RE: [10GBASE-T] latency
With careful design, it should be possible to pick special sequences for
a few functions such as Pause so that latency in detecting these is less
than that for decoding data. I don't know how many such functions are
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[mailto:email@example.com] On Behalf Of Gavin
Sent: Friday, February 20, 2004 6:56 AM
To: Stoltz, Mario
Cc: Stephen Bates; Booth, Bradley; firstname.lastname@example.org
Subject: RE: [10GBASE-T] latency
I've been working on a NIC-on-a-chip for Gigabit Ethernet, and I have
some experience with the way flow control is used in Ethernet
The PAUSE function is used to ensure that the receive buffers in a NIC
do not overflow. A PAUSE packet is triggered when a station's receive
buffer is filled to a high-water mark. When the other station's MAC
processes the PAUSE packet, it stops transmitting. The high-water mark
level is calibrated so that all data potentially transmitted between the
PAUSE packet being sent and it being processed will not overflow the
The total latency between a PAUSE packet transmit being requested by
station A and the PAUSE packet actually pausing the transmitter in
station B determines how much additional data could be received before
the flow stops. The processing time in the receiver is a part of this
delay, along with the propagation delay, the time to send the PAUSE
frame and potentially two maximum-length frames (one on each of station
A & B) (these could be jumbo frames).
So given this upper bound on the response time, it is possible to set
the watermark level so that PAUSE frames will prevent buffer overflow.
If a standard increases the processing latency in the receiver then the
buffer sizes and watermark level would need to be changed in the
controller/switch, as more data would potentially need to be buffered
between the transmit/receive of a PAUSE packet. I do not believe that
this would create a major problem in the design of the controller.
As you say, since the propagation delay of a 3km fiber link is
substantially greater than for 100m UTP (~30,000 bit times compared to
~1140 bit times), the receive buffer size / space above the watermark
level used in fiber controllers should be substantially larger than for
Gigabit over copper. Jumbo frames also change the amount of buffering /
watermark level needed. I think this indicates that an increase in
receiver processing time for 10GBase-T is viable in terms of the PAUSE
operation. There may be other requirements regarding RDMA etc.
On Fri, 2004-02-20 at 11:47, Stoltz, Mario wrote:
> 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
> 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
> 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,
> -----Original Message-----
> From: email@example.com
> [mailto:firstname.lastname@example.org] On Behalf Of
> Stephen Bates
> Sent: Donnerstag, 19. Februar 2004 18:56
> To: Booth, Bradley; email@example.com
> 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
> If anyone has any questions about this please feel free to contact me.
> 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
> > Any thoughts or comments?
> > Cheers,
> > Brad
> > Bradley Booth
> > Chair, IEEE 802.3 10GBASE-T Study Group firstname.lastname@example.org
> > 512-732-3924 (W)
> > 512-422-6708 (C)