# RE: Minimum IPG w/ WIS ?

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Julio,

First off, the minimum IPG does not occur in the WIS case. IPGs at the
receiving RS will tend to be much longer than the minimum because the
receiving PCS will put the data rate back up to 10 Gb/s reinserting the
bytes that the transmitting PCS stripped out (plus or minus a little because
of clock compensation).

So IFS stretch isn't particularly relevant to your question. However, I will
point out that I do not get same results as you for IPG sizes with WIS.
The calculation for IFS stretch is:

ifsStretchSize := (ifsStretchCount + headerSize + frameSize +
interFrameSpacing) div ifsStretchRatio

which is for a min frame the integer part of (64 + 512 + 96 plus 0 to 104 in
ifsStretchCount) divided by 104 which means IFS stretch for a min size frame
is 6 to 7

For a max frame, 512 becomes 1522 * 8 for a stretch of 118 to 119 octets.

These are the IPG sizes going into the RS. Coming out of the RS some IPGs
may be shrunk or enlarged by up to 3 octets to align the Start to lane 0.

These numbers shouldn't be surprising because the payload capacity of WIS is
about 8% less than 10 Gb/s.

So, what does cause the minimum IPG. When attached to a LAN Phy, the MAC
puts out a minimum IPG of 96 BT = 12 octet times.

The RS may delay the start of a packet up to 3 octet times to align it to
lane 0 and it may not need to delay the start of the next packet at all.
Therefore, the RS can shrink any individual IPG by up to 3 characters. Note
that this doesn't change the average of IPG lengths. Those 3 characters will
eventually show up added to IPGs when the RS again delays the start of
packets. It is a jitter in packet start rather than a loss of overall idle
time. Never the less,
minimum IPG out of an RS is 9 characters.

If a sublayer doing clock compensation has just reached the FIFO level where
it wants to delete an idle column, then it will delete it when it gets a
chance. That might be just as that 9 character IPG arrives and it will
delete 4 characters from it leaving a 5 character IPG. As you point out, it
won't need to delete again for quite a few columns. (However, you should
find that it may worst case to delete one in every 50,000 columns rather
than one in 100,000 because the incoming clock worst case is +100 PPM and
the outgoing clock is -100 PPM for a difference of 200 PPM.)

Of course, each XGXS and each PCS in the link may be doing clock
compensation. That is unlikely, but each of these sublayers is allowed to.
So there can be up to 6 sublayers doing clock compensation in a path. It is
possible for each of them to be running slightly slower than the one before
it. Then on a very improbable moment, all their FIFOs could reach the point
where they want to delete idle at the same time. If packets are being sent
with minimum IPG, they won't all get to delete in the first IPG. They will
shorten IPGs to the minimum until each of them has had a chance to delete a
column. Over times, all of the deletions will average to one out of every
50,000 columns or less, but in the worst case, 6 of those deletions could be
clumped close together.

I hope this clarifies the process for you.

Regards,
Pat

-----Original Message-----
From: Julio.Hernandez@xxxxxx [mailto:Julio.Hernandez@xxxxxx]
Sent: Tuesday, February 20, 2001 4:25 PM
To: pat_thaler@xxxxxxxxxxx; stds-802-3-hssg@xxxxxxxx
Subject: RE: Minimum IPG w/ WIS ?

Pat,

I have been tracking this IPG thread, because I too am trying
to understand what is intended for minimum IPG (actually IFG -
Inter Frame Gap, no ?) in the 10Gb/s WIS application, and I
would like to pose the following observation for clarification
by those who understand it better.

If I read the algorithm correctly in section 4.2.8, specifically
page 32, lines 16-22, it looks like the MAC is taking the minimum
IPG/IFG called out in the table in section 4.4.2 (page 41, lines
15-16), into account when counting bits to determine its "stretch
size" (bytes/octets that will be added in the IFG/IPG to adapt for
the WIS rate). As well as, the preamble and SFD bytes (header size).

So, if I understand it all correctly, that should mean that in a
PCS+WIS application, the MAC generated data stream into the XGMII
inputs of the PCS will contain:
1. 12 bytes/octets minimum of IPG/IFG for a minimum size
packet/frame per frame, and
2. 26 bytes/octets minimum of IPG/IFG for a maximum size
packet/frame per frame
In both cases the "interFrameGap" AKA "interFrameSpacing" as
defined in the present version of D2.1, is maintained.

My confusion comes in with the note on page 41, lines 42-45,
in section 4.4.2, as you were discussing earlier;
This notes says that this "interFrameGap shrinkage" to 5
bytes/octets is;
"... as measured at the XGMII receive signals at the DTE."

Keeping in mind that +/-100ppm is only one 4-byte column to
be deleted/added every 10,000 4-byte columns, worst case.
I guess I am not clear on how this would ever bring the IFG/IPG
down to 4-5 bytes (1 column) of IPG/IFG on a frame-by-frame
basis ?

Your clarification on this matter would be sincerely
appreciated. (-:

Thanks, & Best Regards,
Julio
==================================================

pat_thaler@xxxxxxxxxxx on 02/20/2001 01:02:24 PM
From: pat_thaler@xxxxxxxxxxx on 02/20/2001 01:02:24 PM
To:   sanjeev@xxxxxxxxx, pat_thaler@xxxxxxxxxxx, pat_thaler@xxxxxxxxxxx,
stds-802-3-hssg@xxxxxxxx, yariv@xxxxxxxxxxxxx
cc:
Subject:  RE: RS minimum IPG

Sanjeev,

I can't quite parse what you've said. In any case, Shimon has pointed
out
where it is specified in 4.4.2. Note that the numbers in 4.4.2 are

10 Mb/s   47 BT
100 Mb/s  not specified
1 Gb/s    64 BT
10 Gb/s   40 BT (= 5 Phy characters)

As you can see, consistancy between speeds was not particularly a goal.

Pat

-----Original Message-----
From: Sanjeev Mahalawat [mailto:sanjeev@xxxxxxxxx]
Sent: Tuesday, February 20, 2001 12:45 PM
To: pat_thaler@xxxxxxxxxxx; pat_thaler@xxxxxxxxxxx; sanjeev@xxxxxxxxx;
stds-802-3-hssg@xxxxxxxx; yariv@xxxxxxxxxxxxx
Subject: RE: RS minimum IPG

At 01:10 PM 02/20/2001 -0700, pat_thaler@xxxxxxxxxxx wrote:
>I dashed off my response too quickly. I meant to say "I don't know of
>anyplace a minimum receive IPG of 4 is stated for 10 Gb/s."
>
>Pat

The original question was why everybody is talking about
shrinking the IPG to 4 bytes. The receive IPG is never shrinked
to 4 bytes and and it is never less than 5 bytes at RS receive
theoritically,
for explained or unexplaned reasons, it is an issue that
specify the preamble and the min IPG that
are "consistence" with the slower speeds those are 4 bytes
min IPG and SFD at RS. I am not sure of you chasing the the place.

Thanks
-Sanjeev

>
>-----Original Message-----
>From: pat_thaler@xxxxxxxxxxx [mailto:pat_thaler@xxxxxxxxxxx]
>Sent: Tuesday, February 20, 2001 9:48 AM
>To: sanjeev@xxxxxxxxx; stds-802-3-hssg@xxxxxxxx; yariv@xxxxxxxxxxxxx
>Subject: RE: RS minimum IPG
>
>
>
>Sanjeev,
>
>Where do you find such a definition in the standard? I don't know of
>anyplace a minimum receive IPG of 5 is stated. Further, that is a
parameter
>that has varied based on speed so what it was at 100/1000 Mb/s does not
>limit our choice at 10 Gig.
>
>Regards,
>Pat
>
>-----Original Message-----
>From: Sanjeev Mahalawat [mailto:sanjeev@xxxxxxxxx]
>Sent: Monday, February 19, 2001 11:25 AM
>To: stds-802-3-hssg@xxxxxxxx; yariv@xxxxxxxxxxxxx
>Subject: Re: RS minimum IPG
>
>
>
>The min IPG varies from 9 bytes to 15 bytes
>based on the packet size and due to clock compensation
>the PHY may delete a column that will lead to min
>IPG of 5 Bytes. So, thoeritically it should not be
>less than 5 bytes but the spec. always defines it
>to be 4 bytes as this was in the 100/1000 mbps
>specs.
>
>Thanks
>-Sanjeev
>

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