Ethernet network is deployed in two classic topologies, bus and star. The physical topology defines how a node (which could be any connected device, such as a computer, printer, or server) is linked to the network.
A bus topology comprises of nodes linked together by a single long cable. Each node taps into the bus and directly communicates with all other nodes on the bus. The major advantage of this topology is its easy expansion, by adding extra taps, and the lack of a central point of failure. The major disadvantage is that any break in the cable will cause all nodes on the cable to lose their network connection. A star topology links exactly two nodes together on the network. A device called a hub is used as a collection point, where many of the connections come together. The major advantage is any single break only disables one host, and the major disadvantage is the added cost of a hub. In practice, the reliability of most hubs is very high and they are much less likely to fail than a heavily used strand of coaxial cable. Hence, the star topology gained favor with users seeking a highly dependable LAN.
As well as having a physical topology, Ethernets also have a logical topology that is inferred from the way signals flow over the set of media segments that make up the whole system.
Multiple Ethernet segments can be linked together to form a larger network using a signal amplifying and retiming device called a repeater. Through the use of repeaters, multiple segments can grow to form a nonrooted branching tree. This means that each media segment is an individual branch of the complete system. Even though the media segments may be physically connected in a star pattern, with multiple segments attached to a repeater, the logical topology is still that of a single Ethernet channel that carries signals to all stations.
A tree is a formal name for systems like this, and a typical network design actually ends up looking more like a complex concatenation of network segments of both topologies. On media segments that support multiple connections, such as coaxial Ethernet, you may install a repeater and a link to another segment at any point on the segment. Other types of segments known as link segments can only have one connection at each end.
A system of linked segments that may grow in any direction, and does not have a specific root segment, is termed nonrooted. Most importantly, segments must never be connected in a loop. Every segment in the system must have two ends, since the Ethernet system will not operate correctly in the presence of looped paths.
If we have several media segments linked with repeaters and connecting to stations, a signal sent from any one station travels over that station’s segment and is repeated onto all other segments. This way, it is heard by all of the other stations over the single Ethernet channel.
The physical topology may include both bus cables and a star cable layout. In Figure 2.4, we have three segments connected to a single repeater, laid out in the star physical topology.
The point is that no matter how the media segments are physically connected together, there is one signal channel delivering frames over those segments to all stations on a given Ethernet system.
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