[ibis-macro] Re: The PAM4 BIRD 178 is not clear if PAM4_Mapping is Tx only, or should it be allowed for Rx ...

  • From: Mike LaBonte <mlabonte@xxxxxxxxxx>
  • To: <bob.miller@xxxxxxxxxxxxx>, <bob@xxxxxxxxxxxxxxxxx>
  • Date: Wed, 15 Jul 2015 09:24:25 -0400 (EDT)

The RX model itself may have no need for PAM4_Mapping in the same way the TX
may have no need for it. They have only analog interfaces; the EDA tools
deal with the digital interfaces. The PAM4 BIRD175.3 has these notes:



Other Notes: There are two reasons why a mapping is required:

1. The EDA tool needs to convert a symbol error rate into a bit error
rate. For PAM4, each symbol carries two bits of information. So when an
incorrect symbol is received, there can be either one or two bit errors
involved. The EDA tool needs to know how many bits were received in error to
accurately calculate a BER.

2. SerDes designers may choose other mappings for reasons of their own.
The choice of a mapping may affect the bit error rate, but, for example,
might produce error patterns that fall more often into the correctable space
of a particular choice of Forward Error Correction (FEC) code. The mapping
enables SerDes designers to communicate these choices, and for system
developers to evaluate these choices.



Neither reason indicates that an AMI model needs to be treated differently
based on a certain PAM4_Mapping, but the second one hints that a link might
work as designed only with a certain PAM4_Mapping. Does FEC for example
imply some distinction between Tx and Rx?



In any case, even if PAM4_Mapping were more likely to be associated with a
Tx model, maybe it should not be associated as such in directionality
checking tables unless IBIS explicitly associates PAM4_Mapping with TX only.



Mike



From: ibis-macro-bounce@xxxxxxxxxxxxx
[mailto:ibis-macro-bounce@xxxxxxxxxxxxx] On Behalf Of Bob Miller
Sent: Tuesday, July 14, 2015 6:56 PM
To: bob@xxxxxxxxxxxxxxxxx
Cc: Mike Steinberger; <fangyi_rao@xxxxxxxxxxxx>; IBIS-ATM; Walter Katz;
Arpad_Muranyi@xxxxxxxxxx; Mike Steinberger; michael.mirmak@xxxxxxxxx
Subject: [ibis-macro] Re: The PAM4 BIRD 178 is not clear if PAM4_Mapping is
Tx only, or should it be allowed for Rx ...



[quick addendum]: In case it wasn't clear, my current stance on this is "I
don't think I care". Having said that, I generally fall back on
standardizing the most general option that doesn't just add useless clutter.

Bob



On Tue, Jul 14, 2015 at 4:47 PM, Bob Miller <bob.miller@xxxxxxxxxxxxx>
wrote:

I will confess to a certain amount of naivete here w.r.t. what users
actually expect an IBIS-AMI simulation to tell them, but I would generally
regard insuring that the Tx and Rx can use the same PAM4 encoding scheme to
be waaaay down on the list of my customers' simulator priorities. Yes, they
need to be compatible. No, I don't need a simulator to tell me; I just need
to read the documentation assuming it exists. A table in an AMI file (or
even worse, a compiled-in table returned in parameters_out) seems a pretty
obscure place to hide this fairly basic requirement which would determine if
I was even interested in a vendor's serdes.

Indeed I'm not currently exactly sure what the Rx executable would actually
do with the encoding info to alter what it returns via the AMi API
"bung-hole" in either AMI_Init (impulse) or AMI_GetWave (TD wave at the
"decision node"). And the Tx is fed the pre-encoded PAM4 waveform in its
input and pretty much filters it and passes it to the output and otherwise
doesn't care what the encoding is. Not one line of code in my current PAM4
Tx or Rx models currently cares what the encoding is, though perhaps
embedded FEC in the Rx executable (why??) would require knowing.

Like I said -- my ignorance may be showing for all to see. Enlighten me here
or privately if you feel I shall be horribly embarrassed by said
enlightenment. All in good fun<wink>.

Regards,

Bob



On Tue, Jul 14, 2015 at 3:18 PM, Bob Ross <bob@xxxxxxxxxxxxxxxxx> wrote:

All,



We should open this discussion to the ATM reflector for other possible
inputs.



Bob



From: Mike Steinberger [mailto:msteinb@xxxxxxxxxx]
Sent: Tuesday, July 14, 2015 2:14 PM
To: fangyi_rao@xxxxxxxxxxxx
Cc: wkatz@xxxxxxxxxx; Arpad_Muranyi@xxxxxxxxxx; msteinberger@xxxxxxxxxx;
michael.mirmak@xxxxxxxxx; bob@xxxxxxxxxxxxxxxxx
Subject: Re: The PAM4 BIRD 178 is not clear if PAM4_Mapping is Tx only, or
should it be allowed for Rx ...



Fangyi-

What data leads you to believe that PAM4 mapping mismatches are going to be
so rare that we should preclude the possibility of modeling them? I submit
that none of us have enough experience yet with PAM4 to make that
determination.

It might be perfectly reasonable to state that if the receiver mapping has
not been specified, it is assumed that the receiver will use the same
mapping as the transmitter. That would give us the simplicity you're looking
for without precluding other possibilities.

Mike S.

On 07/14/2015 04:08 PM, fangyi_rao@xxxxxxxxxxxx wrote:

My understanding is that PAM4_Mapping is only for Tx. I wouldn’t worry too
much about different mappings between Tx and Rx. It’s possible in theory,
but rare in reality.



Regards,

Fangyi



From: Mike Steinberger [mailto:msteinb@xxxxxxxxxx]
Sent: Tuesday, July 14, 2015 1:52 PM
To: Walter Katz
Cc: RAO,FANGYI (K-USA,ex1); Muranyi, Arpad; Mike Steinberger; Mirmak,
Michael; 'Bob Ross'
Subject: Re: The PAM4 BIRD 178 is not clear if PAM4_Mapping is Tx only, or
should it be allowed for Rx ...



Guys-

What would happen in the real silicon if the Tx used one mapping and the Rx
used another? (Answer: BER=0.5)
In the real silicon, is there any guarantee that the Tx and Rx are going to
use the same mapping? (Nope)

Our simulations should display the same behavior as the silicon.

In short, I strongly suggest that the PAM4 mapping needs to be defined for
both the transmitter and the receiver.

Mike S.

On 07/14/2015 02:31 PM, Walter Katz wrote:

Fangyi,



I have always assumed that PAM4_Mapping would be used in Tx .ami files only.
The BIRD is not explicit on this. Should we clarify this in the Editorial
work to make it Tx only. If we allow it for Rx, then we would somehow need
to make sure the Tx and Rx model used the same PAM4_Mapping. I would prefer
to make PAM4_Mapping Tx only. We would like to clarify this at the IBIS-ATM
meeting next week.



Walter



Walter Katz

wkatz@xxxxxxxxxx

Phone 303.449-2308

Mobile 303.335-6156







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