or more host processors may go through mul- tiple front ends into one or more types of networks involving various types of terminals. A network of this type is called a "distributed network." The electronic transmission of encoded information from one point to another requires various physical elements, devices, and sys- items, as well as standards and procedures. An understanding of these basic elements and concepts can help users of telcom services to take advantage of the communication systems that are now available. 1.2.1 Parallel and Serial Transmission Short distance communications between terminals (data input/out- put devices) and computers may take place in a variety of ways. Some configurations, for example, involve the transmission of data char- acters across multiple lines. This is known as parallel transmission. If you were transmitting one bit of a seven-bit ASCII code character per line, you would need seven lines. This method of transmission is called parallel-by-bit serial-by-character (see Fig. 1-1). This method is often used over short distances. However, over long distances (in- volving miles) serial transmission is preferred because it involves only one data line. In order to send data over a single line, the bits must be trans- mitted in series. This is called (appropriately) serial transmission. It is the most widely used method of transmitting data. In seven-bit ASCII, a "C" would appear as illustrated in Fig. 1-2. 1.2.2 Channels and Facilities A communications link or channel is a path for electrical transmis- sion between two or more stations or terminals. It may be a single wire, a group of wires, a coaxial cable, or a special part of the radio frequency spectrum. The purpose of a channel is to carry informa- THE MEDIUM AND THE MARKET Data source ASC II 00000 Parallel-by-bit Serial-by-character 0 0 0 0 1 0 "B" Fig. 1-1. Parallel-by-bit and serial-by-character transmission. tion from one location to another. All channels have limitations on their information handling abilities, depending