Topology and Network Principles
A good place to start understanding the topology or layout of networks is to ask the question why do we network computers? The main reasons are to

  • share data or information or software programs
  • share physical resources such as printers, modems, hard disk drives
  • backup or save important data for safe keeping
How we achieve this falls into some basic styles. Computers need to talk to each other and how they do this is either in an uncontrolled manner or a controlled manner. Uncontrolled means any computer will talk when they want to and hopefully the other computers are not talking so the information gets through. But this raises problems when other computers are talking and the message gets mixed up with all the other computer chatter. If this approach is taken then there needs to be some rules to dictate what to do when more than one computer is talking. Ethernet takes on this approach and uses rules in the form of the CSMA/CD protocol. (the rules) This is shown above as the "Bus Topology". All computers share a common pathway, a coax cable, and all may talk at any time under the CSMA/CD protocol. (the rules)

Other network protocols rely on having a unique physical pathway to talk on so no one else can interfere with the information transfer. In the star topology above the computers decide who they want to talk to then a switch is used to connect them. No one else interferes with the communication thus a dedicated pathway is established for each conversation between each computer. With a lot of computers this centralised switch can get pretty big and complicated. If the switch fails all computers are cut off. The central switch may even take on a more sophisticated role by controlling the computers hanging off it.

Another approach is the ring topology where all computers pass information to each other in a circulating manner. When one computer wants to talk to another one the message is sent to the nearest computer that in turn passes the information on to the next computer and so on. Eventually the computer who is destined for the information receives the data. This process also needs rules and this is called "Token Ring".

The last topology is to have one big computer acting as a dictator. The computer at the top of the tree is in total control and no other computer may talk until the dictator says you can talk. So the dictator or main computer will ask each computer if they have something to say. (polling) If a computer has something to say the information is processed through the main computer. Banks use this approach to do financial transactions through their central main frame computers.

There are many different approaches but at the end of the day you need some very clear principles in your rules to make it all work such as :
  • Who is in control
  • Who takes control if the main controller dies
  • When may you speak
  • How long may you talk for before others become impatient
  • Do you have permission to talk
  • How will we be connected, copper, fiber, radio
  • Will we connect in a switched, bus, ring environment
  • How do we talk outside our group of computers to other groups
  • How do we talk to the Internet
All these broader parameters and many others are defined by quite a few different international standards such as the OSI or Open Systems Integration protocol. (more rules). What I have been discussing is a broad overview of the many different ways that computers may speak with each other. Of course you appreciate when I say talk or speak I mean transfer data. When data is transferred it is sent in blocks or packets. This means that most circuits are asynchronous. The data is sent as and only when needed. Another name for a block of data is a frame. Within these blocks, packets or frames are built in smaller blocks that contain the rules and other information such as security, clocking, addresses of the sender and the receiver, error detection and correction and so on.

When you are trying to come to grips with all of these different protocols you need to appreciate that the topology's I have discussed, tree, star switched, ring and bus are ways that the computers talk to each other. I have described how they talk and this is called the LOGICAL connection. But as cablers we are more interested in the PHYSICAL connection or how the computers are actually cabled together. The physical cabling of almost all logical connections is done by physically star cabling the computers to a central device, typically a HUB or SWITCH.

It is probably timely to introduce you to the terms software and hardware. Software is the basic code or script that makes computer programs operate. An example of software would be the basic operating systems all computers use. One major manufacturer of this type of software is the Microsoft Corporation. You have heard of WINDOWS 95, WINDOWS 98, WINDOWS 2000, ME, NT4, XP and so on. These are all operating systems and are software programs.

Hardware on the other hand is the physical components already discussed in the "Basic Principles" such as the PC itself, the NIC, the cable, the modem and so on. I wish to explain a little more about some of the other devices you will encounter while cabling. You will come across the term LAN, WAN, MAN, HUB, Repeater, Regenerator, Bridge, Switch, Router and Gateway. You don't need to know every little detail about them or how to program some of them. But they do fit into and connect to our cabling systems so when you strike one it helps to know broadly what they are and what they do.