02. Physical Layer

Physical Layer

1. Communication Basics

• Channel: The physical path for data transmission
• Signal: The carrier of data, divided into analog and digital signals
• Bandwidth: The highest frequency range that a channel can carry, determines the maximum data transmission rate
• Symbol, Baud, Rate:
  • Symbol: The smallest unit of a signal
  • Baud: Number of symbols transmitted per second
  • Rate: Number of bits transmitted per second (bps)
• Source and Sink: The sender and receiver of information

2. Nyquist Theorem and Shannon Theorem

• Nyquist Theorem: Maximum symbol rate for a noiseless channel $C=2W\log_2M$
• Shannon Theorem: Maximum data rate for a noisy channel $C=W\log_2(1+S/N)$

3. Encoding and Modulation

• Encoding: Converting digital signals into signals suitable for transmission (e.g., Manchester encoding)
• Modulation: Converting digital signals into analog signals (ASK, FSK, PSK)

4. Switching Methods

• Circuit Switching: Establishes a dedicated physical path, suitable for telephony
• Message Switching: Store-and-forward, suitable for large volumes of data
• Packet Switching: Divides data into small packets, flexible and efficient, used by the Internet

5. Datagram and Virtual Circuit

• Datagram: Each packet is routed independently
• Virtual Circuit: A logical path is established first, subsequent packets follow the path

6. Transmission Media

• Twisted Pair: Commonly used in LANs, good anti-interference, low cost
• Coaxial Cable: High bandwidth, strong anti-interference, suitable for long distances
• Optical Fiber: Extremely high bandwidth, very strong anti-interference, suitable for backbone networks
• Wireless: Suitable for mobile communications, easily affected by interference

7. Physical Layer Interface Characteristics

• Mechanical Characteristics: Interface shape, size, pins
• Electrical Characteristics: Voltage, current, impedance
• Functional Characteristics: Functions of each pin
• Procedural Characteristics: Signal timing, handshaking, etc.

8. Physical Layer Devices

• Repeater: Amplifies and forwards signals, extends transmission distance
• Hub: Multi-port repeater, implements star topology

Exercises

1. Briefly explain the differences among bandwidth, symbol, baud, and rate.
2. In what scenarios are the Nyquist theorem and Shannon theorem applicable?
3. Compare the characteristics of circuit switching, message switching, and packet switching.
4. List common transmission media and their characteristics.
5. What are the common physical layer devices? What are their functions?