CS 268 Computer Networking L 11 Wireless in the Real World Wireless in the Real World Real world deployment patterns Mesh networks and deployments Assigned reading Modeling Wireless Links Architecture and Evaluation of an Unplanned 802 11b Mesh Network 2 1 Wireless Challenges Force us to rethink many assumptions Need to share airwaves rather than wire Don t know what hosts are involved Host may not be using same link technology Mobility Other characteristics of wireless Noisy lots of losses Slow Interaction of multiple transmitters at receiver Collisions capture interference Multipath interference 3 Overview 802 11 Deployment patterns Reaction to interference Interference mitigation Mesh networks Architecture Measurements 4 2 Characterizing Current Deployments Datasets Place Lab 28 000 APs MAC ESSID GPS Selected US cities www placelab org Wifimaps 300 000 APs MAC ESSID Channel GPS derived wifimaps com Pittsburgh Wardrive 667 APs MAC ESSID Channel Supported Rates GPS 5 AP Stats Degrees Placelab Placelab 28000 APs MAC ESSID GPS APs Max degree Portland 8683 54 San Diego 7934 76 San Francisco 3037 85 Boston 2551 39 50 m 1 2 1 6 3 Degree Distribution Place Lab 7 Unmanaged Devices WifiMaps com 300 000 APs MAC ESSID Channel Channel age 6 51 11 21 1 14 10 4 Most users don t change default channel Channel selection must be automated 8 4 Growing Interference in Unlicensed Bands Anecdotal evidence of problems but how severe Characterize how 802 11 operates under interference in practice Other 802 11 9 Throughput to decrease linearly with interference There to be lots of options for 802 11 devices to tolerate interference Bit rate adaptation Power control FEC Packet size variation Spread spectrum processing Transmission and reception diversity Throughput linear What do we expect Interferer power log scale 10 5 Key Questions How damaging can a low power and or narrow band interferer be How can today s hardware tolerate interference well What 802 11 options work well and why 11 Effects of interference more severe in practice Caused by hardware limitations of commodity cards which theory doesn t model Throughput linear What we see Th eo ry Interferer power log scale 12 6 Experimental Setup Access Point UDP flow 802 11 Interferer 802 11 Client 13 802 11 Receiver Path PHY To RF Amplifiers Amplifier control RF Signal Analog signal ADC 6 bit samples Timing Recovery PHY MAC MAC AGC Barker Correlator Demodulator Descrambler Data includes beacons Preamble Detector Header CRC 16 Checker Receiver SYNC SFD CRC Payload PHY header Extend SINR model to capture these vulnerabilities Interested in worst case natural or adversarial interference Have developed range of attacks that trigger these vulnerabilities 14 7 Timing Recovery Interference Interferer sends continuous SYNC pattern Interferes with packet acquisition PHY reception errors Weak interferer Moderate interferer Log scale 15 Interference Management Interference will get worse Density device diversity is increasing Unlicensed spectrum is not keeping up Spectrum management Channel hopping 802 11 effective at mitigating some performance problems Sigcomm07 Coordinated spectrum use based on RF sensor network Transmission power control Enable spatial reuse of spectrum by controlling transmit power Must also adapt carrier sense behavior to take advantage 16 8 Impact of frequency separation Even small frequency separation i e adjacent 802 11 channel helps 5MHz separation good performance 17 Transmission Power Control Choose transmit power levels to maximize physical spatial reuse Tune MAC to ensure nodes transmit simultaneously when possible Spatial reuse network capacity link capacity AP1 Client2 AP2 Client1 Spa al Reuse 1 Concurrent transmissions increase spa4al reuse AP1 Client2 AP2 Client1 Spa al Reuse 2 18 9 Transmission Power Control in Practice For simple scenario easy to compute optimal transmit power May or may not enable simultaneous transmit Protocol builds on iterative pair wise optimization AP1 d12 AP2 d22 d11 d21 Adjusting transmit power requires adjusting carrier sense thresholds Client2 Client1 Echos Alpha or eliminate carrier sense Altrusitic Echos eliminates starvation in Echos 19 Details of Power Control Hard to do per packet with many NICs Some even might have to re init many ms May have to balance power with rate Reasonable goal lowest power for max rate But finding ths empirically is hard Many power rate combinations and not always easy to predict how each will perform Alternate goal lowest power for max needed rate But this interacts with other people because you use more channel time to send the same data Uh oh Nice example of the difficulty of local vs global optimization 20 10 Rate Adaptation General idea Observe channel conditions like SNR signalto noise ratio bit errors packet errors Pick a transmission rate that will get best goodput There are channel conditions when reducing the bitrate can greatly increase throughput e g if a decrease in bitrate gets you from 90 loss to 10 loss 21 Simple rate adaptation scheme Watch packet error rate over window K packets or T seconds If loss rate threshhigh or SNR etc Reduce Tx rate If loss rate threshlow Increase Tx rate Most devices support a discrete set of rates 802 11 1 2 5 5 11 etc 22 11 Challenges in rate adaptation Channel conditions change over time Loss rates must be measured over a window SNR estimates from the hardware are coarse and don t always predict loss rate May be some overhead time transient interruptions etc to changing rates 23 Power and Rate Selection Algorithms Rate Selection Auto Rate Fallback ARF Estimated Rate Fallback ERF Goal Transmit at minimum necessary power to reach receiver Minimizes interference with other nodes Paper Can double or more capacity if done right Joint Power and Rate Selection Power Auto Rate Fallback PARF Power Estimated Rate Fallback PERF Conservative Algorithms Always attempt to achieve highest possible modulation rate 24 12 Power Control Rate Control summary Complex interactions More power Higher received signal strength May enable faster rate more S in S N May mean you occupy media for less time Interferes with more people Less power Interfere with fewer people Less power less rate Fewer people but for a longer time Gets even harder once you consider Carrier sense Calibration and measurement error Mobility 25 Overview 802 11 Deployment patterns Reaction to interference Interference mitigation Mesh networks Architecture Measurements 26 13
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