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In order to automate the reading process, the SSCC is often encoded in a barcode, generally GS1-128, and can also be encoded in an RFID tag. It is used in electronic commerce transactions. The SSCC comprises an extension digit, a GS1 company prefix, a serial reference, and a check digit. It is all numeric.
For instance, the UPC-A barcode for a box of tissues is "036000241457". The last digit is the check digit "7", and if the other numbers are correct then the check digit calculation must produce 7. Add the odd number digits: 0+6+0+2+1+5 = 14. Multiply the result by 3: 14 × 3 = 42. Add the even number digits: 3+0+0+4+4 = 11.
The check digit is a weighted modulo-103 checksum. It is calculated by summing the start code 'value' to the products of each symbol's 'value' multiplied by its position's weight in the barcode string. The start symbol and first encoded symbol are in position 1. The sum of the products is then reduced modulo 103.
The check digit consists of one numeric digit providing a means of validating the recording and transmission accuracy of the owner code and serial number. To compute the check digit, the letters have to be converted to numbers. This is done in three steps:
The check digit is an additional digit, used to verify that a barcode has been scanned correctly. It is computed modulo 10, where the weights in the checksum calculation alternate 3 and 1. In particular, since the weights are relatively prime to 10, the EAN-13 system will detect all single digit errors.
The check digit is computed as follows: Drop the check digit from the number (if it's already present). This leaves the payload. Start with the payload digits. Moving from right to left, double every second digit, starting from the last digit. If doubling a digit results in a value > 9, subtract 9 from it (or sum its digits).
The simplest checksum algorithm is the so-called longitudinal parity check, which breaks the data into "words" with a fixed number n of bits, and then computes the bitwise exclusive or (XOR) of all those words. The result is appended to the message as an extra word.
Every pair of digits is color-coded, showing the code and the value for each digit. Because digits are encoded by pairs, only an even number of digits can be encoded. Typically an odd number of digits is encoded by adding a "0" as first digit, but sometimes an odd number of digits is encoded by using five narrow spaces in the last digit. [a]