Basic Concepts of Computer Networks Network Topology

Lesson Objectives At the end of this lesson, the learner should be able to achieve the following three objectives: Clearly define the term network topology. Identify the types of network topologies and their individual advantages and disadvantages. How to properly choose the appropriate topology for a given situation.

DEFINITION: What Is a Topology? A network topology describes the arrangement of systems on a computer network. It defines how the computers, or nodes, within the network are arranged and connected to each other. (TechTerms.com, 2007)

Commonly Used Network Topologies The most commonly used network topologies are: Bus Ring Star Mesh

The Bus Topology The bus topology is the simplest and most common. It is often used when a network installation is small, simple, or temporary. It is a passive topology. This means that computers on the bus only “listen” to the data being sent. They are not responsible for “moving” the data from one computer to the next.

The Bus Topology In an active topology network, the computers regenerate signals and are responsible for moving the data through the network. On a bus network, all the computers are connected to a single cable. When one computer sends a signal using the cable, all the computers on the network receive the message, but only the addressee accepts it. The other computers disregard the message.

Advantages of the Bus Topology The bus is simple and very reliable in very small networks. The bus network requires the least amount of cable to connect the computers together and is, therefore, less expensive than other cabling configurations. Failure of one network node does not effect the rest of the network.

Disadvantages of the Bus Topology Heavy network traffic can slow a bus considerably A break in the cable or lack of proper termination can shutdown the entire network. It is extremely difficult to troubleshoot a bus if the entire network shuts down.

Appropriate Use of a Bus Topology The network is small. The network will not be frequently reconfigured. The least expensive solution is necessary. Network growth is not predicted.

The Ring Topology In a ring topology, each computer is connected directly to the next computer in line, forming a circle of cable. It uses a token to pass the information from one computer to the next. Each computer is connected to the next computer in the ring and retransmits what it receives from the previous computer. The message flow around the ring is unidirectional. The ring is classified as an active topology because there is no termination point in the ring.

Token Passing in the Ring Topology Token passing is a method of sending data in a ring topology. A small packet, called the token is passed around the ring to each computer in turn. If a computer has information to send, it modifies the token, adds address information and data and sends it down the ring. The information travels around the ring until it either reaches it’s destination or returns to the sender. A token can circle a ring 200 meters in diameter at a speed of around 10,000 times per second.

Advantages of Ring Topology All the computers have equal access to the network. Even with multiple users, network performance is balanced. There is no “bottle neck”. Allows for error checking and acknowledgement.

Disadvantages of the Ring Topology Failure of one computer can affect the entire network. Data cannot be transmitted successfully. Data packets must pass through every computer. This makes the network slower in nature. It is difficult to troubleshoot the ring network. Adding or removing computers disturbs the network.

Appropriate Uses of the Ring Topology The network must operate reasonably under a heavy load. A higher speed network is required. The network will not be frequently reconfigured.

The Star Topology In a star topology, each device has a dedicated point-to-point link to a central controller, sometimes referred to as a hub, server, or host. Each computer on a star network communicates with the central hub that then resends the message to the appropriate computer(s). The hub can be active or passive in nature. An active hub regenerates the electrical signal and sends it to all the computers connected to it.

Hubs in the Star Topology The type of hub used in a star topology is commonly referred to as a multi-port repeater. Active hubs require electrical power to operate. A passive hub, such as a wiring panel, acts as a connection point and does not amplify or regenerate the signal. Passive hubs do not require electrical power to operate.

Advantages of the Star Topology A star network is easy to modify and add new computers. During the addition or removal of nodes, the network continues to function normally. The central hub can be easily upgraded when the network capacity is exceeded. The central hub provides for centralized monitoring and management of the network, thereby increasing security. A single computer failure does not necessarily bring down the entire star network.

Disadvantages of the Star Topology If the central hub fails, then the entire network ceases to operate. The attached nodes are disabled. There is a larger upfront cost to create a star network in terms of cabling and hardware. A star network requires a dedicated server.

Appropriate Uses of the Star Topology The network requires easy addition or removal of client computers. Star network topologies are recommended for easy troubleshooting ability. The network is large in nature. The network is expected to grow in the future.

The Mesh Topology In a mesh topology, every device has a dedicated point-to-point link to every other device. A fully connected mesh network has n(n – 1)/2 physical channels to link n devices. To accommodate the many links, every device on the network must have n -1 input/output ports.

Advantages of the Mesh Topology Because there is a dedicated link, there is no traffic between computers. Failure of one node computer does not affect the rest of the network. Because of the dedicated link, privacy and security are easier to maintain. Point-to-point links make fault identification and fault isolation easier to determine.

Disadvantages of the Mesh Topology Due to the number of cables and input and output ports, it can be more expensive to establish and maintain. A large amount of space is necessary to run each of the cables.

Review of Common Network Topologies

Conclusions Topologies are the essence of computer network designs. Efficient networks can only be built based on the complete knowledge and understanding of the previously explored topologies. Knowledge of communication devices is of equal importance when determining the best options for network requirements. Resources and time can be wasted when choosing the wrong topology to fit the requirements of the network to be built.

References Web References http://compnetworking.about.com/od/networkdesi gn/ig/Computer-Network-Topologies/ http://www.techterms.com/definition/networktop ology http://en.wikipedia.org/wiki/Network_topology http://www.youtube.com/watch?v=kfEDPQAYH4k