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

2. 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. 3 From Links to Networks Variety of architectures  Single hop networks  Multi-hop networks

4. 4 The Wireless Future … Internet

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

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

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

8. 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. 9 Medium Access Control

10. 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 A C C B B

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

12. 12 The Simple Fix 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 A C C B B Don’t transmit Don’t transmit Can collisions still occur?

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

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

15. 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 The transmitter can DETECT if and when collision occurs

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

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

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

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

20. 20 Calculating SINR A B C

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

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

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

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

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

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

27. 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. 28 CTS = Clear To Send RTS = Request To Send IEEE 802.11 with Omni Antenna D Y S M K RTS CTS

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

30. 30 But is that enough?

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

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

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

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

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

36. 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. 37 Questions?