Data Link: Overview & framing

The layer:

• Transfers frames over a single link
• groups bits into frames
• provides services to network layer
• regulates data flow
• handles transmission errors

Possible services

• unacknowledged connectionless (no connection or error recovery) — e.g. ethernet
• acknowledged connectionless (frame is resent if needed) — e.g. 802.11 WiFi
• acknowledged connection-oriented (conn. established, only for unreliable connections)

Framing

frames — parts of bit stream with a checksum (computed when sent, recomputed to check for correctness when received)

Framing methods

• byte count
  • first byte is length of frame
  • problem is that if length gets fucked up, so rest of data
• byte stuffing
  • contain a flag byte to delimit frames
  • flag byte at end, flag byte at start
  • need something that isn’t used in data (or escaping)
• bit stuffing
  • like byte stuffing, but smaller
  • bit pattern instead of flag byte
  • single bit instead of flag byte