[10GBASE-T] latency
Hi Stephen/all,
I am currently reading this book:
The switch book : The complete guide to LAN switching technology / Rich
Seifert, John Wiley & Sons , [2000]
It makes some comments on the use of the PAUSE function in pp 640-641 and
338-339 I reproduce a few lines below:
'In most environments, it is both common and reasonable practice to
disable the function (PAUSE function)'
So if the author is right it seems the PAUSE function may no longer be
critical.
It also discusses how latency affects buffer requirements in the switches
issuing a PAUSE.....
Regards,
Pedro
>
> Hi Stephen,
>
> The latency requirements in the standard are based on clause 44.3
> (which refers to clause 31 and annex 31B). Underlying reason for
> specifying delay budgets is "Predictable operation of the MAC Control
> PAUSE
> operation" as the standard puts it.
>
> In the 802.3ae days, there was a minor discussion in 2001 around the
> latency budgets summed up in table 44.2 "Round-trip delay constraints".
> Back then, I commented against the latency numbers of draft 3.0 (which
> are now in the standard).
> My argument back then was based on two points: a) the fact that the
> individual delay numbers in table 44.2 seemed to be built assuming
> different semiconductor technologies, and b) the fact that cabling
> delay is several orders of magnitude above sublayer delay anyway if we
> look at the distance objectives of the optical PHYs. For the sake of
> economic feasibility, I proposed relaxing the numbers, but without
> success.
>
> The current situation seems as if the delay budget threatens to inhibit
> technically attractive solutions. What we are probably missing (today
> as well as back then) is some data on the MAC control PAUSE operation
> and how it is really used in the field. That could tell us how
> reasonable it may be to add some slack to the current numbers.
>
> Some data, anyone?
>
> Best regards,
> Mario.
>
> -----Original Message-----
> From: owner-stds-802-3-10gbt@majordomo.ieee.org
> [mailto:owner-stds-802-3-10gbt@majordomo.ieee.org] On Behalf Of Stephen
> Bates
> Sent: Donnerstag, 19. Februar 2004 18:56
> To: Booth, Bradley; stds-802-3-10gbt@ieee.org
> Subject: Re: [10GBASE-T] latency
>
>
>
> Hi Brad and the 10GBASE-T Group
>
> I used to work for Massana (now part of Agere) but am now an Assistant
> Prof at the University of Alberta. I've been talking to some of you
> about this latency issue as I think it has a huge bearing on the
> viability of 10GBASE-T.
>
> I did some work based on the presentation of Scott Powell and others
> that tried to estimate the power consumption of 10GBASE-T components.
> Based on present performance criteria and ADCs featuring in ISSCC this
> year I concur with his results which show that they are, by far, the
> dominant power drain. For this and other reasons I am coming to the
> conclusion that the trade off between the SNR target at the decoder
> input and coding gain is not appropriate at present (I assuming we are
> using the 1000BASE-T code).
>
> Part of my research is involved with coding and decoding in high-speed
> systems with ISI. One area of application is obviously 10GBASE-T. I
> know Sailesh presented some work on LDPC codes. Another coding option
> people have mentioned is a concatenated code. Both of these require
> that the latency budget in 10G be relaxed. In the first case because
> LDPC
> requires an iterative decoder and the second since we must interleave
> between the two codes.
>
> I have heard the figure of 1us being the limit for MAC to MAC latency
> in 10G though I've not heard any justification or reasons for this.
> Even assuming we can 50% of this in the decoder we still only have
> about 400-500 baud periods (and hence clock cycles) to play with. This
> is a very small figure for both the options above.
>
> I think getting a better idea of what the upper bound on latency needs
> to be is very important and I would be interested in hearing people's
> opinion on the coding options for 10GBASE-T. I hope to make another of
> the study group meetings as soon as my teaching commitments are
> concluded.
>
> If anyone has any questions about this please feel free to contact me.
>
>
> Regards
>
>
> Stephen
>
> On Wed, 2004-02-18 at 12:12, Booth, Bradley wrote:
>> I remember Sailesh mentioning that if we are willing to make
>> trade-offs against latency, that we can make use of significantly more
>> powerful techniques to reduce the complexity. I know people have been
>> looking at this as a possible issue. What is an acceptable latency
>> trade-off? Is the current latency requirement for 1000BASE-T creating
>> problems for it in latency sensitive applications?
>>
>> Any thoughts or comments?
>>
>> Cheers,
>> Brad
>>
>> Bradley Booth
>> Chair, IEEE 802.3 10GBASE-T Study Group
>> bbooth@ieee.org
>> 512-732-3924 (W)
>> 512-422-6708 (C)
> --
> ------------------------------------------------------------------------
> --
>
> Dr. Stephen Bates
>
> Dept. of Electrical and Computer Engineering Phone: +1 780 492
> 2691 The University of Alberta Fax: +1 780
> 492 1811 Edmonton
> www.ece.ualberta.ca/~sbates Canada, T6G 2V4
> stephen.bates@ece.ualberta.ca
>
> ------------------------------------------------------------------------
> --