Block length or block format length refers to the structure of the individual data transmission blocks. The structure of the block format depends on the type of transmission, i.e. synchronous transmission or asynchronous transmission.
Block formats for synchronous transmission are defined in DIN 66019 for character-oriented control methods. They can use characters for bit synchronization (BS), character synchronization (SYN) and final PAD character. The block format contains, among other things, the characters for the start of text (STX) and end of text (ETB), the block check string, and the message content.
In the LAN area, the term frame is used instead of block format at the data link layer.