Categorizing Networks by Topology

 Introduction.  Networks are sometimes categorized by the physical topology of the network.  The physical topology refers to the physical shape of the network, the way in which cable is laid out. The logical topology refers to the paths the signals travel as they make their way through the network.  The five most frequently implemented topologies are listed below. Linear bus - Ring - Star Bus - Mesh - Hybrid - We now give a quick summary of these topologies. Linear Bus Topologies.  A linear bus topology is laid out in a straight line.  The line doesn't have to be exactly physically straight, but the connecting cable proceeds from one computer to the next.

 This sort of network requires some sort of termination at each end.  Without it, the signal will bounce and may cause disruption of communications.  One end of the network should also be grounded.  They are usually based on either thick or thin coax cable. When a message is sent on these networks it is transmitted to every device on the network.  Each NIC - network interface card examines the headers of the message to determine whether the message is addressed to that device.  If it isn't then the message is discarded by that particular device. These networks are relatively easy to set up.  They are often relatively inexpensive because they require less cabling.  These are most suitable for relatively small networks. These networks are classified as passive because each device doesn't regenerate the signal and pass it on.  This males the network vulnerable to having the signal degrade or attenuate.  Repeaters can be used to ease this problem. If the cable is broken then not everything is connected.  It may also turn out that two little linear bus networks are created that work that end up bouncing signals due to the new not properly terminated ends. Ring Topologies.  If you were to connect each of the devices to a ring shaped cable with no endpoints you would get a ring topology.  This way each device is directly connected to two other devices.  The signal can end up traveling around the entire loop.

 These are usually built out of coax cable.  A token ring network usually uses STP cable developed by IBM. Messages travel in one direction on a ring.  Every device receives signals from its upstream neighbor and sends them to its downstream neighbor.  This topology is considered an active network because each device regenerates the signal before passing it along to the next.  This topology is usually implemented in token rings and rather than this necessarily being the physical topology, it is the logical topology. These ring topologies are usually relatively easy to troubleshoot and simple to set up.  It requires more cable than a linear topology. If a connection breaks then all network communications come to a halt.  It can also be somewhat difficult to add more devices to these networks.  If you do add even a single device you need to temporarily shut it down while you break the chain and add in two new links. Star Bus Topologies.  This is one of the most popular LAN topologies where each device is attached to central hub or switch.

 A star bus network can be active, passive or intelligent.  If it is passive the hub is just a connection point.  With a well configured switch as the focal point the signal can be specifically directed to a device.Generally, the signal is passed from one device to the hub/switch at which point it directed outward towards other devices.  If it is a hub then the signal is broadcast to everything else on the network.  If it is a switch then it can be configured to send the signal to a specific device based on the MAC address. These networks tend to be much more fault tolerant, in that breaks in lines only disconnect single devices.  Unfortunately, break downs at the hub/switch have much more major impact.  It is usually also quite easy to add more computers.  Diagnosing problems can also be fairly easy if the central directing device is capable of providing diagnostic information. These networks are typically more expensive due to cable and the necessity of having a central directing device.  But, in particular, if you use a switch, they are very good and almost essential for increasing efficient use of available bandwidth. Mesh Topologies.  The mesh topology for LANs is really quite infrequent.  It is much more likely to appear for interconnecting routers and or switches.  Almost every device on the network is directly connected to every other device.

 These redundant options for connection make this topology the most fault tolerant.  If one device is down, the signal can take another path.Unfortunately, these sorts of networks can quickly become complicated and adding devices can be quite difficult. Hybrid Topologies.  Obviously, it is often going to be the case that the LAN doesn't fit any of these categories exactly.  For example, it is becoming quite frequent to see some sort of star hierarchies for LANS.