1 / 18

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

corina
Download Presentation

CPET 355

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. 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

More Related