The following sections will discuss the different types of topologies including bus, star, extended star, ring, mesh, and hybrid. The physical and logical topologies of a network will also be discussed.

Bus Topology
Commonly referred to as a linear bus, all the devices on a bus topology are connected by one single cable. This cable proceeds from one computer to the next like a bus line going through a city. The main cable segment must end with a terminator that absorbs the signal when it reaches the end of the line or wire. If there is no terminator, the electrical signal representing the data bounces back at the end of the wire, causing errors in the network. Only one packet of data can be transmitted at a time. If more than one packet is transmitted, they collide and have to be resent. A bus topology with many hosts can be very slow due to these collisions. This topology is rarely used and would only be suitable for a home office or small business with a few hosts.

Star Topology  
The star topology is the most commonly used architecture in Ethernet LANs. When installed, the star topology resembles spokes in a bicycle wheel. It is made up of a central connection point that is a device such as a hub, switch, or router, where all of the cabling segments actually meet. Each host in the network is connected to the central device with its own cable.

A star topology costs more to implement than the bus topology. This is because more cable is used and a central device is needed such as a hub, switch, or router. However, the advantages of a star topology are worth the additional costs. Since each host is connected to the central device with its own wire, if there is a problem with that cable, only that host is affected. The rest of the network is operational. This benefit is extremely important. It is the reason why virtually every newly designed network has this topology.

When a star network is expanded to include an additional networking device that is connected to the main networking device, it is called an extended star topology. Larger networks, like those of corporations or schools, use the extended star topology. When used with network devices that filter frames or packets, like bridges, switches and routers, this topology significantly reduces the traffic on the wires by sending packets only to the wires of the destination host.

Ring Topology
The ring topology is another important topology in LAN connectivity. It is important to know the advantages and disadvantages of choosing a ring topology. As the name implies, hosts are connected in the form of a ring or circle. Unlike the bus topology, it has no beginning or end that needs to be terminated. Data is transmitted in a way that is unlike the bus or the star topology. A frame travels around the ring, stopping at each node. If a node wants to transmit data, it adds the data as well as the destination address to the frame. The frame then continues around the ring until it finds the destination node, which takes the data out of the frame. The advantage of using a method such as this is that there are no collisions of data packets.

There are two types of rings:

• Single ring – All the devices on the network share a single cable, and the data travels in one direction only. Each device waits its turn to send data over the network. 
• Dual ring – Two rings allow data to be sent in both directions. This creates redundancy (fault tolerance), meaning that in the event of a failure of one ring, data will be still be able to be transmitted on the other ring.

The most common implementation of the ring topology is in Token Ring networks. The IEEE 802.5 standard is the Token Ring access method used. Fiber Distributed Data Interface (FDDI) is a technology that is similar to Token Ring, but it uses light instead of electricity to transmit data. It uses the dual ring.

Mesh Topology
The mesh topology connects all devices (nodes) to each other for redundancy and fault tolerance. It is used in WANs to interconnect LANs and for mission critical networks like those used by governments. Implementing the mesh topology is expensive and difficult.

Hybrid Topology
The hybrid topology combines more than one type of topology. When a bus line joins two hubs of different topologies, this configuration is called a star bus. Businesses or schools that have several buildings, known as campuses, sometimes use this topology. The bus line is used to transfer the data between the star topologies.

Interactive Lab Activity  (Flash, 366 KB)    Identify LAN Topologies and Devices

Worksheet  (PDF, 7 KB)   The Internet