Error detection and Correction

History of data Communication History of data communicationData communication began in 1837 when telegraph was invented (Morse code)Telegraph in English by sir charles wheatstone and sir william cooke in 18441849 first low speed telegraph1860 first high speed telegraph 1876- telephone by Alexander graham bell in bell laboratory 1920 first radio station were installed1949- Diode based computer capable of executing stored program.1951 Mass produced electronic computer Then a number of mainframe computers small business computers, personal computers has increased rapidly

Standards for data communicationInternational standard Organization (ISO)Creates set of rules and standard for graphics, document exchange etc.

Consultative committee for international telephony and telegraph. (CCITT)To set rules for telephone and telegraph communication

Institute of electrical and electronic engineers (IEEE).Organization for electronic and computer communication engineers

American national standard Institute.(ANSI)OFFICIAL STANDARD AGENCY FOR AMERICAFor equipment manufacturers and users of data processing equipments.

Error detection and CorrectionTypes of Error Single bit ErrorMulti bit ErrorBurst Error

Single bit Error

Multi Bit Error

Burst Error

Error Detection TechniqueRedundancy CheckingVertical Redundancy CheckingCheck SumLongitudinal Redundancy CheckingCyclic Redundancy CheckingRedundancy CheckingIt is a technique of sending every data twiceThe receiver will do bit by bit comparison b/w the two versions of the dataIf the same character is not received twice in succession, then the transmission error is occurred

If 1010 is to be transmitted 11001100 this data will be transmitted Check sum

Vertical Redundancy Checking

Longitudinal Redundancy Checking

LRC vs VRC

Cyclic Redundancy CheckIt is a mathematical technique which is applied to the transmitted data.CRC is more effective in catching 99.95% of all transmission errors.

Steps in Calculating CRC(Step 1)G(x) -> Message polynomialP(x) -> Generator Polynomial G(x)is divided by P(x) and the Quotient is discarded.Steps in Calculating CRC(Step 2)Remainder of the binary division is added to the data message polynomial G(x).

(Step 3)At the receiver section, the new data message polynomial G(x) is again divided by P(x)

If the remainder is zero, then there is no error occurred Division in the sender

Division in the Receiver

Error Correction Error CorrectionForward Error CorrectionRetransmissionSymbol substitution Symbol substitution Was designed to be used for human environment i.e. when any human being at the receiving end analyses the data and has to make decisions on its integrity.A unique undefined character such as reverse question mark ( ) is substituted for the error (or) bad characterSuppose if the character error cannot be identified by the operator then Retransmission is necessaryRETRANSMISSIONIt means to resend the data when errors are detected. It is often called as ARQ [automatic request for retransmission]. It is most reliable method of error correction.ARQ is of two types Discrete ARQ It uses acknowledgement to indicate the successful and un successful reception of data. ( Positive ACK( error free ) & Negative ACK (with error).

Continuous ARQ ( selective repeat ARQ)To transmit a particular frame of dataFORWARD ERROR CORRECTIONIt the only error correction method which actually detects and corrects transmission errors at the receive end without calling retransmission.Hamming code

Condition to find number of hamming bits

Example

Cont

Serial Interface RS 232

RS means recommended standardRS 232 Pinout

Parallel InterfaceComputerPrinterData circuitsSTBACKBUSYPOSLCTAFPrimeERRORParallel interfaceData lines - It has 8 data lines d0 to d7 all are uni directionalControl linesStrobe used by computer to direct the printer to accept dataAutofed signal- Prime signal Clear printers memorySelect this line is not frequently used it is grounded oftenStatus tells what the printer is doingACK, BUSY, ERROR,