1.1. Recognizing Logical and Physical Network Topologies

Basically, a topology is a shape, so a network topology is the shape of a network. There is, however, a big difference between a physical network topology and a logical network topology. The physical network topology represents how the network looks to the naked eye—in other words, the way the components are arranged. The logical network topology represents how the flow of information works its way through the network. This may not be the same as it looks to the naked eye. You should understand the main network topologies as well as the difference between a physical network topology and a logical one. In this section, we discuss the most common network topologies.

1.1.1. Critical Information

Recognizing the major differences in regard to the shape and the components that are used in the most common topologies is important. You should be able to recognize these differences given a diagram, schematic, or description. In the following paragraphs, we discuss each network topology in greater detail.

1.1.1.1. Star

A star topology is a group of computers that are connected to a central location such as a hub or a switch. This is the most common topology in use today. The computers may be physically located next to each other or spread throughout an entire building, but the flow of information from each computer to the other computers must go through the central location. In a star topology, each computer has its own cable or connection to the hub. Since each computer has its own connection, the failure of one computer will not affect the other computers in the net-work; however, if the hub or switch should fail, then all of the computers on that hub or switch will be affected. Figure 1.1 is an illustration of a star topology.

Figure 1.1. A star topology

1.1.1.2. Bus

The bus topology was commonly used in earlier networks but is not seen much today. In a bus topology, a single cable connects all the computers. A coaxial cable is used with special connectors called BNC and T connectors. (We will discuss cables and connectors in the next section.) The T connectors provide an independent connection for each computer on the bus. In addition, the bus only works if both ends of the cable have a special resistor installed called a terminator. Figure 1.2 shows a bus topology; Figure 1.3 shows the T connector used to connect the computers to the bus.

Figure 1.2. A bus topology

Figure 1.3. A T connector

1.1.1.3. Mesh

The mesh topology is not often used and is almost never used for individual computers. In a fullmesh topology, all of the components in the mesh have independent connections to all of the other components in the mesh. For example, if four computers are connected with a "full mesh," then the number of connections can be determined by the following formula:

n(n − 1) = total number of connections

In this case:

4(4 − 1) = 12

In other words, there are a total of 12 connections and each computer has to contain three network interface cards.

Actually, any network with multiple or redundant connections to network components can be considered a mesh topology, but because of the expense involved in creating this type of network, they are rarely created for individual computers. A mesh, and even a full mesh, would most likely be found connecting multiple networks in an organization. In fact, the Internet is the best and biggest example of a mesh topology. Figure 1.4 shows a fullmesh topology with four computers.

Figure 1.4. A mesh topology

1.1.1.4. Ring

A ring topology (see Figure 1.5) looks exactly like a star topology to the naked eye. The real difference between a ring topology and a star topology lies in the technology used. Computers in a ring topology generally use an IBM Token Ring technology. Other components can also be arranged in a ring topology and use different technologies. The computers involved in a ring topology are not generally arranged in a physical ring. In fact, just as with a star topology, they can be located next to each other or spread throughout a building. The difference is that the central component that connects them contains the logical ring that facilitates communication on the network using the ring technologies. (We will discuss ring technologies in the next section.)

Figure 1.5. A ring topology

1.1.2. Exam Essentials

Know the difference between a physical topology and a logical topology. The physical topology of the network is simply what it looks like or how the components are arranged. The logical topology, on the other hand, represents the flow of information in the network.

Be able to list the main features of a star topology. Be able to recognize a star topology from a diagram, schematic, or description. A star topology contains a central hub (or switch) and all information must flow through the hub. A star topology is considered fault tolerant, since each device has its own network connection.

Describe the main features of a bus topology. A bus topology uses a coaxial cable, and all computers are connected to the same cable. T connectors are used to attach the computers. Resistors called terminators must be used at both ends of a bus. Keep in mind that the failure of one computer will generally not affect the entire segment, but a break in the cable will.

Summarize the main features of a mesh topology. A mesh topology provides multiple connections for the devices in the mesh. A full mesh requires n(n – 1) connections, where n is the number of devices in the mesh. A mesh topology is most often used with networks and not with individual computers.

Describe the main features of a ring topology. A ring topology looks like a star topology; the real difference is in the way that the information is transferred through the network. The IBM Token Ring is the most common type of ring topology, but is quickly being replaced by the star topology and Ethernet.