Re: [time-nuts] Smart fiber-optic cable ( a reference to Hp's smart clock )

[Hal Murray]

g...@partiallystapled.com said:
> Take a look at CERN's White Rabbit project. They're doing sub-nanosecond
> time transfer over fiber, including compensating for temperature variations.

The other group doing that sort of work that I know about is the radio 
astronomers in Chile.

I don't have a good URL handy.  Something about it was posted here a year or 
3 ago.


-- 
These are my opinions.  I hate spam.



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Re: [time-nuts] Smart fiber-optic cable ( a reference to Hp's smart clock )

[Robert Darlington]

My gut feeling is that it's pretty easy to measure propagation delay over
fiber, especially if it's long.  Is there some reason you can't just bounce
light off the far end of the cable and measure round trip time?  I don't
know the electronics involved in signaling with fiber but surely you could
have a 2nd piece of fiber closely coupled /  run along side the  data cable
with a mirrored end and just measure it with a TDR.

Of course, I've never tried this and there might be some major engineering
challenges.  Any thoughts on this?

-Bob


On Mon, Feb 4, 2013 at 3:39 PM, mike cook  wrote:

> I think someones on it.
>
> check out
> 
>
> Le 4 févr. 2013 à 23:09, Stanley a écrit :
>
> > If a fiber-optic cable had temperature sensors either installed with or
> embedded inside of this could make for better modeling changes in delay
> making more accurate transfer of time and frequency possible. With fiber to
> tower installs now under way to provide more data at cell towers why not
> backup GPS frequency and time transfer with the same medium ? Would this
> also increase the data rate of the cable ? That is faster rates due to the
> better timing uncertainty.
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Re: [time-nuts] Smart fiber-optic cable ( a reference to Hp's smart clock )

[mike cook]

I think someones on it.

check out 


Le 4 févr. 2013 à 23:09, Stanley a écrit :

> If a fiber-optic cable had temperature sensors either installed with or 
> embedded inside of this could make for better modeling changes in delay 
> making more accurate transfer of time and frequency possible. With fiber to 
> tower installs now under way to provide more data at cell towers why not 
> backup GPS frequency and time transfer with the same medium ? Would this also 
> increase the data rate of the cable ? That is faster rates due to the better 
> timing uncertainty. 
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Re: [time-nuts] Smart fiber-optic cable ( a reference to Hp's smart clock )

[Michael Tharp]

On 02/04/2013 05:09 PM, Stanley wrote:

If a fiber-optic cable had temperature sensors either installed with or 
embedded inside of this could make for better modeling changes in delay making 
more accurate transfer of time and frequency possible. With fiber to tower 
installs now under way to provide more data at cell towers why not backup GPS 
frequency and time transfer with the same medium ? Would this also increase the 
data rate of the cable ? That is faster rates due to the better timing 
uncertainty.


Take a look at CERN's White Rabbit project. They're doing sub-nanosecond 
time transfer over fiber, including compensating for temperature variations.


http://www.ohwr.org/projects/white-rabbit/wiki

Here is a demonstration in which fiber is blasted with a heat gun:
https://www.youtube.com/watch?v=ZSRQEExbdq8

The hardware is rather specialized at the moment but it seems there is 
ongoing work on making it more generally available. Certainly if the 
telcos were interested they could make it happen, but probably they are not.


There is quite a lot of useful, open-source material coming out of this 
project including a VHDL implementation of a Vernier TDC. At some point 
I plan to use it to build a high-performance TIC.


-- m. tharp
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Re: [time-nuts] Smart fiber-optic cable ( a reference to Hp's smart clock )

[Jim Lux]

On 2/4/13 2:39 PM, Bob Camp wrote:

Hi

Consider that cost to manufacture the cable goes up as you put stuff in it. You 
not only need sensor packages, you also need to connect them so they can report 
data. Unless the sensors are optically powered and linked, they would 
compromise the inherent lighting immunity the fiber provides.

It's a *lot* easier to do the same thing with a round trip traverse of the 
entire optical link. This is already done by at least one supplier of these 
parts. It may be done by several others as well.

Cable delay is not the limiting issue on most short link systems. It's not 
clear that data rates would be improved by delay modeling.



I took the suggestion as providing a way to provide a timing reference 
at the cell site (used for a variety of purposes E911, etc.) instead of 
GPSDOs and Rb clocks, not so much improving data rates.


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Re: [time-nuts] Smart fiber-optic cable ( a reference to Hp's smart clock )

[Bob Camp]

Hi

Consider that cost to manufacture the cable goes up as you put stuff in it. You 
not only need sensor packages, you also need to connect them so they can report 
data. Unless the sensors are optically powered and linked, they would 
compromise the inherent lighting immunity the fiber provides. 

It's a *lot* easier to do the same thing with a round trip traverse of the 
entire optical link. This is already done by at least one supplier of these 
parts. It may be done by several others as well. 

Cable delay is not the limiting issue on most short link systems. It's not 
clear that data rates would be improved by delay modeling.

Bob


On Feb 4, 2013, at 5:09 PM, Stanley  wrote:

> If a fiber-optic cable had temperature sensors either installed with or 
> embedded inside of this could make for better modeling changes in delay 
> making more accurate transfer of time and frequency possible. With fiber to 
> tower installs now under way to provide more data at cell towers why not 
> backup GPS frequency and time transfer with the same medium ? Would this also 
> increase the data rate of the cable ? That is faster rates due to the better 
> timing uncertainty. 
> ___
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Re: [time-nuts] Smart fiber-optic cable ( a reference to Hp's smart clock )

[Jim Lux]

On 2/4/13 2:09 PM, Stanley wrote:

If a fiber-optic cable had temperature sensors either installed with or 
embedded inside of this could make for better modeling changes in delay making 
more accurate transfer of time and frequency possible. With fiber to tower 
installs now under way to provide more data at cell towers why not backup GPS 
frequency and time transfer with the same medium ? Would this also increase the 
data rate of the cable ? That is faster rates due to the better timing 
uncertainty.



ACtually, you could *measure* the delay in the cable by sending a signal 
down it and then sending it back on another fiber in the same bundle.


I suspect, though, that this strategy would be more expensive than a 
$1000 GPS disciplined OCXO or something.


Setting up a scheme to measure the prop delay (or temp) might be fairly 
expensive, when the GPS strategy is pretty inexpensive and reliable.


That said, radio astronomers and the Deep Space Network do measure fiber 
temperatures and stuff, but those guys and gals are obsessive about 
measuring everything to a fraction of a gnat's eyelash. (and 
compensating for the CTE of the gnat).


 For another thing, I'll bet the same entity doesn't own the cell site, 
the equipment, the fiber, and the other end.  That is, if you're ACME 
cell company, you basically buy a "data connection" at your cellsite, 
and someone else worries about how those bits get to the other end. 
There's some service level agreement that specifies error rates and 
availability.   I don't know, though.  They may just rent dark fiber.


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[time-nuts] Smart fiber-optic cable ( a reference to Hp's smart clock )

[Stanley]

If a fiber-optic cable had temperature sensors either installed with or 
embedded inside of this could make for better modeling changes in delay making 
more accurate transfer of time and frequency possible. With fiber to tower 
installs now under way to provide more data at cell towers why not backup GPS 
frequency and time transfer with the same medium ? Would this also increase the 
data rate of the cable ? That is faster rates due to the better timing 
uncertainty. 
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