Re: HARI Latency
Dan,
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
http://grouper.ieee.org/groups/802/3/10G_study/public/nov99/frazier_1_1199.pdf
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
http://grouper.ieee.org/groups/802/3/10G_study/public/nov99/ritter_1_1199.pdf
pages 15 and 16
Lane 0 wrd0...Kidldddd/ddddKRTdKddS/ddddKidlKddd... Legend: K=Comma
Kidl=Idle
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
economics.
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,
Rich
--
"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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