1. Wireless EE 122: Intro to Communication Networks Fall 2010 (MW 4-5:30 in 101 Barker) Scott Shenker TAs: Sameer Agarwal, Sara Alspaugh, Igor Ganichev, Prayag Narula http://inst.eecs.berkeley.edu/~ee122/ Materials with thanks to Jennifer Rexford, Ion Stoica, Vern Paxson and other colleagues at Princeton and UC Berkeley

2. Review of Last Lecture Peer-to-peer: –Design paradigm: No central contol –Economic model: leverage user nodes As an economic model: –Latest step in war over control –Grassroots way to build large-scale system –But need good coordination mechanism 2

3. P2P Tasks Search: general keyword/wildcard search –Controlled flooding Lookup: given name, get IP address of location –DHT or similar structure Download: –Chunking is used because of asymmetric bandwidth –Self-scaling depends on an incentive mechanism oPrevent free-riders 3

4. 4 Wireless Networks

5. Wireless Communication Standards Cellular –2G: GSM (Global System for Mobile communication), CDMA (Code Division Multiple Access) –3G: CDMA2000 IEEE 802.11 –A: 5.0Ghz band, 54Mbps (25 Mbps operating rate) –B: 2.4Ghz band, 11Mbps (4.5 Mbps operating rate) –G: 2.4Ghz, 54Mbps (19 Mbps operating rate) –N: 2.4/5Ghz, 150Mbps IEEE 802.15 – lower power wireless –802.15.1: 2.4Ghz, 2.1 Mbps (Bluetooth) –802.15.4: 2.4Ghz, 250 Kbps (Sensor Networks) 5

6. Wireless Link Characteristics 6 (Figure Courtesy of Kurose and Ross)

7. What Makes Wireless Different? Signals sent by sender don’t always end up at receiver intact –Complicated physics involved, which we won’t discuss –But what can go wrong? Two quantities we care about: –Signal-to-noise ratio (SNR) oSignal Power/Noise Power –Bit-Error Rate (BER) 7

8. Path Loss Signal power attenuates by about ~r 2 factor for omni-directional antennas in free space – r is the distance between the sender and the receiver The exponent depends on placement of antennas Less than 2 for directional antennas Greater than 2 when antennas are placed on the ground –Signal bounces off the ground and reduces the power of the signal 8

9. Multipath Effects Signals bounce off surface and interfere with one another Self-interference 9 SR Ceiling Floor

10. Interference from Other Sources External Interference –Microwave is turned on and blocks your signal Internal Interference –Hosts within range of each other collide with one another’s transmission (remember Aloha) We have to tolerate path loss, multipath, etc., but we can try to avoid internal interference This is what makes 802.11 interesting…. 10

11. Ideal Radios (courtesy of Gilman Tolle and Jonathan Hui, ArchRock)

12. Real Radios (courtesy of Gilman Tolle and Jonathan Hui, ArchRock)

13. 13 The Amoeboed “cell” (courtesy of David Culler, UCB) Signal Noise Distance

14. Wireless Bit Errors The lower the SNR (Signal/Noise) the higher the Bit Error Rate (BER) We could make the signal stronger… Why is this not always a good idea? –Increased signal strength requires more power –Increases the interference range of the sender, so you interfere with more nodes around you oAnd then they increase their power……. Error Correction schemes can correct some problems 14

15. 15 802.11

16. 802.11 Architecture Designed for limited area AP’s (Access Points) set to specific channel Broadcast beacon messages with SSID (Service Set Identifier) and MAC Address periodically Hosts scan all the channels to discover the AP’s –Host associates with AP (actively or passively) 16 802.11 frames exchanges 802.3 (Ethernet) frames exchanged

17. Wireless Multiple Access Technique? Carrier Sense? –Sender can listen before sending –What does that tell the sender? Collision Detection? –Where do collisions occur? –How can you detect them? 17

18. 18 A and C can both send to B but can’t hear each other –A is a hidden terminal for C and vice versa Carrier Sense will be ineffective – need to sense at receiver Hidden Terminals ABC transmit range

19. 19 Exposed Terminals Exposed node: B sends a packet to A; C hears this and decides not to send a packet to D (despite the fact that this will not cause interference)! Carrier sense would prevent a successful transmission –But we do carrier sense anway (why?) ABC D

20. Key Points No concept of a global collision –Different receivers hear different signals –Different senders reach different receivers Collisions are at receiver, not sender –Only care if receiver can hear the sender clearly –It does not matter if sender can hear someone else –As long as that signal does not interfere with receiver Goal of protocol: –Detect if receiver can hear sender –Tell senders who might interfere with receiver to shut up 20

21. Basic Collision Avoidance Since can’t detect collisions, we try to avoid them Carrier sense: –When medium busy, choose random interval –Wait that many idle timeslots to pass before sending When a collision is inferred, retransmit with binary exponential backoff (like Ethernet) –Use ACK from receiver to infer “no collision” –Use exponential backoff to adapt contention window How would we get ACKs if we didn’t do carrier sense? 21

22. 22 MA with Collision Avoidance (MACA) Before every data transmission –Sender sends a Request to Send (RTS) frame containing the length of the transmission –Receiver respond with a Clear to Send (CTS) frame –Sender sends data –Receiver sends an ACK; now another sender can send data When sender doesn’t get a CTS back, it assumes collision senderreceiver other node in sender’s range RTS ACK data CTS

23. 23 MACA, con’t If other nodes hear RTS, but not CTS: send –Presumably, destination for first sender is out of node’s range … sender receiver other node in sender’s range RTS dat a CTS dat a

24. 24 MACA, con’t If other nodes hear RTS, but not CTS: send –Presumably, destination for first sender is out of node’s range … –… Can cause problems when a CTS is lost When you hear a CTS, you keep quiet until scheduled transmission is over (hear ACK) senderreceiver other node in sender’s range RTS ACK data CTS

25. 25 Overcome hidden terminal problems with contention-free protocol 1.B sends to C Request To Send (RTS) 2.A hears RTS and defers (to allow C to answer) 3.C replies to B with Clear To Send (CTS) 4.D hears CTS and defers to allow the data 5.B sends to C RTS / CTS Protocols (MACA) BCD RTS CTS A B sends to C

26. 26 5 Minute Break Questions Before We Proceed?

27. 27 Channelization of spectrum Typically, available frequency spectrum is split into multiple channels Some channels may overlap 26 MHz100 MHz200 MHz150 MHz 2.45 GHz 915 MHz 5.25 GHz 5.8 GHz 3 channels8 channels4 channels 250 MHz500 MHz1000 MHz 61.25 GHz24.125 GHz122.5 GHz

28. Preventing Collisions Altogether Frequency Spectrum partitioned into several channels –Nodes within interference range can use separate channels –Now A and C can send without any interference! –Aggregate Network throughput doubles 28 A B C D

29. Using Multiple Channels 802.11: AP’s on different channels –Usually manually configured by administrator –Automatic Configuration may cause problems Most cards have only 1 transceiver –Not Full Duplex: Cannot send and receive at the same time Multichannel MAC Protocols –Automatically have nodes negotiate channels oChannel coordination amongst nodes is necessary oIntroduces negotiation and channel-switching latency that reduce throughput 29

30. Wireless Multihop Networks Vehicular Networks –Delay Tolerant (batch) sending over several hops carry data to a base station Common in Sensor Network for periodically transmitting data –Infrastructure Monitoring oE.g., structural health monitoring of the Golden Gate Bridge Multihop networking for Internet connection sharing –Routing traffic over several hops to base station connected to Internet –E.g., Meraki Networks 30

31. Large Multihop Network (courtesy of Sanjit Biswas, MIT) 1 kilometer

32. Multi-Hop Wireless Ad Hoc Networks (Courtesy of Tianbo Kuang and Carey Williamson University of Calgary) R A B C D S

33. 33 Multi-Hop Wireless Ad Hoc Networks R A B C D S

34. 34 Multi-Hop Wireless Ad Hoc Networks S R A B C D 1 2 3 4 5 6 7 8 9 10 11 12 (Assume ideal world…)

35. 35 Multi-Hop Wireless Ad Hoc Networks S R A B C D 1 2 3 4 5 6 7 8 9 10 11 12

36. 36 Multi-Hop Wireless Ad Hoc Networks S R A B C D 1 2 3 4 5 6 7 8 9 10 11 12

37. 37 Multi-Hop Wireless Ad Hoc Networks S R A B C D 1 2 3 4 5 6 7 8 9 10 11 12

38. 38 Multi-Hop Wireless Ad Hoc Networks S R A B C D 1 2 3 4 5 6 7 8 9 10 11 12

39. 39 Multi-Hop Wireless Ad Hoc Networks S R A B C D 1 2 3 4 5 6 7 8 9 10 11 12

40. 40 Multi-Hop Wireless Ad Hoc Networks S R A B C D 1 2 34 5 6 7 8 9 10 11 12

41. 41 Multi-Hop Wireless Ad Hoc Networks S R A B C D 1 2 3 4 5 6 7 8 9 10 11 12

42. 42 Multi-Hop Wireless Ad Hoc Networks S R A B C D 1 2 3 4 5 6 7 8 9 10 11 12

43. 43 Multi-Hop Wireless Ad Hoc Networks S R A B C D 1 2 3 4 5 6 7 8 9 10 11 12

44. 44 Multi-Hop Wireless Ad Hoc Networks S R A B C D 1 2 3 4 5 6 7 8 9 10 11 12

45. 45 Multi-Hop Wireless Ad Hoc Networks S R A B C D 1 2 3 4 5 6 7 8 9 10 11 12

46. 46 Multi-Hop Wireless Ad Hoc Networks S R A B C D 1 2 3 4 5 6 7 8 9 10 11 12

47. 47 Multi-Hop Wireless Ad Hoc Networks S R A B C D 1 2 3 4 5 6 7 8 9 1011 12

48. 48 Multi-Hop Wireless Ad Hoc Networks S R A B C D 1 2 3 4 5 6 7 8 9 10 11 12

49. 49 Multi-Hop Wireless Ad Hoc Networks S R A B C D 1 2 3 4 5 6 7 8 9 10 11 12

50. 50 Multi-Hop Wireless Ad Hoc Networks S R A B C D 1 2 3 4 5 6 7 8 9 10 11 12

51. 51 Multi-Hop Wireless Ad Hoc Networks S R A B C D 1 2 3 4 5 6 7 8 9 10 11 12

52. What Do YOU Think Really Happens? 52

53. 53 Multi-Hop Wireless Ad Hoc Networks S R A B C D 1 2 3 4 5 6 7 8 9 10 11 12 Problem 1: node A can’t use both of these links at the same time - shared wireless channel - transmit or receive, but not both (Reality check…)

54. 54 Multi-Hop Wireless Ad Hoc Networks S R A B C D 1 2 3 4 5 6 7 8 9 10 11 12 Problem 2: S and B can’t use both of these links at same time - range overlap at A

55. 55 Multi-Hop Wireless Ad Hoc Networks S R A B C D 1 2 3 4 5 6 7 8 9 10 11 12 Problem 3: LOTS of contention for the channel - in steady state, all want to send - need RTS/CTS to resolve contention RTS: Request-To-Send CTS: Clear-To-Send

56. 56 Multi-Hop Wireless Ad Hoc Networks S R A B C D 1 2 3 4 5 6 7 8 9 10 11 12 RTS CTS

57. 57 Multi-Hop Wireless Ad Hoc Networks S R A B C D 1 2 3 4 5 6 7 8 9 10 11 12

58. 58 Multi-Hop Wireless Ad Hoc Networks S R A B C D 1 2 3 4 5 6 7 8 9 10 11 12 RTSCTS

59. 59 Multi-Hop Wireless Ad Hoc Networks S R A B C D 1 2 3 4 5 6 7 8 9 10 11 12

60. 60 Multi-Hop Wireless Ad Hoc Networks S R A B C D 1 2 3 4 5 6 7 8 9 10 11 12

61. 61 Multi-Hop Wireless Ad Hoc Networks S R A B C D 1 2 3 4 5 6 7 8 9 10 11 12

62. 62 Multi-Hop Wireless Ad Hoc Networks S R A B C D 1 2 3 4 5 6 7 8 9 10 11 12

63. 63 Multi-Hop Wireless Ad Hoc Networks S R A B C D 1 2 3 4 5 6 7 8 9 10 11 12

64. 64 Multi-Hop Wireless Ad Hoc Networks S R A B C D 1 2 3 4 5 6 7 8 9 10 11 12

65. 65 Multi-Hop Wireless Ad Hoc Networks S R A B C D 1 2 3 4 5 6 7 8 9 10 11 12

66. 66 Multi-Hop Wireless Ad Hoc Networks S R A B C D 1 2 3 4 5 6 7 8 9 10 11 12

67. 67 Multi-Hop Wireless Ad Hoc Networks S R A B C D 1 2 3 4 5 6 7 8 9 10 11 12

68. 68 Multi-Hop Wireless Ad Hoc Networks S R A B C D 1 2 3 4 5 6 7 8 9 10 11 12

69. 69 Multi-Hop Wireless Ad Hoc Networks S R A B C D 1 2 3 4 5 6 7 8 9 10 11 12

70. 70 Multi-Hop Wireless Ad Hoc Networks S R A B C D 1 2 3 4 5 6 7 8 9 10 11 12 Problem 4: TCP uses ACKS to indicate reliable data delivery - bidirectional traffic (DATA, ACKS) - even more contention!!!

71. 71 Multi-Hop Wireless Ad Hoc Networks S R A B C D 1 2 3 4 5 6 7 8 9 10 11 12

72. 72 Multi-Hop Wireless Ad Hoc Networks S R A B C D 1 2 3 4 5 6 7 8 9 10 11 12

73. 73 Multi-Hop Wireless Ad Hoc Networks S R A B C D 1 2 3 4 5 6 7 8 9 10 11 12

74. 74 Multi-Hop Wireless Ad Hoc Networks S R A B C D 1 2 3 4 5 6 7 8 9 10 11 12

75. 75 Multi-Hop Wireless Ad Hoc Networks S R A B C D 1 2 3 4 5 6 7 8 9 10 11 12

76. 76 Multi-Hop Wireless Ad Hoc Networks S R A B C D 1 2 3 4 5 6 7 8 9 10 11 12 1 1 12 2

77. 77 Multi-Hop Wireless Ad Hoc Networks S R A B C D 1 3 4 5 6 7 8 9 10 11 12 1 2

78. 78 Multi-Hop Wireless Ad Hoc Networks S R A B C D 1 3 4 5 6 7 8 9 10 11 12 1 2

79. 79 Multi-Hop Wireless Ad Hoc Networks S R A B C D 1 3 4 5 6 7 8 9 10 11 121 2

80. 80 Multi-Hop Wireless Ad Hoc Networks S R A B C D 1 3 4 5 6 7 8 9 10 11 12 1 2

81. 81 Multi-Hop Wireless Ad Hoc Networks S R A B C D 1 3 4 5 6 7 8 9 10 11 12 1 2

82. 82 Multi-Hop Wireless Ad Hoc Networks S R A B C D 1 3 4 5 6 7 8 9 10 11 12 1 2 2

83. Lesson Multihop wireless is hard to make efficient 83

84. Summary Wireless is a tricky beast –Distributed multiple access problem –Hidden terminals –Exposed terminals –Current protocols sufficient, given overprovisioning Multihop even more complicated 84

85. Rest of Course Next lecture: Security After that: Future of Networking –Why things will look completely different in ten years Review 85