Chapter 6 - Digital Data Communication Techniques 9e

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Chapter 6 - Digital Data Communication Techniques 9e

  1. 1. Digital Data Communications Techniques CEN 220/CIS 192 Advanced Data Communications and Networking Data and Computer Communications, W. Stallings 9/E, Chapter 6
  2. 2. 2 Digital Data Communications Techniques A conversation forms a two-way communication link; there is a measure of symmetry between the two parties, and messages pass to and fro. There is a continual stimulus-response, cyclic action; remarks call up other remarks, and the behavior of the two individuals becomes concerted, co-operative, and directed toward some goal. This is true communication. —On Human Communication, Colin Cherry
  3. 3. 3 Asynchronous and Synchronous Transmission Timing problems require a mechanism to synchronize the transmitter and receiver  receiver samples stream at bit intervals  if clocks are not precisely aligned, drifting will sample at wrong time after sufficient bits are sent Two solutions to synchronizing clocks:
  4. 4. 4 Asynchronous Transmission Data are transmitted one character at a time – each character is 5 to 8 bits in length – receiver has the opportunity to resynchronize at the beginning of each new character Simple and cheap – requires overhead of 2 or 3 bits per character (~20%) The larger the block of bits, the greater the cumulative timing error Good for data with large gaps (keyboard)
  5. 5. 5 Asynchronous Transmission
  6. 6. 6 Synchronous Transmission Block of bits transmitted in a steady stream without start and stop codes Clocks must be synchronized to prevent drift  can use separate clock line  embed clock signal in data Need to indicate start and end of block  use preamble and postamble bit patterns Frame is data plus preamble, postamble, and control information More efficient than asynchronous for large blocks of data
  7. 7. 7 Types of Error An error occurs when a bit is altered between transmission and reception  binary 1 is transmitted and binary 0 is received or binary 0 is transmitted and binary 1 is received
  8. 8. 8 Error Detection Regardless of design you will have errors Can detect errors by using an error-detecting code added by the transmitter • code is also referred to as check bits Recalculated and checked by receiver Still chance of undetected error Parity  parity bit set so character has even (even parity) or odd (odd parity) number of ones  even number of bit errors goes undetected
  9. 9. 9 Parity Check The simplest error detecting scheme is to append a parity bit to the end of a block of data If any even number of bits are inverted due to error, an undetected error occurs
  10. 10. 10 Error Detection Process
  11. 11. 11 Cyclic Redundancy Check (CRC) One of most common and powerful checks For block of k bits transmitter generates an n bit frame check sequence (fcs) Transmits k+n bits which is exactly divisible by some predetermined number Receiver divides frame by that number  if no remainder, assume no error
  12. 12. 12 Error Correction Correction of detected errors usually requires data block to be retransmitted Not appropriate for wireless applications  bit error rate is high causing lots of retransmissions  propagation delay long (satellite) compared with frame transmission time, resulting in retransmission of frame in error plus many subsequent frames Need to correct errors on basis of bits received codeword  on the transmission end each k-bit block of data is mapped into an n-bit block (n > k) using a forward error correction (FEC) encoder
  13. 13. 13 Error Correction Process
  14. 14. 14 How Error Correction Works Adds redundancy to transmitted message  redundancy makes it possible to deduce original message despite some errors Block error correction code
  15. 15. 15 Line Configuration - Topology
  16. 16. 16 Line Configuration - Topology
  17. 17. 17 Line Configuration - Duplex Data exchanges classified as half or full duplex ● Half duplex (two-way alternate) ● only one station may transmit at a time ● requires one data path ● Full duplex (two-way simultaneous) ● simultaneous transmission and reception between two stations ● requires two data paths ● separate media or frequencies used for each direction
  18. 18. 18 Summary Asynchronous and synchronous transmission  asynchronous • data transmitted one character at a time  synchronous • block of bits transmitted in steady stream without start and stop codes Error detection and correction  single bit error and error burst  error detecting codes • parity and cyclic redundancy check (CRC) Line configurations  topology  full duplex and half duplex

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