Ethernet uses a refinement of ALOHA, known as Carrier Sense Multiple Access (CSMA), which improves performance when there is a higher medium utilisation. When a NIC has data to transmit, the NIC first listens to the cable (using a transceiver) to see if a carrier (signal) is being transmitted by another node. This may be achieved by monitoring whether a current is flowing in the cable (each bit corresponds to 18-20 milliAmps (mA)). The individual bits are sent by encoding them with a 10 (or 100 MHz for Fast Ethernet) clock using Manchester encoding. Data is only sent when no carrier is observed (i.e. no current present) and the physical medium is therefore idle. Any NIC which does not need to transmit, listens to see if other NICs have started to transmit information to it.
I am getting confused about CSMA/CD (half-duplex communication) after reading below sentence.
Data is only sent when no carrier is observed (i.e. no current present) and the physical medium is therefore idle.
Is not pulse (link integrity test pulse, normal pulse, fast pulse) is generated when the medium is idle?
One more thing.
In the output from the show interfaces fastethernet , show interfaces gigabitethernet or show interfaces tengigabitethernet command observe the value of the carrier signal counters. The lost carrier counter shows the number of times that the carrier was lost during transmission. The no carrier counter shows the number of times that the carrier was not present during transmission.
Is carrier lost determined by RX of interface?