Error control, parity check, check sum, vrc

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Error control, parity check, check sum, vrc

  1. 1. <ul><li>4.2 </li></ul><ul><li>Error Control </li></ul>
  2. 2. 4.2.1 Importance of Accurate Reception <ul><li>Each member of a code is assigned a unique identity. </li></ul><ul><li>It is extremely important that it be sent and received without errors. </li></ul>
  3. 3. 4.2.1 Importance of Accurate Reception <ul><li>For instance </li></ul><ul><ul><li>The EBCDIC code for c is 10000011 </li></ul></ul><ul><ul><li>An error in bit#1 produces 10000010  which is the code for b. </li></ul></ul><ul><ul><li>An error in bit#2 produces 10000001  which is the code for a </li></ul></ul><ul><ul><li>An error in bit#8 produces 00000011  which is the code for ETX (ETX=end of text). </li></ul></ul>
  4. 4. 4.2.1 Importance of Accurate Reception <ul><li>Clearly, it is important to know whether the message has been corrupted in transmission or not. </li></ul>
  5. 5. 4.2.2 Error Detection <ul><li>Error detection is a cooperative activity between the sender and the receiver. </li></ul><ul><li>In which the sender adds information to the character or frame to assist the receiver is determining whether an error has occurred in transmission or reception. </li></ul><ul><li>Before sending, the sender and receiver must agree on the information that will be added, and how it will be used. </li></ul>
  6. 6. (1) Parity Checking: <ul><ul><li>Simple or Two Dimensional </li></ul></ul><ul><ul><li>In parity check, a parity bit is added to every data unit so that the total number of 1s is even or odd. </li></ul></ul>
  7. 7. (1) Parity Checking:
  8. 8. (1) Parity Checking: <ul><li>Suppose the sender wants to send the word world . In ASCII the five characters are coded as </li></ul><ul><li>1110111 1101111 1110010 1101100 1100100 </li></ul><ul><li>The following shows the actual bits sent </li></ul><ul><li>1110111 0 1101111 0 1110010 0 1101100 0 1100100 1 </li></ul>
  9. 9. (1) Parity Checking: <ul><li>Two-dimensional parity </li></ul><ul><ul><ul><li>In two-dimensional parity check, a block of bits is divided into rows and a redundant row of bits is added to the whole block. </li></ul></ul></ul>
  10. 11. (2) Checksum <ul><ul><li>The sender follows these steps: </li></ul></ul><ul><ul><ul><li>The unit is divided into k sections, each of n bits. </li></ul></ul></ul><ul><ul><ul><li>All sections are added using one’s complement to get the sum. </li></ul></ul></ul><ul><ul><ul><li>The sum is complemented and becomes the checksum. </li></ul></ul></ul><ul><ul><ul><li>The checksum is sent with the data. </li></ul></ul></ul><ul><ul><li>The receiver follows these steps: </li></ul></ul><ul><ul><ul><li>The unit is divided into k sections, each of n bits. </li></ul></ul></ul><ul><ul><ul><li>All sections are added using one’s complement to get the sum. </li></ul></ul></ul><ul><ul><ul><li>The sum is complemented. </li></ul></ul></ul><ul><ul><ul><li>If the result is zero, the data are accepted: otherwise, rejected. </li></ul></ul></ul>
  11. 12. Example Part 1 <ul><li>Suppose the following block of 16 bits is to be sent using a checksum of 8 bits. </li></ul><ul><li>10101001 00111001 </li></ul><ul><li>The numbers are added using one’s complement </li></ul><ul><li>10101001 </li></ul><ul><li>00111001 ------------ Sum 11100010 </li></ul><ul><li>Checksum 00011101 </li></ul><ul><li>The pattern sent is 10101001 00111001 00011101 </li></ul>
  12. 13. Example part 2 <ul><li>Now suppose the receiver receives the pattern sent in Example 7 and there is no error. </li></ul><ul><li>10101001 00111001 00011101 </li></ul><ul><li>When the receiver adds the three sections, it will get all 1s, which, after complementing, is all 0s and shows that there is no error. </li></ul><ul><li>10101001 </li></ul><ul><li>00111001 </li></ul><ul><li>00011101 </li></ul><ul><li>Sum 11111111 </li></ul><ul><li>Complement 00000000 means that the pattern is OK. </li></ul>
  13. 14. (3) VRC Checking: <ul><ul><li>Vertical Redundancy Check </li></ul></ul><ul><ul><li>VRC adds a parity bit to each character. </li></ul></ul><ul><ul><li>Most Often, it is used with ASCII. </li></ul></ul><ul><ul><li>The parity bit occupies bit position number eight. </li></ul></ul><ul><ul><li>VRC can detect all single-bit errors. It can detect multiple-bit or burst errors only the total number of errors is odd. </li></ul></ul>
  14. 15. (3) VRC Checking: <ul><ul><li>ASCII for a is  1000011 </li></ul></ul><ul><ul><li>ASCII for a with even parity is  10000111 p , the parity bit (1 p ) is a 1 so as to make a total of four 1s (an even number of 1s). </li></ul></ul><ul><ul><li>ASCII for a with odd parity is  10000110 p ; the parity bit (0 p ) is a 0 so as to leave a total of three 1s (an odd number of 1s) </li></ul></ul>
  15. 16. (3) VRC Checking: <ul><li>When operating asynchronously, the start (0s) and stop (1s) bit frame the entire eight bits: </li></ul><ul><ul><li>ASCII for a sent asynchronously is  0 s 10000111 s </li></ul></ul><ul><ul><li>ASCII for a with even parity sent asynchronously is  0 s 10000111 p 1 s </li></ul></ul><ul><ul><li>ASCII for a with odd parity sent asynchronously is  0 s 10000110 p 1 s </li></ul></ul>
  16. 17. (3) VRC Checking: <ul><ul><li>The Start and stop bits have no role in the VRC process; they simply frame the data for the convenience for the receiver. </li></ul></ul>

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