1 / 35

IEEE 802.11 Wireless Local Area Networks

IEEE 802.11 Wireless Local Area Networks. The future is wireless Presented by Tamer Khattab and George Wong Prepared for EECE571N - Advanced Networking. WLAN Technology Overview. Physical layer technologies Architecture Transmission Medium access control technologies. Architecture.

davina
Download Presentation

IEEE 802.11 Wireless Local Area Networks

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. IEEE 802.11 Wireless Local Area Networks The future is wireless Presented by Tamer Khattab and George Wong Prepared for EECE571N - Advanced Networking

  2. WLAN Technology Overview • Physical layer technologies • Architecture • Transmission • Medium access control technologies

  3. Architecture • Network connectivity life time • Ad-hoc

  4. Architecture (Cont.) • Network connectivity life time • Infrastructure

  5. Architecture (Cont.) • Connection type • Point-to-point

  6. Architecture (Cont.) • Connection type • Broadcast

  7. Transmission Medium • Radio frequency transmission • Narrow-band transmission • Spread spectrum transmission • direct sequence • frequency hopping • Infrared transmission • Laser diode sources • Light emitting diode sources

  8. Wireless LAN Standards • IEEE 802.11 wireless LAN standards • ETSI HIPERLAN wireless LAN standards

  9. Wireless LAN Standards • IEEE 802.11 wireless LAN standards

  10. Physical Layer IEEE 802.11 Wireless Ethernet

  11. IEEE 802.11 Physical Layer • Medium type • 2.4 GHz FHSS (2400 - 2483.2 GHz) • 2.4 GHz DSSS (2400 - 2483.2 GHz) • Diffused infrared DFIR (850 - 950 nm) • Rates: basic=1 Mbps, enhanced=2 Mbps

  12. FHSS • Band 2400-2483.5 MHz • GFSK (Gaussian Frequency Shift Keying) • Sub-channels of 1 MHz • Only 79 channels of the 83 are used • Slow hopping • 3 main sets each with 26 different hopping sequences

  13. FHSS (Cont.) • Sequences within same set collide at max. on 5 channels • Min. hopping distance of 6 channels. • No CDMA within same BSS • Coexisting BSS in the same coverage area use different sequences from the same hopping set.

  14. Hopping distance >= 6 sub-channels (The distance in frequency between two consecutive hops) Sub-channel FHSS (Cont.) Time 400 ms Frequency 1 MHz

  15. PLCP preamble PLCP header PLCP_PDU Sync pattern 80 bit SFD 16 bit 4 bit Payload data (variable length) HEC 16 bit PLW 12 bit PSF MAC data could be at 1 Mb/s or 2 Mb/s Physical layer header and preamble always at 1 Mb/s FHSS (Cont.)

  16. DSSS • Band 2400-2483.5 MHz • DBPSK (Differential Binary Phase Shift Keying) • Band divided into 11 overlapping channels each with bandwidth 11 MHz • Coexisting BSS in the same coverage area use channels separated by at least 30 MHz. • 11 bit Barker sequence is used for spreading • No CDMA used within one BSS

  17. DSSS (Cont.) 11 MHz Channel number 11 10 9 8 7 6 5 4 3 2 1 Frequency (MHz) 2412 2417 2422 2427 2432 2437 2442 2447 2452 2457 2462

  18. PLCP preamble PLCP header PLCP_PDU SFD 16 bit SR 8 bit HEC 16 bit LN 16 bit Sync pattern 128 bit Payload data (variable length) SG 8 bit MAC data could be at 1 Mb/s or 2 Mb/s Physical layer header and preamble always at 1 Mb/s DSSS (Cont.)

  19. Infra Red • Wave length near visible light 850-950 nm • PPM (Pulse Position Modulation) • Diffused transmission technique used • Only used for indoor transmission

  20. IEEE 802.11a • 5 GHz (5.15-5.25, 5.25-5.35, 5.725-5.825GHz) • OFDM (Orthogonal Freq. Div. Multiplexing) • 52 Subcarriers • BPSK/QPSK/QAM • Forward Error Correction (Convolutional) • Rates: 6, 9, 12, 18, 24, 36, 48, 54 Mbps

  21. IEEE 802.11b • 2.4 GHZ band • DSSS (11-chip) • Rates 5.5 and 11 Mbps • M-arry modulation. • Convolutional Codes • Shorter Preamble

  22. Product Samples 2.4 GHz FHSS ISA 2.4 GHz FHSS PCMCIA 5 GHz DSSS PCMCIA

  23. MAC Layer IEEE 802.11 Wireless Ethernet

  24. Overview of the Protocol Layers • IEEE 802.11 specifies a MAC layer that is designed to operate over wireless channel • IEEE 802.11 is in the same protocol layer as the IEEE 802.3 Network Layer Data Link Layer IEEE 802.2 Logical Link Control (LLC) IEEE 802.3 Ethernet IEEE 802.4 Token Bus IEEE 802.5 Token Ring IEEE 802.11 Wireless Ethernet MAC Layer Physical Layer

  25. IEEE 802.11 – CSMA/CA • CS – Carrier Sense • Each transmitter listens to the physical link before transmitting • MA – Multiple Access • Many nodes are connected to the same physical link. • CA – Collision Avoidance • Methods used to avoid collision

  26. CSMA/CA • Why not CSMA/CD? • Difficult to detect collision in a radio environment • Radio environment is not as well controlled as a wired broadcast medium, and transmissions from users in other LANs can interfere with the operation of CSMA/CD • Radio LANs are subject to the hidden-station problem

  27. Hidden-Station Problem • A knows the existence of B • C knows the existence of B • B knows the existence of A and C • However, A does not know the existence of C A B C

  28. Hidden-Station Problem • Since A and C are sufficiently distant from each other that they cannot hear each other’s transmission (Carrier Sense doesn’t work!) • This condition will result in the transmissions from the two stations, A and C, proceeding and colliding at the intermediate station B (However, A and C cannot hear the collision!)

  29. (a) C A Data Frame B A transmits data frame A C (b) Data Frame B Data Frame C transmits data frame and collides with A at B Hidden-Station Problem • CSMA/CA medium access control was developed to prevent this type of collision

  30. Exposed Node Problem • Suppose B is sending to A. C is aware of this communication because it hears B’s transmission. It would be a mistake for C to conclude that it cannot transmit to anyone just because it can hear B’ transmission • This is not a problem since C’s transmisstion to D will not interfere with A’s ability to receive from B A B C D

  31. Collision Avoidance • IEEE 802.11 address these two problems, hidden-station and exposed node problems, with an algorithms called Multiple Access with Collision Avoidance • Sender and receiver exchange control frames with each other before the sender actually transmit any data • The sender transmits a Request to Send (RTS) frame to the receiver and the receiver then replies with a Clear to Send (CTS) frame

  32. Collision Avoidance • RTS includes a field that indicates how long the sender wants to hold the medium • CTS reserves channel for sender, notifying (possible hidden) station • For Neighbors • See CTS: keep quite • See RTS but no CTS: ok to transmit • Receiver sends ACK when it receives the frame • Neighbors keep silent until seeing ACK

  33. Collision Avoidance A B C D

  34. UBC Wireless Network • UBC is currently deploying the wireless network • http://www.UNP.ubc.ca/ • “It’s changed students’ lives” Christopher Macdonald, School of Architecture • “It was like winning the lottery” Alan Steeves, Research Engineer

  35. Questions?

More Related