In this statement, "switching off"
refers to the frequency modulation applied to the transmitter clock (spread
spectrum clocking) to reduce EMI. Some SATA devices already have SSC enabled
and receiver devices of expanders and initiators that allow attachment
of SATA devices are required in SAS 1.1 to accommodate SSC. On last week's
PHY teleconference, three makers of expanders/initiators indicated that
SSC applied to SAS 1.1 rates would be accepted by their receivers without
issue. So far there has been no case identified where SSC applied to the
signal transmitted by an end device transmitting at SAS 1.1 rates will
cause an issue.
Ralph Weber <roweber@ieee.org> Sent by: owner-t10@t10.org
No Phone Info Available
07/05/2006 08:42 AM
To
t10@t10.org
cc
Subject
Re: SAS-2 Spread spectrum clock
* From the T10 Reflector (t10@t10.org), posted by:
* Ralph Weber <roweber@ieee.org>
*
To be honest, I do not fully follow everything being said in
this thread, but one suggestion sticks out like a sore thumb.
"Why don't we switch off the SSC of the SAS device, if it is
connected to a SAS-1.1 expander?"
Fibre Channel includes features where valid physical connections
are automatically disabled because of protocol interoperability
problems.
These features have generated deep anger in the user community.
They are a black stain in Fibre Channel products for some very
critical customers.
"Switching off" anything that is correctly connected is a very
bad idea. This is not a path to be followed without grave
consideration.
Rob, you will have to twist my are really hard to really hard
to get my vote for this kind of proposal and I suspect you
prefer things that way.
All the best,
.Ralph
Massimo POZZONI wrote:
>* From the T10 Reflector (t10@t10.org), posted by:
>* Massimo POZZONI <massimo.pozzoni@st.com>
>*
>
>The SAS-II device could transmit to a SAS-II expander using +/-2500ppm.
>In this case we have the same SSC for TX and RX and the RX is not
>stressed.
>
>
>
>>SAS-1.1 expanders/HBAs are supposed to support receiving +0/-5000
ppm
>>(I hope they were more compliant than SATA HBAs), but were not
>>expected to tolerate +2500/-2500 ppm. SAS disk drive vendors
would
>>prefer to implement just one behavior regardless of the expander/HBA
>>to which they are attached, so transmitting +0/-5000 ppm is the
>>only safe choice.
>>
>>
>
>Why don't we switch off the SSC of the SAS device, if it is connected
>to a SAS-1.1 expander?
>As you mention, the SAS-1.1 expander is not supposed to receive
>SSC from a SAS device.
>
>
>
>>Disk drive vendors would prefer not to include separate SSC controls
>>per phy. Since there are generally more disk drives than
HBAs or
>>expanders, it makes sense to keep them as simple as possible.
>>
>>
>
>I completely agree on this request. Switching off the SSC in
>case separate SSC controls would be required, can always solve this
>issue.
>
>Massimo
>
>
>-------------------------------------------------------------------
>
>Date: Tue, 4 Jul 2006 15:04:22 -0500
>From: "Elliott, Robert (Server Storage)" <Elliott@hp.com>
Add To Address
>Book
>Subject: RE: SAS-2 Spread spectrum clock -
>To: <t10@t10.org>
>
>
>
>>-----Original Message-----
>>From: owner-t10@t10.org [mailto:owner-t10@t10.org] On Behalf
>>Of Massimo POZZONI
>>Sent: Tuesday, July 04, 2006 2:13 AM
>>To: t10@t10.org
>>Subject: SAS-2 Spread spectrum clock -
>>
>>* From the T10 Reflector (t10@t10.org), posted by:
>>* Massimo POZZONI <massimo.pozzoni@st.com>
>>*
>>The 06-263r2 (SAS-2 Spread spectrum clock) specifies that:
>>
>> - the device trasmitter SSC is 0/-5100 ppm (both SAS and
SAS)
>>
>> On the other hand,
>> - expander transmitter is 0/-5100 ppm if talking
to a SATA device
>> - expander transmitter is + 2500/-2500 ppm if talking to
a
>>SAS device
>>
>> Let's consider the receiver Clock Recovery (CDR) of the
device.
>> Let's assume that the receiver Clock Recovery makes use
of the same
>> clock reference used by its transmitter (to save power and
area
>> in the PHY).
>>
>> As a consequence, this clock reference, used by the receiver
CDR
>> to recover the incoming data, will be SSC modulated as the
>> transmitter.
>>
>> We see now that the receiver CDR is much more stressed than
in SATA.
>> In fact its reference clock can be at -5000 ppm, while the
data
>> it is receiving can be at +2500 ppm. Total 7500 ppm.
>>
>> My concern is for the Clock Recovery. It has been proven
that
>> many problems are already existing in sata devices, where
the
>> clock recovery is stressed up to 5000 ppm.
>>
>>
>
>What problems? I haven't seen any errata to the SATA specification
>trying to reduce the ppm range.
>
>There are many accounts on the Internet about problems arising from
>HBAs not supporting SSC as required. Disk drive vendors that
tried
>turning on SSC have had to provide consumers special programs or
>jumpers to turn SSC off so they will work in non-compliant systems.
>http://www.intel.com/support/motherboards/server/sb/CS-021230.htm
>http://www.xlr8yourmac.com/archives/nov05/111105.html
>http://discussions.apple.com/message.jspa?messageID=2379652
>http://www.barefeats.com/hard49.html
>http://www.seagate.com/support/kb/disc/faq/ssc.html
>http://www.wdc.com/en/library/eide/2579-001037.pdf
>http://www.hitachigst.com/hdd/support/download.htm
>
>
>
>> Now we are proposing to stress it at 7500 ppm !!!.
>>
>> My proposal:
>> If an expander is transmitting to a SATA device: TX @ 0/-5000
>> If an expander is transmitting to a SAS device: TX
@ +2500/-2500
>> (This is not changed)
>>
>> If a SAS device is transmitting to a SAS expander: TX @
+2500/-2500
>> (This is the new !!)
>> If a SATA device is transmitting to a SAS expander: TX @
0/-5000 ppm
>>
>>
>
>SAS-1.1 expanders/HBAs are supposed to support receiving +0/-5000 ppm
>(I hope they were more compliant than SATA HBAs), but were not
>expected to tolerate +2500/-2500 ppm. SAS disk drive vendors
would
>prefer to implement just one behavior regardless of the expander/HBA
>to which they are attached, so transmitting +0/-5000 ppm is the
>only safe choice.
>
>HBAs would like to have as few choices (e.g. clock trees) as possible.
>They must use +0/-0 when attached to a SAS-1.1 phy, and can only
>use +0/-5000 ppm when attached to a SATA phy, so +2500/-2500 ppm
>would require a third clock tree that is not really necessary.
>Nothing should break if they did transmit +2500/-2500 ppm to a
>SAS-2 phy, since all SAS-2 receivers will be required to
>tolerate it.
>
>
>
>> If the SAS expander cannot tolerate the +2500/-2500 from
the
>> SAS device, the SSC is turned off in all the ports of the
device.
>>
>>
>
>Disk drive vendors would prefer not to include separate SSC controls
>per phy. Since there are generally more disk drives than HBAs
or
>expanders, it makes sense to keep them as simple as possible.
>
>
>
>> In summary: is it possible to have the same SSC range for
both the
>> transmitter and the receiver of a SAS device?
>>
>>
>
>I don't think so.
>
>
>
>>Massimo Pozzoni
>>
>>
>
>--
>Rob Elliott, elliott@hp.com
>Hewlett-Packard Industry Standard Server Storage Advanced Technology
>https://ecardfile.com/id/RobElliott
>
>--------------------------------------
>Attachment: smime.p7s (7k bytes) Open
>
>
>------------------------------------------------------------------------
>
>Delete Prev Next Reply/All Forward/Inline
Open Inbox 147 of
>148
>*
>* For T10 Reflector information, send a message with
>* 'info t10' (no quotes) in the message body to majordomo@t10.org
>
>
>
>
>
*
* For T10 Reflector information, send a message with
* 'info t10' (no quotes) in the message body to majordomo@t10.org