[SI-LIST] Re: the right way to choose the scrambler

Gentile Giovanni,
Oh how the double meaning of English words can confuse!
Coding, de-coding, scrambling and unscrambling are all words used in 
cryptography.  They are also used in serial data transmission with 
somewhat different meanings.
Coding is used to reduce the required bandwidth of a link, scrambling is 
used to reduce the spectral peaks emitted by the link.
If  raw data is being serially transmitted the the required bandwidth 
has a minimum of  DC to 0.5 x bit rate.  Coding acts to reduce that 
bandwidth by, typically, reducing the run length.  Thus coding can 
significantly reduce the effects of dispersion in the transmission path 
and thus reduce the deterministic jitter and result in better BER 
figures.    However, if  a constant sequence of bits is continuously 
transmitted then the spectral content of that signal can result in 
significant spectral peaks, with the resulting EMC implications.  In 
order to reduce these peaks the data stream can be scrambled before 
being coded (scrambling after coding destroys the bandwidth limiting 
effect of the coding).  Almost all high speed links (except, for 
example, USB) use the excellent 8b/10b coding scheme, which uses 10 bits 
to transmit 8 bit data and also gives a high degree of error detection.  
Examples are Fibre Channel, Gigabit ethernet, serial ATA and 1394b, all 
of these , except for Fibre Channel also use scrambling.  Properly 
applied scrambling has no impact on BER. Coding has by far the biggest 
effect.

I suspect therefore that your question should be referring to coding 
methods rather than scrambling methods and googling coding schemes 
rather than scrambling might give better results.  For my own part I'd 
choose 8b/10b.
    Bandwidth is in the order of 1/50 data rate to 1/2 data rate.
    Excellent error detection properties ( no error correction though).
    Implementable in Rom, lookup tables.
    Well understood and implemented by silicon vendors.
    Abbastanza the de facto standard for serial data transmission.
    If, as Steve has stated, the IBM patent has lapsed then that's 
another plus.
Regards
Dave Instone


Guasti Giovanni wrote:

>Hi gurus,
>does someone of you know a book or web sites where I could find an =
>analytical description about one way to choose the scrambler for my =
>custom high speed link?.
>
>The known parameters are
>the transmission channel
>the receiver/transmitter behaviours
>the frequency data spectrum
>
>I would like to find out the best scrambler BEFORE to make any measure =
>of BER/quality of signal or simulations.
>Best regards,
>       Giovanni
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-- 
Regards
Dave Instone
Oxford Semiconductor Ltd (oxsemi.com)
+44 (0)1235 824963


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