Modulo is a check digit method used in bar codes and data packet transmission. It is an error-detecting algorithm for a remainder operation in which the integer remainder of an integer division is output as a checksum.
According to the procedure, in modulo the digits of a number are multiplied individually by a certain factor and the sum is formed from the products, which is then divided by the modulo factor. The remainder from the division is the checksum. For 11 modulo 3, the remainder is 2.
- For bar codes, the checksum is determined according to a modulo scheme. If it is a modulo 16 procedure, then the checksum of the digits is divided by 16 and the remainder is subtracted from the modulo value. For example, a cross sum of 57 in modulo 16, results in the value 3 (3x16=48) remainder 9, which is entered as a check digit.
- Indata transmission, a modulo-based procedure is used for the consequent transmission of user information in packet-switched protocols such as X.25 using counters and a window mechanism at the link layer. Each communication system has a transmit as well as receive counter and a defined window size. The modulo defines the counter size used. The ITU-T recommendation X.25 recognizes two modulo variants, the modulo-8 and the modulo-128 counter. The modulo counter defines the available quantity from possible sequence numbers. The modulo-8 counter defines 8, the modulo-128 counter 128 sequence numbers. These numbers are used to number the data blocks consecutively so that the sequence in the target system can be monitored and the window size can be maintained.