Re: [BP] EIT Fun
john,
You bring up some important points and i would like to give my take on them:
1. (first) Usually a range of allowable amplitudes are specified for a
transmitter to allow for normal variations in manufacturing and
operating conditions. In additions, if the manufacturer who has good
control may want to use some of that range to enhance his
specifications, for instance power consumption or ability to blast
through marginal channels. All in all this means that we can count on
any particular amplitude for any transmitter and if we want to be
conscientious we should assume the worst case: min spec amplitude for
the thru channel and max for all aggressors.
2. Synthesized test cases have value: they better represent the
boundaries of the channel specs than real channels can at any reasonable
cost. You complain that they are un-realistic but i have customers
demanding that my Transceivers work on channels which look pretty
un-real to me and i expect that this tendency will continue in the
future. If there is really a useful relationship between thru and xtalk,
beyond ICR, which all "reasonable" channel will have, lets add it to the
channel spec.
3.
4. I think howard covered this pretty well
5. Again you assume a control over Tx amplitude which probably will not
exit.
charles
DAmbrosia, John F wrote:
> All,
>
> Wow, you get sick for a few days and stay offline and you get to miss
> all of the fun!
>
> I will try to go through all of the email and comment –
>
> Howard’s initial email suggested using 800mv for the Tx amplitude of
> the victim and 1200mV for the aggressors. I disagree with this for
> several reasons.
>
> First, per the continuing discussions we have had
> regarding the Tx amplitude, it was agreed that 800mV was
> the “minimum” maximum that could be counted on from
> devices. The suggestion to use 1200mV for aggressors
> suggests that a higher launch amplitude is available.
> Being able to boost the launch amplitude of the victim
> would most certainly help.
>
> Second, I have long been against these “synthesized” test
> cases as they fail to take into account the realities of
> channel synergy. This is especially true for xtalk
> aggressors. Take for example the use of Tyco Case #7. Tyco
> Case#7 is a 13” total length channel (6”LC +1”BP +6” LC).
> So why would this be used as a noise source for a longer
> channel at 1200mV? In terms of FEXT there is already an
> approximate 8 to 10 dB difference at Nyquist. This is then
> being exasperated by increasing the Tx amplitude to its
> maximum (where others have already said that is where they
> would expect the Tx amplitude to be lower. Also, because
> of the shorter distance you have significant ringing over
> a large number of bits that just doesn’t exist in a long
> channel FEXT aggressor.
>
> Third, Rich suggested Next should be the same as victim
> and Fext should be higher. If we were looking for the real
> world worst case scenario, I would expect that the NEXT
> would be different than the victim, as it is originating
> from a different device, and that for FEXT it would be
> dependent on the link being looked at. For FEXT signals at
> the line card I would expect the amplitude of the
> aggressors to probably be the same as the victim, while
> for victims at the switch, the launch amplitude would be
> dependent on whether the signals are originating from the
> same line card or not. However, from the switches we were
> looking at in this study, we saw that neighboring channels
> were similar to the victim channels. (Typically we saw the
> blocks at the switch go from extreme to the other, not
> intermixed). This is not to say that all designs are done
> intelligently, however.
>
> Fourth, I don’t believe we can arbitrarily assign a
> maximum amplitude to a channel. If the receiver needs it,
> it will request that of the Tx. This goes back to my point
> about Tyco Case #7 again. I doubt the forward channel
> response of this channel would require the Tx output
> amplitude to be at maximum output voltage. The amplitude
> that is appropriate is dependent on the needs of that
> specific channel.
>
> Lastly, Pat raises an interesting point – “In the past
> standards I've worked on, we have usually assumed that
> implementations could be running at the limits because we
> don't have a basis for assuming a distribution.” We have
> been judging channels to be working, based on simulation
> results that assumed a max Tx amplitude of 800mV for the
> victim and aggressor. So if a channel needs 1200mV to
> work, but the subsequent crosstalk causes a neighboring
> channel that worked based on 800mV launched amplitudes to
> now fail – which channel is actually the problem? I would
> have to point to the 1200mV channel, as receivers are
> tested to a normative Tx max peak to peak amplitude of
> 800mV in the EIT test.
>
> My two cents.
>
> John
>
>
>
> Cheers!
>
> John D'Ambrosia
>
> Architect, Ethernet EcoSystem
>
> Business Development, Global CC&CE
>
> Tyco Electronics
>
> Tel 717.986.5692
>
> Fax 717.592.2470
>
> Cell 717.979.9679
>
> Email - john.dambrosia@tycoelectronics.com
>
>