Checksum, echo check, ARQ and check digits

Last time

Before we start: last time, in Errors in transmission and parity checks, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.

Four more methods

Parity is not the only check. Cambridge wants four more: the checksum, the echo check, automatic repeat request, and the check digit. The first three check data sent across a network. The check digit checks data typed or scanned in.

The checksum

Before sending, the sender uses an agreed algorithm to calculate a value from the data. This is the checksum. The checksum is sent along with the data. The receiver uses the same algorithm to calculate the checksum again from what arrived. If the two checksums are different, there is an error, and the data is requested again.

The checksum, in a picture

Here is the checksum in a picture: the data and its checksum go across, the receiver works it out again, and asks for the data again if they differ.

A checksum in numbers

Here is a simple example. The data bytes are twelve, nine and four. The sender adds them: twenty-five. That is the checksum, and it is sent too. If the receiver gets twelve, eight and four, it adds them to twenty-four, which does not match twenty-five. So an error is found, and the data is sent again. Real checksums use a cleverer formula, but the idea is the same.