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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