Medium Access Control

1
Medium Access Control

• Onno W. Purbo
• Onno_at_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

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CSMA/CA access mechanism
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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
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MAC retransmission

• Positive acknowledgement
• Retransmission

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MAC retransmission in CSMA/CA
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Fragmentation

• Smaller packet ? higher reliability.
• Reassembly

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RTS/CTS hidden nodes
14
RTS/CTS hidden node
15
Network Topology
16
Ad-hoc network
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Access Point Roaming
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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.

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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.

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Active Scanning Example

• Steps to association
• Station sends Probe

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Active Scanning Example

• Steps to association
• Station sends Probe
• APs send Probe Response.

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Active Scanning Example

• Steps to association
• Station sends Probe
• APs send Probe Response.
• Station selects best AP.

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Active Scanning Example

• Steps to association
• Station sends Probe
• APs send Probe Response.
• Station selects best AP.
• Station sends Association Request to selected AP.

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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.

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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.

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MAC Management Frame ..

• Reassociation Response
• Capability, Status Code, Station ID, Supported
  Rates.
• Dissociation
• Reason Code
• Authentication
• Algorithm, Sequence, Status, Challenge Text.
• Deauthentication
• Reason