Networks in More Detail
The shapes a network is built in
Work out how much cable a bus, star, ring or mesh needs, and what a single break costs each of them.
Once you have more than two machines to join up, you have to decide what shape the cables make, and there are four shapes worth knowing. A bus runs one long cable past every machine, and each machine taps into it. A star gives every machine its own cable to one device in the middle, and everything goes through that device. A ring joins each machine to the two beside it, and the last one back to the first. A mesh gives every machine its own cable to every other machine. That is the whole of the vocabulary; everything else about a shape is something you can work out from the picture.
Count the routes, not the cables
What a break costs is not a fact to learn about each shape — it falls straight out of the picture, and this is the one thing to hold on to. Pick two machines and count the different routes a message could take between them. Where there is only one route, anything on that route is a single point of failure: break it and those two machines lose each other. Where there is more than one route, a break just sends the message the other way and nobody notices. So the question to ask about any shape at all, including one you have never seen before, is how many routes it offers between two machines.
Worked example
Six computers hang off one bus cable. The cable is cut halfway along. What happens?
Before the cut, take any two of the six and look for a route between them. There is exactly one: along the cable.
A bus is the cheapest shape there is, and this is what it is cheap at — it never offers a second route to anybody.
Try it together
Now work out together how many cables a mesh of six computers needs.
Counting cables directly goes wrong almost every time. Count the ends instead.
1.How many cables run from any one of the six computers?
Have a go
Have a go on your own. A star has nine computers around one switch, and one of the outer cables is cut. How many of the nine can still reach the switch?
Ready to practice?
Eight questions on what you have just read. Nothing is timed, and you can play as many times as you like.