Wireless Networking Understanding the departure from wired networks, Case study: IEEE 802.11 (WiFi)

1. Wireless NetworkingUnderstanding the departure from wired networks,Case study: IEEE 802.11 (WiFi)

2. Many Motivations for Wireless • Unrestricted mobility / deployability • Unplugged from power outlet • Significantly lower cost • No cable layout, service provision • Low maintenance • Ease • Direct communication with minimum infratructure

3. From Links to Networks • Variety of architectures • Single hop networks • Multi-hop networks

4. Internet The Wireless Future …

5. No Free Lunch • Numerous challenges • Channel fluctuation • Lower bandwidth • Limited Battery power • Disconnection due to mobility • Security • …

6. Question Is … Can’t we use the rich “wireline” knowledge ? In solving the wireless challenges

7. The Answer Wireless channel: A dispersive medium The PHY and MAC layer completely dissimilar The whole game changes

8. On Our Agenda • Key Physical layer behavior • From Wired to Wireless • The principles of wireless medium access control • Collision avoidance (CSMA/CA) not detection • The emergence of 802.11 (WiFi)

9. Medium Access Control

10. The Channel Access Problem • Multiple nodes share a channel • Pairwise communication desired • Simultaneous communication not possible • MAC Protocols • Suggests a scheme to schedule communication • Maximize number of communications • Ensure fairness among all transmitters A B C

11. The Trivial Solution • Transmit and pray • Plenty of collisions --> poor throughput at high load A B C

12. The Simple Fix Don’t transmit • Transmit and pray • Plenty of collisions --> poor throughput at high load • Listen before you talk • Carrier sense multiple access (CSMA) • Defer transmission when signal on channel A B C Can collisions still occur?

13. CSMA collisions spatial layout of nodes Collisions can still occur: Propagation delay non-zero between transmitters When collision: Entire packet transmission time wasted note: Role of distance & propagation delay in determining collision probability

14. CSMA/CD (Collision Detection) • Keep listening to channel • While transmitting • If (Transmitted_Signal != Sensed_Signal)  Sender knows it’s a Collision  ABORT

15. 2 Observations on CSMA/CD • Transmitter can send/listen concurrently • If (Sensed - received = null)? Then success • The signal is identical at Tx and Rx • Non-dispersive The transmitter can DETECT if and when collision occurs

16. Unfortunately … Both observations do not hold for wireless Leading to …

17. Wireless Medium Access Control C D A B Signal power SINR threhold Distance

18. Wireless Media Disperse Energy A cannot send and listen in parallel C D A B Signal power Signal not same at different locations SINR threhold Distance

19. Collision Detection Difficult • Signal reception based on SINR • Transmitter can only hear itself • Cannot determine signal quality at receiver

20. Calculating SINR B A C

21. Red < Blue = collision Red signal >> Blue signal C D X A B Signal power SINR threhold Distance

22. Important: C has not heard A, but can interfere at receiver B C is the hidden terminal to A C D X A B Signal power SINR threhold Distance

23. Important: X has heard A, but should not defer transmission to Y Y X is the exposed terminal to A C D X A B Signal power SINR threhold Distance

24. Hidden and Exposed Terminal Problems Critical to wireless networks even today

25. Idea! C D X A B Signal power SINR threhold Sensitivity threshold Distance

26. Idea! Do not transmit in this region Will this solve the wireless MAC problem? C D X A B Signal power SINR threhold T Sensitivity threshold Distance

27. The Emergence of 802.11 • Wireless MAC proved to be non-trivial • 1992 - research by Karn (MACA) • 1994 - research by Bhargavan (MACAW) • Led to IEEE 802.11 committee • The standard was ratified in 1999

28. RTS = Request To Send CTS = Clear To Send IEEE 802.11 with Omni Antenna M Y S RTS D CTS K

29. IEEE 802.11 with Omni Antenna silenced M Y silenced S Data D ACK silenced X K silenced

30. But is that enough?

31. RTS CTS RTS/CTS • Does it solve hidden terminals ? • Assuming carrier sensing zone = communication zone E F A B C D E does not receive CTS successfully  Can later initiate transmission to D. Hidden terminal problem remains.

32. Hidden Terminal Problem • How about increasing carrier sense range ?? • E will defer on sensing carrier  no collision !!! RTS E F CTS A B C D Data

33. Hidden Terminal Problem • But what if barriers/obstructions ?? • E doesn’t hear C  Carrier sensing does not help RTS E F CTS A B C D Data

34. Exposed Terminal • B should be able to transmit to A • RTS prevents this E RTS CTS A B C D

35. Exposed Terminal • B should be able to transmit to A • Carrier sensing makes the situation worse E RTS CTS A B C D

36. Thoughts ! • 802.11 does not solve HT/ET completely • Only alleviates the problem through RTS/CTS and recommends larger CS zone • Large CS zone aggravates exposed terminals • Spatial reuse reduces  A tradeoff • RTS/CTS packets also consume bandwidth • Moreover, backing off mechanism is also wasteful The search for the best MAC protocol is still on. However, 802.11 is being optimized too. Thus, wireless MAC research still alive

37. Questions?