A network topology is the physical layout of computers, cables, and other components on a network. There are a number of different network topologies, and a network may be built using multiple topologies.
The different types of network topologies (or layouts) are:
- Bus topology
- Star topology
- Mesh topology
- Ring topology
- Hybrid topology
- Wireless topology
A bus topology uses one cable as a main trunk to connect all of the systems together. A bus topology is very easy to set up and requires no additional hardware such as a hub. The cable is also called a trunk, a backbone, or a segment. With a bus topology, when a computer sends out a signal, the signal travels the cable length in both directions from the sending computer. When the signal reaches the end of the cable length, it bounces back and returns in the direction it came from. This is known as signal bounce. Signal bounce is a problem, because if another signal is sent on the cable length at the same time, the two signals will collide and be destroyed and then must be retransmitted. For this reason, at each end of the cable there is a terminator. The terminator is designed to absorb the signal when the signal reaches the end, preventing signal bounce. If there is no termination, the entire network fails because of signal bounce, which also means that if there is ever a break in the cable, you will have unterminated ends and the entire network will go down.
A bus is a passive topology, which means that the workstations on the bus are not responsible for regenerating the signal as it passes by them. Since the workstations do not play an active role, the workstations are not a requirement of a functioning bus, which means that if a workstation fails, the bus does not fail. But if there is an unterminated end in the bus, the entire network will fail.
Advantages of a Bus Topology
One advantage of a bus topology is cost. A bus topology uses less cable than a star topology or a mesh topology, and you do not need to purchase any additional devices such as hubs. Another advantage of a bus topology is the ease of installation. With a bus topology, you simply connect the workstation to the cable segment or backbone. You need only the amount of cable to connect the workstation to the backbone. The most economical choice for a network topology is a bus topology, because it is easy to work with and a minimal amount of additional devices are required. Most importantly, if a computer fails, the network stays functional.
Disadvantages of a Bus Topology
The main disadvantage of a bus topology is the difficulty of troubleshooting it. When the network goes down, it is usually due to a break in the cable segment. With a large network, this problem can be tough to isolate.
Scalability is an important consideration in the dynamic world of networking. Being able to make changes easily within the size and layout of your network can be important in future productivity or downtime. The bus topology is not very scalable.
In a star topology, all computers are connected through one central device known as a hub or a switch. Each workstation has a cable that goes from the network card to the hub device. One of the major benefits of a star topology is that a break in the cable causes only the workstation that is connected to the cable to go down, not the entire network, as with a bus topology. Star topologies are very popular topologies in today’s networking environments.
Advantages of a Star Topology
One advantage of a star topology is scalability and ease of adding another system to the network. If you need to add another workstation to the network with a star topology, you simply connect that system to an unused port on the hub. Another benefit is the fact that if there is a break in the cable it affects only the system that is connected to that cable.
Centralizing network components can make an administrator’s life much easier in the long run. Centralized management and monitoring of network traffic can be vital to network success. With a star configuration, it is also easy to add or change configurations because all of the connections come to a central point.
Disadvantages of a Star Topology
On the flip side, if the hub fails in a star topology, the entire network comes down, so we still have a central point of failure. But this is a much easier problem to troubleshoot than trying to find a cable break with a bus topology. Another disadvantage of a star topology is cost. To connect each workstation to the network, you will need to ensure that there is a hub with an available port, and you will need to ensure you have a cable to go from the workstation to the hub. Today, the cost is increasingly less of a disadvantage because of the low prices of devices such as hubs and switches.
A mesh topology is not very common in computer networking today, but you must understand the concept for the exam. In a mesh topology, every workstation has a connection to every other component of the network.
Advantages of a Mesh Topology
The biggest advantage of a mesh topology is fault tolerance, meaning that, if there is a break in a cable segment, traffic can be rerouted through a different pathway because there are multiple pathways to send data from one system to another. This fault tolerance means that it is almost impossible for the network to go down due to a cable fault.
Disadvantages of a Mesh Topology
A disadvantage of a mesh topology is the cost of the additional cabling and network interfaces to create the multiple pathways between each system. A mesh topology is very hard to administer and manage because of the numerous connections.
In a ring topology, all computers are connected via a cable that loops in a ring or circle. A ring topology is a circle that has no start and no end. Because there are no ends, terminators are not necessary in a ring topology. Signals travel in one direction on a ring while they are passed from one computer to the next, with each computer regenerating the signal so that it may travel the distance required.
Advantages of a Ring Topology
A major advantage of a ring topology is that signal degeneration is low because each workstation is responsible for regenerating or boosting the signal. With the other topologies, as the signal travels the wire, it gets weaker and weaker as a result of outside interference: eventually, it becomes unreadable if the destination system is too far away. Because each workstation in a ring topology regenerates the signal, the signal is stronger when it reaches its destination and seldom needs to be retransmitted.
Disadvantages of a Ring Topology
The biggest problem with ring topologies is that if one computer fails or the cable link is broken, the entire network can go down. With newer technology, however, this isn’t always the case. The concept of a ring topology today is that the ring will not be broken when a system is disconnected; only that system is dropped from the ring. Isolating a problem can be difficult in some ring configurations. (With newer technologies, a workstation or server will put out a beacon if it notices a break in the ring.)
Another disadvantage is that if you make a cabling change to the network or move a workstation, the brief disconnection can interrupt or bring down the entire network.
It is important to note that it is typical for networks to implement a mixture of topologies to form a hybrid topology. For example, a very popular hybrid topology is a star-bus topology, in which a number of star topologies are connected by a central bus. This is a popular topology because the bus will connect hubs that are spread over distance.
Another very popular hybrid topology is the star-ring topology. The star-ring topology is popular because it looks like a star but acts as a ring. For example, there is a network architecture known as Token Ring that uses a central “hub” type device, but the internal wiring makes a ring. Physically it looks like a star, but logically it acts as a ring topology.
A wireless topology is one in which few cables are used to connect systems. The network is made up of transmitters that broadcast the packets using radio frequencies. The network contains special transmitters called cells, or wireless access points, which extend a radio sphere in the shape of a bubble around the transmitter. This bubble can extend to multiple rooms and possibly floors in a building. The PCs and network devices have a special transmitter-receiver, which allows them to receive broadcasts and transmit requested data back to the access point. The access point is connected to the physical network by a cable, which allows it, and any wireless clients, to communicate with systems on the wired network.
The wireless cells, or access points, are connected to the network by connecting into the hub or switch that has a connection to the rest of the wired network. The clients do not have cables connecting them to the network. These are wireless clients, and they will get access to the network through the wireless cell (or access point).
Another option for wireless networks is the use of a radio antenna on or near the building, which allows one cell to cover the building and the surrounding area. This approach is best in a campus-type arrangement, where many buildings that need to be included in the cell are in a close geographical area. This setup does not easily allow you to connect the buildings by a backbone and physical cables and then to each building containing the required cells for all its PCs and devices.
Wireless networks also can consist of infrared communications, similar to a remote-control TV, but this type of communication is slow and requires a direct line of sight—as well as close proximity—for the communication to work. Infrared mainly is used only between two systems. Infrared is not used often as a complete networking solution and should not be considered even as an option for a whole network; it is useful between laptops or a laptop and a printer.
Advantages of a Wireless Topology
The nice thing about wireless networks is the lack of cabling. The wireless network requires only base backbone segments to connect the wireless cells to the wired network if there is one. Once these are set up, the PC and network devices also need the special transmitter-receiver network interface cards to allow the PCs and devices to communicate with the cell and then through the cell to the servers. Troubleshooting failed devices and cells is very easy and makes failed components easy to find and replace.
Disadvantages of a Wireless Topology
Disadvantages of wireless networks include a greater chance of signal interference, blockage, and interception. Other devices and machinery that emit radio frequencies or “noise” can cause interference and static, which can disrupt the bubble of communication around the cell. Another source of noise is lightning during storms. This noise is the same static you hear when lightning strikes while you are speaking on a phone.
Blockage can occur in structures that are made of thick stone or metal, which do not allow radio frequencies to pass through easily. This drawback usually can be overcome somewhat by changing the frequency used by the devices to a higher frequency. You can determine early if this is going to be a problem in your building by trying to use a radio inside the building to pick up some radio stations. If the radio will not pick them up, the building material is too thick to allow radio frequencies to pass through the walls. This problem can be overcome by installing a cell in each room where a PC or network device will be placed.
Another major disadvantage with wireless is signal interception. Signal interception means unwanted third parties could intercept wireless communications without physically being on the premises; they would simply have to be within the signal range. One of the key steps to securing wireless communication is to limit who can connect to the network and to encrypt the traffic in transit.
Point-to-Point and Point-to-Multipoint
There are two popular layouts for topologies: they are either point-to-point or point-to-multipoint.
A point-to-point topology—also known as host to host—is one system connected directly to another system. In the past these systems would connect directly through the serial ports with a null modem cable, but these days, you could connect them using a crossover cable or a wireless connection.
A point-to-multipoint topology uses a central device that connects all the devices together. This topology is popular with wireless. With point-to-multipoint, when the central device sends data, it is received by all devices connected to the central device. But if one of the devices that are connected sends data, then it is received by only the destination system.
Segments and Backbones
With the various topologies we’ve looked at, you have seen the words segment and backbone mentioned a couple of times. A network segment is a cable length (or multiple cable lengths) that is uninterrupted by network connectivity devices, such as bridges and routers. It is typical that a single network may be broken into multiple network segments through the use of a bridge or router to cut down on network traffic.
You also saw the word backbone mentioned a few times. A backbone is the main cable segment or trunk in the network. In a bus network, you might see a main cable trunk that has smaller cables connecting the workstations. These smaller cables, known as drop cables, connect the workstations to the backbone. One example of a backbone is a satellite linking geographically dispersed local area networks (LANs), making a wide area network (WAN). Such a backbone is an example of a wireless communications network, whereas the previous examples all used cable as the medium.
Published on Fri 17 February 2017 by Daniel White in Networking with tag(s): networks topologies