Re: Speed

It's not really possible to directly relate datarate (like Mb/s) to
signalling speed (like MHz) without including the type of encoding or
format of the data. The datarate refers to the rate of transmission of
the actual "information", while the signalling speed refers to the
signal rate or speed in the media.

For example, consider unencoded NRZ data transmission. NRZ data means
non-return-to-zero. In other words, it means that each bit stays at
its value (1 or 0) for the duration of the bit interval. The highest
frequency of signalling is when you're sending a 1010101010 pattern.
At 100Mb/s, each bit interval is 10ns. The transmitted signal will
look like a square wave with a period of 20ns (a 1 followed by a 0).
Thus, the frequency of the square wave is 50MHz. However, since it is
a square wave and not a sine wave, there are odd harmonic frequencies
present too (150MHz, 250MHz, 250MHz, etc.)

RZ signalling is where each bit returns-to-zero at the middle of the
bit time. Thus a 1 is transmitted as a 1 level or 1/2 the bit time,
then it goes to 0 for the 2nd half of the bit time. A 0 is transmitted
as a zero for the entire bit time. In this format of signalling, the
highest frequency is present in the transmission of a continuous string
of 111111's, because this will be a transmitted as a squarewave at
100MHz if the datarate is 100Mb/s.

The above examples show that even unencoded data, sent at the same
datarate, result in dramatically different signalling rates in the
transmission, depending on the format of the data.

There are many, many encoding schemes that are used to transmit digital
data, and each of them transform the spectrum of the data signal into a
different spectrum for the transmitted signal. In general though, you
don't get anything for free. This means, that the faster the
"information" rate or data rate, the more bandwidth or speed of
signalling will be needed.

The difficulty in achieving high speed digital data transmission is
directly related to the "media" which is used to carry it - i.e.
twisted pair wires, coax, printed circuit traces, fiber optic cable, RF
signals, etc. Each type of media has its own advantages and
limitations when it comes to its use for data transmission.

AoE is a little out of date with respect to this - but you have to take
into account the context of where the statement was made. If they were
talking about driving much greater than 100Mb/s on twisted pair over
long distances - yeah, that's not trivial but it is done everyday.
However, 100Mb/s is child's play for fiberoptics for great distances.
Serial fiber optic transmission is commonly done at 10Gb/s. Many such
10Gb/s signals can be put onto the same fiber simultaneous (at
different optical wavelengths, or "colors"), thus achieving overall
throughput that's much greater. Single channel transmission can also
be done at many hundreds of Gb/s on fiber too - but there isn't really
much widespread use of that yet - since much of the ulta high speed
fiber transmission is standards based, and there aren't popular
standards out there for too much beyond the 10's of Gb/s per channel,
yet...

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