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Medium Access Control

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Medium Access Control

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    1. Medium Access Control Onno W. Purbo Onno@indo.net.id

    2. Main Channel Access Mechanism TDMA CSMA/CA Polling

    3. TDMA Access Mechanism

    4. TDMA OK for phone Predictable Tight / fix length Not OK for data Bursty traffic unpredictable

    5. CSMA/CA access mechanism

    6. CSMA/CA Listen before talk. Contention (wait a random amount of time). Collision avoidance. Slotted 50 us FH 20 us DS

    7. Polling channel access mechanism

    8. Polling Semi TDMA, semi CSMA/CA Base station keeps control No fix length packet

    9. MAC Techniques

    10. MAC retransmission Positive acknowledgement Retransmission

    11. MAC retransmission in CSMA/CA

    12. Fragmentation Smaller packet ? higher reliability. Reassembly

    13. RTS/CTS & hidden nodes

    14. RTS/CTS hidden node

    15. Network Topology

    16. Ad-hoc network

    17. Access Point & Roaming

    18. Roaming, Access Point & MAC Forwading

    19. Scanning Scanning required for many functions. Finding and joining a network. Finding a new AP while roaming. Initializing an independent BSS (ad hoc) network. 802.11 MAC uses a common mechanism for all PHY. Single or multi channel. Passive or active scanning. Passive scanning Find networks simply by listening for Beacon.

    20. Scanning .. Active Scanning On each channel Send a probe, wait for a probe responds. Beacon or Probe Response contains information necessary to join a network.

    21. Active Scanning Example Steps to association Station sends Probe

    22. Active Scanning Example Steps to association Station sends Probe APs send Probe Response.

    23. Active Scanning Example Steps to association Station sends Probe APs send Probe Response. Station selects best AP.

    24. Active Scanning Example Steps to association Station sends Probe APs send Probe Response. Station selects best AP. Station sends Association Request to selected AP.

    25. Active Scanning Example Steps to association Station sends Probe APs send Probe Response. Station selects best AP. Station sends Association Request to selected AP. AP sends Association Response.

    26. MAC Management Frame Beacon Timestamp, Beacon Interval, Capabilities, ESSID, Supported Rates, parameters. Traffic indication map. Probe ESSID, Capabilities, Supported Rates. Probe Responds Timestamp, Beacon Interval, Capabilities, ESSID, Supported Rates, parameters. Same for Beacon except for TIM.

    27. MAC Management Frame .. Association Request Capability, Listen Interval, ESSID, Supported Rates. Association Responds Capability, Status Code, Station ID, Supported Rates. Reassociation Request Capability, Listen Interval, ESSID, Supported Rates, Current AP Address.

    28. MAC Management Frame .. Reassociation Response Capability, Status Code, Station ID, Supported Rates. Dissociation Reason Code Authentication Algorithm, Sequence, Status, Challenge Text. Deauthentication Reason

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