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IEEE 802.11 Wireless LAN Standard

IEEE 802.11 Wireless LAN Standard. Chapter 14. IEEE 802 Protocol Layers. Protocol Architecture. Functions of physical layer: Encoding/decoding of signals Preamble generation/removal (for synchronization) Bit transmission/reception Includes specification of the transmission medium.

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IEEE 802.11 Wireless LAN Standard

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  1. IEEE 802.11 Wireless LAN Standard Chapter 14

  2. IEEE 802 Protocol Layers

  3. Protocol Architecture • Functions of physical layer: • Encoding/decoding of signals • Preamble generation/removal (for synchronization) • Bit transmission/reception • Includes specification of the transmission medium

  4. Protocol Architecture • Functions of medium access control (MAC) layer: • On transmission, assemble data into a frame with address and error detection fields • On reception, disassemble frame and perform address recognition and error detection • Govern access to the LAN transmission medium • Functions of logical link control (LLC) Layer: • Provide an interface to higher layers and perform flow and error control

  5. Separation of LLC and MAC • The logic required to manage access to a shared-access medium not found in traditional layer 2 data link control • For the same LLC, several MAC options may be provided

  6. MAC Frame Format • MAC control • Contains Mac protocol information • Destination MAC address • Destination physical attachment point • Source MAC address • Source physical attachment point • CRC • Cyclic redundancy check

  7. Logical Link Control • Characteristics of LLC not shared by other control protocols: • Must support multiaccess, shared-medium nature of the link • Relieved of some details of link access by MAC layer

  8. LLC Services • Unacknowledged connectionless service • No flow- and error-control mechanisms • Data delivery not guaranteed • Connection-mode service • Logical connection set up between two users • Flow- and error-control provided • Acknowledged connectionless service • Cross between previous two • Datagrams acknowledged • No prior logical setup

  9. Differences between LLC and HDLC • LLC uses asynchronous balanced mode of operation of HDLC (type 2 operation) • LLC supports unacknowledged connectionless service (type 1 operation) • LLC supports acknowledged connectionless service (type 3 operation) • LLC permits multiplexing by the use of LLC service access points (LSAPs)

  10. 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

  11. IEEE 802.11 Services

  12. 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

  13. 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

  14. 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

  15. 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

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

  17. 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 frames exchange • Source issues request to send (RTS) • Destination responds with clear to send (CTS) • Source transmits data • Destination responds with ACK

  18. Access Control (PCF)

  19. DIFS data SIFS ACK IEEE 802.11 MAC Protocol: CSMA/CA 802.11 Sender: 1. if sense channel idle for DIFSthen - transmit entire frame (no CD) 2. if sense channel busy then - start random backoff time - timer counts down while channel idle - transmit when timer expires - if no ACK, increase random backoff interval, repeat 2 802.11 Receiver: if frame received OK - return ACK after SIFS (ACK needed due to hidden terminal problem) sender receiver

  20. RTS/CTS (Dealing with Hidden Terminal) idea: allow sender to “reserve” channel rather than random access of data frames: avoid collisions of long data frames • optional; not typically used • sender first transmits smallRequest-To-Send (RTS) packets to AP using CSMA • RTSs may still collide with each other (but they’re short) • AP broadcasts Clear-To-Send CTS in response to RTS • CTS heard by all nodes • sender transmits data frame • other stations defer transmissions Avoid data frame collisions completely using small reservation packets!

  21. RTS(B) RTS(A) reservation collision RTS(A) CTS(A) CTS(A) DATA (A) ACK(A) ACK(A) Collision Avoidance: RTS-CTS Exchange B A AP defer time

  22. Medium Access Control Logic

  23. 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

  24. 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

  25. 6 4 2 2 6 6 6 2 0 - 2312 frame control duration address 1 address 2 address 3 address 4 payload CRC seq control Address 3: used only in ad hoc mode Address 1: MAC address of wireless host or AP to receive this frame Address 3: MAC address of router interface to which AP is attached Address 2: MAC address of wireless host or AP transmitting this frame 802.11 Frame: Addressing

  26. router AP Internet R1 MAC addr AP MAC addr source address dest. address 802.3frame AP MAC addr H1 MAC addr R1 MAC addr address 3 address 2 address 1 802.11 frame 802.11 Frame: Addressing H1 R1

  27. MAC Frame Format

  28. 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

  29. 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

  30. 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

  31. 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

  32. 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

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

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

  35. Wired Equivalent Privacy

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

  37. Physical Media Defined by Original 802.11 Standard • Direct-sequence spread spectrum • Operating in 2.4 GHz ISM band • Data rates of 1 and 2 Mbps • Frequency-hopping spread spectrum • Operating in 2.4 GHz ISM band • Data rates of 1 and 2 Mbps • Infrared • 1 and 2 Mbps • Wavelength between 850 and 950 nm

  38. IEEE 802.11a and IEEE 802.11b • IEEE 802.11a • Makes use of 5-GHz band • Provides rates of 6, 9 , 12, 18, 24, 36, 48, 54 Mbps • Uses orthogonal frequency division multiplexing (OFDM) • Subcarrier modulated using BPSK, QPSK, 16-QAM or 64-QAM • IEEE 802.11b • Provides data rates of 5.5 and 11 Mbps • Complementary code keying (CCK) modulation scheme

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