star Topology and Extended-Star Topology

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star-topologyThe star topology is the most common physical topology in Ethernet LANs. This topic describes both the star and extended-star topologies.

When installed, the star topology resembles spokes in a bicycle wheel. It is made up of a central connection point that is a device, such as a hub, switch, or router, where all the cabling segments actually meet. Each device on the network is connected to the central device with its own cable.

Star Topology
Although a physical star topology costs more to implement than the physical bus topology, the advantages of a star topology make it worth the additional cost. Each device is connected to the central device with its own wire, so that if that cable has a problem, only that one device is affected, and the rest of the network remains operational. This benefit is extremely important and is the reason why almost every newly designed Ethernet LAN has a physical star topology.

Advantages of a Star Topology

* Easy to install and wire.
* No disruptions to the network then connecting or removing devices.
* Easy to detect faults and to remove parts.

Disadvantages of a Star Topology

* Requires more cable length than a linear topology.
* If the hub or concentrator fails, nodes attached are disabled.
* More expensive than linear bus topologies because of the cost of the concentrators.

The protocols used with star configurations are usually Ethernet or LocalTalk. Token Ring uses a similar topology, called the star-wired ring.

Extended-Star Topology
Extended-star-topologyWhen a star network is expanded to include an additional network device that is connected to the main network devices, the topology is referred to as an extended-star topology.
The problem with the pure extended-star topology is that if the central node point fails, large portions of the network can become isolated.

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