Medium Access Control

							Medium Access Control

       Onno W. Purbo
      Onno@indo.net.id
Main Channel Access
Mechanism
   TDMA
   CSMA/CA
   Polling
TDMA Access Mechanism
TDMA
   OK for phone
       Predictable
       Tight / fix length
   Not OK for data
       Bursty traffic unpredictable
CSMA/CA access mechanism
CSMA/CA
   Listen before talk.
   Contention (wait a random amount of
    time).
   Collision avoidance.
   Slotted
       50 us – FH
       20 us – DS
Polling channel access
mechanism
Polling
   Semi TDMA, semi CSMA/CA
   Base station keeps control
   No fix length packet
MAC Techniques
MAC retransmission
   Positive acknowledgement
   Retransmission
MAC retransmission in
CSMA/CA
Fragmentation
   Smaller packet  higher reliability.
   Reassembly
RTS/CTS & hidden nodes
RTS/CTS hidden node
Network Topology
Ad-hoc network
Access Point & Roaming
Roaming, Access Point &
MAC Forwading
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.
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.
Active Scanning Example
                 Steps to association
                     Station sends Probe
Active Scanning Example
                 Steps to association
                     Station sends Probe
                     APs send Probe
                      Response.
Active Scanning Example
                 Steps to association
                     Station sends Probe
                     APs send Probe
                      Response.
                     Station selects best
                      AP.
Active Scanning Example
                 Steps to association
                     Station sends Probe
                     APs send Probe
                      Response.
                     Station selects best
                      AP.
                     Station sends
                      Association Request
                      to selected AP.
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.
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.
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.
MAC Management Frame ..
   Reassociation Response
       Capability, Status Code, Station ID, Supported
        Rates.
   Dissociation
       Reason Code
   Authentication
       Algorithm, Sequence, Status, Challenge Text.
   Deauthentication
       Reason