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This graph illustrates the problem with extending the limit to 500 MHz. The scaling here is incorrect. The limit is only calculated to 250 MHz, so the response should be scaled to touch the limit at 250 MHz or below. If the response is over the limit above that frequency, that is something that the TIA will have to consider in extending the limits to higher frequencies. Conceivably the slope of the limit will be adjusted above 250 MHz to better fit the data. This is predicted by the connector and cable models.
Sterling Vaden
Albert Vareljian wrote:
Bill, Xiaopeng In fact, the difference between envelop and measured responses could get quite substantial. Pls see attached graph. The NEXT (taken from Terry Cobb data) had been scaled to fit (touch) the envelop used in calculations. The difference in integrated power in the responses is ~6.6 dB. Regards, AlbertWilliam Jones wrote:Xiaopeng Based on Albert's result we only need a small amount of cancellation to provide convincing margin to the system. Let us say on the order of 3 or 4 dB. I know this is achievable. Besides Albert was using the ANEXT envelop. I still claim, we will see a measurable difference in the cancellation requirement when we use measurement data. regards Bill -----Original Message----- From: xichen@marvell.com [mailto:xichen@marvell.com] Sent: Friday, February 21, 2003 11:17 AM To: Albert Vareljian Cc: xichen@marvell.com; William Jones; stds-802-3-10GBT-Cabling@ieee.org; stds-802-3-10GBT-Modeling@ieee.org; stds-802-3-10GBT@ieee.org Subject: Re: [10GBT-Modeling] RE: [10GBT-Cabling] [10GBASE-T] a channel capacity estimation program for your evaluation Albert, Thank you for your input. With the consideration of ANEXT, the Shannon capacity of CAT-6 is 10Gbps by using the models presented in my program. Accordingly, your calculation shows the Shannon capacity is 12Gbps. I admit this is a significant difference. However, even with the 12Gbps Shannon capacity, there is no way we can achieve 10Gbps effective throughput over 100m CAT-6 cable if the ANEXT cannot be suppressed by any meanings. Regards, Xiaopeng Albert Vareljian <albertv@ieee.org>@majordomo.ieee.org on 02/21/2003 01:41:11 PM Sent by: owner-stds-802-3-10gbt@majordomo.ieee.org To: xichen@marvell.com cc: William Jones <wjones@solarflare.com>, stds-802-3-10GBT-Cabling@ieee.org, stds-802-3-10GBT-Modeling@ieee.org, stds-802-3-10GBT@ieee.org Subject: Re: [10GBT-Modeling] RE: [10GBT-Cabling] [10GBASE-T] a channel capacity estimation program for your evaluation Xiaopeng, Just to illustrate how big a potential gap between the measured vs envelope approach could be, I performed capacity computations based on the Cat6 measured characteristics provided by Terry Cobb. A 'test' channel was 'assembled' by combining the worst behaving impairments out of the set of Cat6 measured characteristics for the Channel attenuation, Echo, NEXT and FEXT. The following assumptions were also made: 10 dBm launch power with a flat PSD, Echo, NEXT and FEXT cancellation of 40, 25 and 20 dB respectively, -140 dBm/Hz background. The capacity without ANEXT came out on the order of 29 Gbps. This was reduced to app. 12 Gbps with the ANEXT envelope, as per your code. As the Channel attenuation and Alien NEXT are the major impairments in the system, reliable references in a broad frequency range would be the key. Another source that could have a potential impact on the system, but has not been considered yet, is the RF ingress. Regards, Albert xichen@marvell.com wrote:Bill, You are right. Chris did not provide us with the information on alien xtalk. And the model used in the program is from a presentation on last Nov's meeting. I have given the link of the presentation in the program. You can check it out. I understand that this model has not been accepted by the SG yet. Since there is no other source of ANEXT model, I decidedtouse it temporarily. I believe there will be a lot work to be done on defining a good model of ANEXT. Once we settle down on this, we will have a much more clear picture of what we can do or cannot do. Also, the model Chris gave is only up to 100MHz (for CAT-5E) and 250MHz (for CAT-6). My simple way of extrapolation can be inaccurate. Sinceourstandard has to base on a referable cable model, before we can move ahead, we have to work with cable guys to solve this problem. Regards, Xiaopeng "William Jones" <wjones@solarflare.com> on 02/20/2003 03:47:22 PM To: <xichen@marvell.com> cc: <joseph.chou@realcomtec.com>, <stds-802-3-10GBT-Cabling@ieee.org>, <stds-802-3-10GBT-Modeling@ieee.org>, <stds-802-3-10GBT@ieee.org> Subject: RE: [10GBT-Modeling] RE: [10GBT-Cabling] [10GBASE-T] a channel capacity estimation program for your evaluation Xiaopeng Strictly speaking, Chris provided no information on alien xtalk. Based on what he provided yields a capacity of 17.4 Gbps (using your program with the aforementioned corrections). Of course alien xtalk can't be ignored and must be dealt with accordingly. As a point of clarity, what youareusing to model alien xtalk is the pair-to-pair NEXT limit line out of 568b lowered by 3dB. We provided the NEXT limit for comparison to our measurements only. We can hardly draw any conclusions from the heuristic use of a NEXT limit line applied as an alien xtalk limit line. As to whether scaled measurements will yield capacity resultssubstantiallydifferent then the smooth envelop associated with the measurements, I have this to say. First, by definition, the envelop exceeds the measurement at all frequencies. Therefore, a measurable contribution to capacity will occur at every frequency where the measurement does not touch the limit line. I suggest reducing the frequency step size of the integrationbeforeconcluding whether or not there is an appreciable difference in capacity. Secondly, from our experience and from talking with a number ofindividualswho are actively taking cable measurements, high frequency extrapolation of these limit lines are rather pessimistic. regards Bill -----Original Message----- From: xichen@marvell.com [mailto:xichen@marvell.com] Sent: Thursday, February 20, 2003 1:18 PM To: William Jones Cc: joseph.chou@realcomtec.com; stds-802-3-10GBT-Cabling@ieee.org; stds-802-3-10GBT-Modeling@ieee.org Subject: RE: [10GBT-Modeling] RE: [10GBT-Cabling] [10GBASE-T] a channel capacity estimation program for your evaluation Bill, Sorry I have to clarify my opinions which may not be the same as yours. According to the calculation of the program which is based on the ISO channel model provided by Chris, the target we are talking about (10Gbps over 100m CAT-5E/6 cable) is even theoretically infeasible (channel capacity is <10Gbps). And I don't think that there will be anappreciabledifference in capacity when the measured data (while scaled to the channel model limit) is used. Another thing I want to clarify is about the Shannon limit. The Shannon limit is only theoretically achievable without limit on the complexity of system. With the currently proposed approaches (traditional modulation/coding schemes), I don't think there is any way for us to make it close to the channel capacity. Nevertheless, the implementation constraints will only make the really achievable performance worse. Regards, Xiaopeng "William Jones" <wjones@solarflare.com> on 02/20/2003 11:57:35 AM To: <xichen@marvell.com>, <joseph.chou@realcomtec.com> cc: <stds-802-3-10GBT-Cabling@ieee.org>, <stds-802-3-10GBT-Modeling@ieee.org> Subject: RE: [10GBT-Modeling] RE: [10GBT-Cabling] [10GBASE-T] a channel capacity estimation program for your evaluation Xiaopeng I'm glad you feel now that this is only a difficult problem and not one that is impossible. We have always said that it was a hard problem. To your point that these calculations are theoretical limits that may be practically challenging to realize in implementations is precisely why we presented cycle based time domain simulation results as well as the capacity results. This modeling approach captures the entire effect ofthechannel and crosstalk including phase information which is lost in the frequency domain calculations and which is important for practical considerations. These simulations included finite length filters, adaptation jitter and clock recovery effects among other things thereby accounting for real implementation effects and supporting the implementation feasibility. To the point about the transmit PSD, after correcting for the programming errors, I got an additional 760Mbps in capacity with a flat PSDbandlimitedto 400MHz. I do agree that alien xtalk is a limiting noise source but still dispute the level. If I lower the alien xtalk level by 6 dBaligningit closer with our measured data but still using the ISO cable models Igeta capacity of 10.0Gbps on Cat5e. To Chris' point about using measured data scaled to limit lines, I think this is very important. There is an appreciable difference in capacity when the calculations are used with smooth curves for the xtalk ascomparedwith actual measured data. There is a lot of physically based structure in, for example NEXT, as seen in any of the frequency response curveswhichis further reflected in the impulse responses. We have seen SNR improvements on the order of 3 to 5 dB when using the measurement data as opposed to the envelop. regards, Bill William W. Jones, Ph.D. Director of Systems Engineering SolarFlare Communications, Inc. 949-581-6830, ext. 2550 mobile: 619-405-2445 fax: 949-581-4695 wjones@solarflare.com -----Original Message----- From: xichen@marvell.com [mailto:xichen@marvell.com] Sent: Thursday, February 20, 2003 10:23 AM To: joseph.chou@realcomtec.com Cc: stds-802-3-10GBT-Cabling@ieee.org; stds-802-3-10GBT-Modeling@ieee.org; William Jones Subject: Re: [10GBT-Modeling] RE: [10GBT-Cabling] [10GBASE-T] a channel capacity estimation program for your evaluation Joseph, First, thank you for pointing out the mistake I made on the return loss.Ihave corrected it in the attached program. As you said, it does notaffectthe result much. Also you are correct that the input signal PSD should not be calculated using my simplified approach. Instead, the best thing we can do is to launch as much as signal power in this band while meeting the FCC restriction. However, by my observation, the data I used in the programisvery close to the best thing we can do. So you can think the approach is reasonably near-optimal. The difference between Solarflare's data and the ISO 11801 cable modelswasa topic in the last Jan meeting. Should we follow the standard model or some specific measurements is still a problem. I did not give a careful calculation of the channel capacity based on Solarflare's channel model. But I guess you are right. The major factor that causes the difference between the results based on the standard channel model and Solarflare's model is the strength of the residual alien NEXT since it is the dominant noise source. In my view point, suppressing alien NEXT is very difficult, especially when there are many disturbers. Also I would like to emphasize that the calculated Shannon capacity is the theoretical limit. It is fairly reliable when the frequency-domainchannelmodel is reliable. However, as all of us have known, it is not easy to approach this limit at all. In my opinion, even the Shannon capacity can be 14Gbps based on Solarflare's calculation, to achieve effective 10Gbps channel throughput will be extremely difficult. Regards, Xiaopeng Marvell Semiconductor Inc. (Please check the updated i3e.m) (See attached file: i3e.m) "Joseph Chou" <joseph.chou@realcomtec.com>@majordomo.ieee.org on02/19/200307:28:01 PM Please respond to <joseph.chou@realcomtec.com> Sent by: owner-stds-802-3-10gbt-modeling@majordomo.ieee.org To: <xichen@marvell.com>, <stds-802-3-10GBT-Cabling@ieee.org>, <stds-802-3-10GBT-Modeling@ieee.org> cc: "'William Jones'" <wjones@solarflare.com> Subject: [10GBT-Modeling] RE: [10GBT-Cabling] [10GBASE-T] a channel capacity estimation program for your evaluation Hi Xiaopeng, I am new in this reflector. I looked at your program. Here are some observations: 1. The conclusion that you pointed "the Alien NEXT dominates the capacity" is surely right. 2. The Echo (Return loss) used in i3e.m has wrong sign. The return loss should be negative (loss) when adding to the signals. However, it does not affect the result much. 3. The Input Signal Si in i3e.m should be calculated based on the bandwidth. In the theoretical estimation of channel capacity, the actual baud rate due to coding or modulation is not in the picture. 4. The Alien NEXT model used in i3e.m is about 5 to 10 dB higher than the one used by SolarFlare's presentation of last November. 5. The SolarFlare's presentation also has less Insertion Loss (5 to 10 dB) than what specified in the ISO 11801 Class D. The above observations conclude why you and Bill have different result of channel capacity: 7GH vs. 14GH for Cat5e cable. Best Regards, -Joseph Chou -----Original Message----- From: owner-stds-802-3-10gbt-cabling@majordomo.ieee.org [mailto:owner-stds-802-3-10gbt-cabling@majordomo.ieee.org] On Behalf Of xichen@marvell.com Sent: Wednesday, February 19, 2003 2:13 PM To: stds-802-3-10GBT-Cabling@ieee.org; stds-802-3-10GBT-Modeling@ieee.org; stds-802-3-10gbt@ieee.org Cc: William Jones Subject: [10GBT-Cabling] [10GBASE-T] a channel capacity estimation program for your evaluation Hi, Bill and 10GBASE-T SG members, Sorry for the so late response. I was on a quite long vacation past several week. I have finished a Matlab program to distribute in the 10GBase-T SG for the evaluation of the bottomline channel performance. The channel simulation is based on the models (return loss, FEXT, NEXT, insertion loss) provided by Chris and a presentation (Alien NEXT) last Novermber. Without consideration of any implementation and modulation/coding, the Shannon channel capacity can be calculated only based on the frequency-domain channel model. Please read the comments in the program for details Unfortunately, based on the available channel models, even with a quite optimistic belief in the achievable DSP power, the Shannon capacity tells me that both CAT-5E and CAT-6 cables cannot support 10Gbps throughtput over a distance of 100 meters. Please read the comments in the program for details. Surely, the calculation is completely based on the available channel models. I know more work is needed on setting up a channel model accepted by all of us. But unless some dramatic improvement can be achieved in the alien NEXT suppression, the program tells us pretty much the story. If you have any question about the program itself, you are more than welcome to post on the group reflector for discussion. Best regards, Xiaopeng (See attached file: i3e.m) "William Jones" <wjones@solarflare.com>@majordomo.ieee.org on 01/22/2003 11:18:48 PM Sent by: owner-stds-802-3-10gbt@majordomo.ieee.org To: <stds-802-3-10gbt@ieee.org> cc: Subject: RE: [10GBASE-T] channel model Xiaopeng How are the calculations coming? Are you using the models Chris sent out last week? regards Bill -----Original Message----- From: William Jones Sent: Wednesday, January 15, 2003 4:08 PM To: stds-802-3-10gbt@ieee.org Subject: [10GBASE-T] channel model(part 2) Bill, Sure. Once the common channel model (I only need the frequency domain characteristics) is available, I can begin to work on it. Regards, Xiaopeng ---------- Xiaopeng Would you be willing to sign up for technical feasibility based on an optimal DFE calculation? regards Bill ----------- Bill, I has no problem to use TIA or ISO based channel and noise model once it is standardized. My point is that even using the frequency-domain model we can still give a quite good estimation of the practically achievable distance at a throughtput of 10Gbps over CAT-5E or CAT-6 cables. Regards, Xiaopeng ------------- Xiaopeng We believe, as George discussed in his part of the tutorial in Kauai, that with an adequate amount of crosstalk cancellation, the throughput is achievable. The question then becomes can this level of cancellation be practically achieved, hence, my interest in time domain models. Until we get the models from the channel modeling Ad Hoc, we could argue this point endlessly. Until then, I still believe we should not use DSL models, but, rather something from TIA 568b or ISO 11801. regards Bill