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*To*: stds-802-3-hssg@xxxxxxxx*Subject*: PAM5 and 8B10B and FEC and SAME_BAUD*From*: Kardont@xxxxxxx*Date*: Thu, 2 Mar 2000 02:15:27 EST*Sender*: owner-stds-802-3-hssg@xxxxxxxx

Rich, You sent today an email to the Reflector, that I reproduce below, that left me perplexed. I sent you a fast response, but I would like now to be more detailed. You seem to affirm that you can have it all: use PAM-5 and include 8b/10b and FEC and still be able to transmit all this information at 5 Gbaud (serial). I think that this is not true. I will explain this assuming, for simplicity that we use instead four parallel transmitters at 1.25 Gbaud each. The final result will be the same as if we had used one transmitter at 5 Gbaud and collected together four consecutive PAM-5 symbols to form one unit. The raw data consists of 8 bits at 1.25 Gbaud that come from the MAC layer. These 8 bits represent 2^8 = 256 possible symbols. Using four PAM-5 transmitters at 1.25 Gbaud the number of possible 4-dimensional symbols is 5^4 = 625. That is, in the 800 psec baud interval you can transmit 1 out of 625 possible symbols. One such symbol, for example, could be: {-1, +2, 0, -1} Meaning that the A-transmitter sent a "-1", the B- transmitter sent a "+2", and so on. Since we have 625 possible symbols (or points in this 4-dimensional space) and we only need 256 different symbols for the raw data, we have more than twice the number of points than we really need for raw data: 625 > 2 * 256 = 512 It is this fact that enables to add Trellis coding and get 6 dB coding gain using FEC. Now, if you also want to add the 8b/10b coding, your "raw" data becomes now something more than 512 symbols: 256 for Current RD(-) and another 256 symbols for Current RD(+) plus another 12 symbols corresponding to valid special code groups, like K28.5 and K28.7. In total, your "raw" data after adding 8b/10b consists of 524 different symbols. Since we have 625 possible points (5^4) we still have enough points to cover the needed 524 symbols corresponding to this "raw" data, but we are left with only 625 - 524 = 101 unused points. This redundancy is too small and does not allow us to add Trellis Coding. In other words: we can not get any coding gain or FEC. The only way to get coding gain in this case would be either by going to PAM-6 (to get a total of 6^4 = 1296 symbols, which is greater than 2*524 = 1048) or to increase the baud rate above 1.25 Gbaud. But assume, for instance, that we sacrifice the coding gain altogether and we decide to use just PAM-5 to get 625 possible symbols in order to accomodate the needed 524 symbols of the 8b/10b coding. I do not see any sense in adding the 8b/10b coding , because once we went to 5-level symbols for transmission we lost the two main properties that make the 8b/10b coding attractive: DC balance and low running disparity. How could you add these two properties to a 5-level transmission scheme ? When you use PAM-5 you have to add a scrambler anyway in order to get the "quasi DC balance", as the PCS of the 1000BASE-T does. Summarizing: you can not have a PAM-5 serial at 5 Gbaud (or a PAM-5 4-WDM at 1.25 Gbaud) that also includes the 8b/10b and FEC. Either you must go to PAM-6 or increase the baud rate. And in any case, I do not see the value in adding the 8b/10b coding when we use a 5-level transmission scheme. Jaime Jaime E. Kardontchik Micro Linear San Jose, CA 95131 email: kardontchik.jaime@xxxxxxxxxxx ******************************** In a message dated 3/1/00 6:49:49 PM, rtaborek@xxxxxxxxxxx wrote: > >Ed, > >..... > >You point out that FEC requires a higher line rate, which mitigates the benefits >of FEC. This is true for a traditional Binary signaled system. > >However, for a MultiLevel system such as PAM5, an additional level can easily >accomodate the FEC overhead. Several HSSG PAM5 proposals as well as 1000BASE-T >utilize FEC in this fashion. As an example, my 5 Gbaud PAM5 proposal used 4 of >the levels to respresent 2 bits and the 5th level to represent FEC as well as >special characters. In terms mapping, one 8B/10B code-group could be mapped to >4 >PAM5 symbols creating a PAM5x4 "code-group" symbol. Note that the PAM5x4 symbol >period is 800 ps, and is used to convey 10 bits and that the PAM5 symbol period >is 200 ps = 5Gbaud. You can find more detail on thge latter example in: >http://grouper.ieee.org/groups/802/3/tutorial/july99/mastut.pdf > >It is possible to add FEC to MultiLevel signaling WITHOUT increasing the line >rate. However, adding extra levels does further decrease SNR. The tradeoff then >becomes FEC complexity and decreased SNR vs. effective BER gain. I like the way >this tradeoff leans to the left :-) > >Best Regards, >Rich

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