What is 802.11? • Suite of of physical layer (PHY) and link- layer protocols standardized by IEEE 802.11 Networks for Dummies Professors • Aka “WiFi” (or “wireless ethernet”) Lecture 25 • Wildly successful: hundreds of millions in use May 10, 2010 • Most laptops and smartphones have it today 6.02 Spring 2010 • Multiple possible uses 802.11 (WiFi) physical, link, subnetwork • Cellular wireless LANs layer essentials • Mesh networks Application of 6.02 topics and techniques • Mobile ad hoc networks Example Deployments Cellular Wireless LAN Architecture Wireless local area WiFi hotspots Boston-area WiFi APs beacon periodically networks Client scans, picks an AP To/from ¡Internet ¡ Associates, authenticates Obtains IP address AP wigle.net AP Cell ¡ Cell ¡ Wireless mesh networks Meraki.com MIT roofnet Base picture from Novell Common 802.11 Standards Layered System (Alphabet soup: a, b, g, n, …) Network layer (IP) Not part of 802.11 Access point selection Mobility management Subnetwork layer Mesh routing Framing Stop-and wait rxmit protocol Link layer MAC (mainly CSMA) Bit rate adaptation ( non-std ) Power-saving protocol ( non-std ) Channel (freq) allocations Physical layer (PHY) Modulation (mainly OFDM) Convolutional coding From wikipedia 1
Multiple Frequency Channels In-Phase and Quadrature Transmitter DAC V Digital Bits Modulator In 20 MHz From wikipedia • 802.11b/g/n: up to 14 channels each 20 MHz wide, centered 5 MHz from each other in 2.4 GHz band • North America (1-11), Japan (1-14), most of world DAC (1-13) • b uses “direct sequence spread spectrum”, g uses orthogonal frequency division multiplexing • 802.11a/n: 5 GHz band, 20 channels (in US), OFDM • “Etiquette rules” set power levels and other constraints Multiple Bit Rates In-Phase and Quadrature Receiver ADC V Digital Bits De- out Modulator Convolutional Modulations coding ADC From wikipedia I (cos) and Q (sin) as Constellations Channels and Sub-carriers 20 MHz With OFDM: 52 carriers, 312.5 KHz per carrier 16.25 MHz total used, with some “guard bands” 48 carriers used for data, 4 for control From Mythili Vutukuru 2
Digital Modulator for Orthogonal Frequency Division Layered System Multiplexing (OFDM) Network layer (IP) Not part of 802.11 Access point selection Subnetwork layer Mesh routing Framing Bits Stop-and wait rxmit protocol In Link layer MAC (mainly CSMA) Bit rate adaptation (non-std) Power-saving protocol ( non-std ) Channel (freq) allocations Physical layer (PHY) Modulation (mainly OFDM) Convolutional coding 802.11 MAC Time-varying Channel • Mobility • Radios aren’t wires – inherently broadcast 30 • Change in attenuation SNR (dB) 25 • Receptions aren’t perfect like in Ethernet 20 • Multipath fading 15 10 • Ethernet: either perfect reception or perfect • Adapt redundancy by 5 collision 0 0 2000 4000 6000 8000 10000 picking best 10 s • Wireless: probabilistic receptions Time (Milliseconds) modulation/code • Time-varying channels combination • Interference 25 SNR (dB) 20 15 • How to achieve high throughput? • Needs accurate and 10 5 responsive channel 0 1500 1550 1600 1650 1700 1750 estimates 250 ms Time (Milliseconds) 16 MAC Protocol: Sharing a Wireless Channel Bit-Rate Adaptation: One Approach Collision! Frame-based x u ✔ Problems w/ ✘ Data 802.11 CSMA MAC z v y ACK • MAC: decide who transmits when Es0mate ¡frame ¡loss ¡ • Goal: increase spatial concurrency (reuse) rate ¡at ¡each ¡bit ¡rate ¡ • Carrier Sense Multiple Access (CSMA) • Sender senses “busy” defer Pick bit rate that maximizes throughput: bitrate * (1-lossrate) • “Busy” by energy or preamble detection Problem: Takes a long time, not good for mobile users 17 3
Summary Access point selection Mobility management Subnetwork layer Mesh routing Framing Stop-and wait rxmit protocol Link layer MAC (mainly CSMA) Bit rate adaptation (non-std) Power-saving protocol ( non-std ) Channel (freq) allocations Physical layer (PHY) Modulation (mainly OFDM) Convolutional coding Hundreds ¡of ¡millions ¡of ¡devices ¡ Protocols ¡and ¡designs ¡s0ll ¡evolving ¡ Many ¡open ¡challenges ¡ 4
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