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IE 418/518: Telecommunication Concepts

IE 418/518: Telecommunication Concepts. Lecture Notes #6 Digital Data Communication Techniques. System A. System B. Transmission Modes. Timing problems require a mechanism to synchronize the transmitter and receiver Timing of bits is the key!. ……101101. Controlling Timing.

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IE 418/518: Telecommunication Concepts

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  1. IE 418/518:Telecommunication Concepts Lecture Notes #6 Digital Data Communication Techniques

  2. System A System B Transmission Modes • Timing problems require a mechanism to synchronize the transmitter and receiver • Timing of bits is the key! ……101101

  3. Controlling Timing • Two solutions • Asynchronous Transmission • Each character of data is treated independently • Synchronous Transmission • For sending large blocks of data • Control schemes • Character-oriented • Bit-oriented

  4. Asynchronous Transmission • Data transmitted one character at a time • 5 to 8 bits • Timing only needs to be maintained within each character • Transmitter and Receiver clocks need not be in sync! • Resynchronize at the beginning of each character

  5. Asynchronous Transmission Behavior • In a steady stream, interval between characters is uniform • Length of stop element • In idle state • Receiver looks for transition 1 to 0 • Then samples next seven intervals • Char length • Then looks for next 1 to 0 for next char

  6. Asynchronous Transmission Diagram

  7. Asynchronous Transmission Advantages • Simple • Cheap • Overhead of 2 or 3 bits per char • Approx  20% • Good for data with large gaps • e.g. keyboard

  8. Asynchronous Transmission Frame synchronization • Blocks of printable characters • Encapsulate complete block between two special (non-printable) transmission control characters • STX  Start-of-text • ETX  End-of-text

  9. Asynchronous Transmission Frame synchronization • Binary data • e.g., Contents of a file containing a compiled program • Use of single ETX not sufficient to indicate the end of a file • One of the bytes might be the same as an ETX character • Solution  STX and ETX are preceded by another transmission control character • Data link escape (DLE)

  10. Asynchronous Transmission Frame synchronization • Binary data • Character or byte stuffing

  11. Synchronous Transmission • Two approaches • Character-oriented • Bit-oriented • Both use the same bit synchronization methods • Major difference is in how they achieve character & frame synchronization

  12. Synchronous Transmission Bit Level • Block of data transmitted without start or stop bits • Tx and Rx clocks must be synchronized!!! • Option 1 -- Can use separate clock line • Good over short distances • Subject to impairments • Option 2 -- Embed clock signal in data • Manchester encoding • Carrier frequency (analog)

  13. Synchronous Transmission Character-Oriented • Used for transmission of block of characters • e.g., files of ASCII characters • Character synchronization • Achieved with synchronous idle character (SYN) • Frame synchronization • STX-ETXsequence preceded by SYN char • Once bit-synchronization is achieved • Receiver enters “hunt mode” • Bit stream is interpreted in windows of 8 bits

  14. Synchronous Transmission Character-Oriented Example

  15. Synchronous Transmission Bit-Oriented • Used for transmission of binary data • Preferred control scheme • Point-to-point links • Uses a flag pattern for start and end of frame • Idle bytes  01111111 • Flag pattern  01111110 • Bit stuffing (or zero bit insertion) • Inserts a “0” after five consecutive 1s • LANs use a similar scheme • More efficient (lower overhead) than async

  16. Synchronous Transmission Bit-Oriented Example

  17. Complementary Reading • Halsall, F., Data Communications, Computer Networks and Open Systems, (USA: Addison-Wiley, 1996), pp. 107-125.

  18. ERROR DETECTION

  19. Basic Probability Concepts • Pb Probability a bit is received in error (BER) • P1  Probability a frame arrives with no errors • P2  Probability that, with the use of error detection, a frame arrives with one or more undetected errors • P3  Probability that, with the use of error detection, a frame contains errors and these will be detected

  20. Error Detection • Additional bits added by transmitter for error detection code • Additional bits calculated as a function of the other bits • Parity • Value of parity bit is such that character has even (even parity) or odd (odd parity) number of ones • Even number of bit errors goes undetected • Requires retransmission if an error is detected

  21. Parity Example • Character to be transmitted  “S” • “S”  1010011 • Even Parity  0 1010011 • Odd Parity  11010011 • Transmission w/ one error (odd)  11011011 • Error detected!! • Transmission w/ two errors (odd)  11011111 • No error detected!! • Received character is “_”

  22. Cyclic Redundancy Check (CRC) • Useful to detect error bursts • Process Steps: • For a block of k bits (message), transmitter generates an (n – k)- bit sequence • Known as the Frame Check Sequence (FCS) • Transmit n bits which is exactly divisible by some number • Receiver divides frame by that number • If no remainder, assume no error

  23. Cyclic Redundancy Check (CRC) • T n-bit frame to be transmitted • D  k-bit block of data, or message • The first k bits of T • F  (n – k)-bit Frame Check Sequence (FCS) • The last (n - k) bits of T • P  Pattern of n – k + 1 bits • Predetermined divisor

  24. CRC Example • Calculate the FCS for the message shown below using P = 11001 • D  11100110 • FCS  ?

  25. CRC Example

  26. CRC Standards • CRC-12 • Used when character length is 6 bits • CRC-16 and CRC-CCITT • Used when character length is 8 bits • Used in WANs • CRC-32 • Used in LANs • DoD applications

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