HOW WI-FI WORKS AND WHY IT BREAKS WI-FI MECHANICS

Transcription

1 HOW WI-FI WORKS AND WHY IT BREAKS WI-FI MECHANICS

2 THE WI-FI COMMUNICATION PROCESS MECHANICS

3 MECHANICS: HOW WI-FI COMMUNICATION WORKS WHAT WE LL BE COVERING Brief Discourse on Frame Types Discovery of Wi-Fi Networks Joining Associating with a Network (and Disassociating) General Communication Process

4 THE MECHANICS A BRIEF DISCOURSE ON FRAME TYPES 1. Management 2. Control 3. Data

5 FRAME TYPES MANAGEMENT FRAMES Used to discover, join, and leave a network: Beacon Probes/Probe responses Association requests/response Reassociation request/response Deauthentication Disassociation

6 FRAME TYPES CONTROL FRAMES Manage delivery of traffic on a network Request to Send (RTS) Clear to Send (CTS) Acknowledgement (ACK)

7 FRAME TYPES DATA FRAMES Contain the actual data

8 FINDING NETWORKS TO JOIN DISCOVERY

9 LISTEN FOR BEACON FRAMES PASSIVE SCANNING

10 PASSIVE SCANNING BEACON FRAMES Access Points continually transmit Beacon Frames, usually at a rate of roughly 10 per second. Essentially an announcement of presence and capabilities.

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13 SEND PROBE REQUESTS ACTIVE SCANNING

14 ACTIVE SCANNING PROBE REQUESTS AND RESPONSE WHO S OUT THERE? I am here, and here is the information about me.

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16 DISCOVERY EMPHASIZING THE IMPORTANCE OF CHANNELS APs send beacon frames only on the channel they are configured to use. A client STA that s listening for beacons listens on a single channel, then switches, then listens some more. A client STA sending a problem sends it on a single channel, then switches channels and sends another.

17 Wi-Fi Frequency Band Channel Width Channels Total 2.4 GHz ISM 22 MHz 1,2,3,4,5,6,7,8,9,10,11 11 (3 non-overlapping) 5 GHz U-NII-1 20 MHz 36,40,44, GHz U-NII-2 20 MHz 52,56,60, GHz U-NII-3 20 MHz 149,153,157,161, GHz U-NII-2 Extended 20 MHz 100,104,108,112,116,120, 124,128,132,136,140,144 12

18 JOINING NETWORKS AUTHENTICATION AND ASSOCIATION

19 AUTHENTICATION AND ASSOCIATION WHAT IS WI-FI AUTHENTICATION? Wi-Fi Authentication is essentially confirmation that the devices at each end are capable devices. For actual data protection of data integrity and privacy, stay tuned.

20 AUTHENTICATION AND ASSOCIATION ASSOCIATION Occurs after authentication A client STA sends an association request to an AP which requests permission to join that AP s Basic Service Set (BSS) The AP sends an association response back to the client STA containing an Association ID (AID)

21 AUTHENTICATION AND ASSOCIATION AUTHENTICATED AND ASSOCIATED. NOW WHAT? In the case of an open network, the client STA can start to work with the rest of the network stack, e.g. obtaining a DHCP lease and beginning to use applications.

22 AUTHENTICATION AND ASSOCIATION AGAIN Protection of data privacy and integrity now comes at a different stage.

23 LEAVING A NETWORK DISASSOCIATION

24 DISASSOCIATION A NOTIFICATION, NOT A REQUEST Either an AP or a client STA can choose to leave a BSS. The device leaving sends a disassociation frame, which is a statement of occurrence.

25 RULES OF THE WI-FI ROAD COMMUNICATION PROCESS

26 IN WI-FI, ONLY ONE DEVICE AT A TIME CAN TRANSMIT ON A CHANNEL HALF-DUPLEX TECHNOLOGY

27 HOW DOES THAT EVEN WORK? CARRIER SENSE MULTIPLE ACCESS WITH COLLISION AVOIDANCE

28 CSMA/CA STEPS TO DETERMINE CONTROL OF THE MEDIUM 1. Carrier Sense Virtual Physical 2. Random Backoff Timer 3. Request to Send (RTS) and Clear to Send (CTS)

29 CSMA/CA VIRTUAL CARRIER SENSE Transmitted frames contain a Duration/ID field Duration field on the frame header tells a STA how long the medium will be busy

30 I EXPECT THE NETWORK TO BE BUSY FOR ABOUT X AMOUNT OF TIME NAV TIMER

31 CSMA/CA PHYSICAL CARRIER SENSE The client listens to the network; it tells the client things: 1. Whether there is traffic inbound for the STA? 2. Whether the medium is busy using Clear Channel Assessment.

32 CSMA/CA RANDOM BACKOFF TIMER Different from the NAV Timer Performs a separate countdown

33 CSMA/CA RANDOM BACKOFF TIMER This final check uses two factors to define a countdown time: 1. Contention Window (CW): random 2. Slot Time: defined by the PHY mode

34 WAIT. THERE S MORE! REQUEST TO SEND (RTS) CLEAR TO SEND (CTS)

35 TRANSMISSION PERMISSION REQUEST TO SEND/CLEAR TO SEND (RTS/CTS) 1. Client STA transmits a Request to Send (RTS) 2. AP receives the RTS, then transmits a Clear to Send (CTS) 3. Clients associated to the AP use the information contained in the CTS to reset their NAV timers Essentially, the client STA is asking for control of the medium, and the AP is notifying everyone in range that the client has control of the medium.

36 TRANSMIT!

37 IT S NOT OVER, NOT YET

38 TRANSMITTER HAS TO WAIT FOR ACKNOWLEDGEMENT (ACK)

39 RETRANSMISSIONS ACKNOWLEDGEMENT IS CRITICAL Receiver sends ACKs to verify that the receipt of unicast frames. Acknowledgement is the only way the sender knows the transmission was received. Lack of acknowledgement is interpreted as a collision and failure of delivery.

40 IF THERE S NO ACKNOWLEDGEMENT? RETRANSMISSION

41 UNDERSTAND THE PROBLEM FOR SUPERIOR TROUBLESHOOTING TECHNIQUE HOW IT BREAKS

42 HOW IT BREAKS THREE FUNDAMENTAL WI-FI PROBLEMS 1. Excessive Retransmissions 2. Medium Contention 3. Not the Wi-Fi (Blame it Anyway )

43 DO OR DO NOT, BUT THERE IS TRY, TRY, TRY, TRY, TRY EXCESSIVE RETRANSMISSIONS

44 EXCESSIVE RETRANSMISSIONS VISUALIZATION Analysis tools can show you evidence for congestion. Wireshark Metageek Eye P.A.

45 FUNDAMENTAL PROBLEM #1: EXCESS RETRANSMISSIONS EXCESS RETRANSMISSIONS Could be caused by Low Signal to Noise Ratio RF Interference Adjacent Channel Interference

46 FUNDAMENTAL PROBLEM #1: EXCESSIVE RETRANSMISSIONS LOW SIGNAL TO NOISE RATIO Signal to noise ratio is the most important metric we care about Subtract RSSI level from Noise level to derive SNR Lower SNR means higher likelihood of collisions or retransmissions

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49 LOW SIGNAL TO NOISE

50 SENSITIVE TO PHYSICS ATTENUATION Loss of signal amplitude (intensity) Materials/obstacles Distance

51 SIGNAL LEVELS AND COVERAGE TRANSMIT POWER ASYMMETRY 1. I AM HERE! 2. HELLO AP! CAN WE TALK?

52 SIGNAL LEVELS AND COVERAGE TRANSMIT POWER ASYMMETRY 3. BY ALL MEANS 4. HOW ABOUT THAT LOCAL SPORTS FRANCHISE?

53 SIGNAL LEVELS AND COVERAGE TRANSMIT POWER ASYMMETRY 2. HELLO AP! CAN WE TALK? 3. HELLO AP! CAN WE TALK? 4. HELLO AP! CAN WE TALK? 1. I AM H

54 EXCESSIVE RETRANSMISSIONS RADIO FREQUENCY INTERFERENCE

55 EXCESSIVE RETRANSMISSIONS RADIO FREQUENCY INTERFERENCE Something that modifies or disrupts a signal as it travels Unwanted signal added to wanted signal

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57 ADJACENT CHANNEL INTERFERENCE

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59 ADJACENT CHANNEL INTERFERENCE ADJACENT CHANNEL INTERFERENCE Use non-overlapping channels. In the 2.4 GHz band, use only channels 1, 6, and 11.

60 WHO GETS TO TALK? MEDIUM CONTENTION

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62 FUNDAMENTAL PROBLEM #2: MEDIUM CONTENTION VISUALIZING MEDIUM CONTENTION Some Wi-Fi equipment reports channel utilization via the BSS beacon frame.

63 FUNDAMENTAL PROBLEM #2: MEDIUM CONTENTION MEDIUM CONTENTION ROOT CAUSES Co-Channel Interference (CCI) Network Capacity Airtime Issues

64 MANY DEVICES COOPERATING VIA CSMA/CA CO-CHANNEL INTERFERENCE

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67 Channel 44 Channel 44

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69 Channel 36 Channel 44

70 CO CHANNEL INTERFERENCE REMEDIATIION Design your network for a minimal channel overlap.

71 AGAIN, MANY DEVICES COOPERATING VIA CSMA/CA NETWORK CAPACITY

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74 NETWORK CAPACITY CAPACITY REMEDIATION More Access Points Smaller Coverage Cells Band Steering and Load Balancing* *Here Be Dragons, though.

75 THE MOST COUNTERINTUITIVE OF ALL AIRTIME

76 TEXT

77 FUNDAMENTAL PROBLEM #3 NOT THE WI-FI (BLAME IT ANYWAY ) DNS DHCP Slow Internet Connection VLAN Configuration Issues