Intro

Topology is the geometric configuration of networks both physically and logically. This includes connecting computers and other devices on networks. There are four basic topologies: Bus, Ring, Star, and Mesh.

Sometimes, especially in larger networks, the topology may be a hybrid of the basic topologies.

Bus

DIAGRAM: Bus topology
A bus topology (aka physical bus or linear bus) has each station connected to the main cable or backbone. The backbone is terminated at each end. The backbone is typically thinnet coaxial cable or greater.

Any station can send a packet at any time although only one station at a time can do so. The packet goes to all the other stations down the backbone. The other stations discard the packet unless the packet is addressed to it.

Bus networks are often used for temporary LANs because the set up is so fast and simple. However if the backbone goes down, the entire LAN goes down. If there is a problem at a station, it may be difficult to isolate it.

Ring

DIAGRAM: Ring topology
A ring topology has a physical and logical ring. This is opposed to a hybrid star ring topology which appears to have a physical star but actually has a logical ring in its hub.

Any station can send a packet around the ring but only the station with the single logical token can do so. The token is passed around the ring giving all stations an opportunity to speak. In some instances some stations are given higher priority to use the token.

As a packet passes around the ring, each station acts as a repeater and boosts the packet before passing it on, unless the packet is addressed to that station. This packet regeneration enables very large rings.

This is a very fast and simple network. However if any part of the ring goes down, the entire LAN goes down. If there is a problem at a station, it may be difficult to isolate it.

See also my definition of Token Ring.

Star

DIAGRAM: Star topology
A star topology (aka pinwheel topology) has each station connected to a central hub.

Any station can send a packet at any time although only one station at a time can do so. A packet is sent from a station to the hub. The hub retransmits the packet to the other stations. The other stations discard the packet unless the packet is addressed to it.

This is probably the easiest topology to configure although a star topology requires more cabling than either bus or ring topologies. If a station goes down, it does not bring down the entire LAN. However if the hub goes down, then it brings down the entire LAN.

Mesh

DIAGRAM: Mesh topology
A mesh topology has each station connected to the others.

A mesh topology is more difficult to set up and requires more media than the other topologies so far. However it has great fault tolerance: If a segment of the network is broken, a packet can find an alternate route to its destination.

Hybrid

There are many different ways the basic topologies can be combined. The World Wide Web itself is a giant hybrid topology.

  • A hybrid star bus topology is a bus topology where at least one of the stations is replaced with the hub a star topology network.
  • A hybrid star ring topology looks like a star network except that the hub is wired as a logical ring. Such a hub is much easier to implement than a physical ring topology.
  • A hybrid mesh topology is any hybrid topology where some of the key computers are connected in a mesh fashion. The WWW has its domain name servers as part of a hybrid mesh network.


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