Data Link Layer

1
Data Link Layer

• Here, youll learn about
• Basic design of the data link layer
• The provision of a wirelike channel
• Algorithms for providing reliable and efficient
  communication
• Detection and correction of errors

2
Data Link Layer

• Two sublayers
• The Media Access Control (MAC)
• The Logical Link Control (LLC)

3
Data Link Layer Issues

• Providing a well-defined service interface to the
  network layer
• Determining how the bits of the physical layer
  are grouped into frames
• Dealing with transmission errors
• Regulating the flow of frames so that slow
  receivers are not swamped by fast senders

4
Services Provided to the Network Layer

• The principal service is transferring data from
  the network layer on the source machine to the
  network layer on the destination machine.
• Three different service levels
• unacknowledged connectionless service
• acknowledged connectionless service
• acknowledged connection-oriented service

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Framing
6
Framing

• Different framing methods for networks
• Character count
• Starting and ending characters, with character
  stuffing
• Starting and ending flags, with bit stuffing
• Physical layer coding violations

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Framing

• Character Stuffing

• Bit Stuffing

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Framing

• Physical layer coding violations - applicable
  only to networks in which encoding on the
  physical medium contains some redundancy

9
Error Control

• Making sure all frames are eventually delivered
  to the network layer at the destination, and in
  the proper order
• Positive and negative acknowledgements
• Timing mechanisms
• Handling multiple sending of frames

10
Flow Control

• Managing fast senders for slow receivers
• You may send me n frames now, but after they
  have been sent, do net send any more until I have
  told you to continue.

11
Error Detection and Correction

• Error occur in both wired and wireless
  communication
• Wireless communication is becoming more common
• Error rates of wireless communication are orders
  of magnitude worse than wired communication

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Error Correcting Codes
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Error Correcting Codes

• Introduction of redundancy to allow the receiver
  to deduce that an error occurred and what the
  transmitted character must have been
• Use exclusive-or (XOR) to determine how many bits
  differ

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Hamming Code

• Hamming distance the number of bit positions in
  which two codewords differ
• To correct d errors, you need a distance of 2d1
  code because that way the legal codewords are so
  far apart than even with d changes, the original
  codeword is still closer than any other codeword,
  so it can be uniquely determined

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Hamming Code

• Example of an error-correcting code with only
  four valid codewords
• 0000000000, 0000011111,
• 1111100000, and 1111111111
• Code has a distance of 5
• Code can correct 2 errors

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Parity Bit

• Given the message is 100101, what is the parity
  bit of this message?
• Even Parity if the number of set (1) bit is even
  (parity bit is 0)
• Odd Parity if the number of set (1) bit is odd
  (parity bit is 1)

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Error Detecting Codes
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Error Detecting Codes

• Only include enough redundancy to allow the
  receiver to deduce that an error has occurred,
  but which error, and have it request an
  retransmission
• Requires less bits to detect errors compared to
  Hamming code (error correcting code)
• To detect d errors, you need a Hamming distance
  of d1

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Error Detecting Codes

• Convert the message to a matrix of bits to detect
  long error bursts
• Polynomial code or Cyclic redundancy code (CRC)

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CRC

• Polynomial Arithmetic

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CRC
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CRC

• Polynomials used in International Standards for
  CRC
• CRC-12 X12 X11 X3 X2 X1 1
• For character length of 6 bits
• CRC-16 X16 X15 X2 1
• For character length of 8 bits
• CRC-CCITT X16 X12 X5 1
• For character length of 8 bits