A physical topology defines the way in which computers, printers, and other devices are connected to a network. A logical topology describes how the hosts access the medium and communicate on the network. The type of topology determines the capabilities of the network, such as ease of setup, speed, and cable lengths.

Physical Topologies
Figure 1 shows the common LAN physical topologies:

• Bus
• Ring
• Star
• Hierarchical or extended star
• Mesh

Bus Topology
In the bus topology, each computer connects to a common cable. The cable connects one computer to the next, like a bus line going through a city. The cable has a small cap installed at the end, called a terminator. The terminator prevents signals from bouncing back and causing network errors.

Ring Topology
In a ring topology, hosts are connected in a physical ring or circle. Because the ring topology has no beginning or end, the cable does not need to be terminated. A specially-formatted frame, called a token, travels around the ring, stopping at each host. If a host wants to transmit data, the host adds the data and the destination address to the frame. The frame then continues around the ring until the frame stops at the host with the destination address. The destination host takes the data out of the frame.

Star Topology
The star topology has a central connection point, which is normally a device such as a hub, switch, or router. Each host on a network has a cable segment that attaches the host directly to the central connection point. The advantage of a star topology is that it is easy to troubleshoot. 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 remains operational.

Hierarchical or Extended Star Topology
A hierarchical or extended star topology is a star network with an additional networking device connected to the main networking device. Typically, a network cable connects to one hub, and then several other hubs connect to the first hub. Larger networks, such as those of corporations or universities, use the hierarchical star topology.

Mesh Topology
The mesh topology connects all devices to each other. When every device is connected to every other device, a failure of any cable does not affect the network. The mesh topology is used in WANs that interconnect LANs.

Logical Topologies
The two most common types of logical topologies are broadcast and token passing.

In a broadcast topology, each host addresses either data to a particular host or to all hosts connected on a network. There is no order that the hosts must follow to use the network – it is first come, first served for transmitting data on the network.

Token passing controls network access by passing an electronic token sequentially to each host. When a host receives the token, it can send data on the network. If the host has no data to send, it passes the token to the next host, and the process repeats itself.