the difference between "lost carrier" and "no carrier"??

Unanswered Question
Jan 20th, 2009


I wonder that the difference between "lost carrier" and "no carrier" error counter in output of the show int fastethernet.

"no carrier" means

Number of times the carrier was not present during the transmission.

"lost carrier" means

Number of times the carrier was lost during transmission.

what is the difference?

duration interval of the lost carrier or level of the voltage?

I have this problem too.
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Yudong Wu Wed, 01/21/2009 - 12:51

The carrier is an electrical signal that Ethernet devices use to detect whether the wire is currently being used by another transmitting station.

The lost carrier counter increases each time a carrier sense loss occurs. This happens when the hardware is transmitting a frame onto the wire and does not see its own carrier wave on the Ethernet.

The absence of the carrier signal increments the no carrier counter.

Sanghee Han Wed, 01/21/2009 - 18:31

Thanks for your response.


I don't understand "no carrier" a little.

If there is no carrier in media,

is that like to link-down?

what's the difference between no carrier and link down?

sorry for many questions.

marikakis Thu, 01/22/2009 - 02:14


It seems to me that not all devices support detection of both of these error types, so you can meet a variety in definitions if you search those online. As far as I understand, typically, "lost carrier" is when we cannot hear our own message on the medium and "no carrier" is when we hear "nothing".

Those descriptions seem vague if you are prepared to dig deep down the OSI layer stack :-) As far as I understand (after some reading of the IEEE 802.3 standard, which is not an easy for me document to read and is quite big to read all of it), "nothing" is defined based on the actual physical medium.

Typically, the remote transmitter can transmit actual data towards the local receiver or some link integrity pulses. If local receiver sees no data and no pulses (for a specific small period of time), it senses "no carrier" and link goes down ("no carrier" is an event that cases the link to go into a "down state"; this is cause-consequence type of relationship between "no carrier event" and "link down state"). So, "no carrier" is a situation detected by local receiver and probably has to do with either the receive path of the cable or the remote transmitter.

The "lost carrier" is when we do sense "something" coming towards the local receiver, but we cannot see our own data looped back on the medium. This is also detected by the local receiver and probably indicates a problem in the transmit direction of the cable or the loopback circuitry at the remote side.

I am not sure I understand this perfectly and in all cases of media, but it seems to make sense :-)

Kind Regards,


marikakis Thu, 01/22/2009 - 02:45

I post some part of IEEE 802.3 for documentation purposes:

" Link Integrity Test function requirements

In order to protect the network from the consequences of a simplex link segment failure, the MAU shall monitor the RD circuit for RD_input and link test pulse activity. If neither RD_input nor a link test pulse is received for a time “link_loss,” the MAU shall enter the Link Test Fail state and cause the input_idle message to be sent on the DI circuit and the TD_idle message to be sent on the TD circuit (Figure 14-6). The value of “link_loss” shall be between 50 ms and 150 ms. When RD_input or a number “lc_max” of consecutive link test pulses is received on the RD circuit, the MAU shall exit the Link Test Fail state. The value of “lc_max” shall be between 2 and 10 inclusive."

This is not an easy read, but the general idea is simpler than the actual details.


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