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Computer Networks

PHY. Computer Networks. Physical Layer. Mechanical Medium Connectors Electrical/optical Band Modulation Procedural Timing, etc. Practical Limits. Nyquist Noiseless Dual of Sampling Theorem C (bps) = 2H (Hz) log 2 M (symbol el’ts) Shannon White noise (e.g., thermal)

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Computer Networks

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  1. PHY Computer Networks CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman

  2. CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman Physical Layer • Mechanical • Medium • Connectors • Electrical/optical • Band • Modulation • Procedural • Timing, etc.

  3. CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman Practical Limits • Nyquist • Noiseless • Dual of Sampling Theorem C (bps) = 2H (Hz) log2 M (symbol el’ts) • Shannon • White noise (e.g., thermal) C (bps) <= H (Hz) log2 (1+S/N) S = signal power, N = noise power

  4. CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman Decibels • Logarithmic measure • Addition of logs = multiplication • SNR in dB SNR (dB) = 10 log10 (S/N)

  5. CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman Mechanical • Medium • Ribbon, UTP, STP, Cat 5, … • Coax (50 , 75 ) • Optic fiber (SM, MM, …) • Wireless • Connectors • BNC, RJ11, RJ45, Coax, Fiber termination,…

  6. CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman Dividing Bands • SDM – space (different wire, direction,…) • FDM - frequency • TDM – time • CDM – code division multiplexing (spread spectrum) • MF/TDM – First divide frequency, then time TDM MF/TDM FDM chan A frequency frequency frequency chan 1 chan 1 chan 4 A2 A3 A1 chan 2 chan 3 A1 chan B B1 B2 B1 B2 chan C C1 chan D time time time

  7. CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman Electrical/Optical • Band • Range of frequencies used • Amplitude range • Modulation • Analog signal (AM, FM) • Digital • ASK • FSK • PSK amplitude phase

  8. CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman Modulation • ASK – amplitude • Angle Shift Keying • FSK – frequency • PSK - phase • QAM – hybrid combination of PSK, ASK • OFDM – Orthogonal FDM • PPM – Pulse Position Modulation

  9. CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman Constellations 01 Q-PSK Q-ASK 00 01 11 10 11 00 10 Note use of the Gray Code to minimize received bit errors in the presence of noise 8-QAM

  10. CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman OFDM 2.5 Sub-carrier n – BPSK 2.3 Sub-carrier m – not used Frequency 1.5 Sub-carrier l - QPSK 0.7 Sub-carrier k - BPSK 0.5 Sub-carrier j – 8-QAM Time Multiple subcarriers Especially useful for frequency selective fading channels Select modulation, which subcarriers to use, FEC rate Bit loading – select modulation per subcarrier

  11. CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman Pulse Position Modulation • One pulse per symbol frame • Uses position of pulse in symbol frame • If N positions, then log2 N bits/pulse • Very power efficient Symbol = position 0xA7 0x1F 0x02 … … … … Symbol Frame i Symbol Frame i+1 Time

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