gic must be built in to end devices SO that they can detect errors and request a retransmission in order to cor- rect for the errors. If these two prerequisites-understandability and error control-are met, then communication can occur. We communicate by using data devices over what is generically termed a transmission line. There are five main types of transmission lines-circuits, channels, lines, trunks, and virtual circuits-each of which has a specific meaning. (Keep in mind that these transmission lines can be composed of either wired or wireless media, and in fact, these days, we're seeing more and more use of wireless facilities to accomplish networking, from simple mobile phones to the Interplanetary Internet.) The following sections describe each of these types of transmission lines in detail. Circuits A circuit is the physical path that runs between two or more points. It terminates on a port (i.e., a point of electrical or optical interface), and that port can be in a host computer, on a multiplexer, on a switch, or in another device, as discussed later in this chapter. In and of itself, a circuit does not define the number of simultaneous conversa- tions that can be carried; that is a function of the type of circuit it is. For example, a simple, traditional telephone circuit is designed to carry just one conversation over one physical pathway. (But note that I specifically reference "traditional telephone circuit"; digital broadband facilities, such as DSL, permit multiple channels, such as separate voice and data channels, over the one physical pathway.) However, con- verting to a digital circuit makes it possible to extract or derive multiple channels over that circuit, subsequently facilitating multiple simultaneous conversations. The circuit is therefore the measure of the connectivity between two endpoints. There are two types of circuits: two-wire circuits and four-wire circuits. Two-Wire Circuits A two-wire circuit has two insulated electrical conductors: One wire is used for transmission of the 