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IEEE 802.11 DCF Mechanisms for Efficient Wireless Communication

Learn about CSMA/CA in IEEE 802.11, Backoff Intervals, Binary Exponential Backoff, Handoff Scanning, and Solutions. Explore how nodes manage contention and optimize transmissions with dynamic window adjustments. Discover key latency factors in handoff processes and effective AP selection methods.

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IEEE 802.11 DCF Mechanisms for Efficient Wireless Communication

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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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