A digital signal is a sequence of discrete, discontinuous voltage pulses. Each pulse is a signal element. Binary data are transmitted by encoding each data bit into signal elements. In the simplest case, there is a one-to-one correspondence between bits and signal elements. An example is shown in Figure 3.16, in which binary 1 is represented by a lower voltage level and binary 0 by a higher voltage level. We show in this section that a variety of other encoding schemes are also used.

The transmission of a stream of bits from one device to another across a transmission link involves a great deal of cooperation and agreement between the two sides. One of the most fundamental requirements is synchronization. The receiver must know the rate at which bits are being received so that it can sample the line at appropriate intervals to determine the value of each received bit.Two techniques are in common use for this purpose. In asynchronous transmission, each character of data is treated independently. Each character begins with a start bit that alerts the receiver that a character is arriving.The receiver samples each bit in the character and then looks for the beginning of the next character. This technique would not work well for long blocks of data because the receiverΓÇÖs clock might eventually drift out of synchronization with the transmitterΓÇÖs clock. However, sending data in large blocks is more efficient than sending data one character at a time. For large blocks, synchronous transmission is used. Each block of data is formatted as a frame that includes a starting and an ending flag. Some form of synchronization, such as the use of Manchester encoding, is employed.