Hubs do not read any of the data passing through them, and they are
not aware of the source or destination of the frame. Essentially, a hub simply receives
incoming bits, ampli?¬?es the electrical signal, and transmits these bits through all its ports
to the other devices connected to the same hub.
A hub extends, but does not terminate, an Ethernet LAN. The bandwidth limitation of a
shared technology remains. Although each device has its own cable that connects to the
hub, all devices of a given Ethernet segment compete for the same amount of bandwidth.
Collisions
Collisions are part of the operation of Ethernet, occurring when two stations attempt to
communicate at the same time. Because all the devices on a Layer 1 Ethernet segment
share the bandwidth, only one device can transmit at a time. Because there is no control
mechanism that states when a device can transmit, collisions can occur as shown in
Figure 2-2.
Hub
100 Meters 100 Meters
142 Chapter 2: Ethernet LANs
Figure 2-2 Ethernet Collision
Collisions are by-products of the CSMA/CD method used by Ethernet. In a sharedbandwidth
Ethernet network, when using hubs, many devices will share the same physical
segment. Despite listening ?¬?rst, before they transmit, to see whether the media is free,
multiple stations might still transmit simultaneously.
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