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8.2 | |
Collision Domains and Broadcast Domains | |
| 8.2.3 | |
Segmentation |
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The history of how Ethernet handles
collisions and collision domains dates back to research at the
University of Hawaii in 1970. In its attempts to develop a wireless
communication system for the islands of Hawaii, university researchers
developed a protocol called Aloha. The Ethernet protocol is actually
based on the Aloha protocol.
One important skill for a networking
professional is the ability to recognize collision domains.
Layer 2 devices segment or divide
collision domains.
These smaller collision domains will
have fewer hosts and less traffic than the original domain.
Layer 3 devices, like Layer 2 devices, do not forward collisions. Because of this, the use of Layer 3 devices in a network has the effect of breaking up collision domains into smaller domains. Layer 3 devices perform more functions than just breaking up a collision domain. Layer 3 devices and their functions will be covered in more depth in the section on broadcast domains.
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Connecting several computers to a single shared-access medium that has
no other networking devices attached creates a collision domain. This
situation limits the number of computers that can use the medium, also
called a segment. Layer 1 devices extend but do not control
collision domains. 
Controlling frame propagation using the MAC address assigned to every
Ethernet device performs this function. Layer 2 devices, bridges,
and switches, keep track of the MAC addresses and which segment they
are on. By doing this these devices can control the flow of traffic at
the Layer 2 level. This function makes networks more efficient by
allowing data to be transmitted on different segments of the LAN at
the same time without the frames colliding. By using bridges and
switches, the collision domain is effectively broken up into smaller
parts, each becoming its own collision domain. 
The fewer
hosts that exist in a collision domain, the more likely the media will be
available. As long as the traffic between bridged segments is not too
heavy a bridged network works well. Otherwise, the Layer 2 device can actually slow down
communication and become a bottleneck itself. 
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