e parity bit, is set to be even with the help of the parity bit. In the case of odd parity, the parity bit is set to "1" or "0" SO that the total number of "1" bits in the block is odd. To detect possible transmission errors, the receiver determines whether the received number of "1" bits is even or odd depend- ing on the parity agreed. We will see later that this parity check method is a simple example of a data link layer protocol. Asynchronous transmission is used for the transmission of ASCII char- acters in conventional terminal-mainframe computer communications. For larger information blocks it is used in some file transfer protocols such as KERMIT and X-LINK. In these protocols special "start of block" characters are sent at the beginning. Then information follows as asynchronous words and at the end special "end of block" characters are sent. Synchronous transmission is a more modern principle for transmitting a large amount of information in a frame (see Figure 6.2). Each frame starts with a special start-of-frame bit sequence and the frame may contain more than 1,000 bytes of information. Each frame also contains error control words and an end-of-frame sequence. The receiver uses the error control sec- tion of the frame to detect if errors have occurred in transmission. The most common detection method for error detection is a cyclic redundancy check (CRC). It is much more reliable than the parity check method discussed pre- viously. In the case of errors the transmitter retransmits the frame in error. In the most common protocols the receiver sends an acknowledgment to the Data Communications transmitter in the other transmission direction for received error-free frame or frames. If errors have occurred, the frame is not acknowledged in a prede- fined period of time and the transmitter sends it again. In asynchronous transmission the start bit provided the required tim- ing information for each byte of data. Most synchronous transmission meth- ods are so-called "bit-oriented" protocols in which data bl