RE: WWDM vs. 10Gb/s serial

[elg@xxxxxxxxxxx (Ed Grivna)]

> 
> 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
> >
> >
> >