Overview of Link Layer

• Responsibility: transfer datagram from one node to physically adjacent node over link
• Flow control
• Error detection
• Error correction
• Could be full or half duplex (can only send one at a time)
• Implemented at network interface (NIC)

Error detection

• Single bit parity
• 2D bit parity, detect and correct single bit errors
• CRC32

Access protocols

• P2P, Broadcast
• TDMA, FDMA, WDM
• Random access protocols:
  • Slotted ALOHA (on collision, retransmit with probability p: 37%)
  • Pure ALOHA (no clock, less efficiency: 18%)
• CSMA (carrier sense multiple access)
  • Idle, transmit; busy, defer
• CSMA/CD (with collision detection)
  • If collision, abort