[time-nuts] inexpensive fiber optic distribution

[Julien Goodwin]

On 21/3/19 5:27 pm, Hal Murray wrote:
> 
> jimlux at earthlink.net said:
>> It seems like you could probably figure out how to interface to these  things
>> and use them to distribute timing signals. 
> 
> There are two big advantages of fibers:
>   They work well for long distances.
>   No ground loops.

Pure optical amplification, and multiplexing is also possible, less
useful for pure time & frequency use though.

However for long distance you start to get into fun issues like thermal
affects etc, and running something like Sync-E might actually make more
sense than a pure signal.

> The target market for those contraptions is networking.  The signal is 
> digital, and they get to work with a "nice" signal, probably 8B/10B.  That's 8 
> bits of data in 10 bits on the wire.  Roughly, they pick 256 out of 1024 
> patterns that have minimal long strings of 0s and 1s and use a few more 
> patterns for control - things like idle, and begin/end of packet.  It works 
> fine if AC coupled.

8b/10b in the older signals up to 1g, 64b/66b in 10g. These days native
25g, 50g and more are combined to make 100g / 200g / 400g Ethernet.

> I don't know how well they will work for something like a PPS.  Somebody 
> should try it.  The signal pattern is mostly trying to be friendly to the PLL 
> that does clock recovery and/or the AGC that sets up the switching level.
> 
> If I wanted to send a PPS, I'd use something simple like Manchester encoding.  
> That assumes you can line your transmit clock up with the PPS.  It won't work 
> if you just want a link-extender for a PPS.
> 
> If you are connecting to a FPGA, use whatever they support for high-speed 
> serial interface.  Again, that assumes you can line your signal up with the 
> transmit clock.
> 
> I'm pretty sure the signaling is differential, either PECL or one of the newer 
> standards like LVDS.

Yep, all diff I/O, details are in the spec. Random sample:
http://www.kt.agh.edu.pl/~lason/SFP/TRX100007/SCP6Gx8c.pdf

>> They take a standard singlemode duplex fiber and have a pinout suitable  for
>> plugging into a Cisco, etc. switch. Apparently, there's some ...
> 
> I'm pretty sure none of the programming is on the high speed path.  Maybe 
> changing the signal levels.  I don't know how much of it is total BS, aka 
> vendor lock in, as compared to actually doing something useful.  I'd be 
> surprised (but not much) if they didn't work fine with no programming.

Correct, the control path is out of band, and just I2C.

The lock in is almost entirely vendor profiteering, optical monitoring &
wavelength switching are standards (where implemented), but plenty of
vendors use (almost always trivially bypassable) lock-in to sell SFPs
they rebrand from the same OEMs at up to 100x markup.

The folk in the big telcos (or in my case, big content providers) who
run the global backbones have plenty of stories.