Tuesday, 14 November 2017

Network Topologies

A topology defines both the physical and logical structure of a network. 

Topologies come in a variety of configurations, including:

• Bus

• Star

• Ring

• Full or partial mesh

Ethernet supports two topology types – bus and star.

Ethernet Bus Topology

In a bus topology, all hosts share a single physical segment (the bus or the backbone) to communicate:

A frame sent by one host is received by all other hosts on the bus. However, a host will only process a frame if it matches the destination hardware address in the data-link header.

Bus topologies are inexpensive to implement, but are almost entirely deprecated in Ethernet. There are several disadvantages to the bus topology: 

• Both ends of the bus must be terminated, otherwise a signal will reflect back and cause interference, severely degrading performance.

• Adding or removing hosts to the bus can be difficult.

• The bus represents a single point of failure - a break in the bus will affect all hosts on the segment. Such faults are often very difficult to troubleshoot.
Ethernet Star Topology
In a star topology, each host has an individual point-to-point connection to a centralized hub or switch:
A hub provides no intelligent forwarding whatsoever, and will always forward every frame out every port, excluding the port originating the frame.
As with a bus topology, a host will only process a frame if it matches the destination hardware address in the data-link header. Otherwise, it will discard the frame.

A switch builds a hardware address table, allowing it to make intelligent forwarding decisions based on frame (data-link) headers. A frame can then be forwarded out only the appropriate destination port, instead of all ports.

Hubs and switches are covered in great detail in another guide.

Adding or removing hosts is very simple in a star topology. Also, a break in a cable will affect only that one host, and not the entire network.

There are two disadvantages to the star topology:

• The hub or switch represents a single point of failure.

• Equipment and cabling costs are generally higher than in a bus topology.

However, the star is still the dominant topology in modern Ethernet networks, due to its flexibility and scalability. Both twisted-pair and fiber cabling can be used in a star topology.

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