Lecture 13 Wireless Networking 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 Ethernet 802 3 Dominant wired LAN technology 10BASE5 vampire taps 10BASE T 100BASE TX 1000BASE T Frame format Physical Preamble SFD Link Src Dest Type Len 7 x 10101010 10101011 6 bytes 6 bytes 2 bytes Layer 3 Link Payload CRC 46 1500 bytes 4 bytes Gap 96 ns 960 ns 9600 ns 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 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 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 Signal Strength Over Space Directional Antennas 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 Analog Signals Amplitude Wavelength Specifying the Signal Modulation 1 0 1 On Off Keying OOK 1 0 1 Amplitude Shift Keying ASK Modulation Continued 1 0 1 Frequency Shift Keying FSK 1 0 1 Phase Shift Keying PSK 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 I Q Modulation in I Q Plots Q Q Q Q Q I OOK ASK FSK BPSK QPSK Example measurements from 16 QAM 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 Signal Plus Noise 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 Bit Error Rates There is a theoretical limit on how much information a channel can carry Shannon limit Bit error rate depends on the SINR and the modulation This is why wireless link layers use more complex chip bit encoding If signal is strong high SINR have few chip errors can use low encoding If signal is weak low SINR have many chip errors use higher encoding to recover from errors Symbols Bits Chips 0000 0001 0010 0011 11011001110000110101001000101110 11101101100111000011010100100010 00101110110110011100001101010010 00100010111011011001110000110101 1111 11001001011000000111011110111000 Application Note 9804 Example Theoretical Bit Error Rates W4 Z 1 1 0E 02 1 0E 03 MSK PSK 1 0E 04 DBPSK DQPSK 1 0E 05 COHERENT OOK OFSK 1 0E 06 INCOHERENT OOK OFSK 1 0E 07 0 Wn AL EQUALIZER OUT ZER 1 0E 01 BE s fed through a ummed together ularly effective in smission over ement in rapidly fices and homes g in relation to ct on system cost expensive to 1 2 3 4 5 6 7 8 9 Eb No dB 10 11 12 13 14 15 FIGURE 4 PROBABILITY OF BIT ERROR FOR COMMON MODULATION METHODS For the purposes of link budget analysis the most important aspect of a given modulation technique is the Signal toNoise Ratio SNR necessary for a receiver to achieve a Variable Bit Rates 802 11b supports 1 2 5 5 and 11Mbps 2 5 5Mbps and 11Mbps are QPSK To support this the signal field says what the data rate is 00001010 1Mbps 11 chips bit barker code 00010100 2Mbps 11 chips bit barker code 00110111 5 5Mbps 2 chips bit CCK 01101110 11Mbps 1 chip bit CCK So the header is still at 1Mbps even if the data is at 11Mbps Collisions are not so simple A 40dB B 60dB C If A transmits first B can still decode its packet If C transmits first A will corrupt its packet and B can t decode C s packet What if AB and BC are both 60dB Signal strength matters this is the RF capture effect 802 11 Packet Loss Rates 802 11 Packet Loss Rates at 11Mbps 100 Links 80 60 40 20 0 0 0 0 2 0 4 0 6 0 8 Packet Reception Ratio How does this affect TCP 1 0 Wireless PHY Summary Can t control or limit the channel Need to deal with weak signals interference etc Signal strength affects collisions Many different kinds of modulation amplitude frequency phase Use robust encodings when needed use fast speeds when possible Lots of intermediate packet delivery ratios 2 minute break Wireless Link Layers MAC Layer Responsibilities Arbitrate control of the channel One node should be able to use 100 Multiple nodes should get a fair share Want high utilization under contention CA versus CD Collision detect CD is hard in wireless Local signal is much stronger than anything received Protocols use collision avoidance CA by sensing the channel Simple MAC CSMA CA 1 Wait a small random period check the channel 2 If the channel is busy go to 1 maybe longer wait 3 Transmit packet S B1 B2 B3 B4 TX 802 11b MAC CSMA CA Maintain a waiting counter c For each time step channel is idle c When c 0 transmit If packet is not acknowledged layer 2 pick a new larger c Use lack of layer 2 ack as collision detect S B1 TX B2 ACK Problems with CSMA CA Want to know state of channel at receiver not transmitter But wireless is not transitive A hears B A hears C B and C may not hear each other B and C can only sense their channel but need to know if A s channel is clear Hidden Terminal Problem B A C B and C can t hear each other A can hear both B and C sense a clear channel transmit and collide at A B is a hidden terminal to C and C is a hidden terminal to B Exposed Terminal Problem B A C D A transmits to B C hears the transmission backs off even if it wants to transmit to D C is an exposed terminal to A