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Physical & Logical Topology: Definition & Characteristics

Instructor: David Whitsett

David has taught computer applications, computer fundamentals, computer networking, and marketing at the college level. He has a MBA in marketing.

How do we connect to network resources? In this lesson, we'll discuss physical and logical topologies, which are different ways of describing network connections. We'll also look at characteristics of some commonly used topologies.

What Are Physical & Logical Topologies?

People often draw diagrams or pictures to help describe complicated things. Maps are pictures of routes to get from point A to point B. Instruction manuals use diagrams to help us figure out how to put things together.

When we use the word 'topology' in this lesson, we are referring to how things in a network are connected, as in a configuration.

  • A physical topology describes how network devices are physically connected - how devices are actually plugged into each other. We're talking about cables, wireless connectivity, etc.
  • A logical topology describes how network devices appear to be connected to each other.

For example, in a logical diagram of your office network, you may show a connection between city A and city B. But in the actual physical network, maybe your data goes through switching points in several other cities as well. The logical path is a high-level representation; the physical path is the actual route.

Signal Topologies

Logical topology can also refer to the 'signal topology' of the network, which is how the signals are passed across the physical network. How signals are set up and transmitted is governed by network protocols. The most common signal topologies are shared media access and token-based access.

  • In a shared media access network, all the devices on the network can transmit at any time, which sometimes results in collisions if two nodes transmit at once. Ethernet is based on shared media access.
  • In a token-based access network, nodes have to wait their turn with the network token before they can transmit.

Common Types of Physical Topologies

The common types of physical topologies for networks are star, bus, ring, and mesh.

A diagram of the types of logical topologies.
diagram

The most widely-used physical topology in businesses today is the star topology, in which all the network devices are connected back to a central hub or switch.

In a bus topology, network devices are connected to a central cable in a linear direction, like stops on a bus route.

In most offices and many home networks, the underlying network technology used is Ethernet. Ethernet (shared media access) is based on a family of standards that has been around for almost 50 years and is not brand-specific to any one manufacturer. Devices can can connect to Ethernet through wired or wireless connections. Ethernet networks are logically a bus topology and physically a star topology.

Logical topology diagram

A ring topology means that the devices in the network are connected in a circular fashion, and every piece of data is sent around the ring until it finds the intended recipient. IBM had a version of a ring topology called Token Ring that was a popular choice for office networks for a while, but the network electronics were more expensive than Ethernet, and as Ethernet speeds grew, Token Ring fell out of favor. IBM Token Ring was a token-based access network, just as the name implies.

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