[vortex] Sorry to trouble you, I wonder if you could help me.
Thu Apr 18 08:44:01 2002
On Thu, 18 Apr 2002, BIWU.XIE wrote:
> Exactly how to do remote loopback control is an issue. somebody is
> trying to decide. In general, a signal is sent to the other end of the
> link. That signal may be carried within the preamble bits or within an
> Ethernet control frame. The signal is intepreted by the far end as an
> indication to go into loopback mode, where either bits or frames are
> received and then echoed out the interface.
Err, in the previous message Donald said that this is a hardware device
which just links Rx to Tx (I've used one of those some years ago, just
didn't know how it's called). Now you say (if I understand your statement)
that this should be a feature _on the card_ which can be controlled
remotely. So I don't understand anymore...
Anyway, there is nothing in the documentation about what you are asking. I
think that this would be a _very big_ design mistake as that would allow
anybody on the local network to just stop the networking on any
remote computer by putting it in loopback mode. Plus, I don't understand
what would be the purpose: you'd be able to test cabling and maybe some
intermediate device (hub/switch) ? This can probably be better done in
software, by receiving a packet and putting it as early as possible back
into the Tx queue; however, that means that what is going over the wire
should be a valid packet.
> How to decide the MAC loopback test and the PHY loopback test success or
There are some diagnostic bits which can give at least partly information
about the health status of the card, check the documentation. In the
AutoSelect sequence there is even a description for a check for media
where the presence of the link cannot be determined easily (like a link
beat) and where a valid packet has to be sent and some answer received;
you could use it for loopback too.
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