Physical Topologies
A topology is basically a map of a network. The physical topology of a network describes the layout of the cables and workstations and the location of all network components. Often, physical topologies are compared to logical topologies, which define how the information or data flows within the network. The topologies are usually similar. It is important to note, however, that a network can have one type of physical topology and a completely different logical topology. You'll learn more about this later in this chapter.
The cables or connections in a physical topology are often referred to as network media (or physical media). Choosing how computers will be connected in a company's network is critical. A wrong decision in the physical topology makes the media difficult to correct, because it is costly and disruptive to change an entire installation once it is in place. The typical organization changes the physical layout and physical media of a network only once about every 10 years, so it is important to choose a configuration that you can live with and that allows for growth.
In this section, we'll look at the five most common topologies:
- Bus
Ring
Star
Mesh
Wireless
Bus
In a bus topology, all computers are attached to a single continuous cable that is terminated at both ends, which is the simplest way to create a physical network. Originally, computers were attached to the cable with wire taps. This did not prove practical, so drop cables are now used to attach computers to the main cable.
When communicating on a network that uses a bus topology, all computers see the data on the wire. This does not create chaos, though, because the only computer that actually accepts the data is the one to which it is addressed. You can think of a bus network as a small party. David is already there, along with 10 other people. David would like to tell Joe something. David yells out, 'Joe! Will you grab me a cup of coffee, please?' Everyone in the party can hear David, but only Joe will respond. A star network with a hub, which you'll read about later in this section, also operates in this manner.
Real World Scenario: A bus sounds good, but . . .
Despite the simplicity of the bus topology, there are some inherent disadvantages to this design. For example, what happens if the wire breaks or is disconnected? Neither side can communicate with the other, and signal bounce occurs on both sides. The result is that the entire network is down. For this reason, bus topologies are considered to have very little fault tolerance.
Sometimes, because a cable is inside a wall, you cannot physically see a break. To determine if a break has occurred, you can use a tool known as a Time Domain Reflectometer, or TDR (also called a cable tester). This device sends out a signal and measures how much time it takes to return. Programmed with the specifications of the cable being tested, it determines where the fault lies with a high degree of accuracy. We'll discuss cable testers in Chapter 6, 'Network Installation and Upgrades.'
As with most things, there are pros and cons to a bus topology. On the pro side, a bus topology:
Is simple to install
Is relatively inexpensive
Uses less cable than other topologies
On the con side, a bus topology:
Is difficult to move and change
Has little fault tolerance (a single fault can bring down the entire network)
Is difficult to troubleshoot
Star
Unlike those in a bus topology, each computer in a star topology is connected to a central point by a separate cable. The central point is a device known as a hub. Although this setup uses more cable than a bus, a star topology is much more fault tolerant than a bus topology. This means that if a failure occurs along one of the cables connecting to the hub, only that portion of the network is affected, not the entire network. It also means that you can add new stations just by running a single new cable
The design of a star topology resembles an old wagon wheel with the wooden spokes extending from the center point. The center point of the wagon wheel would be considered the hub. Like the wagon wheel, the network's most vulnerable point is the hub. If it fails, the whole system collapses. Fortunately, hub failures are extremely rare.
As with the bus topology, the star topology has advantages and disadvantages. The increasing popularity of the star topology is mainly due to the large number of advantages, which include the following:
New stations can be added easily and quickly.
A single cable failure won't bring down the entire network.
It is relatively easy to troubleshoot.
The disadvantages of a star topology include the following:
Total installation cost can be higher because of the larger number of cables, but prices are constantly becoming more and more competitive.
It has a single point of failure, the hub.
Ring
In the ring topology, each computer is connected directly to two other computers in the network. Data moves down a one-way path from one computer to another The good news about laying out cable in a ring is that the cable design is simple. The bad news is that, as with bus topology, any break, such as adding or removing a computer, disrupts the entire network. Also, because you have to 'break' the ring in order to add another station, it is very difficult to reconfigure without bringing down the whole network. For this reason, the physical ring topology is seldom used.
Note Although its name suggests a relationship, Token Ring does not use a physical ring topology. It instead uses a physical star, logical ring topology
A few pros and many cons are associated with a ring topology. On the pro side, the ring topology is relatively easy to troubleshoot. A station will know when a cable fault has occurred because it will stop receiving data from its upstream neighbor.
On the con side, a ring topology is:
Expensive, because multiple cables are needed for each workstation.
Difficult to reconfigure.
Not fault tolerant. A single cable fault can bring down the entire network.
Mesh
A path exists from each station to every other station in the network. While not usually seen in LANs, a variation on this type of topology-the hybrid mesh-is used on the Internet and other WANs in a limited fashion. Hybrid mesh topology networks can have multiple connections between some locations, but this is done only for redundancy. Also, it is not a true mesh because there is not a connection between each and every node, just a few for backup purposes
a mesh topology can become quite complex as wiring and connections increase exponentially. For every n stations, you will have n(n -1)/2 connections. For example, in a network of 4 computers, you will have 4(4-1)/2 connections, or 6 connections. If your network grows to only 10 computers, you will have 45 connections to manage! Given this impossible overhead, only small systems can be connected this way. The payoff for all this work is a more fail-safe, or fault-tolerant, network, at least as far as cabling is concerned.
Today, the mesh topology is rarely used, and then only in a WAN environment and only because the mesh topology is fault tolerant. Computers or network devices can switch between these multiple, redundant connections if the need arises. On the con side, the mesh topology is expensive and, as you have seen, quickly becomes too complex.
Wireless
Radio frequency (RF) systems are being used all over corporate America. The RF networking hardware that is available today makes it easy for people to connect wirelessly to their corporate network as well as to the Internet.
The most popular example of an RF network today is what is known as an ad hoc RF network. These networks are created when two or more entities with RF transceivers that support ad hoc networking are brought within range of each other. The two entities will send out radio waves to each other, and they will both recognize that there is another RF device close by that they can communicate with. These ad hoc networks allow people with laptops or handheld devices to create their own networks on the fly and transfer data
The other example of an RF network is a multipoint RF network. This type of RF network has many stations, each with an RF transmitter and receiver, and each station communicates with a central device known as a wireless bridge. A wireless bridge (known as a wireless access point, or WAP, in RF systems) is a device that provides a transparent connection to the host LAN via an Ethernet or Token Ring connection and uses some wireless method (e.g., infrared, RF, or microwave) to connect to the individual nodes. This type of network is mainly used for two applications: office 'cubicle farms' and metropolitan-area wireless Internet access. Each of these applications requires that the wireless bridge be installed at some central point and the stations that are going to access the network be within the operating range of the bridge device
Note that the workstations at the top of the figure can communicate wirelessly to the server and printer connected to the same network as the bridge device.
There are many different brands, makes, and models of RF LAN equipment. This used to be a source of difficulty with LAN installers. In the infancy of RF wireless networking, every company used different frequencies, different encoding schemes, different antennas, and different wireless protocols. The marketplace was screaming for a standard to be proposed. For this reason, the IEEE 802.11 standard was developed. 802.11 is a networking standard that specifies various protocols for wireless networking. It does in fact specify that either infrared or RF can be used for the wireless network, but most RF systems are the only ones advertising IEEE 802.11 compliance.
Backbones and Segments
With complex networks, we must have a way of intelligently identifying which part of the network we are discussing. For this reason, we commonly break networks into backbones and segments
Understanding the Backbone
A backbone is the part of the network to which all segments and servers connect. A backbone provides the structure for a network and is considered the main part of any network. It usually uses a high-speed communications technology of some kind, such as FDDI (Fiber Distributed Data Interface) or 100-Megabit Ethernet (Fast Ethernet). All servers and all network segments typically connect directly to the backbone so that any segment is only one segment away from any server on that backbone. Because all segments are close to the servers, the network is more efficient.
Understanding Segments
Segment is a general term for any short section of the network that is not part of the backbone. Just as servers connect to the backbone, workstations connect to segments. Segments are connected to the backbone to allow the workstations on them access to the rest of the network.
Selecting the Right Topology - Ease of installation
- Ease of maintenance
- Cable fault tolerance
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