LAN Topologies

• Recognize the following logical or physical topologies, given a schematic diagram or description:

  • Star/hierarchical

  • Bus

  • Mesh

  • Ring

  • Wireless

The term network topology refers to the layout of a network. The type of topology affects what networking method is used, as well as what cable types and network devices are required. Topologies are very important, and they serve as the foundation for the information you'll learn in the following sections. You will certainly be asked about topologies on the Network+ exam.

Before we look at the different types of topologies, we must first examine one of the most confusing networking principles: the difference between physical and logical topologies. Then we'll examine the specific physical LAN topologies in use today: bus, star, ring, mesh, and wireless.

Physical and Logical Topologies

Network topologies can be defined on a physical level or on a logical level. The physical topology refers to how a network is physically constructed—that is, how it actually looks. The logical topology refers to how a network looks to the devices that use it—in other words, how it actually functions. In a number of commonly implemented network models, the physical topology differs from the logical topology. It can be difficult to appreciate what that means, so let's use an example.

The most commonly implemented network model is a physical star/logical bus topology. In this configuration, computers are connected to a central device, called a hub or switch, which gives the network the appearance of a star (or a reasonable facsimile thereof). However, the devices attached to the star see the network as a linear bus topology and use the topology based on its logical characteristics.

The Bus Topology

The bus network topology is also known as a linear bus because the computers in such a network are linked together using a single cable called a trunk, or backbone. Computers are connected to this backbone as shown in Figure 1.6.

The computers can be connected to the backbone by a cable, known as a drop cable, or, more commonly, directly to the backbone, via T connectors. At each end of the cable, terminators prevent the signal from bouncing back down the cable. In addition, one end of the cable should be grounded. More information on the specific connectors and connections used in different networks is provided in Chapter 2, “Cabling and Connectors.”

Bus topologies are easy and inexpensive to implement because a single-segment bus topology doesn't require any special networking equipment. However, they are notoriously difficult to troubleshoot, and a single break in the network cable renders the entire segment useless. For this and a number other reasons, such as limited speed capacity, bus topologies have been largely replaced with the physical star topology. The main features, advantages, and disadvantages of bus topologies are provided in Table 1.1.

Table 1.1. Features, Advantages, and Disadvantages of the Linear Bus Topology

Features	Advantages	Disadvantages
Uses a single length of cable.	It is inexpensive and easy to implement.	It cannot be expanded easily. Doing so may render the network inaccessible while the expansion is performed.
Devices connect directly to the cable.	It doesn't require special equipment.	A break in the cable renders the entire segment unusable.
The cable must be terminated at both ends.	It requires less cable than other topologies.	It is difficult to troubleshoot.

The Star Topology

In a star topology, each device on the network connects to a centralized device via a single cable. This arrangement creates a point-to-point network connection between the two devices and overall gives the appearance of a star. Figure 1.7 shows an example of the star topology.

Because each device must have its own cable, a star topology requires far more cable than other topologies such as a physical linear bus. In addition, special equipment is required to create the hub of the star layout, adding to the cost of implementing a star topology. (Chapter 3, “Networking Components and Devices,” explains the function of network devices such as hubs and switches that are used in a star topology.)

Multiple stars can be combined into a tree-like structure known as a hierarchical star. The hierarchical star allows for high levels of flexibility and expandability. Depending on the networking equipment used, it also makes it possible to manage traffic and isolate high-traffic areas of the network. Figure 1.8 shows an example of a hierarchical star topology.

One of the biggest advantages of the star topology is that computers can be connected to and disconnected from the network without affecting any other systems. Thus it's very easy to add systems to or remove systems from the network. In addition, the failure of a system or the cable it uses to attach likewise generally has no effect on other stations on the network. However, in the star topology, all devices on the network connect to a central device, and this central device creates a single point of failure on the network.

The star topology is the most widely implemented network design in use today; you will definitely encounter it in the real world. Working with and troubleshooting a star topology can be a tricky matter, however, and you need to know what to look for and where to look. For more information on troubleshooting star networks and other specific network topology errors, see Chapter 15, “Troubleshooting Procedures and Best Practices.”

The features, advantages, and disadvantages of the physical star topology are provided in Table 1.2.

Table 1.2. Features, Advantages, and Disadvantages of the Physical Star Topology

Features	Advantages	Disadvantages
Devices connect to a central point.	It can be easily expanded without disruption to existing systems.	It requires additional networking equipment to create the network layout.
Each system uses an individual cable to attach.	A cable failure affects only a single system.	It requires considerably more cable than other topologies, such as the linear bus.
Multiple stars can be combined to create a hierarchical star.	It is easy to troubleshoot.	Centralized devices create a single point of failure.

The Ring Topology

In the ring topology, the network layout forms a complete ring. Computers connect to the network cable directly or, far more commonly, through a specialized network device.

On a ring network, data travels in one direction, passing from one computer to the next until it reaches the intended destination. Figure 1.9 shows an example of the ring topology.

Ring topologies are more difficult to install and configure than other topologies because breaking the loop disrupts the entire network. Even if network devices are used to create the ring, the ring must still be broken if a fault occurs or the network needs to be expanded. Ring topologies are relatively uncommon; the physical star layout is by far the most popular topology. For this reason, you are unlikely to actually install a ring topology. Table 1.3 shows the features, advantages, and disadvantages of the ring topology.

Table 1.3. Features, Advantages, and Disadvantages of the Ring Topology

Features	Advantages	Disadvantages
Devices are connected in a closed loop or ring.	It is easy to troubleshoot.	A cable break can disrupt the entire network.
Dual-ring configuration can be used for fault tolerance.	Can be implemented in a fault tolerant configuration.	Network expansion creates network disruption.

Mesh Topology

The mesh topology is unique: It requires each computer on the network to be individually connected to every other device. This configuration provides maximum reliability and redundancy for the network. If one cable or link fails, the data can use an alternate path to get to its destination. Figure 1.10 shows an example of the mesh topology.

Given the relative ease with which the other topologies can be created and the complexity of the mesh layout, you should not be surprised to learn that networks using the mesh layout are few and far between. In fact, you are extremely unlikely to see a mesh layout in a LAN setting. The mesh topology is sometimes adopted in WAN configurations that require direct connections between each and every geographic site.

However unlikely you are to see a mesh topology, it is mentioned in the Network+ exam objectives, so be sure you can identify a mesh layout should the need arise. Table 1.4 lists the features, advantages, and disadvantages of the mesh topology.

Table 1.4. Features, Advantages, and Disadvantages of the Mesh Topology

Features	Advantages	Disadvantages
A true mesh uses point-to-point connectivity between all devices.	Multiple links provide fault tolerance and redundancy.	It is difficult to implement.
A hybrid mesh uses point-to-point connectivity between certain devices, but not all of them.	The network can be expanded with minimal or no disruption.	It can be expensive because it requires specialized hardware and cable.

Wireless Topology

The wireless topology is the newest type of network layout. The wireless topology is hard to define because, as the name implies, it uses no wires.

There are many types of wireless networks, but this section focuses on the kind found in a LAN. These systems, which are relative newcomers to the networking scene, use a centralized device known as a wireless access point (WAP) that transmits signals to devices with wireless NICs installed in them. Figure 1.11 shows an example of such a layout.

Wireless networking has advantages such as the ability to move around without worrying about physical cable access points on the network. This flexibility can be seen in many real-world scenarios. For example, on a university campus, students using laptops can move from classroom to classroom without disconnecting from the network or worrying about network connection points. In a hospital, doctors can do rounds and have real-time access to medical records. These examples make it clear why wireless networking is sure to become increasingly popular.

From an installation and troubleshooting perspective, wireless networks have some unique advantages over standard wired topologies. No cables means no faulty cables or cable breaks. Although the WAPs represent central points of failure, if a WAP develops a fault, systems connected to that WAP can simply relocate to another area to access a different WAP.

Wireless networking is not without drawbacks, though. Wireless networks are currently slower than conventional cable-based networks. In addition, security concerns discourage many people from using wireless networking, although many of these issues have now been addressed. Table 1.5 outlines the features, advantages, and disadvantages of wireless topologies.

Table 1.5. Features, Advantages, and Disadvantages of the Wireless Topology

Features	Advantages	Disadvantages
No physical connections are required.	It provides flexible network access.	It is still relatively new and expensive.
It can be used in LAN or WAN environments.	It can be used in environments where physical access is not possible.	It has potential security issues. Speed is limited in certain implementations.