sent voice, video, or data. Analog systems are different for each application because of different performance requirements. For example, a telephone connection requires channels with approximately 4-kHz bandwidth, but television signals require 5-MHz bandwidth with a much better S/N. In digital systems the corre- sponding characteristic is the data rate. For example, an analog telephone sig- nal requires 64 Kbps and video with a much wider bandwidth requires 2 to 140 Mbps depending on the coding scheme in use. We can use one high- data-rate system for a single video channel or a large number of speech channels. Digital technology provides efficient multiplexing for sharing capacity in high-data-rate connections. This makes high-capacity digital networks and systems flexible. The same system, if it provides a high enough data rate, can be used for any application. 3.4.3 Examples of Messages In the previous sections, we described the characteristics of the digital and analog signals and systems. Now we look at some simple examples of Signals Carried over the Network information sources that produce messages that are transmitted through the network. There are many different information sources, including machines as well as people, and messages or signals appear in various forms. As for sig- nals we can identify the two main distinct message categories: analog and digital. 3.4.3.1 Information, Messages, and Signals The concept of information is central to communication. However, informa- tion is a loaded word, implying schematic and philosophical notions and, therefore, we prefer to use the word message instead. Message means the physical manifestation of information produced by a source. Systems han- dling messages convert them into electrical signals suitable, for example, for a certain transmission media. 3.4.3.2 Analog Message An analog message is a physical quantity that varies through time, usually in smooth and continuous fashion. Examples of analog messages are acoustic pressure produced when you speak or l