|  | 3.1 | Basic LAN Devices
| 3.1.1 | The teaching topology
Topology defines the structure of the network. There are two parts to the topology definition, the physical topology, which is the actual layout of the wire (media), and the logical topology, which defines how the media is accessed by the hosts. The physical topologies that are commonly used are the Bus, Ring, Star, Extended Star, Hierarchical, and Mesh. These are shown in the graphic. A bus topology uses a single backbone segment (length of cable) that all the hosts connect to directly. A ring topology connects one host to the next and the last host to the first. This creates a physical ring of cable. A star topology connects all cables to a central point of concentration. This point is usually a hub or switch, which will be described later in the chapter. An extended star topology uses the star topology to be created. It links individual stars together by linking the hubs/switches. This, as you will learn later in the chapter, will extend the length and size of the network. A hierarchical topology is created similar to an extended star but instead of linking the hubs/switches together, the system is linked to a computer that controls the traffic on the topology. A mesh topology is used when there can be absolutely no break in communications, for example the control systems of a nuclear power plant. So as you can see in the graphic, each host has its own connections to all other hosts. This also reflects the design of the Internet, which has multiple paths to any one location. The logical topology of a network is how the hosts communicate across the medium. The two most common types of logical topologies are Broadcast and Token-passing. Broadcast topology simply means that each host sends its data to all other hosts on the network medium. There is no order the stations follow to use the network, it is first come, first serve. This is the way that Ethernet works and you will learn much more about this later in the semester. The second type is token-passing. Token-passing controls network access by passing an electronic token sequentially to each host. When a host receives the token, that means that that host can send data on the network. If the host has no data to send, it passes the token to the next host and the process repeats itself. The diagram in the graphic shows many topologies. It shows a LAN of moderate complexity that is typical of a school or a small business. It has many symbols, and it depicts many networking concepts that will take time to learn. This LAN is typical of a small campus, and represents of most of the devices that you will study for your CCNA.
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