4.2.CSMA/CD
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Consider Figure below , which depicts an Ethernet network using a shared bus topology.
Before a device in this network transmits, it listens to the wire to see if there is currently any traffic being transmitted. If no traffic is detected, the networked device transmits its data.
However, what if two devices simultaneously had data to transmit?
If they both listen to the wire at the same time, they could simultaneously, and erroneously, conclude that it is safe to send their data. However, when both devices simultaneously send their data, a collision occurs.
Ethernet was designed with a mechanism to detect collisions and allow the devices whose transmissions collided to retransmit their data at different times.
After the devices notice that a collision occurred, they independently set a random back off timer . Each device waits for this random amount of time to elapse before again attempting to transmit.
Here’s the logic:
Because each device almost certainly picked a different amount of time to back off from transmitting, their transmissions should not collide the next time these devices transmit, as illustrated in Figure below .
