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 So even though some information covered in this chapter is obsolete or nearly so, your understanding of these older technologies will help you better understand current and future technologies.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

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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 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.

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

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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 sta- tions 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 con- nected 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, regener- ates 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 com- munication. 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 communi- cation 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.

A physical star is the topology of choice for these reasons and more. With a central device, communication options are available that simply aren’t possible with a physical bus. For example, the central device can be a 100 Mbps hub, which increases a physical bus’s top

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speed tenfold, or a switch, making it possible for multiple communication sessions to occur simultaneously and in full-duplex mode.

As a budding network administrator, being able to monitor and manage your network with a central device is a big advantage over what was possible with a physical bus topology. Today’s hubs and switches can include software that collects statistics about your network traffic patterns and even alerts you when excessive errors or unusually high traffic rates are occurring on your network. You don’t get these features in a $19.99 hub or switch, but enterprise-level devices can be equipped with several network management tools.

As long as your current cabling and installed NICs support it, your network can be upgraded quickly and easily from a ponderous 10 Mbps hub-based LAN to a blazing fast 100 Mbps or even 1000 Mbps switched network simply by replacing the central device. In addition, if your NICs must also be upgraded, you can upgrade in steps because most devices support multiple speeds. So if you want to upgrade from 100 Mbps to 1000 Mbps, you can replace the central device with a switch that supports both speeds, and then upgrade NICs as time and money allow. The switch transmits and receives on each port at the speed supported by the NIC connected to that port.

What happens if the number of workstations you need to connect exceed the number of ports on the central device? In this case, you can connect hubs or switches, as you learned in Chapter 2. When several hubs or switches must be connected, usually one device is used as the central connecting point, forming an extended star.

Extended Star The extended star topology, shown in Figure 3-3, is the most widely used in networks containing more than just a few computers. As the name implies, this topology is a star of stars. A central device, usually a switch, sits in the middle. Instead of attached computers forming the star’s arms, other switches (or hubs) are connected to the central switch’s ports. Computers and peripherals are then attached to these switches or

Switch

Figure 3-2 A physical star topology network Courtesy of Course Technology/Cengage Learning

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Physical Topologies 113

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hubs, forming additional stars. The extended star is sometimes referred to as a “hierarchical star” because there are two or more layers of stars, all connecting back to the central star.

The extended star can be used to connect many computers, with the central device running at a very fast speed to shuttle data between the LAN’s outer stars. This topology is most effective when the center of the star is running at a much faster speed than other devices; for example, the central device can run at 1000 Mbps while other devices run at 100 Mbps.

How Data Travels in a Physical Star The details of how data travels from com- puter to computer in a physical star depend on the type of central device. Data transmission starts at a device at the end of one of the central device’s arms. From there, it travels along the network medium’s length until it arrives at the central device. As you know from learn- ing how hubs and switches work, the transmission path differs, depending on the device. Other devices, such as multistation access units (MAUs) used in token ring networks, move data differently. The type of central device, therefore, determines the logical topology, discussed later in this chapter.

Physical Star Disadvantages With all the clear advantages of a physical star, you might wonder whether there are any disadvantages. None outweigh the advantages, but it’s worth mentioning that the central device represents a single point of failure. In other words, if the hub or switch fails or someone kicks the power cord out of the outlet, down goes the entire network. Thankfully, these devices tend to be reliable and are usually placed out of the way of everyday foot traffic. That being said, they do fail from time to time, and having a spare on hand is a good idea.

When a physical bus was still the norm and the physical star was just coming on the net- working scene in the late 1980s, it was often argued that because each computer must be

Switch

Switch

SwitchSwitch

Switch

Figure 3-3 An extended star topology network Courtesy of Course Technology/Cengage Learning

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cabled directly to the central device, instead of a bus’s daisy-chain arrangement, more cable was required to connect computers. This point is indeed true, and at the time, the amount of cabling needed was a factor in designing a network with a bus or star arrangement. By the time the star network’s advantages were fully realized in the mid-1990s, however, the cabling cost difference had diminished substantially, and the advantages clearly outweighed the minor cost disadvantage.

Physical Ring Topology A physical ring topology is like a bus, in that devices are daisy-chained one to another, but instead of terminating each end, the cabling is brought around from the last device back to the first device to form a ring. This topology had little to no following in LANs as a way to connect computers. It was used, however, to connect LANs with a technology called Fiber Distributed Data Interface (FDDI). FDDI was most often used as a reliable and fast network backbone, which is cabling used to communicate between LANs or between hubs or switches. In Figure 3-4, the devices used to connect buildings form a ring, but computers on each LAN are connected with a physical star topology.

The physical ring also had reliability issues because data had to be forwarded from one station to the next. Unlike a bus, inwhich data travels in all directions and is terminated at both ends, a ring doesn’t have any beginning or end. So each station must reproduce data and pass it along to the next station until it reaches the destination or the originator of the data. In other words, data always travels in one direction. If any station in the ring fails, data can no longer be passed along, and the ring is broken.

Technologies such as FDDI overcome some problems with a physical ring network by creating a dual ring, in which data can travel in both directions so that a single device failure doesn’t break the entire ring. However, this technology is costly, and although it was used extensively in the 1990s and early 2000s because it was fast (100 Mbps) and reliable, 100 Mbps and 1000 Mbps Ethernet have largely supplanted it with an extended star technology.

Building A Building B

Building C

LAN switch FDDI hub

Figure 3-4 A physical ring topology is usually used to connect LANs Courtesy of Course Technology/Cengage Learning

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Physical Topologies 115

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Point-to-Point Topology As its name implies, a point-to-point topology is a direct link between two devices. It’s most often used in WANs, in which a device on a business’s network has a dedicated link to a telecommunication provider, such as the local phone company. The connection then hooks into the phone company’s network to provide Internet access or a WAN or MAN link to a branch office. The advantage of this type of topology is that data travels on a dedicated link, and its bandwidth isn’t shared with other networks. The disadvantage is that this topology tends to be quite expensive, particularly when used as a WAN link to a distant branch office.

Point-to-point topologies are also used with wireless networks in what’s called a wireless bridge. This setup can be used to connect two buildings without using a wired network (see Figure 3-5) or to extend an existing wireless network.

A rudimentary LAN can also be set up with a point-to-point topology by connecting a cable between the NICs on two computers. Of course, this method allows only two computers on the network, but it can be used effectively for transferring files from one computer to another in the absence of a hub or switch.

So as you can see, point-to-point topologies are used for specialized purposes. They aren’t commonly used in LANs; they’re used more often in WANs and large internetworks.

Mesh Topology Amesh topology connects each device to every other device in a network. You can look at a mesh topology as multiple point-to-point connections for the purposes of redundancy and fault tolerance. Figure 3-6 shows a full mesh topology between four locations, with the switch in each location providing connectivity to multiple computers. Each switch is connected to every other switch, which is called a “full mesh.” If each switch were connected to only two other switches, it would be called a “partial mesh.” In either case, the purpose of creating a mesh topology is to ensure that if one or more connections fail, there’s another path for reaching all devices on the network. For example, in Figure 3-6, two connections could fail, but all devices could still communicate with one another. This type of topology is used mostly commonly in large internetworks andWANs, where routers or switches in multiple buildings or towns are connected in a partial or full mesh. Parts of the Internet are also designed with a partial mesh topology, in which major ISPs are connected so that even if one ISP’s network fails, data can bypass this part of the network to get to its destination.

Mesh topologies, although reliable, are also expensive because of the additional cabling and ports required. In most cases, the ports used to connect devices are the highest speed available, such as 1 Gbps or 10 Gbps, and they often use expensive fiber-optic cabling for connecting buildings.

Figure 3-5 A point-to-point wireless topology Courtesy of Course Technology/Cengage Learning

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Logical Topologies As mentioned, a network’s logical topology describes how data travels from computer to com- puter. In some cases, as with a physical bus and physical ring, the logical topology mimics the physical arrangement of cables. In other cases, as with a physical star, the electronics in the central device determine the logical topology.

A network’s logical topology reflects the underlying network technology (covered later in “Network Technologies”) used to transfer frames from one device to one another. Table 3-1 summarizes the main logical topologies, the technologies using them, and the physical topolo- gies for implementing them.

Chicago

Los Angeles Phoenix

New York

WAN link

Figure 3-6 Switches in each building are connected in a full mesh topology Courtesy of Course Technology/Cengage Learning

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Table 3-1 Logical topologies and associated network technologies and physical topologies

Logical topology

Network technology

Physical topology Description

Bus Ethernet Bus or star A logical bus topology can be implemented as a physical bus (although this topology is nowobsolete).When a logical bus is implemented as a physical star usingwired Ethernet, the center of thestar is anEthernethub.Whatever thephysical topology is, data transmitted from a computer is received by all other computers.

Wireless LANs Star Wireless LANs use a physical star topology because they connect through a central access point. However, only one device can

Logical Topologies 117

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You have seen what a logical bus looks like when implemented as a physical bus. All computers are daisy-chained to one another, and network signals travel along the cable’s length in all directions, much like water flowing through interconnected pipes. When a logical bus is implemented as a physical star, the sameprocess occurs, but the pathways are hidden inside the central hub. Figure 3-7 shows what a logical bus might look like when implemented with a hub.

transmit at a time and all devices hear the transmission, so a wireless LAN can be considered a logical bus topology.

Ring Token ring Star Token ring networks use a central device called a multistation access unit (MAU or MSAU). Its electronics form a logical ring, so data is passed from computer to computer in order, until it reaches the destination device.

FDDI Ring As discussed, FDDI devices are connected in a physical ring, and data passes from device to device until it reaches the destination.

Switched Ethernet Star A switched logical topology using a physical star topology running Ethernet is by far themost common topology/technology combination now and likely will be well into the future. A switched topology creates dynamic connections or circuits between two devices whenever data is sent. This topology is sometimes considered a switched point-to-point topology because a circuit is established between two points as needed to transfer data (like turning on a switch), and then the circuit is broken when it’s no longer needed (like turning off a switch).

Signal Signal

Signal Signal

Logical bus inside a network hub

Figure 3-7 A logical bus implemented as a physical star Courtesy of Course Technology/Cengage Learning

Table 3-1 Logical topologies and associated network technologies and physical topologies (continued )

Logical

topology Network technology

Physical topology Description

118 Chapter 3

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A logical bus is sometimes called a “shared media topology” because all stations must share the bandwidth the media provides.

A logical ring using a physical star implements the ring inside the central device’s electronics, which is an MAU in the token ring technology. Data is passed from one node or computer to another until it reaches the destination device (see Figure 3-8). When a port has no device connected to it, it’s simply bypassed, and data is sent out the next connected port.

A switched topology works something like what’s shown in Figure 3-9. Although there’s always an electrical connection between the computer and switch, when no data is being transferred, there’s no logical connection or circuit between devices. However, when the switch receives a frame, a logical circuit is made between the source and destination devices until the frame is transferred.

Figure 3-8 A logical ring implemented as a physical star Courtesy of Course Technology/Cengage Learning

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PC 6PC 5PC 4

No packets being transmitted

PC 1 PC 2

PC 1 and PC 6 communicate while PC 2 and PC 5 communicate

PC 3

Figure 3-9 The logical functioning of a switch Courtesy of Course Technology/Cengage Learning

Logical Topologies 119

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To better understand how these logical topologies work, it helps to know the network technology that drives each topology (discussed later in “Network Technologies”).

Hands-On Project 3-1: Building a Physical Star Topology Network

Time Required: 20 minutes

Objective: Build a physical star topology network.

Required Tools/Equipment: Three workstations named Computer1, Computer2, and Com- puter3; a hub; and three patch cables. Workstations should be configured with an IP address or automatic IP address assignment. Each station should have Wireshark installed.