Lecture 17: Wireless Networking

Transcription

1 Lectre 17: Wireless Networking CSE 222A: Compter Commnication Networks Alex C. Snoeren Thanks: Lili Qi, Nitin Vaidya

2 Lectre 17 Overview Project discssion Intro to WiFi Jigsaw discssion CSE 222A Lectre 17: Wireless Networking 2

3 Project pdate Second checkpoint de Tonight 1-2 page smmary of yor progress since last checkpoint Project presentations Wedsday 3/21 Sign p for a 25-minte slot between 8:00am and 3:30pm» 20 minte presentation (inclding all project members)» 3-4 mintes for qestions Yo will be expected to attend at least 3 presentations and actively participate in discssion Reports de Friday 3/23 by midnight CSE 222A Lectre 17: Wireless Networking 3

4 Carrier Sense Mltiple Access CSMA: listen before transmit If channel sensed idle: transmit entire packet If channel sensed bsy, defer transmission Persistent CSMA: retry immediately with probability p when channel becomes idle (may case instability) Non-persistent CSMA: retry after random interval Bt what abot collisions? CSE 222A Lectre 17: Wireless Networking 4

5 CSMA/CA Impossible to hear collision w/half-dplex radio Wireless MAC protocols often se collision avoidance techniqes, in conjnction with a (physical or virtal) carrier sense mechanism Collision avoidance Nodes negotiate to reserve the channel. Once channel becomes idle, the node waits for a randomly chosen dration before attempting to transmit. CSE 222A Lectre 17: Wireless Networking 5

6 Hidden Terminal Problem A B C B can commnicate with both A and C A and C cannot hear each other Problem When A transmits to B, C cannot detect the transmission sing the carrier sense mechanism Soltion If C transmits, collision will occr at node B Hidden sender C needs to defer CSE 222A Lectre 17: Wireless Networking 6

7 RTS/CTS (MACA) A B C When A wants to send a packet to B, A first sends a Reqest-to-Send (RTS) to B On receiving RTS, B responds by sending Clear-to- Send (CTS), provided that A is able to receive the packet When C overhears a CTS, it keeps qiet for the dration of the transfer Transfer dration is inclded in both RTS and CTS CSE 222A Lectre 17: Wireless Networking 7

8 Backoff Interval Problem: With many contending nodes, RTS packets will freqently collide Soltion: When transmitting a packet, choose a backoff interval in the range [0, CW] CW is contention window Wait the length of the interval when medim is idle Cont-down is sspended if medim becomes bsy Transmit when backoff interval reaches 0 Need to adjst CW as contention varies CSE 222A Lectre 17: Wireless Networking 8

9 Jigsaw: What Really Happens Measre real large wireless networks Collect every possible information» PHY/Link/IP/TCP/App layer trace» Collect every single wireless packet Need many sniffers for 100% coverage Atomatically merge traces Global view of wireless networks across time, locations, channels, and protocol layers Enables manal and atomatic diagnosis CSE 222A Lectre 17: Wireless Networking 9

10 Or lab: EBU3b 150k sqare feet 4 floors 500+ occpants 150 faclty/staff 350 stdents Bilding-wide WiFi 39 access points b/g» Channel 1, 6, active clients anytime Daily Traffic ~5GB CSE 222A Lectre 17: Wireless Networking 10

11 Jigsaw Deployment Overlays existing WiFi network Series of passive sniffers Blanket deployment over 4 floors 39 sensor pods (156 radios) 4 radios per pod, cover all channels in se Captres all activities» Inclding CRC/PHY events Stream back over wired network to centralized storage

12 Jigsaw design Traces synchronization and nification L2 state reconstrction TCP flow reconstrction CSE 222A Lectre 17: Wireless Networking 12

13 Synchronization o o o Frames carry TSF time-stamps Need a global clock Estimate the offset between TSF and the global clock for each sniffer

14 Synchronization Create a virtal global clock To keep nification working Critical evidence for analysis» If A and B are transmitting at the same time they cold interfere» If A starts transmitting after B has started then A can t hear B Reqire fine time-scales (10-50s) NTP is +100 sec accracy HW clocks (TSF) have 100PPM stability TSF diff (s) TSF diff of two sniffers Time (s) CSE 222A Lectre 17: Wireless Networking

15 Ideal Unification Time CSE 222A Lectre 17: Wireless Networking

16 Real Unification Jigsaw nified trace JFrame 1 JFrame 2 Time JFrame 3 JFrame 4 JFrame 5 CSE 222A Lectre 17: Wireless Networking

17 Challenge: sync at large-scale T o s t 1 t 3 How to bootstrap? Goal: estimate the offset between TSF and the global clock for each sniffer Time reference from one sniffer to the other Sync across channels Dal radios on same sniffer slaved to same clock (creates a de facto time linkage) Manage TSF clock skews Continosly re-adjst offsets when merging frames

18 Jigsaw in Action Jigsaw merges 156 traces into one global trace Covers 99% of AP frames, 96% of client frames Starts Jan 24,2006 (Tesday) Dration Total APs 24 hr CSE Clients 1026 Active CSE clients anytime 107 (39 CSE) Total Events 2,700M PHY/CRC Errors 48% Valid Frames 52% CSE 222A Lectre 17: Wireless Networking JFrames Events per Jframe 530M 2.97

19 Jigsaw syncs 99% frames < 20s o Measre sync. qality by max dispersion per Jframe o 20 s is important threshold n back-off time is 20s n inter frame time is 50 s n Sfficient to infer many events CSE 222A Lectre 17: Wireless Networking

20 L2-ACK Beacon

21 Hidden Terminals CSE 222A Lectre 17: Wireless Networking

22 Mlti-hop roting packet A packet B packet src dst Identify a rote, forward over links Abstract radio to look like a wired link C CSE 222A Lectre 17: Wireless Networking 22

23 For Next Class Read ExOR paper Sbmit checkpoint by tonight! CSE 222A Lectre 17: Wireless Networking 23