechanism: case 3

  22. Figure 15.14Addressing mechanism: case 4

  23. Bluetooth • WLAN technology designed to connect devices of different functions such as telephone, PDA, camera, printers, mouse, earpiece etc. • It is an ad hoc network– formed spontaneously • Bluetooth devices/gadgets find each other and make a network called a piconet • Bluetooth LAN can even be connected to the Internet if one of the gadgets has this capability • Standard : IEEE 802.15 that defines wireless personal area network (PAN) operable in an area the size of a room or a hall

  24. Network architecture: Piconet • Piconet can have up to 8 station with one of them function as master • The slave devices synchronize their clocks and hopping sequence (hop) with the master • Communication between master-slave can be 1-1 or 1-many • Slave can change from active to parked state and vice versa

  25. Network architecture: Scatternet Piconet 1 Piconet 2

  26. Scatternet • Piconet can be combined to form scatternet • Station slave in a piconet can be master in another piconet • This Station can receives message from master (first piconet) and acting as master (second piconet ) to deliver them to slave in second piconet

  27. Bluetooth device • Bluetooth device has short distance radio transmitter • Data rate: 1Mbps with bandwidth 2.4-GHz, therefore may be disruption can happen between WLAN IEEE 802.11b and LAN Bluetooth

  28. Bluetooth layer • Bluetooth uses layers that do not exactly match TCP/IP model • Radio layer ~ physical layer • Bluetooth devices are lo-power and have a range of 10 m • Band: use 2.4-GHz ISM band divided into 79 channels of 1 MHz each

  29. Bluetooth layer • Bluetooth use FHSS method in the physical layer to avoid interference from other devices or networks • Bluetooth hops 1600 per second, which means that each device changes its modulation frequency 1600 times per second • To transform bits to a signal, use GFSK modulation(Gaussian) • Baseband layer: equivalent to MAC sublayer in LAN with access method is Time Divison Duplexing-TDMA (half-duplex communication) • Communication only between master-slave • No direct communication between slave-slave

  30. Single-slave communication

  31. Multiple-slave communication

  32. Frame Format

  33. L2CAP Packet Data Format