4.3.Collision Domain
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Devices on a shared Ethernet segment are said to belong to the same collision domain .
One example of a shared Ethernet segment is a 10BASE5 or 10BASE2 network with multiple devices attaching to the same cable.
On that cable, only one device can transmit at any one time. Therefore, all devices attached to the thicknet or thinnet cable are in the same collision domain.
Similarly, devices connected to an Ethernet hub are, as shown in Figure below , in the same collision domain.
A hub is considered to be a Layer 1 device and does not make forwarding decisions.
Instead, a hub takes bits in on one port and sends them out all the other hub ports except the one on which the bits were received.
When multiple devices are connected to the same shared Ethernet segment such as a Layer 1 hub, CSMA/CD must be enabled. As a result, the network must operate in half-duplex mode, which means that only a single networked device can transmit or receive at any one time.
Ethernet switches, dramatically increase the scalability of Ethernet networks by creating multiple collision domains.
In fact, every port on an Ethernet switch is in its own collision domain.
Because a switch port is connecting to a single device, there is no chance of having a collision. With no chance of collision, collision detection is no longer needed. With collision detection disabled, network devices can operate in full-duplex mode rather than halfduplex mode.
