You have a network connected using a physical star topology

chapter3

Network Topologies and Technologies

After reading this chapter and completing the exercises, you will be able to:

● Describe the primary physical networking topologies in common use ● Describe the primary logical networking topologies in common use ● Describe major LAN networking technologies

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Not so long ago, there was a real choice to be made between available network topologies and technologies when designing and building a new internetwork. Thankfully, this area of networking has gotten simpler rather than more complex, mainly because the choices have narrowed, with inferior or costly solutions becoming obsolete.

This chapter discusses network topologies, which describe both the physical arrangement of cabling or pathways between network devices and the logical manner in which data is trans- ferred from device to device. Next, you learn about network technologies or architectures that describe the methods computers use to transmit data to the networking medium in an orderly fashion. As you’ll see, the topology and technology are often tightly coupled, as certain technologies can be used only with certain topologies. The choices have been limited because only a few technologies and topologies remain as viable options. As is often the case, however, it helps to know where networking started to get an idea of where it might be heading. So even though some information covered in this chapter is obsolete or nearly so, your understanding of these older technologies will help you better understand current and future technologies.

Physical Topologies The word “topology,” for most people, describes the lay of the land. A topographic map, for example, shows the hills and valleys in a region, whereas a street map shows only the roads. A network topology describes how a network is physically laid out and how signals travel from one device to another. However, because the physical layout of devices and cables doesn’t necessarily describe how signals travel from one device to another, network topologies are categorized as physical and logical.

The arrangement of cabling and how cables connect one device to another in a network are considered the network’s physical topology, and the path data travels between computers on a network is considered the network’s logical topology. You can look at the physical topology as a topographic map that shows just the lay of the land along with towns, with only simple lines showing which towns have pathways to one another. The logical topology can be seen as a street map that shows how people actually have to travel from one place to another. As you’ll see, a network can be wired with one physical topology but pass data from machine to machine by using a different logical topology.

All network designs today are based on these basic physical topologies: bus, star, ring, and point-to-point. A bus consists of a series of computers connected along a single cable segment. Computers connected via a central device, such as a hub or switch, are arranged in a star topology. Devices connected to form a loop create a ring. Two devices connected directly to one another make a point-to-point topology. Keep in mind that these topologies describe the physical arrangement of cables. How the data travels along these cables might represent a dif- ferent logical topology. The dominant logical topologies in LANs include switching, bus, and ring, all of which are usually implemented as a physical star (discussed later in “Logical Topologies”).

Physical Bus Topology The physical bus topology, shown in Figure 3-1, is by far the simplest and at one time was the most common method for connecting computers. It’s a continuous length of cable

110 Chapter 3

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Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.

connecting one computer to another in daisy-chain fashion. One of this topology’s strengths is that you can add a new computer to the network simply by stringing a new length of cable from the last computer in the bus to the new machine. However, this strength is countered by a number of weaknesses:

● There’s a limit of 30 computers per cable segment. ● The maximum total length of cabling is 185 meters. ● Both ends of the bus must be terminated. ● Any break in the bus brings down the entire network. ● Adding or removing a machine brings down the entire network temporarily. ● Technologies using this topology are limited to 10 Mbps half-duplex communication

because they use coaxial cabling, discussed in Chapter 4.

Because of the preceding limitations, a physical bus topology is no longer a practical choice, and technology has moved past this obsolete method of connecting computers. However, the original Ethernet technology was based on this topology, and the basis of current LAN technol- ogy has its roots in the physical bus. So your understanding of bus communication aids your general understanding of how computers communicate with each other across a network.