1. March 22, 2004 Prof. Paul Lin 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

2. March 22, 2004 Prof. Paul Lin 2 Wireless LAN Layers Data Link Layer Data Link Layer Logical Link ControlLogical Link Control MAC sub-layerMAC sub-layer CSMA/CA – Collision Avoidance CSMA/CA – Collision Avoidance Two configurations Two configurations with Access Points (base stations)with Access Points (base stations) without an Access Pointwithout an Access Point Physical (PHY) Layers Physical (PHY) Layers Signal modulation schemes and CodingSignal modulation schemes and Coding 802.11 Infrared, FHSS, DSSS; 802.11a OFDM, 802.11b HR-DSSS, and 802.11g ODFM802.11 Infrared, FHSS, DSSS; 802.11a OFDM, 802.11b HR-DSSS, and 802.11g ODFM

3. March 22, 2004 Prof. Paul Lin 3 The 802.11 Protocol Stacks Computer Networks, 4 th ed, Andrew Tanenbaum, page 293

4. March 22, 2004 Prof. Paul Lin 4 The 802.11 Protocol Stacks 802.11 Infrared 802.11 Infrared 802.11 FHSS (Frequency Hopping Spread Spectrum) 802.11 FHSS (Frequency Hopping Spread Spectrum) 802.11 DSSS (Direct Sequence Spread Spectrum) 802.11 DSSS (Direct Sequence Spread Spectrum) 802.11a OFDM (Orthogonal Frequency Division Multiplexing) 802.11a OFDM (Orthogonal Frequency Division Multiplexing) 802.11 b HR-DSSS (High Data Rate Direct Sequence Spread Spectrum) 802.11 b HR-DSSS (High Data Rate Direct Sequence Spread Spectrum) 802.11g OFDM 802.11g OFDM

5. March 22, 2004 Prof. Paul Lin 5 802.11 Infrared 0.85-0.95 microns 0.85-0.95 microns Two speeds: 1 Mbps and 2 Mbps Two speeds: 1 Mbps and 2 Mbps Encoding Schemes Encoding Schemes Gray Code – 1 MbpsGray Code – 1 Mbps

6. March 22, 2004 Prof. Paul Lin 6 802.11 FHSS (Frequency Hopping Spread Spectrum) 79 Channels, 1 MHz wide, 2.4-G ISM band 79 Channels, 1 MHz wide, 2.4-G ISM band Pseudorandom no generator – sequence of frequencies Pseudorandom no generator – sequence of frequencies Dwell time - time spent at each frequency Dwell time - time spent at each frequency Adjustable, and less than 400 m-secAdjustable, and less than 400 m-sec Main Disadvantage – low bandwidth Main Disadvantage – low bandwidth Advantages Advantages Security (dwell time, frequency hopping)Security (dwell time, frequency hopping) Good resistance to multipath fading – long distanceGood resistance to multipath fading – long distance Insensitive to radio interference – building-to-building linksInsensitive to radio interference – building-to-building links

7. March 22, 2004 Prof. Paul Lin 7 IEEE 802.11a OFDM (Orthogonal Frequency Division Multiplexing) Up to 54 Mbps in the wider 5-GHz ISM band Up to 54 Mbps in the wider 5-GHz ISM band 52 Frequencies – 48 for data and 4 for synchronization 52 Frequencies – 48 for data and 4 for synchronization Encoding schemes Encoding schemes phase-shift modulation – up to 18 Mbpsphase-shift modulation – up to 18 Mbps QAM modulation - above 18 MbpsQAM modulation - above 18 Mbps At 54 Mbps – 216 data bits are encoded into 288-bit symbols At 54 Mbps – 216 data bits are encoded into 288-bit symbols Good spectrum efficiency – bits/Hz Good spectrum efficiency – bits/Hz

8. March 22, 2004 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 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 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 Supported data rate: 1, 2, 5.5 and 11 Mbps Walsh/Hadamard codes – for faster data rates Walsh/Hadamard codes – for faster data rates Longer communication range – 7 times greater than 802.11a Longer communication range – 7 times greater than 802.11a

9. March 22, 2004 Prof. Paul Lin 9 IEEE 802.11g OFDM (Orthogonal Frequency Division Multiplexing) Approved in November 2001 Approved in November 2001 Operates in the narrow 2.4 GHz ISM band Operates in the narrow 2.4 GHz ISM band Up to 54 MBps Up to 54 MBps

10. March 22, 2004 Prof. Paul Lin 10 802.11 MAC Sublayer Protocol Two problems Two problems The hidden station problemThe hidden station problem The exposed station problemThe exposed station problem Two Modes of Operations – to take care the two problems Two Modes of Operations – to take care the two problems DCF (Distributed Coordination Function)DCF (Distributed Coordination Function) PCF (Point Coordination Function) – use base station or access pointPCF (Point Coordination Function) – use base station or access point

11. March 22, 2004 Prof. Paul Lin 11 DCF (Distributed Coordination Function) Similar to Ethernet Similar to Ethernet Uses CSMA/CA (Collision Avoidance) Uses CSMA/CA (Collision Avoidance) Use Virtual Channel Sensing – Negative Acknowledge Vector Use Virtual Channel Sensing – Negative Acknowledge Vector Stop-and-Wait Protocol Stop-and-Wait Protocol Fragment burst Fragment burst

12. March 22, 2004 Prof. Paul Lin 12 PCF (Point Coordination Function) Use base station or access point to polls (broadcasts) the other stations Use base station or access point to polls (broadcasts) the other stations Beacon Frame (10 to 100 times per sec)Beacon Frame (10 to 100 times per sec) Hopping f and Dwell time, Clock sync.Hopping f and Dwell time, Clock sync. Invite new stations to sign upInvite new stations to sign up Ask any frames to send? Ask any frames to send? Completely controlled transmission order Completely controlled transmission order No collision ever occur No collision ever occur

13. March 22, 2004 Prof. Paul Lin 13 802.11 Inter-frame Spacing SIFS (Short Interframe Spacing) SIFS (Short Interframe Spacing) Inform that a control frame or next frame may be sentInform that a control frame or next frame may be sent PIFS (PCF Interframe Spacing) PIFS (PCF Interframe Spacing) Inform that a PCF frame may be sentInform that a PCF frame may be sent DIFS ( DCF Interframe Spacing) DIFS ( DCF Interframe Spacing) Inform that a DCF frame may be sentInform that a DCF frame may be sent EIFS (Extended Interframe Spacing) EIFS (Extended Interframe Spacing) Inform that a Bad Frame recovery done hereInform that a Bad Frame recovery done here

14. March 22, 2004 Prof. Paul Lin 14 802.11 Inter-frame Spacing SIFS (Short Interframe Spacing) SIFS (Short Interframe Spacing) PIFS (PCF Interframe Spacing) PIFS (PCF Interframe Spacing) DIFS ( DCF Interframe Spacing) DIFS ( DCF Interframe Spacing) EIFS (Extended Interframe Spacing) EIFS (Extended Interframe Spacing)

15. March 22, 2004 Prof. Paul Lin 15 The 802.11 Data Frame ….. Frame Ctrl Addr.3 Dura- tion Addr.1Addr.2 ….. Addr.4Seq. Data Check- sum …. byte 2 2 6 6 6 2 6 0-2312 4 Versi- on Type Sub- Type From DS MorePwr To DS MF W Re try O Bits 2 2 4 1 1 1 1 1 1 1 1

16. March 22, 2004 Prof. Paul Lin 16 The 802.11 Frame Control (11-sub fields) Version – Protocol Version – Protocol Type: data, control, or management Type: data, control, or management Subtypes – RTS or CTS Subtypes – RTS or CTS To DS (distributed system) To DS (distributed system) From DS From DS MF – More fragment will come MF – More fragment will come Retry - retrnasmission Retry - retrnasmission Pwr – Power management (in/out sleep mode) Pwr – Power management (in/out sleep mode) More – frames More – frames W – using WEP (Wired Equivalency Privacy Encryption) W – using WEP (Wired Equivalency Privacy Encryption) O – Strictly in order O – Strictly in order

17. March 22, 2004 Prof. Paul Lin 17 802.11 Data Frame Duration Field Duration Field How long the frame and ACK will occupy the channel, NAV managementHow long the frame and ACK will occupy the channel, NAV management Address 1 - Source Address 1 - Source Address 2 - Destination Address 2 - Destination Address 3 Address 3 Intercell traffic, source base stationsIntercell traffic, source base stations Address 4 Address 4 Intercell traffic, destination base stationsIntercell traffic, destination base stations Sequence – for numbering fragments Sequence – for numbering fragments

18. March 22, 2004 Prof. Paul Lin 18 802.11 Services Five Distribution Services Five Distribution Services Association – connect mobile stations to APAssociation – connect mobile stations to AP DisassociationDisassociation Re-associationRe-association Distribution – Frame routingDistribution – Frame routing Integration – with non-802.11 networkIntegration – with non-802.11 network Four Station Services Four Station Services Authentication – Secrete key (password)Authentication – Secrete key (password) De-authenticationDe-authentication Privacy – RC4 encryptionPrivacy – RC4 encryption Data Delivery - reliabilityData Delivery - reliability