Physical Layer

The physical layer is the lowest layer in the OSI reference model. It is responsible for the transmission of raw bits over a physical medium, defining the electrical, optical, or wireless signaling that carries the data. Its functions include bit-level transmission, modulation, synchronization, and dealing with noise and interference.

1. Basic Concepts of the Physical Layer

  • Defines the hardware means of sending and receiving data (cables, switches, radios, etc.).
  • Transmits data in the form of electrical signals, light pulses, or radio waves.
  • Provides data rate, bit representation, and signal transmission mode (simplex, half-duplex, full-duplex).

2. Fundamentals of Data Communication

Data communication at the physical layer is based on converting digital data into signals that can travel over the medium.
Key concepts include:

  • Analog vs Digital Signals: Continuous vs discrete representation of data.
  • Bandwidth: The capacity of a channel, usually measured in Hz.
  • Noise: Unwanted signals that affect transmission quality.
  • Data Rate: The speed at which data is transmitted, usually measured in bps (bits per second).

3. Nyquist Theorem and Shannon Capacity

  • Nyquist Theorem: In a noiseless channel with bandwidth B, the maximum data rate is 2B log2(M) bits per second, where M is the number of discrete signal levels.
  • Shannon Capacity: In a channel with bandwidth B and signal-to-noise ratio (SNR), the maximum data rate is C = B log2(1 + SNR).
    These formulas establish the theoretical limits of data transmission.

4. Transmission Media

Physical media can be categorized into guided and unguided transmission:

  • Guided Media

    • Twisted Pair Cable: Inexpensive, widely used in LANs.
    • Coaxial Cable: Better shielding, used for cable networks.
    • Optical Fiber: High bandwidth, long-distance, immune to EMI.

  • Unguided Media

    • Radio Frequency (RF): Common in Wi-Fi and Bluetooth.
    • Microwave: Point-to-point communication, used in backbone links.
    • Satellite Communication: Long-range global coverage.

Physical Layer Protocols

  • Electrical Medium Access Control (MAC): Manages reliable physical transmission.
  • Digital Signal Processing (DSP): Converts bits into signals and vice versa.
  • Optical Fiber Communications (OFC): Transmission over fiber optics.
  • Radio Frequency Communications (RF): Wireless data transmission.

Physical Layer Standards

  • IEEE 802.3 (Ethernet)
  • IEEE 802.11 (Wi-Fi)
  • IEEE 802.15.1 (Bluetooth)

Physical Layer Technologies

  • Frequency-Division Multiplexing (FDM)
  • Time-Division Multiplexing (TDM)
  • Code-Division Multiple Access (CDMA)
  • Orthogonal Frequency-Division Multiplexing (OFDM)
  • Single-Carrier Frequency-Division Multiplexing (SC-FDM)