[vortex] Sorry to trouble you, I wonder if you could help me.

Bogdan Costescu bogdan.costescu@iwr.uni-heidelberg.de
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
> failed?

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.

Bogdan Costescu

IWR - Interdisziplinaeres Zentrum fuer Wissenschaftliches Rechnen
Universitaet Heidelberg, INF 368, D-69120 Heidelberg, GERMANY
Telephone: +49 6221 54 8869, Telefax: +49 6221 54 8868
E-mail: Bogdan.Costescu@IWR.Uni-Heidelberg.De