Medium Access Control Onno W Purbo Onnoindo net
Medium Access Control Onno W. Purbo Onno@indo. net. id
Main Channel Access Mechanism n n n TDMA CSMA/CA Polling
TDMA Access Mechanism
TDMA n OK for phone n n n Predictable Tight / fix length Not OK for data n Bursty traffic unpredictable
CSMA/CA access mechanism
CSMA/CA n n Listen before talk. Contention (wait a random amount of time). Collision avoidance. Slotted n n 50 us – FH 20 us – DS
Polling channel access mechanism
Polling n n n Semi TDMA, semi CSMA/CA Base station keeps control No fix length packet
MAC Techniques
MAC retransmission n n Positive acknowledgement Retransmission
MAC retransmission in CSMA/CA
Fragmentation n n 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 n Scanning required for many functions. n n 802. 11 MAC uses a common mechanism for all PHY. n n n Finding and joining a network. Finding a new AP while roaming. Initializing an independent BSS (ad hoc) network. Single or multi channel. Passive or active scanning. Passive scanning n Find networks simply by listening for Beacon.
Scanning. . n Active Scanning n On each channel n n Send a probe, wait for a probe responds. Beacon or Probe Response contains information necessary to join a network.
Active Scanning Example n Steps to association n Station sends Probe
Active Scanning Example n Steps to association n n Station sends Probe APs send Probe Response.
Active Scanning Example n Steps to association n Station sends Probe APs send Probe Response. Station selects best AP.
Active Scanning Example n Steps to association n n Station sends Probe APs send Probe Response. Station selects best AP. Station sends Association Request to selected AP.
Active Scanning Example n Steps to association n n 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 n Beacon n Probe n n Timestamp, Beacon Interval, Capabilities, ESSID, Supported Rates, parameters. Traffic indication map. ESSID, Capabilities, Supported Rates. Probe Responds n n Timestamp, Beacon Interval, Capabilities, ESSID, Supported Rates, parameters. Same for Beacon except for TIM.
MAC Management Frame. . n Association Request n n Association Responds n n Capability, Listen Interval, ESSID, Supported Rates. Capability, Status Code, Station ID, Supported Rates. Reassociation Request n Capability, Listen Interval, ESSID, Supported Rates, Current AP Address.
MAC Management Frame. . n Reassociation Response n n Dissociation n n Reason Code Authentication n n Capability, Status Code, Station ID, Supported Rates. Algorithm, Sequence, Status, Challenge Text. Deauthentication n Reason
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