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*To*: stds-802-3-hssg@xxxxxxxx*Subject*: Re: PAM-5, what are your BERs ?*From*: Jaime Kardontchik <kardontchik.jaime@xxxxxxxxxxx>*Date*: Mon, 28 Feb 2000 10:07:41 -0800*Organization*: microlinear corporation*References*: <4.1.20000228093354.00d902f0@smbmail3>*Sender*: owner-stds-802-3-hssg@xxxxxxxx

JR Rivers wrote: > Not that I object to your analysis; however... > > I believe that the MLT-3 used in FDDI-TPPMD and 100BASE-TX can also be > considered multi-level. If you agree, then it can serve as an example of a > multilevel coding scheme signalled over copper with a BER of 10e-12. I > also believe that the 8B6T signalling scheme in 100BASE-T4 is multi-level; > however, I don't know what it's BER ended up being. > > JR > Hello JR, 100BASE-TX is surely included. However, its target BER seems to be more 2.5*10(-10) than 10^(-12). The 100BASE-TX standard does not include explicitly the BER objective under the "PCS objectives", as it usually does for the other Ethernet Copper standards. The target BER seems to be 2.5*10^(-10), according to ANSI X3.166-1990, under paragraph 8, page 20, "Media signal interfce: "The parameters speciified in this clause are based on a requirement that the bit error rate contributed by the repetition though an FDDI attachment shall not exceed a bit error rate of 2.5 x 10^(-10) under all conditons of clause 8, including the minimum Active Input Interface power level. In addition, the FDDI attachment shall not exceed a bit error rate of 10^(-12) when the Active Input Interface power level is 2 dB or more above the minimum level." This BER interpretation of 2.5 * 10^(-10) for 100BASE-TX is also consistent with what I found in Howard W. Johnson's book "Fast Ethernet, down of a new network". Howard writes on page 140, under Error Robustness: "The residual probablity of an X receiver [100BASE-X] failing to detect any combination of errors is extremely small. Assuming a uniform line error of one part in 2.5*10^10, the mean time between undetected packet errors is 31 million billion years." The interpretation of "uniform line error" as the usual BER for 100BASE-TX receives further confirmation when Howard says in page 121, on Error Robustness referring to 100BASE-T4: "The residual probability of a T4 receiver failing to detect any combination of errors is extremely small. Assuming a uniform line error rate of one part in 10^8, the mean time between undetected packet errors would be 307 million billion years." A BER of 10^8 is exactly what is stated in the Ethernet standards under the "Objectives" section in the PCS for the 100BASE-T4 Standard. Jaime Jaime E. Kardontchik Micro Linear San Jose, CA 95131 > > At 04:36 PM 2/27/00 -0800, Jaime Kardontchik wrote: > > > >Hello 10G'ers, > > > >Edward Chang was right on target. > >He wrote on Feb 23: > > > >> In the past, the multiple voltage-level coding > >> was adopted by two LAN standards, ATM and > >> Ethernet. Both of them were twisted-pair > >> applications, and the BER were 10^(-10). > >> I proposed BER of 10^(-12) in both working > >> groups to be consistent with the LAN optical > >> links' BER; however, for some reason, they > >> remain 10^(-10), officially in both standards. > > > >What was the reason ? > > > >Multilevel voltage coding was not the reason. > >On the contrary, it was a remedy. The real reason > >was the strong ISI at the maximum link lengths > >that these Standards wanted to support. > >Multi-level voltage coding was adopted to > >lower the baud rate or the frequency content > >of the signal in order to make the ISI smaller > >and get a better BER. > > > >However, the remaining ISI in these Copper links > >remained high enough that the eye at the > >input of the receiver remained still > >completely closed. It is this closed eye that > >limited the achievable BER in the Copper media > >to 10^(-10). Post-equalizers, no matter how > >sophisticated they were, are not able to > >completely reverse the effects of eye closening > >on the achievable BER. > > > >---> Conclusion ? In order to get a BER of > > 10^(-12) you have to have a clear > > open eye at the input of the receiver. > > The maximum supported link length > > is set accordingly to meet this basic > > condition: open eye. > > > >Now we can go to the basic two PAM-5 proposals: > > > >1) PAM-5 serial at 5 Gbaud > > > >Using 1300 nm lasers lasers at 5 Gbaud the > >optical eye is completely closed already at > >~170 meters due to heavy ISI (installed MMF, > >500 MHz*km bandwidth). > > > >Oscar claimed a support of 500 meters of > >installed MMF and pointed out that > >DFEs (equalizers) have been successfuly used > >in 100 and 1000 Mbps Copper networks. > >However, the specs and experience of the 100 Mbps > >and 1000 Mbps links over Copper - where the eye > >at the input of the receiver is completely > >closed at the target link length due to ISI - > >put the achievable BERs around 10^(-10) only. > >Even using sophisticated equalizers. > > > >What is the experimental support that a BER > >of 10^(-12) could be achieved when a strong ISI > >closes completely the eye at the input of the > >receiver ? I think that none and that the > >experience points to the contrary. Why then > >could an equalizer running at 5 GHz achieve > >here what sophisticated equalizers running > >40 times slower (125 MHz) were not able to achieve > >in the Copper Ethernets ? Parallel processing > >could enable perhaps to meet the timing > >constraints of the design by running multiple > >equalizers at a lower clock, but will not > >eliminate the basic limitation on achievable > >BERs once the eye at the input of the receiver > >is already completely closed. > > > >Let us see now the case PAM-5 at 1.25 Gbaud: > > > >2) PAM-5 4-WDM at 1.25 Gbaud > > > >Let us compare this approach to two other well > >known on-off approaches: 1 GbE and 4-WDM at > >3.125 Gbaud using the 8b/10b coding. Let us > >assume again 1300 nm lasers and installed > >500 MHz*km MMF. > > > >The figures below (I hope will not get distorted > >during transmission) show the power levels of > >the three systems. For 1 GbE I assumed no ISI. > >For PAM-5 I assumed 400 meters link length > >(there is no ISI up to this distance, ISI= 0 dB). > >For 4-WDM at 3.125 Gbaud I assumed 300 meters link > >length and about 3 dB optical loss due to ISI. > > > >| | > >| | > >| | 1 GbE, 1.25 Gbaud, no ISI > >| | > >| | > > > > > >| | | | | > >| | | | | > >| | | | | PAM-5 4-WDM at 1.25 Gbaud > >| | | | | no ISI. > >| | | | | > > > >|**** ****| > >|**** ****| > >|**** ****| 8b/10b 4-WDM at 3.125 Gbaud > >|**** ****| 3 dB ISI loss > >|**** ****| > > > >( the asterisks denote closening of the eye due > >to ISI) > > > >2a) 1 GbE > > > >By definition, the optical power difference > >between the '0' and '1' levels in the 1 GbE > >case is 1: > > > > optical signal power = 10*log(1) = 0 dB > > > > optical SNR = 0 dB (reference) > > > >And we get a BER of 10^(-12). > > > >2b) PAM-5 4-WDM at 1.25 Gbaud > > > >In the PAM-5 case, notice that using the > >"open fiber control" method that I described > >in a previous email, we get the same launched > >power per channel as in the 1 GbE Ethernet. > >However, in PAM-5 the power difference > >between levels is 0.25 (this is the well > >known 6 dB optical power penalty loss of > >the PAM-5 modulation). Let us now add > >the 6 dB electrical coding gain provided > >by the Viterbi decoding: the effective distance > >between levels doubles. Summarizing, in PAM-5 > >4-WDM at 1.25 Gbaud, the effective optical > >signal power difference between levels is: > > > > effec optical signal power diff = > > 10*log(2*0.25) = - 3 dB > > > >where the factor 2 inside the log comes > >from the coding gain. > > > >The noise power at the input of the > >receiver is the same as in 1 GbE because > >we use the same baud rate. Hence, > > > > effec optical SNR = - 3 dB > > > >This is not bad compared to the 0 dB of 1 GbE. > >Furthermore, notice that we could even bring > >back the SNR for PAM5 to 0 dB (the 1 GbE > >reference) if a new laser safety proposal > >to move the maximum safe power from -4 dBm > >to -1 dBm is accepted. See P. Kolesar et al > >presentation in the next March meeting. > > > >2c) 8b/10b 4-WDM at 3.125 Gbaud > > > >In this case the eye is half closed due > >to ISI. Hence the optical power difference > >is: > > > > optical signal power = 10*log(0.5) > > = - 3 dB > > > >and the electrical noise power at the input > >of the receiver is larger, since the receiver > >needs more bandwidth: > > > > elec noise power = 10*log[(3.125/1.25)^2] > > = 8 dB > > > >Hence, the optical SNR is > > > > optical SNR = - 3 - 8/2 = -7 dB > > > >This case is clearly worse than the 1 GbE > >case in terms of optical SNR. And the > >Task Force considers that this system > >can achieve the needed 10^(-12) BER. > > > >Summarizing: > > > > PAM-5 4-WDM at 1.25 Gbaud has no > > ISI, a clear and wide open eye at > > the input of the receiver, an optical SNR > > only slightly less than the SNR of present > > 1 GbE transceivers and better than > > the proposed 8b/10b 4-WDM at 3.125G > > transceivers. > > > > Hence, it is a good choice if one > > wants to reach the required BER of > > 10^(-12). > > > >Note: the above analysis, based on simple > > static SNRs, is not a substitute for a > >much more careful analysis regarding > >the viabibility of any PAM-5 approach. > >However, a simple back-of-the-envelope > >analysis is very useful to make a quick > >comparison between different PAM-5 proposals > >in order to find out which PAM-5 architecture, > > > > a) one working under strong ISI conditions > > that close the optical eye completely; or > > > > b) another using a lower baud rate > > and working with minimal ISI so that > > the optical eye at the input of the > > receiver is widely open, > > > >which one has more chance to meet the required > >BER of 10^(-12), and discard accordingly the > >one that does not have a chance. > > > >Jaime > > > >Jaime E. Kardontchik > >Micro Linear > >San Jose, CA 95131 > >email: kardontchik.jaime@xxxxxxxxxxx > > > > > >

**Follow-Ups**:**Re: PAM-5, what are your BERs ?***From:*JR Rivers

**References**:**Re: PAM-5, what are your BERs ?***From:*JR Rivers

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