The layout of a network is typically called its "topology," a term that refers both to the physical arrangement of the components and how those components communicate with each other. Network topologies have evolved over the past half century from simple coaxial cable with data transfer speeds of one to three megabytes per second to complex switched fiber optic networks that move up to ten gigabytes per second. There are several different network topologies available, and choosing the right one for your network is critical.
Physical vs. Logical Topologies
Network topologies have two facets: logical and physical. "Physical" refers to the way the components are actually connected, including the media that is used and the placement of devices on the media. "Logical" refers to the way the devices on the network communicate or how the signal moves over the network. Understanding the difference is important as you plan your network.
Bus Topology
The physical bus topology is simple in that it consists of a wire, typically coaxial cable, running from one machine to another. A terminator is placed at each end of the cable to prevent "signal bounce," which can cause the network to become jammed with echoing data transmissions. A physical bus topology is, with rare exception, implemented as a logical bus topology. Each computer sends data out on the wire and each other computer receives the signal. The computer that is the destination processes the data and the others simply discard the data. Of course, if multiple computers transmit data at the same time, collisions occur on the wire and a process begins of waiting and retransmitting.
Ring Topology
The physical ring topology consists of a network very similar to the bus topology, with the one difference being that there are no ends of the cable to terminate because each computer is connected to a literal ring of cable. Although physical rings are rarely used with the exception of data center network backbones, logical rings are the more prevalent implementation of a ring topology. In a logical ring the signal still moves around in a "circle" from one machine to the next, but the physical network is more likely to be implemented as a "star," using a hub as the wiring connector. All of the computers plug into the hub and the signal travels from one computer through the hub to the next computer in line.
Star Topology
The term "star" refers to an actual physical topology over which both logical bus and logical ring topologies can operate. All computers or devices on the network are cabled directly into a hub or switch so that in a diagram the layout looks like a star, with a central point and rays (of cable) extending outward. This design typically requires more cable, but is much easier to troubleshoot in the event of problems.
Switched Topology
A switched topology is neither a logical ring nor a logical bus and is always implemented as a physical star. The switch device creates circuits between devices "on the fly," giving each connection the full bandwidth of the network. This topology also allows multiple "conversations" to be in progress at a given point in time between pairs of devices. The switched topology is used in almost every new local area network design. The difference between a switch and a hub is that switches are "smart" and direct the signal only to the destination device, while hubs are "dumb," sending the signal to all devices that are connected to it.
Wireless Topologies
Wireless topologies do not require cabling, so no physical topology is visible. There are two different modes for wireless networks: infrastructure mode and ad hoc mode. Infrastructure mode refers to the use of a wireless access point that is wired into the physical network, either into a switch or hub. Ad hoc mode refers to a peer-to-peer arrangement where two or more computers are network with each other without need for an access point. Ad hoc networks resemble a physical and logical bus network, while infrastructure wireless network resemble a physical star and a logical bus network.
Tags: physical topology, each other, logical ring, access point, coaxial cable, devices network, from machine