Computer Networks PHY CEN 5501C - Computer Networks - Spring 2007 - - PowerPoint PPT Presentation

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

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

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

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

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

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 frequency chan A frequency frequency chan 1 chan 2 chan 1 chan 3 chan 4 A1 A2 A3 A1 chan B B1 B2 B1 B2 chan C C1 chan D time time time CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman 6

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

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

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 CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman 9

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 CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman 10

Pulse Position Modulation • One pulse per symbol frame • Uses position of pulse in symbol frame • If N positions, then log 2 N bits/pulse • Very power efficient Symbol = position 0xA7 0x1F 0x02 … … … … Symbol Frame i Symbol Frame i+1 Time CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman 11