SLIDE 1
CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman 1
CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman 2
Physical Layer
- Mechanical
– Medium – Connectors
- Electrical/optical
– Band – Modulation
- Procedural
Computer Networks PHY CEN 5501C - Computer Networks - Spring 2007 - - - PowerPoint PPT Presentation
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 -
SLIDE 2
SLIDE 3
CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman 3
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
SLIDE 4
CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman 4
Decibels
- Logarithmic measure
– Addition of logs = multiplication
- SNR in dB
SNR (dB) = 10 log10 (S/N)
SLIDE 5
CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman 5
Mechanical
- Medium
– Ribbon, UTP, STP, Cat 5, … – Coax (50 Ω, 75 Ω) – Optic fiber (SM, MM, …) – Wireless
- Connectors
– BNC, RJ11, RJ45, Coax, Fiber termination,…
SLIDE 6
CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman 6
Dividing Bands
- SDM – space (different wire, direction,…)
- FDM - frequency
- TDM – time
- CDM – code division multiplexing (spread spectrum)
- MF/TDM – First divide frequency, then time
time frequency time frequency time frequency chan A chan B chan C chan D chan 1 chan 2 chan 3 chan 1 chan 4 A1 A2 A3 A1 C1 B1 B2 B1 B2 FDM TDM MF/TDM
SLIDE 7
CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman 7
Electrical/Optical
- Band
– Range of frequencies used – Amplitude range
- Modulation
– Analog signal (AM, FM) – Digital
- ASK
- FSK
- PSK
phase amplitude
SLIDE 8
CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman 8
Modulation
- ASK – amplitude
- Angle Shift Keying
– FSK – frequency – PSK - phase
- QAM – hybrid combination of PSK, ASK
- OFDM – Orthogonal FDM
- PPM – Pulse Position Modulation
SLIDE 9
CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman 9
Constellations
00 01 11 10 00 01 11 10 Q-ASK Q-PSK 8-QAM
Note use of the Gray Code to minimize received bit errors in the presence of noise
SLIDE 10
CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman 10
OFDM
Frequency Time
Sub-carrier k - BPSK Sub-carrier j – 8-QAM Sub-carrier m – not used 2.3 0.7 0.5 Sub-carrier l - QPSK 1.5 Sub-carrier n – BPSK 2.5
Especially useful for frequency selective fading channels Select modulation, which subcarriers to use, FEC rate Bit loading – select modulation per subcarrier
Multiple subcarriers
SLIDE 11
CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman 11
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 Frame i
… … … …
0xA7 0x1F Symbol = position Time Symbol Frame i+1 0x02