Lecture 4: Ethernet Layers (Data Link and Physical)

Data Link Layer (Layer 2)

• Encapsulates Layer 3 packets into Frames and manages access to the physical media.
• Split into LLC Sublayer (communicates with upper software layers) and MAC Sublayer (handles data encapsulation and media access control).
• Performs Error Detection to reject corrupted frames.

Media Access Control & Duplex Modes

• Half-Duplex: Only one device can send or receive at a time. Legacy Ethernet uses CSMA/CD (Collision Detection) and Wireless uses CSMA/CA (Collision Avoidance).
• Full-Duplex: Devices can send and receive simultaneously (e.g., Ethernet switches).

Physical Layer (Layer 1)

• Receives frames and encodes them into bit streams (0s and 1s) for transmission over media.
• Involves Encoding (like Manchester or 4B/5B) and Signaling (electrical voltages, light pulses, or radio waves).
• Bandwidth is the capacity to carry data (measured in bps, Mbps, etc.). Latency is the delay, Throughput is actual transfer speed, and Goodput is throughput minus traffic overhead.

Network Media

• Copper Cabling: UTP, STP, Coaxial. Cheap and easy but suffers from Attenuation (signal loss over distance) and EMI/RFI (interference). Crosstalk is mitigated by twisting wire pairs.
• Fiber-Optic Cabling: Uses glass cores to transmit light (Single-mode or Multimode). Transmits over long distances at high bandwidths and is completely immune to EMI/RFI, but is expensive and difficult to install.
• Wireless Media: Transmits via radio/microwave frequencies. Provides great mobility but suffers from interference, security vulnerabilities, and shared bandwidth limitations. Key standards include Wi-Fi (802.11), Bluetooth (802.15), and Zigbee (802.15.4).