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CSMA/CA in IEEE 802.11

CSMA/CA in IEEE 802.11. Physical carrier sense, and Virtual carrier sense using Network Allocation Vector (NAV) NAV is updated based on overheard RTS/CTS/DATA/ACK packets, each of which specified duration of a pending transmission Nodes stay silent when carrier sensed (physical/virtual)

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CSMA/CA in IEEE 802.11

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  1. CSMA/CA in IEEE 802.11 • Physical carrier sense, and • Virtual carrier sense using Network Allocation Vector (NAV) • NAV is updated based on overheard RTS/CTS/DATA/ACK packets, each of which specified duration of a pending transmission • Nodes stay silent when carrier sensed (physical/virtual) • Backoff intervals used to reduce collision probability

  2. Backoff Interval • When transmitting a packet, choose a backoff interval in the range [0,cw] • cw is contention window • Count down the backoff interval when medium is idle • Count-down is suspended if medium becomes busy • When backoff interval reaches 0, transmit RTS

  3. B1 = 25 B1 = 5 wait data data wait B2 = 10 B2 = 20 B2 = 15 DCF Example B1 and B2 are backoff intervals at nodes 1 and 2 cw = 31

  4. Backoff Interval • The time spent counting down backoff intervals is a part of MAC overhead • Choosing a large cwleads to large backoff intervals and can result in larger overhead • Choosing a small cw leads to a larger number of collisions (when two nodes count down to 0 simultaneously)

  5. Since the number of nodes attempting to transmit simultaneously may change with time, some mechanism to manage contention is needed • IEEE 802.11 DCF: contention window cw is chosen dynamically depending on collision occurrence

  6. Binary Exponential Backoff in DCF • When a node fails to receive CTS in response to its RTS, it increases the contention window • cw is doubled (up to an upper bound) • When a node successfully completes a data transfer, it restores cw to Cwmin • cw follows a sawtooth curve

  7. 802.11 Handoff

  8. Scanning Phase in Current 802.11Handoff Process • AP discovery by scanning • Explicitly scan all channels • Passive scan • The client switches to a candidate channel and listens for periodic beacon packets from APs. • Active scan • The client actively broadcasts a probe packet on each channel to force APs to respond immediately. • New AP selection • Compare the signal strength

  9. Problems in 802.11 Handoff • Approximate latencies for current 802.11 handoff • Scanning: 350-500ms • Authentication: < 10 ms • (Re)association: < 10 ms (w/o IAPP) • The client only monitors the signal to the current AP and handoffs only after the service degrades below an acceptable threshold. • Solutions • Continuously monitoring the proximity of nearby 802.11 APs ! • Maintain AP neighboring graph

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