[RPRWG] Careful Use of the 802.3 PHY

["Robert D. Love" <rdlove@xxxxxxxxx>]

In order to look at possible stumbling blocks ahead of us at Sponsor Ballot time, I sent a note to Geoff Thompson and to Bob Grow with the following query:

 

Do either of you see any problem with 802.17 using a "standard 802.3 PHY" and supporting frame sizes less than 64 Bytes", especially if the data rates of interest are 1 and 10 Gbits/s?  Would you expect anyone in 802.3 to be upset with 802.17 requiring the PHY to support the small frame sizes?  (I realize that you can't know for sure how everyone will respond to this question.  I am just looking for your gut reaction here.  Thanks.) 

As I look ahead at the Sponsor level ballot, I want to steer around stumbling blocks and want to get an early peek at whether there is any reasonable standards conformance based reason to want to avoid requiring 802.17 PHYs from supporting these small frame sizes.  I expect that we will have a fair number of 802.3 voters in our sponsor ballot pool.

 

As it turns out, both Bob and Geoff have strong concerns with what we are doing. 

 

Geoff's has kindly given me permission to post his reply which was:

"A "standard" 802.3 PHY can not be used unaltered if it has Auto-Negotiation.

                (i.e. you can't just buy Ethernet chips and use them in 802.17 applications)

        We haven't even looked at the ramifications of having radically different path lengths between transmit and receive

        You lose a great deal of your resiliency if you use 1000BASE-T  links in an RPR ring.

Basically, I have grave reservations about using Ethernet chips in an application where you are throwing out a significant number of the basic design assumptions that were in hand when the PHYs were designed. These assumptions are not necessarily documented in any other fashion than that it was an Ethernet chip being designed. That, in and of itself, sets a large number of conditions. I don't think you get to use an Ethernet chip without examining each and every departure from Ethernet assumptions on an engineering basis at the detail level.

Do I know off the top of my head what ALL of the considerations are? No way! I haven't ever had the occasion to look at the appropriate level of depth that is required to develop product. I'm probably not even the right guy to do it as I am not a PHY designer at this point in my life. Can I think of a few off the top of my head? Yup, see above."

 

Bob Grow's reservations were even stronger.  There are two conclusions we should draw from Geoff's remarks.

(1) We should be studying very carefully, not only the requirements that we get from reading the specifications for the PHYs that we pick up, but the assumptions that went into the development of those PHYs

(2) We need to do comprehensive analysis to justify any change to the assumptions used in developing those PHYs.

 

The second conclusion implies that we need to do a great deal of work before assuming that the PHYs can be used for small frame size (less than 64 Bytes).  We may also want to ask the question as to whether the use of short frames is worth the risk of breaking Ethernet PHYs, especially is we are unable to do a comprehensive review of the Ethernet PHY requirements.  We should also look at the implications of large frame size, even though large frame sizes are used in proprietary products today. 

 

Note that problems uncovered during Sponsor Ballot can completely derail the standard and the schedule.  We don't want to be making any significant changes to the draft at sponsor ballot time.  Certainly changing the length of control frames would be highly significant.  We have work to do.  Let's decide how to best address it early enough to stay within our present schedule.

 

 

Best regards,

 

Robert D. Love
President, Resilient Packet Ring Alliance
President, LAN Connect Consultants
7105 Leveret Circle     Raleigh, NC 27615
Phone: 919 848-6773       Mobile: 919 810-7816
email: rdlove@xxxxxxxx          Fax: 208 978-1187