Network topologies are configuration strategies, or how a Local Area Network (LAN) is designed. There are three basic designs.


            node1      node3      node5
              |          |          |
                    |          |
                  node2      node4

In the Bus Topology, each computer station or node is connected to a communication medium, such as a 50ohm coaxial cable. Both ends of the cable are terminated with a 50ohm resistor to absorb signal reflections. Each node has a unique address. When node1 sends information to node5, it adds the address of the node the data is meant for to the data packet. The data is received by all of the nodes. If the data is not meant for their node, the data is ignored.


                        /     \
                   node2       node4
                        \     /

In the Ring Topology, the communication medium is formed into a closed loop. Each node has a unique address. A token packet is passed between nodes. If a node has data to send, it attaches it to the token, and the entire package is sent to the next node. If node1 had data for node3, it would wait for the token to arrive. If the token was free (no other node had data attached to it for another node), node1 would add the data to the token and pass it along to the next node. Node2 would receive the package, examine it to see if the data was meant for it, then send it on it's way to the next node. If node2 had data to send, it would wait for the next token because this one was carrying data for node3. Node3 would receive the package, determine that the data was meant for it, then remove the data from the token. If node3 had data, it could then attach it to the token, else it would send the empty token to the next node. Should a message be sent to a non-existant node, such as node5, when it arrived back at node1 it would remove the data from the token to free it for use. This topology is sometimes called a Token Ring network.


                node2\  |  /node5
                      \ | /
                      /   \
                node3/     \node4

In a Star Topology, the nodes are distributed from a center point. Located at the center is a hub or other device that connects the nodes to each other. A wide range of center hubs can be employed, from simple electrical connections that attach each node cable to one another to an intelligent hub that directs incoming data directly to the destination address, or to another hub/network if one is attached. Star topologies are the most common of the basic arrangements because of their flexability. You can connect one hub to another (as if you removed node4 and replaced it with a hub) to allow more nodes to be connected to each other.

No soy products were harmed in the rescuing of this nodeshell

Interestingly, Bus and Ring topologies usually look like Stars.

Most Token Ring implementations, for example, make the logical ring look a lot like a star. If you unscrewed the Mau and layed out all the wires, it actually still forms a star, but the network cables congregate in a central point to give the advantages of a Star.

But TallRoo, Why?

  • Why is a Bus bad?
    Well, cut it in the middle and its useless. An no it won't become two smaller busses like some kind of worm-cut-in-half. You need terminators on the end to prevent the bus getting clogged with reflected signals.
  • Why is a ring Bad?
    As with a bus, cut it in the middle and it's broken. To add a new computer you have to break the ring to put a new connection into it. (Clever relays in type 1 Token Ring actually solve this, but you get the idea).
  • Why is a star Good?
    It's not a bus, and it's not a ring. No, seriously, it gives the advantage of central configuration. Plus there's no need to run around the building to add another machine into the network. Just run a cable out from the middle.
  • And, finally Why is a star Bad?
    Of course, one hub also means a single point of failure.
Usually, a combination of different topologies is used.

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