s Data Transmission Over longer distances we use serial transmission either in an asynchronous or synchronous transmission mode. Serial transmission over long distance requires that the timing information for the receiver be transmitted together with the data SO that a separate clock signal is not required. In asynchronous transmission only a small number of bits are transmit- ted at a time, usually 8 bits that correspond to one ASCII character. In the beginning of each block of 8 bits of data, a start bit is sent to indicate to the receiver that it should prepare to receive 8 bits of data (see Figure 6.2). For synchronization the receiver has to know the data rate, which has to be set in advance, SO that when it detects the start bit it is able to receive the few fol- lowing bits. After these bits a stop bit is sent that terminates the 8-bit data block. The next block of data is synchronized independently with the help of a new start bit preceding the data bits. In asynchronous transmission, a simple error-detecting scheme called parity can be used. We may use even or odd parity error checking. If even Introduction to Telecommunications Network Engineering Start Data bits Asynchronous transmission: Information is sent in short P bit(s) + Voltage blocks which contain: - start bit - 7 or 8 information bits - Voltage - parity bit (optional) - stop bit(s). The timing of the receiver is based on the time instant of the start bit. Asynchronous blocks on the line Synchronous transmission: A long data block (thousands of bits) Start of Error End of called a frame starts with a special frame check frame start of frame bit sequence followed by information bits. Information bits Frame ends with the error check code and a special end of frame bit sequence. For synchronization of the receiver a Frame length is variable up to thousands of bytes line code such as a Manchester code is used. Figure 6.2 Asynchronous and synchronous transmission. parity is used, the total number of "1" bits in the block, including data bits and th