1 Wireless Networking Understanding the departure from wired networks, Case study: IEEE (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 The Wireless Future … Internet

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 (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 A C C B B

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

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 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 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 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 A B C D Distance Signal power SINR threhold

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

20 Calculating SINR A B C

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

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 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 Hidden and Exposed Terminal Problems Critical to wireless networks even today

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

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 The Emergence of Wireless MAC proved to be non-trivial research by Karn (MACA) research by Bhargavan (MACAW) Led to IEEE committee  The standard was ratified in 1999

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

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

30 But is that enough?

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

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

36 Thoughts ! 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, is being optimized too. Thus, wireless MAC research still alive

37 Questions?