Terminology

• Node - link that is part of an end-to-end path
  • Refers to hosts and routers
  • Does not matter if node is a router or end node
• Link - communication channel that connects adjacent nodes along communication path
  • e.g. wired links, wireless links, LANs
• Frame - refers to a layer-2 packet
  • Encapsulates datagram (i.e. network level data)

The data-link layer has the responsibility of transferring a datagram from one node to a physically adjacent node over a link.

A datagram can be transferred by different link protocols over different links.

• e.g. Ethernet on first link, frame relay on intermediate links, 802.11 on last link

Each link protocol provides different services.

• e.g. RDT may or may not be provided over a given link

Transportation Analogy

Link Layer Services

• Framing
  • Involves encapsulating a datagram into a frame + adding header/trailer
  • MAC addresses are used in frame headers to identify source/destination
    • These are different from IP addresses
• Link Access
  • Channel access if shared medium is used
  • Communication may be over a shared link - need to provide method to mitigate collisions (multiple access protocol)
• Reliable delivery between adjacent nodes
  • Mainly provided in wireless links (have high error rates)
  • Less errors on low bit-error links (e.g. fiber, etc.)
• Error detection
  • Includes errors caused from signal attenuation, noise
  • Faster to detect errors at link layer
  • When receiver detects errors, it will signal sender to retransmit or drop frame
• Error correction
  • A receiver can identify and correct bit error(s) without resorting to retransmission