Framing

• Special bit strings
• Frame length
• HDLC
• FDDI
• ATM
• Ethernet

The Framing Problem

Frame Structure

• header
• data (payload)
• trailer
• packet length: fixed, variable

The Framing Problem, restated

• Identify the header in a sequence of bits
• Determine the packet length:
  • fixed length
  • header contains length
  • trailer contains mark
  • trailer contains checksum
• Recover from errors or discard erroneous frames.
• Strip headers and trailers and deliver data to the higher layer

HDLC

• High-Level Data Link Control
• class of protocols
• Special Flag is bit pattern 0111 1110
• Must escape the flag in the data:
  • When sending, add a 0 bit after every sequence of 5 " 1"s
  • When receiving, rewrite the sequence 111110 to 11111
  • When receiving, the sequence 111111 is the flag
• Frame structure:
  

  8	8	8	var.	16	8
  flag	address	control	data	cks	flag

HDLC Control field

• N(S): 3-bit sequence number
• N(R): acknowledgement number (N(R) = 3 means received packet 2)
• I frame (Information -- carries data):

  1	3	1	3
  0	N(S)	P/F	N(R)

• S frame (Supervisory):

  2	2	1	3
  10	S	P/F	N(R)

• U frame (Unnumbered):

  2	2	1	3
  11	A	P/F	B

HDLC Modes

• Normal Response mode: primary node polls secondary nodes, which respond
• Asynchronous Response mode (most widely used): secondary node can initiate transfer
• Asynchronous Balanced mode: symmetric

HDLC Asynchronous Response protocol

• U frames used to connect and disconnect
• S frames: ACK
  • Ready to Receive
  • Not Ready to Receive
  • Reject (Negative Ack)
• P/F = 1 requires an immediate response (ack)
• I frames: data

HDLC Example

LAPB

• Asynchronous Balanced Mode of HDLC

FDDI Framing

• 4B/5B uses 16 of 32 possible symbols for data
• 17th symbol is "idle symbol"
• other symbols are control symbols
• first non-idle, non-control symbol is start of frame
• first idle symbol after frame marks end of frame

ATM Framing

• fixed-size cells
• where does the first cell start?
• HEC (Header Error Correction):
  • 5th byte of 53-byte cell
  • CRC of first 4 bytes
  • Compute repeatedly until CRC matches
  • Compute repeatedly every 53 bytes to make sure it is not a coincidence
  • Thereafter, keep checking CRC to detect errors

ATM Errors

• An error may make the CRC not match
• types of errors:
  • corrupted contents
  • shifted boundaries (framing error)
• For the first few CRC errors, keep the presumed cell boundaries
• After a sufficient number of consecutive CRC errors, start looking for the cell boundaries again.