| Thread Links | Date Links | ||||
|---|---|---|---|---|---|
| Thread Prev | Thread Next | Thread Index | Date Prev | Date Next | Date Index |
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
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