We learned that data can be carried by two types of signals: analog and digital. The term "broadband" describes an analog transmission system that can carry multiple simultaneous signals, each modulated on a different carrier frequency (channel). In contrast, "narrowband" analog transmission, like your home telephone line, may carry only one or two channels.

The term "baseband" describes digital transmission. Baseband transmission can only carry one channel of information at a time; however, it is fast and technically simpler to implement. Thus, baseband is commonly used in LANs, such as the popular 10BaseT standard for Ethernet networks.

While all data networks use baseband signaling, their physical layouts or topologies can vary widely. The topology of a network determines the direction and pattern of signal flow. When network professionals discuss a network design, they usually mean its logical topology, which shows how signals flow in the network. In contrast, a physical topology shows how individual devices are connected to each other.

The bus topology, using coaxial cable, was one of the first small network designs. The bus is a coaxial cable "spine." All nodes attach to that cable and communicate by sending information along the common bus. The bus topology has been largely replaced by the more flexible star configuration.

Star topology is currently the most popular way to wire a network. A signal-sharing device, called a "hub," forms the physical and logical center of the network. Each network node is connected to the hub by a separate twisted pair cable. Because each node is isolated on its own cable, a problem with one node does not necessarily affect the other computers attached to that hub.

The bus and star topologies are both broadcast networks. A hub serves the same logical function as a coaxial bus. All nodes connected to the hub (or bus) receive every signal transmitted; however, a node ignores any transmission not addressed to it.

In a ring network, the logical topology requires each node to receive each signal from its neighbor on one side, then relay the signal to its neighbor on the other side. Ring networks can use one of two physical topologies. In a "pure" ring, point-to-point connections link the nodes in a circle, like beads on a necklace. A break in one of the links, or a malfunction of one of the nodes, can take the entire network down.

In a star ring topology, all nodes are connected to a central MAU that distributes signals to each station in turn. If one node fails, the MAU can simply bypass it and maintain communication with the other nodes.

A mesh topology is a system of point-to-point links that connect each node to every other node. This topology is most often used to link separate locations in MANs or WANs. When the number of nodes in a mesh network grows, most network designers abandon the mesh topology in favor of a subscription to a switched "cloud" network owned by a telecommunications service provider (telephone company).

A backbone is any part of a network that connects other networks or network segments. A network backbone can use any of the topologies we have discussed in this unit. However, the data-carrying capacity (bandwidth) of the backbone must be much higher than the segments it connects, just as a major highway carries more traffic than a neighborhood road.

All of these topologies can be implemented by using any, or all, of three types of physical transmission media: copper cable, fiber optic cable, or radio waves. At the beginning of this unit, we stated that there is no one perfect transmission medium. The Media Characteristics Table makes this clear, by comparing the features of twisted pair, coaxial cable, fiber optic cable, microwave, and satellite transmission.

Media Characteristics