1.3 Variations on the Standard Topologies
All of the topologies looked at have their own advantages and disadvantages and a network designer must choose the topology best for purpose. Although ring, star and bus are the most commonly found topologies they are not the only ones. It is possible to combine common topologies to create a hybrid topology. Common hybrids are Star Bus and Star Ring.
Star Wired Ring
The traditional ring topology is not generally used but rather a hybrid is implemented. This hybrid is physically a star because the computers connect to a hub like device called a Multi Station Access Unit (MSAU), however, it is logically a ring because of the way it operates using a token in a similar way to the traditional ring. You can think of the MSAU as been a ‘ring in a box’ because this is where the ring is implemented.
In token passing networks the fact that each node is involved in passing the token and that the token must return to the sender before release of the token means that the sender has acknowledgement that the data has been received.
Token networks have additional management overhead. One of the nodes acts as the manager of the network and determines the order of passing. If a node is switched off or on then the ring is re-established to take account of the changes
Traditional copper token ring networks are legacy IBM networks. Fibre Distributed Dual Interface (FDDI) is an updated version of token passing that makes use of fibre optic cable and is capable of fast data communication over a few kilometres. FDDI has dual rings, although only one is used for data communication with the other providing a backup should the first ring fail.
This hybrid topology is also referred to as a star ring topology.
As its name suggests this is a combination of a star and a bus topology. There is still a linear bus but attached to this are several star networks. This means that a failure of one computer will not affect the rest of the network because the other nodes can still communicate.
An extended star topology has a main star with additional hubs at the end of each link, which have, further nodes connected in a star topology.
A mesh topology is the most robust topology. Not only does it have the capabilities of a star but each of the nodes has a direct connection to all other nodes. This provides alternative links should there be failure on the network.