Physical vs. Logical Topologies

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Each network has both a physical and a logical topology. The physical topology of a network refers to the physical layout of the devices and cabling. The term node is commonly used when discussing topology diagrams. For networking topology diagrams, a node is a device.
Two networks might have the same physical topology, but distances between nodes, physical interconnections, transmission rates, or signal types may be different. A physical topology must be implemented using media that is appropriate for it. In wired networks, recognizing the type of cabling used is important in describing the physical topology. The figure represents some of the physical topologies that you may encounter.
The following are the primary physical topology categories:
  • Bus: In a bus topology, every workstation is connected to a common transmission medium, a single cable, which is called a backbone or bus. Therefore, each workstation is directly connected to every other workstation in the network. In early bus topologies, computers and other network devices were connected to a central coaxial cable via connectors.
  • Ring: In a ring topology, computers and other network devices are cabled in succession and the last device is connected to the first one to form a circle or ring. Each device is connected to exactly two neighbors and has no direct connection to a third. When one node sends data to another, the data passes through each node that lies between them until it reaches the destination.
  • Star: The most common physical topology is a star topology. In this topology, there is a central device to which all other network devices connect via point-to-point links. This topology is also called the hub and spoke topology. There are no direct physical connections among spoke devices. This topology includes star and extended star topologies. In an extended star topology, one or more spoke devices is replaced by a device that has its own spokes. In other words, it is composed of multiple star topologies, whose central devices are connected between each other.
  • Mesh: In a mesh topology, a device can be connected to more than one other device. For one node to reach others there are multiple paths available. Redundant links increase reliability and self-healing. In a full mesh topology, every node is connected to every other node. In partial mesh, certain nodes do not have connections to all other nodes.
The logical topology is the path along which data travels from one point in the network to another. The diagram depicts the logical topology between PC A and the Server. In this example, data does not follow the shortest physical path, which would go through two switches. The logical topology requires data to also travel through the router in order for the two devices to communicate. The same could be true for all other end devices. Logical topology would then be a star, where the router is a central device.
It is possible for the logical and physical topology of a network to be of the same type. However, physical and logical topologies often differ. For example, an Ethernet hub is a legacy device that functions as a central device to which other devices connect in a physical star. The characteristic of a hub is that it “copies” every signal received on one port to all other ports. So a signal sent from one node is received by all other nodes. This behavior is typical of a bus topology. Because data flow has the characteristics of a bus topology, it is a logical bus topology.
The logical topology is determined by the intermediary devices and the protocols chosen to implement the network. The intermediary devices and network protocols both determine how end devices access the media and how they exchange data.
A physical star topology in which a switch is the central device is by far the most common in implementations of LANs today. When using a switch to interconnect the devices, both the physical and the logical topologies are star topologies.