Re: PAM-5, what are your BERs ?
The X3.166 is the fiber PMD. I remember the BER goal of the TP-PMD
committee to be 10e-12 (it's reference in the jitter budget appendix). Was
anyone else out there involved in TP-PMD?
At 10:07 AM 2/28/00 -0800, Jaime Kardontchik wrote:
>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.
>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
> "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
> "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
> "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 E. Kardontchik
>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
>> > 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.
>> > 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 E. Kardontchik
>> >Micro Linear
>> >San Jose, CA 95131
>> >email: kardontchik.jaime@xxxxxxxxxxx