Physical Layer Topology

A network topology refers to the layout of the transmission medium and devices on a network. As a networking professional for many years I can honestly say about the only time network topology has come up is for certification tesing. Here are some basic definitions.

Physical Topology:

Bus: Uses a single main bus cable, sometimes called a backbone, to transmit data. Workstations and other network devices tap directly into the backbone by using drop cables that are connected to the backbone.

This topology is an old one and essentially has each of the computers on the network daisy-chained to each other. This type of network is usually peer to peer and uses Thinnet(10base2) cabling. It is configured by connecting a "T-connector" to the network adapter and then connecting cables to the T-connectors on the computers on the right and left. At both ends of the chain the network must be terminated with a 50 ohm impedance terminator.

Advantages: Cheap, simple to set up.
Disadvantages Excess network traffic, a failure may affect many users, Problems are difficult to troubleshoot.

Star: Branches out via drop cables from a central hub (also called a multiport repeater or concentrator) to each workstation. A signal is transmitted from a workstation up the drop cable to the hub. The hub then transmits the signal to other networked workstations.

The star is probably the most commonly used topology today. It uses twisted pair such as 10baseT or 100baseT cabling and requires that all devices are connected to a hub.

Advantages: centralized monitoring, failures do not affect others unless it is the hub, easy to modify.
Disadvantages If the hub fails then everything connected to it is down.

Ring: Connects workstations in a continuous loop. Workstations relay signals around the loop in round-robin fashion.

The ring topology looks the same as the star, except that it uses special hubs and ethernet adapters. The Ring topology is used with Token Ring networks, (a proprietary IBM System).

Advantages: Equal access.
Disadvantages Difficult to troubleshoot, network changes affect many users, failure affects many users.

Mesh: Provides each device with a point-to-point connection to every other device in the network.

Mesh topologies are combinations of the above and are common on very large networks. For example, a star bus network has hubs connected in a row(like a bus network) and has computers connected to each hub.

Cellular: Refers to a geographic area, divided into cells, combining a wireless structure with point-to-point and multipoint design for device attachment. 

Logical Topology:

Ring: Generates and sends the signal on a one-way path, usually counterclockwise.

Bus: Generates and sends the signal to all network devices.

Physical topology defines the cable's actual physical configuration (star, bus, mesh, ring, cellular, hybrid). Logical topology defines the network path that a signal follows (ring or bus).