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CSE401n: Computer Networks

CSE401n: Computer Networks. Lecture 16 Wireless Link & LANs WS: ch-14 KR: 5.7. 802.11b operate at 2.4 GHz, 11 Mbps widely deployed 802.11a 5-6 GHz range up to 54 Mbps 802.11g 2.4 GHz up to 54 Mbps All have base-station and ad-hoc network versions

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CSE401n: Computer Networks

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  1. CSE401n:Computer Networks Lecture 16 Wireless Link & LANs WS: ch-14 KR: 5.7

  2. 802.11b operate at 2.4 GHz, 11 Mbps widely deployed 802.11a 5-6 GHz range up to 54 Mbps 802.11g 2.4 GHz up to 54 Mbps All have base-station and ad-hoc network versions All use CSMA/CA for multiple access IEEE 802.11 Wireless LANs

  3. IEEE 802.11 Wireless LAN • IEEE 802.11 standard: • unlicensed frequency spectrum: 900Mhz, 2.4Ghz, 5.1Ghz, 5.7Ghz and 802.11b 802.11a

  4. Frequency Band

  5. IEEE 802.11 Wireless LAN • Basic Service Set (BSS) (a.k.a. “cell”) contains: • wireless station (WS) • access point (AP): base station • BSS’s combined to form distribution system (DS) • Two operation modes: • Infrastructure mode • everything through AP • Peer-to-peer mode • called ad hoc network

  6. IEEE 802.11 Architecture • Distribution system (DS) • Access point (AP) • Basic service set (BSS) • Stations competing for access to shared wireless medium • Isolated or connected to backbone DS through AP • Extended service set (ESS) • Two or more basic service sets interconnected by DS

  7. Ad Hoc Network approach • No AP • No connection to the outside world • Wireless hosts communicate with each other • Applications: • “laptop” meeting in conference room, car

  8. Distribution of Messages Within a DS • Distribution service • Used to exchange MAC frames from station in one BSS to station in another BSS • Integration service • Transfer of data between station on IEEE 802.11 LAN and station on integrated IEEE 802.x LAN

  9. Transition Types Based On Mobility • No transition • Stationary or moves only within BSS • BSS transition • Station moving from one BSS to another BSS in same ESS • ESS transition • Station moving from BSS in one ESS to BSS within another ESS

  10. Association-Related Services • Association • Establishes initial association between station and AP • Reassociation • Enables transfer of association from one AP to another, allowing station to move from one BSS to another • Disassociation • Association termination notice from station or AP

  11. Access and Privacy Services • Authentication • Establishes identity of stations to each other • Deathentication • Invoked when existing authentication is terminated • Privacy • Prevents message contents from being read by unintended recipient

  12. 802.11 – Traffic and Access Methods • Two types of traffic services • Asynchronous Data Service (mandatory) • exchange of data packets based on “best-effort” • Time-Bounded Service (optional) • Two types of coordination function • DCF (Distributed Coordination Function) • PCF (Point Coordination Function) • Access methods (why not CSMA/CD?) • DCF-CSMA/CA (collision avoidance) • collision avoidance via randomized back-off mechanism (if sense busy, random backoff even if no collision) • ACK packet for acknowledgements (not for broadcasts) • DCF-RTS/CTS • avoids hidden terminal problem • PCF • access point polls 7.12.1

  13. IEEE 802.11 Medium Access Control • MAC layer covers three functional areas: • Reliable data delivery • Access control • Security

  14. Reliable Data Delivery • More efficient to deal with errors at the MAC level than higher layer (such as TCP) • Frame exchange protocol • Source station transmits data • Destination responds with acknowledgment (ACK) • If source doesn’t receive ACK, it retransmits frame • Four frame exchange • Source issues request to send (RTS) • Destination responds with clear to send (CTS) • Source transmits data • Destination responds with ACK

  15. The Hidden Terminal Problem • A is sending to B, but C cannot receive from A • Friis Law (power decay proportional to distance square) • Therefore C sends to B, without detecting the transmission from A to B • In summary, A is “hidden” for C • Implication: How to do carrier sense and collision detection? A B C

  16. The Exposed Terminal Problem • B is sending to A, C intends to send to D • C senses an “in-use” medium, thus C waits • But A is outside the radio range of C, therefore waiting is not necessary • In summary, C is “exposed” to B • Implication: false carrier sense A B C D

  17. RTS CTS CTS Basic Solution: RTS-CTS • Short signaling packets • RTS (request to send) and CTS (clear to send) • Contain sender address, receiver address, transmission duration • Example: solve the hidden terminal problem A B C D

  18. Access Control

  19. Medium Access Control Logic

  20. Interframe Space (IFS) Values • Short IFS (SIFS) • Shortest IFS • Used for immediate response actions • Point coordination function IFS (PIFS) • Midlength IFS • Used by centralized controller in PCF scheme when using polls • Distributed coordination function IFS (DIFS) • Longest IFS • Used as minimum delay of asynchronous frames contending for access

  21. IFS Usage • SIFS • Acknowledgment (ACK) • Clear to send (CTS) • Poll response • PIFS • Used by centralized controller in issuing polls • Takes precedence over normal contention traffic • DIFS • Used for all ordinary asynchronous traffic

  22. DIFS DIFS PIFS SIFS medium busy contention next frame t 802.11 – Inter Frame Spacing • Defined different inter frame spacing • SIFS (Short Inter Frame Spacing) • highest priority, for ACK, CTS, polling response • PIFS (PCF IFS) • medium priority, for time-bounded service using PCF • DIFS (DCF, Distributed Coordination Function IFS) • lowest priority, for asynchronous data service direct access if medium is free  DIFS

  23. 802.11 – RTS/CTS + ACK • Sender sends RTS with NAV (Network allocation Vector, i.e. reservation parameter that determines amount of time the data packet needs the medium) after waiting for DIFS • Receiver acknowledges via CTS after SIFS (if ready to receive) • CTS reserves channel for sender, notifying possibly hidden stations • Sender can now send data at once, acknowledgement via ACK • Other stations store NAV distributed via RTS and CTS DIFS RTS data sender SIFS SIFS SIFS CTS ACK receiver DIFS NAV (RTS) data other stations NAV (CTS) t defer access new contention

  24. 802.11 Collision Avoidance: RTS-CTS + ACK DIFS: Distributed Inter-Frame Spacing SIFS: Short Inter-Frame Spacing

  25. 802.11: PCF for Polling SIFS PIFS D D point coordinator SIFS U polled wireless stations NAV NAV contention free period t medium busy contention period D: downstream poll, or data from point coordinator U: data from polled wireless station

  26. MAC Frame Format

  27. MAC Frame Fields • Frame Control – frame type, control information • Duration/connection ID – channel allocation time • Addresses – context dependant, types include source and destination • Sequence control – numbering and reassembly • Frame body – MSDU or fragment of MSDU • Frame check sequence – 32-bit CRC

  28. Frame Control Fields • Protocol version – 802.11 version • Type – control, management, or data • Subtype – identifies function of frame • To DS – 1 if destined for DS • From DS – 1 if leaving DS • More fragments – 1 if fragments follow • Retry – 1 if retransmission of previous frame

  29. Frame Control Fields • Power management – 1 if transmitting station is in sleep mode • More data – Indicates that station has more data to send • WEP – 1 if wired equivalent protocol is implemented • Order – 1 if any data frame is sent using the Strictly Ordered service

  30. Control Frame Subtypes • Power save – poll (PS-Poll) • Request to send (RTS) • Clear to send (CTS) • Acknowledgment • Contention-free (CF)-end • CF-end + CF-ack

  31. Data Frame Subtypes • Data-carrying frames • Data • Data + CF-Ack • Data + CF-Poll • Data + CF-Ack + CF-Poll • Other subtypes (don’t carry user data) • Null Function • CF-Ack • CF-Poll • CF-Ack + CF-Poll

  32. Management Frame Subtypes • Association request • Association response • Reassociation request • Reassociation response • Probe request • Probe response • Beacon

  33. Management Frame Subtypes • Announcement traffic indication message • Dissociation • Authentication • Deauthentication

  34. Wired Equivalent Privacy

  35. Authentication • Open system authentication • Exchange of identities, no security benefits • Shared Key authentication • Shared Key assures authentication

  36. IEEE 802.11 DSSS: direct sequence spread spectrum FHSS: frequency hopping spread spectrum OFDM: orthogonal frequency division multiplexing

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