[time-nuts] Frequency from telco

[Brian Kirby]

When I worked for a telco (I left 3 years ago) , we used 2 sources of 
timing.  The first was a timing signal, a T1 from AT&T.  This is a 
special T1, it does not carry any voice or data over it.  This was the 
primary timing we used for our networks.

For backup, we used GPS-Disciplined rubidiums on the switches and the DACs.

I personally would not consider telephone network timing for any 
precision work.  The paths/routing constantly change,  there is bit 
stuffing/droppings at SONET and ATM levels, etc., and thats on good 
circuits.  Throw in a bad circuit and you have bit errors and now other 
framing and timing problems....

I cannot remember when/where, but I believe AT&T removed a lot of their 
cesium clocks and they may only reference to about 15 or so sources 
now.  I read a article were they paired GPS discipline rubidiums with 
their few cesiums  and propagated the signal down hill to other GPS 
disciplined rubidiums that compared to their higher references and GPS 
and voted/averaged their outputs.



Hal Murray wrote:
> Has anybody tried using the phone company as a frequency reference?
>
> Are the telco master clocks locked to GPS or national frequency sources?  Or 
> do they just use their own Cesium box?
>
> Is that a useful way to get a good clock without a GPS antenna?
>
>
> I worked with T1 many years ago.  It wouldn't be hard to extract a signal to 
> feed to a PLL.
>
> Has anybody done that and collected data?  I'd expect lots of short term 
> jitter and wander but the long term should be pretty good.
>
>
> Does anybody know how DSL works?  If I poke around with a scope in my DSL 
> modem/router, will I find a clock locked to the telco's master clock?
>
> How many different versions of DSL are there?
>
>
>