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Re: WIS Loopbacks


Please excuse me for jumping in this discussion. I did not participate
in the WIS claue review due to my presence being required in other
clause meeting in Tampa. I'd like to offer my view on the loopback issue
since it's fairly generic to all Ethernet PHY types. My comments are
interspersed below.

Tim Warland wrote:
> With respect to the proposed loopback mode for the WIS, it
> is currently defined that the WIS shall loop data received from
> the PCS layer through the WIS transmitter back to the WIS
> receiver to the PCS.  The question in today's WG was what
> to send over to the PMD.
> To clarify my suggestion, the PCS data received by the WIS would
> by forwarded to the PMD using the (one and only) WIS transmitter
> block.  However, some signalling would have to be imposed to
> ensure the far end was aware of a local loopback condition (likely
> the reason for the loopback and it is also probable that there would
> not be a far end in this condition).  I suggested the WIS could
> set an AIS condition in the SONET overhead which would
> signal the far end to ignore the data in the SPE. It may however be
> a requirement for the PCS layer or higher which called for the
> loopback to signal the far end for this condition.

A PHY at any level in loopback is basically talking to itself. It's
difficult to carry on multiple simultaneous conversations as would be
the case if a PHY had to talk to itself and its link partner
simultaneouly. Clearly,a PHY in loopback can only listen to itself while
its talking to itself. Therefore, while in loopback, the local device
will summarily ignore any traffic its link partner. The recommended
solution is to disable the optical transmitter whenever the associated
PHY is in any loopback mode. 

If the appropriate management capability exists, it is generally
valuable for a local device to notify its link partner of any alternate
modes of operation (e.g. loopback), configuration changes (e.g. going
offline). It is recommended that such notification should be performed
in advance. I believe that such notification is outside the scope of
P802.3ae and is within the scope of protocols such as SNMP. This
notification, if performed, may be specified to se the AIS condition as
you suggest as part of the protocol. 

> This is in contrast to the suggestion that the (one and only)
> transmitter
> block be configured to send all ones or all zeros to the PMA for
> it to figure out during a loopback condition.
> In addition (to throw it on the reflector) as far as I know, there is
> no test pattern available to test the WIS in osolation.  This nullifies
> the requirement to develop a WIS test pattern.

Any Loopback test patterns are generally well outside the scope of the
> Tim Warland
> Nortel Networks.


Best Regards,
Richard Taborek Sr.                 Phone: 408-845-6102       
Chief Technology Officer             Cell: 408-832-3957
nSerial Corporation                   Fax: 408-845-6114
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Santa Clara, CA 95054