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*To*: elg@xxxxxxxxxxx (Ed Grivna), mick@xxxxxxxxxxx, Paul Bottorff <pbottorf@xxxxxxxxxxxxxxxxxx>, elg@xxxxxxxxxxx, stds-802-3-hssg@xxxxxxxx*Subject*: RE: WWDM vs. 10Gb/s serial*From*: pbottorf@xxxxxxxxxxxxxxxxxx (Paul Bottorff)*Date*: Wed, 12 May 1999 10:33:16 -0700*Sender*: owner-stds-802-3-hssg@xxxxxxxxxxxxxxxxxx

Ed: The 2n bits is wrong. To zero out a scrambler of order n, you need a pattern of (2**n - 1) bits. For instance, lets consider the seventh-order frame synchronized scrambler that is being used by SONET. The pseudo-random sequence generated by the scrambler repeats itself every (2**7 - 1 = 127) bit periods. Since the pseudo-random output of the seventh register is XORed with the payload data, a user could transmit a pattern that continuously repeats the 127-bit pattern from the seventh register of the SONET scrambler. When the pattern from the seventh register is aligned with the 127-bit pattern from the malicious user, the line will see an all zeros pattern. So, to zero out the scrambler, the user 1- must know the order of the scrambler and its polynomial 2- must have access to the whole payload envelop 3- must use a pattern of (2**n - 1) bits generated by the polynomial 4- must repeat its pattern enough until it gets in phase with the output of the scrambler Paul At 07:06 AM 5/11/99 -0500, Ed Grivna wrote: > >> >> Mick: >> >> It is 1 / (2**70) for randomized data. A properly designed scrambler system >> produces completely random line data independent from the data being >> transmitted. A constant string of zeros or any fabricated packet can be >> encoded but the line data will still random. >> >> Paul > >The problem is we are not dealing with randomized data. With reference >to a "properly designed scrambler," I can guarantee you that given >any scrambler based on a polynomial of degree N, you can zero it out with >the correct string of around 2N bits. This is not boasting or grandstanding, >this is mathmatical fact. The SMPTE scrambler polynomial has a high order >term of x^9, and it can be cleared with two characters of data. > >This doesn't say that scrambling can't be used. But it does mean that >a long polynomial needs to be employed to reduce (not remove) the >probability of zeroing out the scrambler. The longer the polynomial, the >greater the latency of encode and decode, and the longer to re-sync if >it gets confused by bad data. Also, the longer the polynomial, the greater >the error propagation; i.e., a one bit error in the serial stream will mess >up more characters before its effects have propagated out of the descrambler. > >The basic SONET polynomial has a high order term of only X^7, while the ATM >srambler is X^43+1. The SONET scrambler can be cleared quite easily, while >the ATM scrambler is quite difficult to clear. The protocol overheads in >these interfaces contain data that will keep the scrambler seeded, it is >the associated data field that can clear it out. > >Then you have the issue with the lack of special characters. With respect >to the 802.3 signal stream, yes scrambling has been used before, but at the >physical layer there has always been a mechanism to indicate non-data >signalling; i.e., a five level code where four levels are used for data >and the fifth is for signalling. > >This is difficult in an NRZ stream unless you change the stream from >character based to add an overhead bit every so many characters. The >telcos use this ALL the time. In a T3 system they add 1 bit out of >every 170 for synchronization and framing. > >Regards, > >Ed Grivna >Cypress Semiconductor > >> >> At 03:50 PM 5/10/99 -0700, Mick Seaman wrote: >> >A string of 70 zero's does not have a 1/2**70 chance of occuring. >> >Zeros and other particular repeating patterns are really much more common >> >than that, as a look at carefully initialized memory will show. There should >> >be no restriction on transferring memory maps on any other particular data >> >around. >> >Any statistical argument has to very carefully assess assumptions as to >> >distribution and independence of variables. >> > >> >Mick >> > >> > >> > > >

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