CS3502: Data and Computer Networks DATA LINK LAYER 1

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CS3502Data and Computer NetworksDATA LINK
LAYER - 1

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data link layer objectives

• thorough understanding of DL layer --
• where/how it fits into network/other layers
• service it provides (to higher layers)
• services it uses (from lower layer)
• synchronous/asynchronous transmission
• error detection and correction
• describe flow control protocols sliding window
• specify/verify basic DL protocols
• elementary performance analysis of DL protocols

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data link layer

• phys. layer subject to errors not reliable and
  only moves information as bits, which alone are
  not meaningful. DL layer adds these, and combines
  bits into frames, or messages.
• purpose of DL transform unreliable physical bit
  stream into reliable data communications link...
• PHY DL DATA COMMUNICATIONS
• MAC layer (media access control) - takes place of
  DL layer in LANs (together with LLC)

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data link layer functions

• framing and frame synchronization
• frames marked by sync/async technique
• error control
• flow control
• addressing
• control information, data on same link (unlike
  EIA232)
• link management (3 phases)

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data link layer framing

• bits must be grouped into frames or messages
• frames marked by synchronous transmission frame
  starts, ends with a special flag pattern
• Meathods
• Character Count
• Start End Characters
• Flags
• Physical Layer Code Violation
• Finite state machine for Bit stuff flag 011110

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data link layer framing

• allows bits to be grouped into fields, subgroups
  two main types are data and control bits.
• several types of control bits some for
• error detection and/or correction
• addressing
• flow control
• other control type information

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data link layer error control

• 3 basic techniques in this course (more complex
  techniques exist)
• parity checking
• very simple and easy error detection
• CRC - cyclic redundancy check
• more complex, but very effective and efficient
• Hamming code
• limited error correction based on complex
  combinations of parity checks

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data link layer parity checking

• to a group of data bits add a single extra bit,
  known as the parity bit. This bit is chosen to
  make the total number of 1s in the group even (or
  odd). Called even (odd) parity checking.
• example data bits 0011001 add parity bit 1
  ---gt00110011.
• exercise construct a FSM to
• (1) output correct parity bit
• (2) read a string and decide parity

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data link layer parity checking

• what is the problem with simple parity checking
  as described? (show how to fool it)
• X X X P
• LRC - double parity checks improve this
• X X X X P
• X X X X P
• X X X X P
• P P P P P
• show how to fool this error check
• improves error probability by factor of 102 - 104

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data link layer error probabilities

• let PB Prob single bit error
• then (1 - PB ) Prob no error
• for group of Nb bits, define
• P1 Probno errors
• P2 Probundetected error
• P3 Probdetected error
• By definition,
• P1 P2 P3 1

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data link layer error probabilities

• examples
• in a 10 bit word, what is Ponly bit 3 wrong?
• Pexactly 1 error? Pexactly 3 errors?
• Pat most 3 errors? P3 errors or more?
• P3 bit burst error, with other 7 bits correct?

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data link layer error probabilities

• Case 1 no error detection
• Then what are these 3 probabilities for Case 1?
• P1 Pr no error
• P2 Pr undetected error
• P3 Pr detected error
• example

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data link layer error probabilities

• Case 2 error detection using parity bit
• for even parity, even errors goes undetected
  so P2 is Pr(even of errors)
• P1 (1 - PB)Nb
• P2 ? NbC2a Pb 2a (1-Pb)Nb-2a
• P3 1- P2 - P1
• hint what is Pr(1 error)? Pr(2 errors)? ... k
  errors?

Nb/2
a1
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data link layer error probabilities

• frame probabilities, parity checking
• given a frame, sent as a sequence of
  words/bytes, each with a parity check. What are
  Pf1, Pf2, and Pf3?
• Pf1 probability of no error in the whole
  frame
• Pf2 probability of an undetected error and no
• detected error anywhere else in the frame
• Pf3 probability of an detected error

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data link layer error probabilities

• Nb no.bits/word Nc no.words/frame.
• Pf1 P1Nc
• where P1 is the probability of no error in a
  word
• Pf2 ? NcCi P2 i (1-Pb)NbNc -i
• where P2 is the probability of undetected
  error in a word
• Pb is the probability there is an
  error in a bit
• Pf3 1 - Pf1 - Pf2

Nc
i1
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error checking CRC

• stronger error check needed
• idea insert a group of bits in the frame, which
  serve as as more powerful check.
• added bits (frame check sequence, FCS) cause the
  resulting frame to be exactly divisible by a
  predetermined number.
• modulo-2 arithmetic used binary addition with no
  carries
• examples

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error checking CRC

• let M denote data message, k bits long
• to M, add F, the FCS, which is n bits long
• resulting transmitted frame is T,
• T 2n M F M F
• is evenly divisible by some pattern P, a sequence
  of (n 1) bits.
• Q how is F calculated from M and P?

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error checking CRC

• F is n bits, pattern P is (n 1) bits
• 1st, last bits of P must be 1
• FCS F computed from M, P
• F remainder R, when dividing
• 2n M / P Q R
• note this division is modulo-2, no carries
• example let M 110011 k 6 n 3 p 1001.
  Find F and T. (answer next page)

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error checking CRC

• quotient Q 110101 remainder R 101 so T
  110011101.
• check divide T by P, R should be 0.
• example M 1010001101 P 110101. Find F, T.
  Then check it.

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error checking CRC

• CRC summary
• all single and double bit errors
• all odd numbers of errors
• all burst errors smaller than n
• most larger burst errors
• If error detected, frame is retransmitted does
  NOT correct error.
• both CRC and parity checking widely used CRC
  used in many network protocols, in addition to
  data link layer, including most LANs.
• CRC can be implemented efficiently in hardware...
  computation done bit by bit

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error checking CRC implementation

• shift register, XOR gates
• 1 XOR gate for each 1 in pattern P, minus 1.
• (n-1) 1-bit shift registers
• example show logic circuit for P - 110101.

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error checking Hamming code

• correct a single bit error
• detect multiple errors
• extended parity checking ie, redundant parity
  bits
• Idea
• parity bits appear in positions corresponding to
  the nodes of a binary tree data bits appear in
  positions corresponding to the leafs. If a bit is
  in error, the other bits point to it by their
  related positions in the tree.

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error checking Hamming code

• each parity bit appears in a position
  corresponding to a power of 2 k
  0,1,2,4,8,16,...
• bits checked by parity bit in position n are
• 1. itself
• 2. continue for next n bits (including itself)
• 3. off (dont check) next n bits
• 4. on (check) the next n bits
• and continue to end of message.
• Example for the data 1101101, show Hamming coded
  mesage.

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error checking Hamming code

• How is single error bit detected?
• example suppose 111100010101 is received.
• 1st parity bit, position 1
• 2nd parity bit, position 2
• 3rd parity bit, position 4
• 4th parity bit, position 8
• 0 parity bit, position 0
• result

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error checking Hamming code

• example suppose 0111011 received is it correct?
• Q for a n-bit message, approximately how many
  bits are needed?
• Summary, error correction
• more complex codes correct multiple errors
• require more redundancy, overhead. Also more
  time... so not widely used in communications.
• do have a place in longer distance
  communications -- eg, deep space, etc. In fact
  could be critical to long distance (time)
  communications