CS3505: DATA LINK LAYER

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CS3505DATA LINK LAYER

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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
• error control
• flow control
• addressing
• link management

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data link layer framing

• bits must be grouped into frames ( messages)
• frames marked by synchronous transmission
  starts, ends with a special flag pattern
• groups bits 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
• parity checking
• simple and easy, but not very effective
• 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

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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 N bits, define
• P1 Probno errors
• P2 Prob undetected error
• P3 Probdetected error
• By definition,
• P1 P2 P3 1

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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
  P1, P2, and P3?
• NB no.bits/word NC no.words/frame.
• P1
• P2
• P3

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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

• 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 n
  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 1101100101, show Hamming
  coded mesage.

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error checking

• 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

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DL layer flow and error control

• purpose control flow of data from sender S to
  receiver R, so R is not overwhelmed nor kept
  idle
• secondary purpose also used to avoid swamping
  the network or link with traffic.
• technique send control information between S
  and R, synchronizing on buffer space,
  transmission rates, etc.
• protocols
• stop-and-wait, alternating bit
• sliding window

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data link layer stop-and -wait protocol

• send 1 frame, then stop, and wait for an
  acknowledgment before sending the next.

X
R
data
ack
data
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data link layer stop and wait protocols

• what is the efficiency of this SW protocol?
  i.e., of the total time spent, how much is
  actually spent sending the data?
• variables
• tD, time spent transmitting the data
• tprop, propagation delay
• tproc, processing time
• tack, time spent transmitting the ack.
• U, utilization or efficiency of the protocol

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DL layer stop and wait

• finite state machine specification of the
  protocol
• assuming no channel errors
• reachability analysis of the protocol
• what happens if a message is lost?

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DL stop and wait

• to tolerate losses, must add timeouts (TO) and
  retransmissions
• data loss
• ack loss
• finite state machine specification
• timeout and retransmission
• reachability analysis

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DL stop and wait

• what is the efficiency of the stop and wait
  protocol? (what of time actually Xmitting data)
• assuming no channel losses
• with channel losses
• what happens if ack is lost? (what problem does
  this cause for the receiver?)
• what is the obvious solution?

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DL alternating bit protocol

• add a number to data frames, to uniquely
  identify enable repeated messages to be safely
  discarded
• CFSM specification, AB protocol
• reachability analysis
• efficiency

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protocol specification

• protocols are algorithms, critical to network
  operation these are/will be standardized, and
  must be clearly specified, without ambiguity, so
  that
• implementations by different vendors will be the
  same, and
• protocol can be thoroughly analyzed (checked for
  errors)
• communicating finite state machines (CFSM) - a
  very often used tool for protocol specification

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link utilization of AB protocol

• satellite link

earth
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sliding window protocol

• suppose w 63
• what if d61 lost?

d0
d1
d62
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HDLC high level data link control

• ISO standard for a data link protocol
• other DL standards exist, but are very similiar
  e.g., PPP
• HDLC combines various functions of the DL layer -
  flow control, error control, sequencing, framing,
  etc. - into a single protocol standard
• HDLC standard is broad, covering several
  different cases

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HDLC

• frame types and formats

• I-frame (info/data)
• S-frame (supervisory)
• U-frame (data)