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ECEN4533 Data Communications Lecture #7 23 January 2013 Dr. George Scheets

ECEN4533 Data Communications Lecture #7 23 January 2013 Dr. George Scheets. Read 3.3 - 3.5 Problems Web 3-11, 2010 Quiz #1 Design #1 due 1 February (Live) 8 February ( Async DL) Late = -1 per working day Quiz #1 Lecture 12, 4 February (Live)

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ECEN4533 Data Communications Lecture #7 23 January 2013 Dr. George Scheets

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  1. ECEN4533 Data CommunicationsLecture #7 23 January 2013Dr. George Scheets • Read 3.3 - 3.5 • Problems Web 3-11, 2010 Quiz #1 • Design #1 due 1 February (Live) • 8 February (Async DL) • Late = -1 per working day • Quiz #1 • Lecture 12, 4 February (Live) • < 11 February (Async Distance Learning)

  2. Read: 4.1 - 4.3 Problems: Quiz #1 for '11 & '12 Design #1 due 1 February (Live) 8 February (Async DL) Late = -1 per working day Quiz #1 Lecture 12, 4 February (Live) < 11 February (Async Distance Learning) ECEN4533 Data CommunicationsLecture #8 25 January 2013Dr. George Scheets

  3. Multiplexing • How a chunk of frequency bandwidth gets broken up into smaller channels... • FDM • TDM • Stat Mux • CDM • Each channel can support one conversation.

  4. Direct Sequence Spread Spectrum • The transmitter multiplies the bit stream with a high-speed pseudo-random spreading signal. • If the receiver has a replica of the spreading signal, properly aligned, the original message can be recovered.

  5. DSSS - Transmit Side +1 Traffic (9 Kbps) time -1 Spreading Signal 27 Kcps +1 +1 +1 time -1 -1 -1 +1 +1 +1 +1 time Transmitted Signal 27 Kcps -1 -1

  6. Wireless X 27 Kcps Square Pulses RF Transmitter BPSK output 27 Kcps 90% of power in 54 KHz BW centered at fc Hertz cos(2πfct) RCVR Front End BPSK input 27 Kcps + noise Low Pass Filter 27 Kcps Square Pulses + filtered noise X cos(2πfct)

  7. +1 +1 +1 +1 time Received Signal 27 Kcps -1 -1 Despreading Signal 27 Kcps +1 +1 +1 time -1 -1 -1 Recovered Traffic 9 Kbps +1 time DSSS-Receiver -1

  8. Receiver Bit Detection • Following the despread operation, a Bit Detector will examine the noisy & distorted waveform and decide whether a 1 or 0 was transmitted in each message bit interval T. • Single Sample Detector • Samples each bit once, usually near the middle. • Compares sampled value to waveform's average (DC) value • If sample > DC value, decide Logic 1 • If sample < DC value, decide Logic 0 • Susceptible to noise burst.

  9. Receiver Bit Detection • Multiple Sample Detector • Samples each bit interval numerous times. • Compares average of sampled values to entire waveform's Mean (DC) value (a.k.a. threshold) • If sample average > Mean, decide Logic 1 • If sample average < Mean, decide Logic 0 • Less susceptible to noise burst. • Best would be to sample each message bit interval an infinite # of times- equivalent to integration.

  10. Detection (OSI Level 1) Recovered Traffic 9 Kbps +1 time • Best Detector computes average value every Tbit seconds & compares to threshold. Here... • If positive, says Logic 1 • If negative, says Logic 0 DSSS-Receiver -1 Tbit Tbit

  11. Someone else talking? • Would be using a different spreading code. • Output from our despread operation would be random high speed chips • Bit Detector expecting lower speed bits • Would output a random sequence of low speed bits • If a voice system, these random bits map to random voltages • Would get static if played on loudspeaker • Real world system squelches this static

  12. Someone else talking? +1 +1 time Signal #2 27 Kcps -1 -1 -1 -1 Our Despreading Signal 27 Kcps +1 +1 +1 time -1 -1 -1 Recovered Garbage from Signal #2 + +1 +1 time -1 -1 -1 -1

  13. Someone else talking? Recovered Garbage from Signal #2 + +1 +1 time -1 -1 -1 -1 Message Bit Detector looks at message bit intervals. Tbit Tbit Would output random sequence of 1's & 0's (2 logic 0's here). time -1 -1

  14. Two signals active? • Receiver Detector sees sum of our signal and unwanted signal. • Still able to detect message bits in example that follows. • But bit errors may be more likely.Note average value of first bit (2/3) is now closer to the threshold of 0 volts. • Unwanted signals have effect similar to noise. Message bit detection errors become more common.

  15. Two signals active. Our Traffic 9 Kbps +1 time -1 Recovered Garbage from Signal #2 + +1 +1 time -1 -1 -1 -1 +2 time sum -2

  16. Our Traffic is Still Recovered +2 time sum -2 Tbit Tbit Positive Area: Output a Logic 1 Negative Area: Output a Logic 0 Detector Output 9 Kbps +1 time -1

  17. Switching: How long & in what manner will a user get to use a channel? • For the duration of the conversation? Circuit Switching • For a tiny, variable length, portion of the conversation? Packet Switching • For a tiny, fixed length, portion of the conversation? Cell Switching

  18. MULTIPLEXING StatMux TDM FDM CDM PSTN Circuit (GSM Mobile Phones) Ethernet Internet WiFi SWITCHING WiFi WiFi Packet Cell ATM ATM Mobile Any combo of Switching & Multiplexing is theoretically possible. Shown are some of the most common.

  19. TDM & Circuit Switching frequency 1 1 byte 2 PSTN 3 1/8000 th second time 1 etc.

  20. StatMux & Packet Switching Data Networks frequency 1 Internet Packets 47 bytes to 1507 bytes 3 1 time 2

  21. TDM or StatMux & Cell Switching frequency 1 Empty ATM Cells 53 bytes 3 1 Empty Empty time 2

  22. Different channels use some of the frequency all of the time. FDM frequency 1st Generation Cell Phones 1 2 3 4 5 time

  23. Combo of TDM & FDM(GSM) TDMA/FDMA frequency 2nd Generation Cell Phones 10 1 4 7 11 2 5 8 3 6 9 12 time 10 1 4 7 etc.

  24. Different channels use all of the bandwidth all of the time. Example: CDMA1 & CDMA2000 CDMA frequency 2nd & 3rd Generation Cell Phones Channels use different codes. Other channels cause noise-like interference. time

  25. Wired Telecom Black Boxes OSI Layer 1 LAN Hubs (obsolete), a.k.a. Shared Hubs WAN Regenerative Repeaters OSI Layer 1 & 2 LAN Bridges (obsolete) LAN Switches, a.k.a. Switched Hubs WAN Switches OSI Layer 1, 2 & 3 Router Generally moving pulses

  26. Repeater or Hub • Operates at OSI Level 1 • Bit Aware • Unaware that packets exist • Repeater: Single input, Single OutputUsually only used on WAN's • Will regenerate & retime symbols (a.k.a Regen) • Hubs: Multiple input, Multiple OutputUsually only used on LAN's • May regenerate & retime symbols • Obsolete

  27. Black Box Performance... From Node A To Node A OSI Level 1 LAN Hub Node B Node B Node C Node C Two packets simultaneously show up at input...

  28. Black Box Performance... From Node A To Node A OSI Level 1 LAN Hub Node B Node B Node C Node C ... will overwrite each other, i.e. garbage out. a.k.a. Shared Hub

  29. WAN SwitchLAN Switched Hub or Bridge • Operates at OSI Layers 1 & 2 • Frame aware • Media Access Control (MAC) aware • Bridge: single input, single output Hub • Two packets arrive simultaneously for same output? • Momentarily store one, ship the other. • Requires CPU & Memory

  30. Black Box Performance... OSI Layer 1-2 Switch Two packets simultaneously show up at input...

  31. Black Box Performance... OSI Layer 1-2 Switch OSI Layer 1-3 Router ... one will be transmitted, the other momentarily buffered and then transmitted.

  32. Internet Router Operates at OSI Layers 1 - 3 Packet aware Media Access Control (MAC) aware More "network aware" than a switch Exchange connectivity info with peers Two packets arrive simultaneously for same output? Momentarily store one, ship the other. Requires CPU & Memory

  33. OSU Campus Network (> 2001) LAN LAN LAN OneNet Ethernet Switch 802.3 LAN 1 & 10 Gbps Ethernet 802.3 LAN 802.3 LAN Routers

  34. Wireless Telecom Black Boxes OSI Layer 1 & 2 Access Point Generally transmitting RF EM sinusoids Between user & Access Point Wired or Wireless Transport Between Access Point & World Limited Deployment To Date & R&D Wireless Radio Protocols (Layers 1-3)

  35. Binary ASK 1.5 1 MHz 0 -1.5 .00001 0 Two different Amplitudes are transmitted Shown is a 1 MHz center frequency. 5 cycles/symbol = 200 K bits/second in(t) X t cos(2πfct)

  36. Binary FSK 1.5 0 -1.5 .00001 0 Two different frequencies are transmitted Symbol #1) 5 cycles/.000005 seconds = 1 MHz Symbol #2) 10 cycles/.000005 seconds = 2 MHz 1.5 MHz Average (center) Frequency 2 symbols in .00001 seconds = 200 K bits/second in(t) VCO fc = 1.5 MHz t

  37. Binary PSK 1.5 1 MHz 0 -1.5 .00001 0 Two different phases are transmitted 10 cycles/.00001 seconds = 1 MHz 5 cycles/symbol = 200 K bits/second in(t) X t cos(2πfct)

  38. Binary PSK in(t) in(t)cos(2πfct) X t cos(2πfct) y(t) Low Pass Filter At receiver... X cos(2πfct) y(t) = 0.5in(t)[cos (0) +cos(2π2fct)] Wiped out by LPF.

  39. POTS Connectivity (1920) Copper Local Loop Copper Local Loop Copper Long Haul CO CO Phone Phone Analog

  40. POTS Connectivity (1970) Copper Local Loop Copper Local Loop Copper Long Haul CO CO Phone Phone Digital TDM 64 Kbps Analog Analog

  41. POTS Connectivity (1990) Copper Local Loop Copper Local Loop Fiber Optic Trunk CO CO Phone Phone Digital TDM 64 Kbps Analog Analog

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