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CPET 355

CPET 355. 4. The Medium Access Control Sublayer – Wireless LAN Paul I-Hai Lin, Professor Electrical and Computer Engineering Technology Purdue University, Fort Wayne Campus. Wireless LAN Layers. Data Link Layer Logical Link Control MAC sub-layer CSMA/CA – Collision Avoidance

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CPET 355

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  1. CPET 355 4. The Medium Access Control Sublayer – Wireless LAN Paul I-Hai Lin, Professor Electrical and Computer Engineering Technology Purdue University, Fort Wayne Campus Prof. Paul Lin

  2. Wireless LAN Layers • Data Link Layer • Logical Link Control • MAC sub-layer • CSMA/CA – Collision Avoidance • Two configurations • with Access Points (base stations) • without an Access Point • Physical (PHY) Layers • Signal modulation schemes and Coding • 802.11 Infrared, FHSS, DSSS; 802.11a OFDM, 802.11b HR-DSSS, and 802.11g ODFM Prof. Paul Lin

  3. The 802.11 Protocol Stacks Computer Networks, 4th ed, Andrew Tanenbaum, page 293 Prof. Paul Lin

  4. The 802.11 Protocol Stacks • 802.11 Infrared • 802.11 FHSS (Frequency Hopping Spread Spectrum) • 802.11 DSSS (Direct Sequence Spread Spectrum) • 802.11a OFDM (Orthogonal Frequency Division Multiplexing) • 802.11 b HR-DSSS (High Data Rate Direct Sequence Spread Spectrum) • 802.11g OFDM Prof. Paul Lin

  5. 802.11 Infrared • 0.85-0.95 microns • Two speeds: 1 Mbps and 2 Mbps • Encoding Schemes • Gray Code – 1 Mbps Prof. Paul Lin

  6. 802.11 FHSS (Frequency Hopping Spread Spectrum) • 79 Channels, 1 MHz wide, 2.4-G ISM band • Pseudorandom no generator – sequence of frequencies • Dwell time - time spent at each frequency • Adjustable, and less than 400 m-sec • Main Disadvantage – low bandwidth • Advantages • Security (dwell time, frequency hopping) • Good resistance to multipath fading – long distance • Insensitive to radio interference – building-to-building links Prof. Paul Lin

  7. IEEE 802.11a OFDM (Orthogonal Frequency Division Multiplexing) • Up to 54 Mbps in the wider 5-GHz ISM band • 52 Frequencies – 48 for data and 4 for synchronization • Encoding schemes • phase-shift modulation – up to 18 Mbps • QAM modulation - above 18 Mbps • At 54 Mbps – 216 data bits are encoded into 288-bit symbols • Good spectrum efficiency – bits/Hz Prof. Paul Lin

  8. IEEE 802.11 b HR-DSSS (High Data Rate Direct Sequence Spread Spectrum) • Use the Chip Rate of 11 million chips/sec to achieve 11 Mbps in the 2.4-GHz band • Chip Rate: In Direct Sequence Modulation Spread Spectrum systems, the rate at which the information signal bit are transmitted as a pseudorandom sequence of chips • Supported data rate: 1, 2, 5.5 and 11 Mbps • Walsh/Hadamard codes – for faster data rates • Longer communication range – 7 times greater than 802.11a Prof. Paul Lin

  9. IEEE 802.11g OFDM(Orthogonal Frequency Division Multiplexing) • Approved in November 2001 • Operates in the narrow 2.4 GHz ISM band • Up to 54 MBps Prof. Paul Lin

  10. 802.11 MAC Sublayer Protocol • Two problems • The hidden station problem • The exposed station problem • Two Modes of Operations – to take care the two problems • DCF (Distributed Coordination Function) • PCF (Point Coordination Function) – use base station or access point Prof. Paul Lin

  11. DCF (Distributed Coordination Function) • Similar to Ethernet • Uses CSMA/CA (Collision Avoidance) • Use Virtual Channel Sensing – Negative Acknowledge Vector • Stop-and-Wait Protocol • Fragment burst Prof. Paul Lin

  12. PCF (Point Coordination Function) • Use base station or access point to polls (broadcasts) the other stations • Beacon Frame (10 to 100 times per sec) • Hopping f and Dwell time, Clock sync. • Invite new stations to sign up • Ask any frames to send? • Completely controlled transmission order • No collision ever occur Prof. Paul Lin

  13. 802.11 Inter-frame Spacing • SIFS (Short Interframe Spacing) • Inform that a control frame or next frame may be sent • PIFS (PCF Interframe Spacing) • Inform that a PCF frame may be sent • DIFS ( DCF Interframe Spacing) • Inform that a DCF frame may be sent • EIFS (Extended Interframe Spacing) • Inform that a Bad Frame recovery done here Prof. Paul Lin

  14. 802.11 Inter-frame Spacing • SIFS (Short Interframe Spacing) • PIFS (PCF Interframe Spacing) • DIFS ( DCF Interframe Spacing) • EIFS (Extended Interframe Spacing) Prof. Paul Lin

  15. byte 2 2 6 6 6 2 6 FrameCtrl Dura-tion ….. Addr.1 Addr.2 Addr.3 Seq. Addr.4 0-2312 4 Data …. Check-sum Sub-Type ToDS FromDS Retry Versi-on Type Pwr More W O MF Bits 2 2 4 1 1 1 1 1 1 1 1 The 802.11 Data Frame ….. Prof. Paul Lin

  16. Version – Protocol Type: data, control, or management Subtypes – RTS or CTS To DS (distributed system) From DS MF – More fragment will come Retry - retrnasmission Pwr – Power management (in/out sleep mode) More – frames W – using WEP (Wired Equivalency Privacy Encryption) O – Strictly in order The 802.11 Frame Control (11-sub fields) Prof. Paul Lin

  17. 802.11 Data Frame • Duration Field • How long the frame and ACK will occupy the channel, NAV management • Address 1 - Source • Address 2 - Destination • Address 3 • Intercell traffic, source base stations • Address 4 • Intercell traffic, destination base stations • Sequence – for numbering fragments Prof. Paul Lin

  18. 802.11 Services • Five Distribution Services • Association – connect mobile stations to AP • Disassociation • Re-association • Distribution – Frame routing • Integration – with non-802.11 network • Four Station Services • Authentication – Secrete key (password) • De-authentication • Privacy – RC4 encryption • Data Delivery - reliability Prof. Paul Lin

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