Unit 2
Physical Layer Fundamentals
Role: Converts bits into signals and vice versa. It manages bit synchronization (clocking), physical topology (bus, star), and standards such as RS-232, Ethernet PHY, and Wi-Fi PHY.
Key Metrics
| Metric |
Description |
Formula / Example |
| Bandwidth | Max data rate (Hz or bps) | 100 MHz = ~200 Mbps (Nyquist) |
| Data Rate | Bits/sec capacity | Shannon: C = B log₂(1 + SNR) |
| Throughput | Actual speed after overhead | 1 Gbps link → 940 Mbps real |
| Latency | Propagation delay | Speed of light: 200,000 km/s in fiber |
Example: Ethernet cable carries 1s as +5V pulses, 0s as 0V, at 10⁹ pulses/sec for Gigabit.
Digital Transmission & Line Coding
Digital Transmission sends data as discrete binary signals (square waves). Unlike analog, digital signals regenerate cleanly via repeaters.
| Scheme |
How It Works |
Pros / Cons |
Use Case |
| NRZ | 1=high, 0=low | Simple / No sync for long 0s | USB 1.0 |
| NRZI | Change=1, No change=0 | Better sync / DC imbalance | USB 2.0 |
| Manchester | Mid-bit transition | Self-clocking / 50% rate loss | Ethernet 10Base-T |
| 4B/5B | 4 bits → 5-bit code | Error detect / 25% overhead | Fast Ethernet |
| MLT-3 | 3 levels (+1,0,-1) | Low EMI / Complex | 100Base-TX |
Diagram (Digital Signal Flow):
Bits: 1 0 1 1 0
NRZ: ____ ____ ____ → Repeater (Regenerate) → ____ ____
Manchester: _/‾\_ ‾\_/ _/‾\_ _/‾\_ ‾\_/
Transmission Modes
| Mode | Direction | Example |
|---|
| Simplex | One-way | Keyboard → PC |
| Half-Duplex | Two-way, one at a time | Walkie-talkie |
| Full-Duplex | Simultaneous two-way | Phone call |
Real Example: Wi-Fi PHY encodes bits into OFDM symbols (64 subcarriers) for 802.11ac (1.3 Gbps).
Multiplexing
| Type |
Works On |
How / Pros |
Example |
| FDM | Analog | Split bandwidth / No sync needed | AM/FM Radio, Cable TV |
| TDM | Digital | Time slots / Full bandwidth use | PCM telephony (E1/T1) |
| WDM | Optical | Different wavelengths / Huge capacity | Fiber backbone (DWDM) |
| CDM | Digital/Analog | Unique codes / Secure | 3G Mobile (CDMA) |
| STATMUX | Digital | Dynamic slots / Efficient | ISDN, Frame Relay |
Diagram (FDM):
Source1 --+ f1 Modulator --> Composite Signal (f1+f2+f3) --> f1 Demod --> Dest1
Source2 --+--> f2 ~~~~/\/\/~~~~/\/\/~~~~ f2 Demod --> Dest2
Source3 --+ f3 Modulator --> Guard Bands Prevent Overlap f3 Demod --> Dest3
TDM Example (Frame):
Time Slots: | Ch1 | Ch2 | Ch3 | Ch1 | Ch2 | Ch3 | (Sync bit at start)
Transmission Media
Guided Media (Wired):
| Type | Bandwidth | Distance | Attenuation | Use Case |
|---|
| Twisted Pair | 100 MHz - 1 GHz | 100m | High | LAN (Ethernet) |
| Coaxial | 1 GHz | 500m | Medium | Cable TV |
| Fiber Optic | 100 THz | 100km+ | Very Low | Internet Backbone |
Unguided Media (Wireless):
| Type | Frequency | Range | Issues | Example |
|---|
| Radio | 3 KHz - 40 GHz | 10km | Interference | Wi-Fi, Bluetooth |
| Microwave | 2 - 40 GHz | 50km | LOS, Weather | Cellular Towers |
| Infrared | 300 GHz - 400 THz | 10m | Walls block | TV Remote |
Diagram (Media Comparison):
Twisted Pair: [RJ45]--//~~//~~--[PC] (Twists reduce crosstalk)
Coaxial: [Core][Shield]--===--[TV]
Fiber: [Core][Cladding]--***--[Router] (*=Light pulses)
Circuit Switched Networks
Establishes a dedicated end-to-end path before data transfer. Phases: 1. Setup, 2. Data Transfer, 3. Teardown.
| Aspect | Circuit Switching | Packet Switching |
|---|
| Resource Use | Dedicated (wasteful if idle) | Shared (statistical mux) |
| Delay | Fixed (predictable) | Variable (queuing) |
| Efficiency | Poor for bursty data | High |
Diagram (Circuit Switch Network):
Station A --+-- Switch1 --+-- Switch2 --+-- Station D
| Ch1 | Ch1 | Ch1
Station B --+-- Ch2 +-- Ch2 +-- Station E
Path A→D: Dedicated Ch1 across switches (reserved till hangup)
Real Example: Old landline call (64 Kbps circuit). Modern VoIP uses Packet Switching.