● 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 transferred 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 So even though some information covered in this chapter is obsolete or nearly so, your understanding of these older technologies will help you better understand current and future technologies.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 different 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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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 technology 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.
How Data Travels in a Physical Bus Two properties inherent in a physical bus are signal propagation and signal bounce. In any network topology, computers communicate with each other by sending information across the media as a series of signals. When copper wire is the medium, as in a typical physical bus, these signals are sent as a series of electrical pulses that travel along the cable’s length in all directions. The signals continue traveling along the cable and through any connecting devices until they weaken enough that they can’t be detected or until they encounter a device that absorbs them. This traveling across the medium is called signal propagation. However, even if a signal encounters the end of a cable, it bounces back and travels in the other direction until it weakens or is otherwise impeded.
When a signal hits the end of a cable and bounces back up the cable’s length, it interferes with signals following it, much like an echo. Imagine if you were trying to communicate
Figure 3-1 A physical bus topology network Courtesy of Course Technology/Cengage Learning
Physical Topologies 111
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in an empty room with hard walls that caused your voice to echo continuously. The echo from the first words out of your mouth would garble the sound of words that followed, and your message would be unintelligible. The term used when electricity bounces off the end of a cable and back in the other direction is called signal bounce or reflection. To keep signal bounce from occurring, you do what you would to keep excessive echo from occurring; you install some type of material at both ends of the medium to absorb the signal. In a physical bus, you install a terminator, which is an electrical component called a resistor that absorbs the signal instead of allowing it to bounce back up the wire.
Physical Bus Limitations Now that you know more about how a physical bus works, the previous list of weaknesses needs some additional explanation. The limitation of 30 stations per cable segment means only 30 computers can be daisy-chained together before the signal becomes too weak to be passed along to another computer. As an electrical signal encounters each connected workstation, some of its strength is absorbed by both the cabling and the connectors until the signal is finally too weak for a computer’s NIC to interpret. For the same reason, the total length of cabling is limited to 185 meters, whether there’s 1 connected station or 30 connected stations. The network can be extended in cable length and number of workstations by adding a repeater to the network, which, as you know, regenerates the signal before sending it out.
At all times, both ends of the bus must be terminated. An unterminated bus results in signal bounce and data corruption. When a computer is added or removed from the network, both ends are no longer terminated, resulting in an interruption to network communication.
For a small network of only a few computers, you might think a bus topology is fine, until you consider the last weakness listed: maximum bandwidth of 10 Mbps half-duplex communication. A physical bus uses coaxial cable (a cabling type discussed in Chapter 4, similar to what’s used in cable TV connections), which is limited to a top speed of 10 Mbps and communication in only half-duplex mode. Most of today’s networks use twisted-pair cabling, which can operate at 100 Mbps or faster and run in full-duplex mode, so communication between devices is much faster.
For all these reasons, the physical bus topology has long since fallen out of favor and been replaced largely by the star topology, discussed next.
Physical Star Topology The physical star topology uses a central device, such as a hub or switch, to interconnect computers in a LAN (see Figure 3-2). Each computer has a single length of cable going from its NIC to the central device.
Some advantages of a physical star topology are the following:
● Much faster technologies are used than in a bus topology.
● Centralized monitoring and management of network traffic is possible.
● Network upgrades are easier.