Sunday, June 5, 2011

Network Topology - Star, Bus, Mesh, Ring Topologies

Network topology - Star, Bus, Mesh, and Ring topologies

Topology of a network is its physical layout. You should be able to identify, based a picture or description, the star, bus, mesh, and ring topology.

Star topology is the most popular network topology in businesses today. It consists of nodes connected to a central switch or hub. If you have a home network, you are probably using the star topology.

Bus topology is one which consists of all of the workstations connected to a single cable. This topology is frequently in coax, or 10Base2, networks. The bus network has a terminator on each end of the network. If a terminator is not present or if there is a problem in the line (e.g. NIC card failure, network disconnected from PC), all workstations on that line lose connectivity.

Mesh topology is one which has all of the workstations connected to each other. This topology is typically only used when high availability is a requirement. It is expensive to maintain and troubleshoot.

Ring topology is one which all of the computers are connected in a loop and data is passed from one workstation to another. This is most common in a token ring environment where a "token" is sent with data from one node to another until it finds its destination.

Network Topology is the physical layout of the network. This concept is the foundation for understanding corporate networks and the technologies used to make them function.

In this section, you will learn:
  • What a network topology is;
  • How to identify the different network topologies.
What is a Network Topology?

There are basically two components to a network. Devices on the network that want to share resources or information and the medium which allows the communication to occur. A Network Topology is the physical layout of the computers, servers, and cables. There are four topologies mentioned in this learning item: star, bus, mesh, and ring. You can add wireless to the list as a growing popular option for network topology.

Understanding Network Topologies

Your corporate network may be a combination of several of these topologies. You may have one topology in your data center, a different in your offices, and a third in your conference rooms. You need to be familiar with each of the topologies, their characteristics, and what they look like drawn out.

Star Topology

The most popular topology for business today - the star topology consists of all of the nodes on a network connected to a central switch or hub. A node is a device attached to the network - such as a computer.

star topology

Each node on the network has a cable back to the central switch. If one cable fails to a node, only that node (computer) is affected. You can combine several switches or hubs to create several stars, all connected together.

The Star topology is very inexpensive to maintain versus other topologies. 10BaseT is an example of Star topology. Think of the star topology as a big wheel. At the center of the wheel is a switch or hub and each spoke going out from the center goes to a node.

Bus Topology

Bus topology is one which all of the devices on the network are connected with a single cable with terminators on each end. This single cable is often referred to as a backbone or trunk.

bus topology

The typical Bus network uses coax as its cable. Coax is a cable similar to what you use for your cable TV. Coax is also referred to as 10Base2.

The upside to using coax is that it is inexpensive, easy to install, and is not as susceptible to electromagnetic interference as twisted pair cable is.

The downside for a coax network is the speed is limited to 10 Mbps (Megabits per second) and that is an interruption occurs in the cable, all of the nodes (workstations) on the cable will lose connectivity. If a NIC fails or a cable is disconnected at any of the points in the network, it will not be terminated properly so all of the computers will lose connectivity to the network.

Mesh Topology

A mesh topology is one which all of the nodes are directly connected with all of the other nodes.

mesh topology

A mesh topology is the best choice when you require fault tolerance, however, it is very difficult to setup and maintain.

There are two types of mesh network: full mesh and partial mesh. A full mesh is one which every workstation is connected to the other ones in the network. In a partial mesh, the workstations have at least two NICs with connections to other nodes on the network. Mesh networks are commonly used in WANs.

Ring Topology

The ring topology is one which the network is a loop where data is passed from one workstation to another.

ring topology

Commonly, you find the ring topology with token ring networks. Token ring networks are defined by IEEE 802.5 and were primarily developed by IBM. The token ring network is designed to transmit a token, or a special frame, designed to go from node to node around the ring. As the frame passes, if a workstation needs to transmit data, it modifies the frame, attaches its data and sends it on. If the data is intended for the next workstation on the network, it receives the data and the information stops at that workstation. If it is intended for somewhere else on the network, the data is retransmitted around the ring until it finds its intended location. Once the data finds its new home, a blank token is transmitted and another workstation can attach data and then that data travels around the ring.

There is a token holding timer to prevent a workstation from transmitting too much data. This protocol ensures all workstations on the network get an opportunity to send data. The original specification could only operate up to 16 Mbps though newer Fast Token Ring networks can transmit up to 1 Gbps (gigabit per second).

Advantages for token ring networks include a 4k maximum frame size, longer distance capabilities than Ethernet, and each station is guaranteed access to a token at some point. Ethernet is a shared access medium meaning each workstation has equal access to the available bandwidth at any given time.

The recommended distance for Type 1 cabling on a token ring network is 300 meters, on Unshielded Twisted Pair (UTP) cabling, about 150 meters. More details will be discussed about token ring shortly.

Introduction to Networking
By: Azhar Qureshi
September 2010

Peetabeck Academy, Fort Albany, Ontario