Re: HARI Latency
The latency figure that you gave is that of data through fiber. What is the latency
through the gates at each end of the Hari encode and decode/alignment gates? What
would be the latency if the PCS encoding were not 8B10B?
Rich Taborek wrote:
> Sorry about not responding previously to your point. I've included your original
> question below also.
> Relative to the latency of the media, amounting to approximately 5 us per km,
> the added latency of Word-Striping over Byte/Column-Striping can be considered
> to be negligible when applied to a PMD interface.
> I don't agree about its simplicity relative to that of Byte/Column-Striping.
> I don't understand your comment about Word-Striping preserving the 8B10B coding
> used for GigE since the Word-Striping proposal for 10 GbE convolutes the
> translation of information form the MAC through the PCS, whereas
> Byte/Column-Striping preserves the coding quite well. I've previously
> illustrated this as follows:
> D<0:7> wwww...IISddddd/dddddIIISddd/ddddIIISdddd... Legend: I=Idle
> D<8:15> rrrr...IIdddddd/ddddTIIIdddd/ddddIIIddddd... S=SOP
> D<16:23> dddd...IIdddddd/ddddIIIIdddd/ddddIIIddddd... T=EOP
> D<24:31> 3210...IIdddddd/ddddIIIIdddd/dddTIIIddddd... d=data
> Figure 1 - Parallel 10 GMII stream
> Proposed Byte striping for 10 GbE is shown in figure 2 using Howard
> Frazier, Cisco, mapping per
> page 15
> Lane 0 wwww...KRSddddd/dddddKRKSddd/ddddKRKSdddd... Legend: K=Comma/Idle
> Lane 1 rrrr...KRdddddd/ddddTKRKdddd/ddddKRKddddd... R=Idle
> Lane 2 dddd...KRdddddd/ddddRKRKdddd/ddddKRKddddd... S=SOP
> Lane 3 3210...KRdddddd/ddddRKRKdddd/dddTKRKddddd... d=data T=EOP
> Figure 2 - Byte/Column Striping proposal for 10 GbE
> Proposed Word striping for 10 GbE is shown in figure 3 using Mark
> Ritter, IBM, mapping per
> pages 15 and 16
> Lane 0 wrd0...Kidldddd/ddddKRTdKddS/ddddKidlKddd... Legend: K=Comma
> Lane 1 wrd1...Kidldddd/ddddKidlKddd/ddddKidldddd... R=Idle
> Lane 2 wrd2...KddSdddd/ddddKidldddd/ddddKidldddd... S=SOP
> Lane 3 wrd3...Kddddddd/ddddKidldddd/TdddKddSdddd... d=data T=EOP
> Figure 3 - Word-Striping proposal for 10 GbE
> The Byte/Column-Striping proposal code-group stream is essentially identical to
> the Parallel 10 GMII stream, whereas the Word-Striping proposal code-group
> stream changes dimensions from vertical to horizontal in 4-byte increments. Also
> note that for Word-Striping the Ethernet Start-of-Packet does not occur in lane
> 0 and the relative complexity of determining the last byte of a packet.
> In summary, striping, although worse, is not the paramount reason to select
> Byte/Column-Striping over Word-Striping. However, I've identified many others in
> my striping evaluation criteria. In addition, other industry efforts utilizing
> Hari are significantly affected by this latency penalty. The primary purpose of
> Hari was to determine whether Hari can be specified in a protocol independent
> manner. The opinion of that group is that it can be so specified. Hari can also
> be PMD independent. As an architect, I would rather specify Hari in a protocol
> and independent manner for the sake of simplicity, interoperability and
> If InfiniBand has chosen Byte/Column-Striping for latency reasons, among others,
> and Byte/Column-Striping better meets 10 GbE requirements including simple
> mapping... and Byte/Column-Striping can be simply mapped for 10 Gigabit Fibre
> Channel, then I see no advantage to Word-Striping and propose
> Byte/Column-Striping for Hari for all protocols and PMDs.
> Best Regards,
> "DOVE,DANIEL J (HP-Roseville,ex1)" wrote:
> > Hi Rich,
> > Would you respond to my earlier point about HARI byte vs word striping
> > and the negligible latency impact when applied to a PMD interface?
> > I believe that word striping offers simplicity and preserves the 8B10B
> > coding used for GigE at the expense of additional latency, but I also
> > believe that this latency has virtually ZERO impact when applied to the
> > PMD implementations which are going to swamp it.
> > On the other hand, I believe that word striping latency would seriously
> > impact backplane implementations, however, this is not the stated objective
> > for HARI, is it? If not, why should we not go to word striping?
> > Dan Dove
> "DOVE,DANIEL J (HP-Roseville,ex1)" wrote:
> > I agree with Mr. Widmer.
> > The only area that I missed in his discussion involves the additional
> > latency of word-striping versus byte-striping.
> > I can see where in a backplane implementation, latency is a concern.
> > However, in the context of a MAC/PHY interface, the media latency will
> > dominate so heavily in the equation, a word-striped interface's latency
> > would be of no concern. Given that fact, for a MAC/PHY interface, all of
> > the benefits he points out should prevail.
> > Best Regards,
> > Dan Dove
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