[ofa-general] DDR and SDR

[Todd Rimmer]

> From: Koen Segers
> Sent: Monday, May 07, 2007 12:49 PM
> To: openib-general at openib.org
> Subject: [ofa-general] DDR and SDR
> 
> A simple question:
> 
> Is it possible to connect a SDR HCA to a DDR switch?

Yes, at the time of Link Layer training, the link speed and width are
negotiated down to the highest common speed/width.  Hence when an SDR
HCA is connected to a DDR switch, the HCA's link and the corresponding
switch port will run at SDR speeds.

> If so, what happens with the data that is send from a DDR HCA to the
SDR
> HCA?
In IB every Path Record, Multicast group and Address Vector has a
"static rate".  This represents the speed of the path between 2 nodes.
When a DDR HCA sends to an SDR HCA, it should have obtained a static
rate from the SA showing a 10Gb/s rate (for a 4x SDR path).  In which
case, the DDR HCA will pace its sending to not exceed SDR speeds.

Todd Rimmer
Chief Architect 
QLogic System Interconnect Group
Voice: 610-233-4852     Fax: 610-233-4777
Todd.Rimmer at QLogic.com  www.QLogic.com