ay to extend the band- width of existing Ethernets. LAN Topologies The final characteristic of a LAN is its topology-the physical and logical mapping of the network. The most common LAN topologies are the tree, bus, ring, and star topologies, which are illustrated in Figure 6.6. The tree topology is commonly used in broadband LANs, and the bus, ring, and star topologies are used in baseband LANs. In today's environment, the most common physical topology is the star, and the most common logical topology-that is, how the signals are exchanged between stations-is the bus. Tree Topology In the tree topology, the root of the tree is the headend, or the central retransmis- sion facility. The trunk cable is attached to this root. Various branch cables are Headend Figure 6.6 LAN topologies Chapter 6 Local Area Networking attached to the trunk cable. From there, user devices can be connected. Although most broadband networks use a single cable, some use a dual-cable system-one cable for each direction to and from the headend. All transmissions must pass through the headend because each device transmits on one frequency and receives on another. The headend is responsible for translating the device's transmit fre- quency to the receive frequency of another device, and this frequency translation is called remodulation. Bus Topology The bus topology, which is the oldest topology, functions like a multipoint circuit. All stations are attached via cable taps or connections to a single length of cable-a wire with two open ends that are terminated by a resistor. The single cable is referred to as a trunk, backbone, or segment. Each station can detect and place signals on the bus according to the access method. Only one computer at a time can send data on a bus network. Signals travel in both directions along the bus, from the point of insertion. Any other sta- tion on the network can receive a transmission from the station. Because the data is sent to the entire network, it travels from one end of the cable to the other. If the s