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From: Daku on 6 Aug 2010 06:42
Thanks to all for your feedback. I had thought
about the baud rate idea that Tim elaborated
on, but the context in which I asked this question
was Ethernet bits going from the MAC layer
to the PHY layer at say 1 Gb/s or more (for the
newer standards).



On Aug 6, 3:25 am, "k...(a)att.bizzzzzzzzzzzz" <k...(a)att.bizzzzzzzzzzzz>
wrote:
> On Fri, 06 Aug 2010 06:49:33 +1000, Grant <o...(a)grrr.id.au> wrote:
> >On Thu, 5 Aug 2010 16:19:00 -0400, "Tom Biasi" <tombi...(a)optonline.net> wrote:
>
> >>"Daku" <dakup...(a)gmail.com> wrote in message
> >>news:1f08b0d9-9d6a-4cf3-93cc-19803e81b278(a)y32g2000prc.googlegroups.com...
> >>>I understand that they are different domains (Hz - analog and bits/sec
> >>> - digital), how would one
> >>> convert e.g., 1Gbit/s to corresponding Hertz
> >>> frequency ?
>
> >>Hi,
> >>Tim is quite knowledgeable on these subjects and gave some usefull info but
> >>maybe your question is not quite so deep.
> >>Hertz is used to describe cycles/second (cycles per second). If you allow it
> >>to mean events per second then a bit can be an event.
> >>So 1Gb/sec is 10^9bits / sec or 1 Ghz.
> >>This is for mental clarity and the actual units may vary.
>
> >Yes, Tim touches on the old baud rate (symbol rate) vs frequency that
> >you're suggesting as the simple answer.
>
> >Both are not incorrect (double negative only to show grey area).
>
> >For example 100Mbps network link bangs four symbols on each of 1/25MHz
> >to get the speed, and Gbps copper network links don't run at 1GHz, they
> >run slower speed, using a couple pairs in each direction.
>
> >So, in this case, context matters.
>
> >Where I said symbols, Tim said constellation, Tim is more accurate :)
>
> Not really. Symbols are the stars in a constellation of stars. The symbol is
> the basic unit of information transferred. A constellation is a
> representation of the possible physical values (voltage, frequency, phase,
> whatever).
>
> A little more specifically, a symbol may be a specific value in the
> constellation or a transition from the current point to another.

From: Cydrome Leader on 6 Aug 2010 11:26
Tim Wescott <tim(a)seemywebsite.com> wrote:
> On 08/05/2010 08:04 AM, Daku wrote:
>> I understand that they are different domains (Hz - analog and bits/sec
>> - digital), how would one
>> convert e.g., 1Gbit/s to corresponding Hertz
>> frequency ?
>
> Without knowing more, one can't. In general, for a given modulation
> scheme, increasing the data rate involves scaling everything down
> equally in time, which scales everything up equally in frequency.
>
> But there are so many different ways to encode data onto a signal that
> the bandwidth vs. data rate spread is pretty wide. There are practical
> systems that have 16-point constellations (four bits/symbol) that can
> pack in four bits/Hz; there may be practical (or at least fitfully
> practical) systems that can do six bits/Hz. On the other hand, there
> are systems that are vastly spectrally inefficient; with those the sky's
> the limit as far as wasted bandwidth.

MIME encoded XML is one serious contender for wasted bandwidth.
From: krw on 6 Aug 2010 20:30
On Fri, 6 Aug 2010 03:42:15 -0700 (PDT), Daku <dakupoto(a)gmail.com> wrote:

>Thanks to all for your feedback. I had thought
>about the baud rate idea that Tim elaborated
>on, but the context in which I asked this question
>was Ethernet bits going from the MAC layer
>to the PHY layer at say 1 Gb/s or more (for the
>newer standards).

Again, you need to know how many bits per symbol are used. IIRC GBE is four
bits (4-pairs in use).
From: Grant on 7 Aug 2010 00:17
On Fri, 06 Aug 2010 19:30:03 -0500, "krw(a)att.bizzzzzzzzzzzz" <krw(a)att.bizzzzzzzzzzzz> wrote:

>On Fri, 6 Aug 2010 03:42:15 -0700 (PDT), Daku <dakupoto(a)gmail.com> wrote:
>
>>Thanks to all for your feedback. I had thought
>>about the baud rate idea that Tim elaborated
>>on, but the context in which I asked this question
>>was Ethernet bits going from the MAC layer
>>to the PHY layer at say 1 Gb/s or more (for the
>>newer standards).
>
>Again, you need to know how many bits per symbol are used. IIRC GBE is four
>bits (4-pairs in use).

I think 100Mbps symbol rate is 25Mhz, 4 bits/symbol, 1 pair each
direction, but no idea what magic's done for 1Gbps over the 4 pairs.

Grant.
From: Gareth on 7 Aug 2010 13:30

On 05/08/2010 16:04, Daku wrote:
> I understand that they are different domains (Hz - analog and bits/sec
> - digital), how would one convert e.g., 1Gbit/s to corresponding Hertz
> frequency ?

According to the Shannon-Hartley theorem, the highest possible data rate is:

C = B*log2(1 + S/N)

Where C is the channel capacity in bits per second

B is the channel bandwidth in Hz

Log2 is base 2 logarithm

and S/N is the signal to noise power ratio (as a linear ratio not in dB)

In practice you will always get less than this, though some modulation
schemes can get quite close.

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