l13 Wireless Networking

l13 Wireless Networking - Lecture 13: Wireless Networking...

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Lecture 13: Wireless Networking
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Outline Wireless physical layer challenges - Signal, noise, modulation - A little bit of EE goes a long way Wireless link layers - Hidden terminals, exposed terminals - CSMA/CA - RTS/CTS Wireless routing and throughput Wireless trends today
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Ethernet: 802.3 Dominant wired LAN technology - 10BASE5 (vampire taps) - 10BASE-T, 100BASE-TX, 1000BASE-T Frame format: Preamble Type/ Len Payload SFD 10101011 Src 6 bytes 2 bytes 46-1500 bytes CRC 4 bytes Gap 96 ns, 960 ns, 9600 ns Physical Link Link Layer 3 Dest 6 bytes
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Physical Layer (Layer 1) Responsible for specifying the physical medium - Category 5 cable (Cat5): 8 wires, twisted pair, RJ45 jack - WiFi wireless: 2.4GHz Responsible for specifying the signal - 100BASE-T: 5-level pulse amplitude modulation (PAM-5) - 802.11b: Binary and quadrature phase shift keying (BPSK/QPSK) Responsible for specifying the bits - 100BASE-T: 4-to-6 bit-to-chip encoding, 3 chip symbols - 802.11b: Barker code (1-2Mbps), complementary code keying (5.5-11Mbps)
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Wireless is Different Variable: signal attenuates over space Interference: other RF sources can interfere with signal Multipath: signal can self-interfere Distributed: nodes cannot detect collisions To address these differences, wireless link layers use slightly different mechanisms Also, can’t just abstract away the physical and link layers: need a brief introduction to underlying EE
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Attenuation Over Space Signal weakens as distance from transmitter increases Reflections, obstructions, etc. complicate the attenuation Depending on the antenna, not uniform in all directions Much more complex than the wired model
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Signal Strength Over Space
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Directional Antennas
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Interference In unlicensed bands (e.g., 802.11), there are lots of transmitters - 802.11 cards - 802.15.1 (Bluetooth) - 802.15.4 (ZigBee) - 2.4GHz phones - Microwave ovens This interference can be stronger or weaker than the signal, and can prevent successful reception
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Analog Signals Amplitude Wavelength
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Specifying the Signal: Modulation On-Off Keying (OOK) 1 0 1 Amplitude Shift Keying (ASK) 1 0 1
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Modulation, Continued Frequency Shift Keying (FSK) 1 0 1 Phase Shift Keying (PSK) 1 0 1
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I/Q Modulation I: in-phase, Q: quadrature Sum of two sines is a sine Show what the carrier looks like compared to a simple, unmodulated signal Use I/Q because this is how it’s actually done in hardware
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I+Q
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Modulation in I/Q Plots Q Q Q Q I Q OOK ASK FSK BPSK QPSK
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Example measurements from 16-QAM
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Signal, Noise, and Interference Signal: energy of desired transmission Noise/Noise floor: energy of hardware thermal effects Interference: energy of other transmitters Usually measured in dBm/dBW: 0dBm = 1mW, 0dBW = 30dBm = 1W - Note dB is a logarithmic scale: 10dBm = 10mW, 20dBm = 100mW
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Signal Plus Noise
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SINR Signal to Interference-and-Noise Ratio Measured in dB: | S | | N + I | - S = -50dBm, N+I = -95dBm, SINR = 45dB - S = -89dBm, N+I = -93dBm, SINR = 4dB SINR is particularly critical in wireless because of attenuation over space
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Bit Error Rates There is a theoretical limit on how much
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l13 Wireless Networking - Lecture 13: Wireless Networking...

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