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Error Detection, Correction and Control

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Title: Error Detection, Correction and Control


1
Error Detection, Correction and Control
  • Rong Wang
  • CGS3285
  • Spring 2004

2
RECOMMENDED READING
  • From textbooks
  • Page 117 123 of Data Communications From
    Basics to Broadband, 3rd Edition by William J.
    Beyda (ISBN 0-13-096139-6)
  • Chapter 10 11 of Data Communications and
    Networking, 3rd Edition, Behrouz A. Forouzan
    (ISBN 0-07-251584-8)

3
TRANSMISSION ERROR
  • A bit is altered between transmission and
    reception
  • Where does errors come from?
  • Electrical Interference
  • Power surges
  • Noise, static, echoes, cross-talk
  • Signal degradation
  • Type of error
  • Single-bit error isolated, does not affect
    nearby error (e.g., white noise)
  • Burst error a cluster of bits involved (faded
    signal)

4
ERROR DETECTION METHODS
  • Echo Checking
  • Receiver returns copy of data back to sender
  • Ignore Parity Checking
  • Used if sender or receiver unable to process
    parity bits
  • Primitive Parity Checking
  • Mark - Always send a 1
  • Space - Always send a 0
  • 12.5 chance of detecting a 1-bit error
  • Simple Parity Checking
  • Even - Make of 1s in character even
  • Odd - Make of 1s in character odd
  • 50 chance of detecting a 1-bit error

5
ASCII CHARACTER TRANSMISSION WITH DIFFERENT
PARITIES
6
EVEN PARITY CHECKING AND BIT ERRORS
7
ERROR DETECTION METHODS (cont.)
  • Multi-bit Parity
  • Cyclical Parity
  • Uses two (2) parity bits
  • Fooled only by errors in certain bit combinations
  • Increases overhead in both data transmitted and
    processing time

8
FIGURE 6-3 CYCLICAL ODD PARITY CHECKING
9
ERROR DETECTION METHODS (cont.)
  • Multi-bit Parity (continued)
  • Hamming Code (forward error correction)
  • Form of forward error correction
  • Can detect and repair single-bit errors
  • Uses seven (7) data bits
  • Bit Positions 3, 5, 6, 7, 9, 10, 11
  • Uses four (4) parity bits
  • Bit Position 1 Bits 3, 5, 7, 9, 11
  • Bit Position 2 Bits 3, 6, 7, 10, 11
  • Bit Position 4 Bits 5, 6, 7
  • Bit Position 8 Bits 9, 10, 11
  • Sum of parity bits with errors is bad data bit
  • Increases overhead in both data transmitted and
    processing time

10
FIGURE 6-4 HAMMING w/ EVEN PARITY
11
ERROR DETECTION METHODS (cont.)
  • Synchronous Block Methods
  • Checksum
  • Use simple arithmetic to generate a checksum
    remainder which is appended to end of message
  • Sender creates checksum as block is transmitted
    then appends to end of block
  • Receiver generates checksum as block is received
    then compares with senders checksum
  • If error, receiver asks for re-transmission of
    block
  • Requires checksum and block sequence number
  • Cyclical Redundancy Check (CRC)
  • Similar to check sum but uses more complex
    mathematical formulas to catch/exaggerate errors

12
ERROR DETECTION / CORRECTION SUMMARY
  • Error detection correction designed to preserve
    the INTEGRITY of transmitted data -- not SECURITY
  • Detection is relatively easy
  • Correction is harder
  • Retransmission is simplest approach
  • Both sender and receiver must use same
    detection/correction method
  • Type of method and algorithms specified by
    protocol

13
DATA LINK ERROR CONTROL (CONT)
  • Error control
  • Based on automatic repeat request, which is the
    retransmission of data
  • Type of error handled
  • Lost frame
  • Damaged frame
  • Techniques for error control
  • Error detection
  • Positive acknowledgement
  • Retransmission after timeout
  • Negative acknowledgement and retransmission
  • Note data link layer divides the stream of bits
    received into manageable data units called
    frames.

14
DATA LINK ERROR CONTROL (CONT)
  • Stop Wait ARQ (Half Duplex)
  • Receiver responds to each block (ACK / NAK /
    WACK)
  • Sender waits for response before sending further
    blocks
  • Continuous ARQ (Full Duplex)
  • Blocks sent continuously without waiting for
    confirmation
  • If error message returned to sender,
  • Types of continuous ARQ
  • Go-Back-N restart transmission at the Nth frame
  • Selective Repeat retransmit the selected frame
    only

15
STOP-AND-WAIT ARQ
  • Note To avoid duplication, data frames are
    alternately labeled with 0 or 1, thus
    acknowledgement (ACK) frames are also numbered 0
    and 1 alternately.
  • Normal Operation
  • Sender sends frame 0 and waits to receive ACK 1
  • When ACK 1 is received, it sends frame 1 and then
    waits to receive ACK0
  • repeat from step 1 and 2 and so on.
  • The ACK must be received before the time set for
    each frame expires

16
STOP-AND-WAIT ARQ (CONT)
  • Lost or Damaged Frame
  • A lost or damaged frame is handled in the same
    way by the receiver
  • When the receiver receives a damaged frame, it
    discards it
  • The receiver remains silent about a lost frame
    and keeps its value of R
  • After the timer at the sender site expires,
    another copy of frame 1 is sent

17
STOP-AND-WAIT ARQ (CONT)
  • Lost or Damaged Acknowledgement
  • If the sender receives a damaged ACK, it discards
    it.
  • When the timer for a frame expires, the sender
    retransmits the frame.

18
STOP-AND-WAIT ARQ (CONT)
  • Delayed Acknowledgement
  • An acknowledge can be delayed at the receiver or
    by some problem with the link
  • If receiver receives a duplicated frame, it
    discards the duplicated frame

19
STOP-AND-WAIT ARQ (CONT)
  • Piggybacking
  • A method to combine a data frame with an
    acknowledgement in case both sender and the
    receiver have data to send.

20
GO-BACK-N ARQ
  • Sequence Numbers
  • Frame from a sending station are numbered
    sequentially. If the header of the frame allows m
    bits fro the sequence number, the sequence
    numbers range from 0
  • Sender Sliding Window
  • Sender uses window to hold the outstanding frames
    until they are acknowledged
  • Frames to the left of the window are those that
    have already been acknowledged and can be purged
  • Frames to the right of the window can not be sent
    until the window slides them

21
GO-BACK-N ARQ (CONT)
  • Receiver Sliding Window
  • Size of the window is always 1
  • The receiver is always looking for a specific
    frame to arrive in a specific order

22
GO-BACK-N ARQ (CONT)
  • Control Variables
  • The sender has three variables
  • S holds the sequence number of the recently sent
    frame
  • holds the sequence number of the last
    frame in the window
  • Holds the sequence number of the first
    frame in the window
  • Size of the window
  • The receiver only has one variable R, that holds
    the sequence number of the frame it expects to
    receive

23
GO-BACK-N ARQ (CONT)
  • Normal Operation

24
GO-BACK-N ARQ (CONT)
  • Damaged or Lost Frame

25
GO-BACK-N ARQ(CONT)
  • Sender window size

26
GO-BACK-N ARQ(CONT)
  • Problem
  • No out-of-order frames
  • Not efficient
  • Multiple frame may be retransmitted

27
SELECTIVE REPEAT ARQ
  • Only damaged frame is resent
  • Sender and receiver windows
  • Size of the senders window must be one half of
  • Size of the receivers window must be same as the
    senders
  • Negative acknowledgement (NACK) is used to define
    damaged frame

28
SELECTIVE REPEAT ARQ (CONT)
  • Lost Frame

29
SELECTIVE REPEAT ARQ (CONT)
  • Sender window size
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