[SI-LIST] Re: purpose of 8b/10b encoding

  • From: "Raja Patil" <raja.patil@xxxxxxxxx>
  • To: si-list@xxxxxxxxxxxxx
  • Date: Thu, 13 Sep 2001 10:11:22 -0700

Hi List members,

Thanks for the educative discussion on 8b10b.

If I mistake not, we were taught that DC balance is required to prevent X-mer 
saturation (and I am NOT 100 yrs old). Then I see this DC balance again coming 
up in Fibre channel discussions and I was puzzled. As usual (and most 
conveniently) I blamed it on Mr. Bill Gates(I mean the ctrl+C => ctrl+V 
methodology) but looks like there is another aspect to it which has/had nothing 
to do with a X-mer. Folks blame it on my Prof, I had been a good student! 
May be believed a bit too much in those guys.

Okay so, I again read about the encoding and still not able to understand it. I 
mean you take 8 bits, then split them in lower 5 and upper 3. Then you write 
them as /Dx.y/ where x is the decimal value of ...Then you have comma 
character..and somewhere I am lost. 
Ref: http://www.iol.unh.edu/training/ge/8B10BEncoding.html

Okay the que is:
If I am to write a C/Matlab code for converting a *raw* data to 8b10b encoded 
data what exactly I need to do? or in other words, Pat how did you
*look at* the spetrum of raw and encoded data. 
How did you generate both the streams?
I hope C/Matlab are sufficient to get a feel for it.
or was is it back-of-the-envelope calculation.

Another thing:
As is quite evident from my question, I am new to this field (come from Instru 
where we are concerned about 
low freq, drift etc. aspects) So I would like to know some book(s) which will 
introduce me to ISI, Jitter,
Clock recovery and other such Networking/ Communication related physical layer 
signalling problems/phenomenon. Most of the books on Jitter are too 
mathematical to give a feel for the physical activity. Also what is this 
subject called? I searced "signalling" on Amazon.com w/o any luck.

All/any comments appreciated.

regards,
- Raja


--

On Wed, 12 Sep 2001 21:38:53  
 zanella, fabrizio wrote:
>
>We've been using serdes devices with 8b/10b encoding and phase locked loops,
>and have found the PLLs to be very sensitive to low frequency noise (<2MHz)
>at low VCC.  Does anyone know if devices which use DLLs (delay locked loops)
>are less sensitive to low frequency noise?
>thanks,  
>
>Fabrizio Zanella
>Signal Integrity
>EMC Corporation
>508-435-2075, x14645
>fzanella@xxxxxxx
>
>
>-----Original Message-----
>From: Mike Jenkins [mailto:jenkins@xxxxxxxx]
>Sent: Tuesday, September 11, 2001 8:50 PM
>To: si-list@xxxxxxxxxxxxx
>Subject: [SI-LIST] Re: purpose of 8b/10b encoding
>
>
>Pat,
>
>In addition to zero DC content and guaranteed transitions to which 
>a PLL can lock, 8B10B (that's the way IBM wrote it originally) code 
>has the string ...0011111... or it's inverse which occurs only as the 
>first 7 characters of a 10-bit character, making byte alignment to 
>these sync characters easy.  Alignment for more efficient codes (e.g., 
>64b/66b) is much more complex and statistical.
>
>The spectral content comes from limiting "running disparity" (the 
>ratio of 0's to 1's) which limits the low frequency content more 
>than, say, scrambled data, allowing the use of smaller blocking 
>capacitors.  (At these high frequencies, good capacitors with large 
>values are not cheap, if they can be gotten at all.)
>
>Lastly, 6B10B also provides a reasonably high error detection 
>capability.
>
>Besides Infninband, 8B10B has been selected by Fibre Channel, 1G and
>10G Ethernet, SerialATA, 3GIO, and, I suspect, more.  Some of the 
>popularity may be momentum, but that's a lot of people going through 
>the same cost/benefit analysis as you.  They all decided it was worth 
>the costs.
>
>Regards,
>Mike
>
>
>"Zabinski, Patrick J." wrote:
>> 
>> Jeffrey,
>> 
>> Thanks for the feedback.
>> 
>> Looking at the spectrum of an un-encoded/raw data stream versus
>> an 8b/10b-encoded data stream, I can see how the power
>> spectral density will be increased at a frequency
>> equal to the data rate/2, which would provide more
>> information for a PLL to synch onto.
>> 
>> If this is the case, is there a way of analyzing exactly how
>> much better a PLL can synch when using 8b/10b vs when
>> not using 8b/10b?  If there is, can the analysis be
>> generalized to allowing me to determine how often
>> transitions need to be in order for a PLL to lock?
>> 
>> Thanks,
>> Pat
>> 
>> >
>> > # What's the purpose of 8b/10b encoding?
>> >
>> > Generally as I understand, it is to be able to recover the clock by
>> > ensuring some number of transitions per period (8 per 10?).
>> > Clocks change
>> > period frequently when working at high speeds, due to
>> > temperature, etc... not
>> > to meantion the fact that otherwise the sending clock would
>> > inevitably be
>> > out of phase, plus period, of your receiver's clock.
>> >
>> >
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>
>-- 
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