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Chapter 15 Wireless LANs

Chapter 15 Wireless LANs. IEEE 802 Standards Working Groups. WiMAX, an acronym that stands for W orldwide I nteroperability for M icrowave Acc ess(802.16). WiFi (Wireless Fidelity) : 802.11. TAG(Technical Advisory Grooup). Introduction. Architecture. Physical Layer. MAC Layer.

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Chapter 15 Wireless LANs

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  1. Chapter 15Wireless LANs

  2. IEEE 802 Standards Working Groups WiMAX, an acronym that stands for Worldwide Interoperability for Microwave Access(802.16) WiFi (Wireless Fidelity) : 802.11 TAG(Technical Advisory Grooup)

  3. Introduction • Architecture • Physical Layer • MAC Layer • Addressing Mechanism

  4. 15.1 IEEE 802.11 - Architecture • Basic Service Set (BSS) • Extended Service Set (ESS)

  5. Basic Service Set • made up of stationary or mobile wireless stations, and possibly a central base station (access point) • BSS without an AP is stand-alone, cannot send data to other BSSs

  6. ESS – Extended Service Set • Made up of two or more BSSs with APs – connected through a distribution system (usually a wired LAN)

  7. Station Types • No-transition mobility – either stationary or moving only inside a BSS • BSS-transition mobility – can move from one BSS to another, confined to one ESS • ESS-transition mobility – can move from one ESS to another

  8. Physical layer specifications • 802.11 defines the specs for the conversion of bits to a signal

  9. FHSS – Frequency hopping spread spectrum • sender sends on one carrier frequency for a short amount of time, hops to another frequency for the same time, then another, and so on, eventually the cycle is repeated • makes it difficult for unauthorized person to make sense of data • 2.4 GHz industrial scientific, and medical (ISM) band (2.4 ~ 2.48Ghz) – 79 subbands of 1MHz • modulation is FSK (1 or 2 bits/buad, two-level or four-level) : date rate of 1 or 2 Mbps

  10. Frequency-Hopping example

  11. DSSS – direct sequence spread spectrum • Each bit sent is replaced by a chip code (similar to CDMA, but this is at the physical layer, not a multiple-access data link layer method) • 2.4 GHz ISM band • Modulation is PSK (BPSK or QPSK)

  12. 802.11 • 802.11 Infrared • Two capacities 1 Mbps or 2 Mbps. • Range is 10 to 20 meters and cannot penetrate walls. • Does not work outdoors. • 802.11 FHSS (Frequency Hopping Spread Spectrum) • The main issue is multipath fading. • 79 non-overlapping channels, each 1 Mhz wide at low end of 2.4 GHz ISM band. • Same pseudo-random number generator used by all stations. • Dwell time: min. time on channel before hopping (400msec).

  13. 801.11a,b,g • 802.11a OFDM – orthogonal frequency division multiplexing • 5-GHz ISM band • 48 subbands for sending 48 groups of bits at a time, 4 subbands for control information • PSK for 18Mbps and QAM for 54 Mbps • 802.11b HR-DSSS – high-rate direct sequence spread spectrum • 2.4 GHz band • similar to DSSS except for encoding, complementary code keying (CCK), encodes 4 or 8 bits to one CCK symbol • modulation BPSK and QPSK • 4 data rates (1, 2, 5.5, and 11 Mbps) : DSSS for 1, 2 Mbps, BPSK for 5.5, QPSK for 11 Mbps. • 802.11b and 11a are incompatible!!

  14. 801.11a,b,g (cont’d) • 802.11g OFDM • 2.4 GHz band • 54-Mbps data rate • Is backward compatible with 802.11b.

  15. MAC layers in IEEE 802.11 standard PCF : Point Coordination Function DCF : Distributed Coordination Function

  16. CSMA/CA • Wireless LAN cannot implement CSMA/CD: • collision detection implies the ability to send and receive at the same time, which implies costly stations and increased bandwidth requirements • collision may not be detected because of a hidden terminal • distance could cause signal fading which could prevent a station from hearing a collision

  17. Hidden and Exposed Terminals (a) The hidden station problem (b) The exposed station problem

  18. Another Example for Exposed Terminal Problem

  19. CSMA/CA flowchart

  20. CSMA/CA and NAV DIFS – distributed interframe space NAV – network allocation vector SIFS – short interframe space collision only occurs during RTS – request to send handshake period (RTS, CTS) CTS – clear to send

  21. Frame format FC : frame control

  22. Subfields in FC (frame control) field

  23. Control frames

  24. Values of subfields in control frames • type field : 01

  25. Subfields in FC field

  26. Addressing mechanism: case 1 • To DS = 0 and From DS = 0 • Frame is not going to a distribution system • Frame is going from one station in a BSS to another • ACK sent to original sender

  27. Addressing mechanism: case 2 • To DS = 0 and From DS = 1 • Frame is coming from a distribution system • Frame is coming from an AP and going to a station • ACK sent to AP

  28. Addressing mechanism: case 3 • To DS = 1 and From DS = 0 • Frame is going to a distribution system • Frame is coming from a station to an AP • ACK sent to original station • address 3 contains final destination of frame

  29. Addressing mechanism: case 4 • To DS = 1 and From DS = 1 • Distribution system is wireless • Frame is coming from an AP going to another AP

  30. 15.2 Bluetooth • Architecture • Radio Layer • Baseband Layer • L2CAP Layer • Other Upper Layers

  31. Architecture - Two Types of Networks • Piconets • Scatternet

  32. Piconet • up to eight stations, one is the master, rest are slaves • slaves synchronize their clocks and hopping sequence with master • only one master • communication between master and slave can be 1-1 or 1-many • can have up to eight additional slaves in parked state • slave in parked state is synchronized, but cannot communicate

  33. Scatternet • scatternet – combined piconets • slave in one piconet can be master in another

  34. Bluetooth layers

  35. Radio Layers • roughly equivalent to the physical layer of the Internet model • 2.4-GHz ISM band • FHSS – frequency hopping spread spectrum • modulation – sophisticated version of FSK, called GFSK (FSK w/ Gaussian bandwidth filtering)

  36. Baseband Layer • roughly equivalent to MAC sublayer in LANs • access method is TDMA • TDD-TDMA (time division duplexing TDMA) – a kind of half duplex communication • single slave communication – master uses even number slots, slave uses odd numbered • multiple slave communication – master uses even number slots, slave sends in the next odd-numbered slot if the packet in the previous slot was addressed to it

  37. Single-slave Communication

  38. Multiple-slave communication

  39. Physical Links • SCO – synchronous connection oriented link – avoiding latency is more important than integrity – damaged packet is never retransmitted • ACL – asynchronous connectionless link – data integrity is more important than avoiding latency – if packet is damaged, it is retransmitted

  40. Frame Format Types

  41. L2CAP data packet format • Logical link control and adaptation protocol – roughly equivalent to the LLC sublayer in LANs • used for data exchange on an ACL link • specific duties: multiplexing, segmentation and reassembly, quality of service, and group management

  42. Questions !

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