2. Wireless Networks share a non deterministic medium: Possible Problems: Noise ⇒ discard the packet Collision ⇒ discard the packet

3. Collision Noise

4. Predict Channel Quality and choose the “right” rate/coding Prediction based on Acknowledgments (ACKs): Successful Transmission ⇒ increase rate Failed Transmission ⇒ decrease rate Why? Noise is the sole reason for transmission failure, because: 802.11 -> 1997 – 1999 CSMA/CA -> collision avoidance

5. Carrier Sensing Multiple Access/Collision Avoidance Avoidance: users perceive the medium differently Backoff Timer: [0, CW] “Packet Fair” Problems: too many nodes hidden terminal capture effect

6. Lower encoding = lower transmission rate  Lower throughput for the node itself  Lower throughput for the SYSTEM Price of “Fairness” – 802.11 Anomaly 1 Mbps 54 Mbps 54 & 1 Mbps

7. Packet Frame FCS: Frame Check Sequence Packet destroyed by Noise:

8. Collision scenarios: i) Visible Nodes ii) Hidden Terminal

9. Collision scenarios in Hidden Terminal:

10. If Power of Packet A ≫ Packet B 1. Packet A received correctly 2. Packet B lost due to collision

11. A more sophisticated checksum function Smaller Segments (1500bytes/numOfSegs) ⇒ smaller CRC (16bits instead of 32) ⇒ small overhead in data transmitted and in CPU usage Why distributed?

12. Pr[Collision | Noise]: too noisy(P0) Pr[Noise | Collision]: small collision(P1) Count the erroneous CRCs big threshold ⇒ Po && P1 small threshold ⇒ Po && P1 Improvement Count the successive erroneous CRCs lower threshold & more accurate prediction

13. The receiver knows the cause of failure but the sender must be informed in time Successful Transmission ⇒ ACK ⇒ rate “Noisy” Transmission ⇒ NAK ⇒ rate Collision ⇒ - ⇒ - NAK per node and when needed Hardware limitations ⇒ Implemented in software Sent according to SampleRate specs

14. Collision ⇒ at least 1/2 of the headers are destroyed Receiver keeps per node statistics ⇒ header is needed ⇒ header small size (24bytes) ⇒ small possibility to be destroyed by noise

16. Congested NetworksHidden Terminal

17. Capture EffectCapture Effect with Half-hidden Terminal