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Is the signal in a fiber optics digital or analog ?

Posted on 2011-05-11
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Is the signal transmitted over a fiber digital or analog ? I understand the content of signal it self may be digital like some document or a mp3 over the SONET or the fiber over HFC network.
But is the data signal be modulated and be transmitted as light (some form of EMW) which is analog signal ?
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Question by:wilsonchtam
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by:Dave Baldwin
ID: 35743043
Technically, all signals on a wire or a fiber are analog because they are affected by their environment.   It's how they are used that determines whether they are considered analog or digital.  For something to be a signal, there have to be changes which can be considered modulation.
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by:lewisg
ID: 35743180
For short haul the signal can be directly modulated so you could say the pulses were digital. For longer distances the laser may operate in continuous wave mode with other modulation schemes. While this could be considered analog it really isn't. The actual signal, not the carrier, would still be digital.
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Author Comment

by:wilsonchtam
ID: 35743233
DaveBaldwin:
I don't quite follow the logic that all signals on a wire or a fiber are analog. Like I raised a question earlier in this community earlier, I got another expert answer that the signal in a LANs cable should be digital as 802.3 specifies that the signal transmitted in a cat5e cable should be in a square wave form which is digital.
My understanding is that the carrier signal on the wire between 2 computers should be digital.



 
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by:wilsonchtam
ID: 35743246
Lewisg:
My question is 'Is the carrier signal (not the content) analog or digital ?'.
You comment remind me something I forgot.
Do you mean the carrier signal in the fiber can be digital or analog depending if it haul length of the fiber ?

Thanks
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by:lewisg
ID: 35743379
'Is the carrier signal (not the content) analog or digital ?'

Is light analog? It depends...

In the case of a carrier it could be thought of as a "one note" analog signal, not very interesting and incapable of carrying information on it's own. By definition an analog signal must vary to carry information.


"Do you mean the carrier signal in the fiber can be digital or analog depending if it haul length of the fiber?"

There are many schemes to transmit data over any given medium. The issue with using some "simple digital" signaling like turning a signal on and off is that over distance things tend to get fuzzy and the transitions loose their crispness. This not only effects reliability but slows the maximum possible data rate. The "cure" is to leave the signal "on" as a carrier for a higher frequency path. I think it would be a mistake to describe any of these signal carrying schemes as analog since they quickly translate to 1&0, on&off, high&low.

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Author Comment

by:wilsonchtam
ID: 35743502
Lewisg:
I agree that sometime it is difficult to distinguish analog and digital. Light can be considered as EM wave but can also be considered as quantum.
Maybe I rephrase my question.
 Does the optics fiber use the on&off to transmit the 1/0 digital signal ?
Or the light is used as carrier to modulate with the digital content to transmit through the fiber (similar to a modem modulate the digital signal from a computer to transmit through the HFC network).

It seems your previous comment implies both can be true depending on length of the haul.
Thanks
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by:lewisg
ID: 35743687
My understanding is that either method can, and has, been used.
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by:wilsonchtam
ID: 35743770
Do you know which method is used on SONET or SDH ?
Thanks
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by:lewisg
ID: 35743801
Since both are multiplexed I would assume they use carriers.
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by:Dave Baldwin
ID: 35744089
Digital is the realm of 1s and 0s.  On any cable or fiber, there are no ones and zeros, only more and less, and the degree of more and less is affected by the environment.  Longer distances means lower signal levels at the far end.

Now as long as the levels can be determined well enough, you can interpret them as 1s and 0s.  Note that 100BaseTX network signals are actually a 25MHz signal that is phase modulated to send the equivalent of 4 bits at a time.  You can also read this article on SONET http://en.wikipedia.org/wiki/Synchronous_optical_networking and see all the analog artifacts that have to be dealt with to get the proper digital interpretation.

Although these are 'digital signals' because they are used that way, on the wire and on the fiber, they are analog in that they are not 1s and 0s but simply more and less.  It is the job of the circuits connected to them to make sure they can be interpreted as digital signals with 1s and 0s.  Even modern high speed motherboards have to be analyzed for their analog properties such as capacitance and resistance to make sure they can be used as digital signal paths.
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by:NetFixr-Dani
ID: 35746579
Hrm, this is a most interesting discussion.  From what all the experts above have said, it seems that the answer can be summarized as:

The actual transmission on the medium (fiber or HFC) is an analog carrier, because it is a waveform that has a multitude of values/properties (amplitude, phase, frequency, etc).  The data signal that is transmitted on that carrier is digital, because it typically consists of simply two possible values (0,1), and in some cases 4 values.

The digital data signal of 0's and 1's is modulated onto the carrier when it leaves the host that is transmitting the data (PC, router, switch, etc.), and then demodulated when it is received at the remote end to the exact match of 0's and 1's that were originally sent (provided there is no disruption/degradation on the medium).  If the signal were analog, then it would be acceptable to have an inexact version at the remote end, and nobody would be happy if that were the case.

Specific to "HFC" - that's why the data termination equipment at the end of CATV-based Internet is called a "cable modem" to do the MOdulation/DEModulation.

- Dani
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by:NetFixr-Dani
ID: 35746657
Hrm, after re-reading the original question and my previous comment, I wanted to add two more things. . .

1) At the demodulation side, the analog carrier is interpreted to re-create a fresh copy of the original digital signal.

2) Back to the original question, by definition the "signal" (actual useful data) is 100% digital on fiber and HFC.  It is the carrier (lightform or radio frequencies, respectively) that is in fact analog.  If the carrier were digital, then it would be a series turning a light source on/off (in the case of fiber) or transmitting and then stopping to transmit a signal (on the HFC).  As the others have mentioned, environmental issues such as dropouts, latency, degradation as well as timing, would make a series of signal on / signal off (e.g. Morse code) ineffective at high data rates.

- Dani
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by:Dave Baldwin
ID: 35748896
If you understood the circuits that are used, you would see that 'digital' states are defined by the analog values (usually volts) of the signals.  This http://en.wikipedia.org/wiki/Digital_signal is an ok description of 'Digital signals'.  This one http://en.wikipedia.org/wiki/Analog-to-digital_converter is interesting because it shows how an analog signal can represent many digital states.
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Author Comment

by:wilsonchtam
ID: 35751613
I agree that the amplitude value of a analog signal can be used to represent digital value.
Let me re-phase my question.
Does the SONET employs a series turning a light source on/off to transmit the hundreds of Mbits signal in the fiber ?
Or the light is modulated to transit across the SONET and the receiver has some component to demodulate the light to get back the digital content ?

Thanks
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by:Dave Baldwin
ID: 35751709
On and Off is modulation.  This is so far the best reference I think I can provide you: http://icwic.cn/icwic/data/pdf/cd/cd077/Sonet%20Optical%20Transceiver,%20Rx,%20Tx/a/480586.pdf   It is the specs for a SONET electro-optical interface assembly.  Note all the analog specs that are used to describe it's operation.
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Accepted Solution

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NetFixr-Dani earned 1000 total points
ID: 35752502
SONET does not turn the laser on/off.  The light is always on, and the digital signaling, as well as timing and path information, is modulated into variations in the analog light carrier.

- Dani
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Author Closing Comment

by:wilsonchtam
ID: 35761105
I think the question itself is complicated and I didn't define it well at the beginning. Two members provided a lot of feedback and this last comment act like a summary in answer the question.
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by:lewisg
ID: 35761216
So, "Two members provided a lot of feedback", but you accept the solution from only one? Actually there were three members that provided intelligent information. Thanks!

The best answer to your original question is still the first one you got from me. Because that IS the way it works. If you wanted to know how SONET works you should have asked THAT question or simply gone to Wikipedia.
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