So calculating a checksum may bea simple method for detecting errors, but doesn't give much more protection than the parity bit, independent ofthe length of the checksum. Even if we had useda four byte long checksum we would not have detected this transmission error.
They often fail in bursts, or due to electrical spikes.Let us assume that in our example array the lowest significant bit of the character' L' is set, and the lowest significant bit of charcter ' a' is lost during communication.The receiver will thansee the arrayrepresenting the string ' M`mmert'.The checksum for this newstring is still 210, but the result is obviously wrong, only after two bits changed. In practice, bitsdo not change purely random during communications. Seems rather good, but this is only theory. We might conclude that with a four byte checksum the chance that we accidentily donot detect an error is less than 1 to 4 billion. Using a two byte checksum willresult in 65,536 possible different checksum values and when a four byte value is used there are more than fourbillion possible values. In this example we have used a one byte long checksum which gives us256 different values. You can use the calculator above to check this result. The onebyte checksum of this array can be calculated by adding all values, than dividing it by 256 and keeping theremainder. Lets take an example string and calculate a one byte checksum.The example string is ' Lammert' which converts to theASCIIvalues. It is certainly easier to calculatea checksum, but checksums do not find all errors. One might think, that using a checksum can replace proper CRC calculations. The answer is simple, they are powerful, detect many types of errors and are extremly fast to calculateespecially when dedicated hardware chips are used. Modern computer world cannot do without these CRC calculation. Also each data block on your harddisk has a CRCvalue attached to it. Allpackets sent over a network connection are checked with a CRC. Nowadays CRC calculations are used in all types of communications. The CRC calculationor cyclic redundancycheck was the result of this. To overcome this problem people havesearched for mathematical sound mechanisms to detect multiple false bits.
#HEX FILE CRC 16 CALCULATOR SERIAL#
For serial data they came up withthe solution to attach aparity bitto each sent byte.This simple detection mechanism works if an odd number of bits in a byte changes, but an evennumber of false bits in one byte will not be detected by the parity check.
Since the beginning of computerscience, people have been thinking of ways to deal with this type of problem. Introduction on CRC calculationsWhenever digital data is stored or interfaced, data corruption might occur.
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