Flags (F) are bit patterns that are transmitted before or after the userdata and signal some state. Thus flags can indicate the beginning of the block(preamble) or the end of the block, the status or the transmission mode, the arithmetic operation or the overflow. As block delimiters, flags have several bits; in contrast, in computer technology they are usually only 1 bit long and indicate certain system states, e.g. that an amount is zero or negative, that an error or an interrupt has occurred. In High Level Data Link Control(HDLC), DÜ blocks begin and end with the block boundary. The remote operating units of the
