Checksums

2. A Checksum. This number is the total of all the bits (or bytes) of a data. For example, it could be:
   • The sum of all the 1 bits.
   • The sum of all the bytes' values (e.g., by converting the characters/bytes into their corresponding ASCII or Unicode values.) Here are a Text to ASCII and a Text to Binary converter web apps.
   • [The Checksum in a TCP packet's header] The 16-bit ones' complement of the ones' complement sum of all 16-bit bytes in the header and content of a packet.
The checksum calculation method is provided to the system ahead of time.

Example: the ASCII-byte (decimal) checksum of 01100101, the letter e in ASCII, is 101, while the checksum of 01001110, the letter N in ASCII, is 78, The parity bit for each of these letters, however, is the same: 0 (Why is the parity bit 0?).

Although a checksum is more reliable than a parity bit, the chance that a transmission error would result in obtaining the same checksum for a modified data isn't small.