Is there another name for Baseline Wander ?

On Fri, 17 Apr 2015 13:14:30 +0000, glen herrmannsfeldt wrote:

> Tim Wescott <seemywebsite@myfooter.really> wrote:
> 
> (snip, someone wrote)
>>> Ethernet uses 8b/10b encoding to remove DC offsets.
>  
>>> http://en.wikipedia.org/wiki/8b/10b_encoding
>  
>> Is it used in Gigabit?  Gigabit uses a multi-level coding scheme, so
>> just encoding the raw data as 8b/10b wouldn't necessarily result in a
>> zero-DC signal.
> 
> The fiber ethernets use it, 1000baseSX and such.
> 
> They use codes with either 5 ones and 5 zeros, or pairs of codes with 4
> ones and six zeros, or the other way around. That, and a running
> disparity, allows one to keep close enough to balanced. They also don't
> use codes that will result in more than four consecutive ones or zeros. 
> After the code pairs for 256 data values are taken, there are still some
> left for other uses, such as an idle code when no data is being sent.
> 
> There are 252 codes with five 0s and five 1s, 210 pairs of codes with
> four and six, enough to remove those with too many zeros or ones in a
> row.

That sounds an awful lot like regular 10base-T and 100base-T.

I assume that the multi-level coding schemes (why does my brain want to 
replace "coding" with "marketing"?) also have some sort of zero DC 
property built into them.  I just don't know.

-- 

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Tim Wescott <seemywebsite@myfooter.really> wrote:

(snip regarding 8b10b coding)
(then I wrote)
>> The fiber ethernets use it, 1000baseSX and such.
 
>> They use codes with either 5 ones and 5 zeros, or pairs of codes with 4
>> ones and six zeros, or the other way around. That, and a running
>> disparity, allows one to keep close enough to balanced. They also don't
>> use codes that will result in more than four consecutive ones or zeros. 
>> After the code pairs for 256 data values are taken, there are still some
>> left for other uses, such as an idle code when no data is being sent.
 
>> There are 252 codes with five 0s and five 1s, 210 pairs of codes with
>> four and six, enough to remove those with too many zeros or ones in a
>> row.
 
> That sounds an awful lot like regular 10base-T and 100base-T.

10baseT is Manchester on UTP. Different collision detect from
coax, but the signals are the same. 
 
> I assume that the multi-level coding schemes (why does my brain 
> want to replace "coding" with "marketing"?) also have some sort 
> of zero DC  property built into them.  I just don't know.

100baseTX uses MLT3, which reduces the bandwidth, but requires
a better S/N. There are three different voltage levels on the wire,
instead of two.

I don't know of a good reference for the PAM5 coding for 1000baseT.

-- glen

On Fri, 17 Apr 2015 17:08:07 +0000, glen herrmannsfeldt wrote:

> Tim Wescott <seemywebsite@myfooter.really> wrote:
> 
> (snip regarding 8b10b coding)
> (then I wrote)
>>> The fiber ethernets use it, 1000baseSX and such.
>  
>>> They use codes with either 5 ones and 5 zeros, or pairs of codes with 
4
>>> ones and six zeros, or the other way around. That, and a running
>>> disparity, allows one to keep close enough to balanced. They also 
don't
>>> use codes that will result in more than four consecutive ones or 
zeros. 
>>> After the code pairs for 256 data values are taken, there are still 
some
>>> left for other uses, such as an idle code when no data is being sent.
>  
>>> There are 252 codes with five 0s and five 1s, 210 pairs of codes with
>>> four and six, enough to remove those with too many zeros or ones in a
>>> row.
>  
>> That sounds an awful lot like regular 10base-T and 100base-T.
> 
> 10baseT is Manchester on UTP. Different collision detect from
> coax, but the signals are the same. 
>  
>> I assume that the multi-level coding schemes (why does my brain 
>> want to replace "coding" with "marketing"?) also have some sort 
>> of zero DC  property built into them.  I just don't know.
> 
> 100baseTX uses MLT3, which reduces the bandwidth, but requires
> a better S/N. There are three different voltage levels on the wire,
> instead of two.
> 
> I don't know of a good reference for the PAM5 coding for 1000baseT.

802.3?  ;)

It's trellis coded.
Whilst it's tempting to say that the PAM5 coding has five levels, each 
four dimensional symbol actually consists of five levels on each of the 
four pairs.  There are 5^4 = 625 possible symbols used to transmit 8 bits 
of user data at 125MBaud.  The 625 symbols are divided into two sets that 
are used every other symbol to improve the hamming distance.

I am not aware that there is any spectral control at the coding layer, 
i.e. there isn't a null at 0Hz (as would be expected with something like 
8B10B).
I may have misinterpreted the specification though.

Regards,
Allan

Allan Herriman <allanherriman@hotmail.com> wrote:

(snip, I wrote)
>> 100baseTX uses MLT3, which reduces the bandwidth, but requires
>> a better S/N. There are three different voltage levels on the wire,
>> instead of two.
 
>> I don't know of a good reference for the PAM5 coding for 1000baseT.
 
> 802.3?  ;)

Yes, but I meant ones understanable by ordinary engineers.
 
> It's trellis coded.
> Whilst it's tempting to say that the PAM5 coding has five levels, each 
> four dimensional symbol actually consists of five levels on each of the 
> four pairs.  There are 5^4 = 625 possible symbols used to transmit 8 bits 
> of user data at 125MBaud.  The 625 symbols are divided into two sets that 
> are used every other symbol to improve the hamming distance.
 
> I am not aware that there is any spectral control at the coding layer, 
> i.e. there isn't a null at 0Hz (as would be expected with something like 
> 8B10B).

Seems to me that 8B10B goes asymptotially to zero, but close enough
not to worry about it.  More important is the upper edge of the band,
which both 100baseTX and 1000baseT keep below 100MHz. At 125Mbaud,
there should be signal to 62.5MHz, and decreasing above that.

From the DSP side, using more levels allows for a reduced bandwidth,
though requires more S/N. As I understand it, the coding, especially
the increased Hamming distance noted above, recovers the lost S/N.

But also interesting for DSP is the echo cancelation required, due
to using all pairs both directions. When you consider the attenuation
over 100m of cable, the receiver has to extract the smaller signal
coming in, while a strong signal is going out. Reflections from
impedance discontinuities, such as connectors, also have to be
subtracted.  At 125Mbaud, the pulses are around 2m long on the cable.  

As far as I know, it doesn't work at all without DSP.

-- glen