atcomsystems.ca/forum Forums Wiring - Telecom Carriers - Trunks & Services T1's, CSU's and DSU's Frequency Drift?

So, I have a clinic with a Plain old T1. Our equipment, a Nortel Option 11, is continuously slipping. It will run for a few hours to a few days, and then spend a couple of hours with static, cutoffs, channels lockout, ect.

When we look at the span it is running 4 hz fast and drifts to 5 hz, then back to 4. The telco tech says this is normal?
Doesn't the span need to be exactly 1.544MHz? Not +4 or 5?

Never heard of frequency shift causing slips, plus that's not much. If you're getting slips and only slips it's a timing problem on your end, assuming it's not a point to point T-1.

If you ran a T1 at exactly 1.544Mbs any slight line impairement would cause the T1 to dip below 1.544Mbs and you would lose channels (not enough bandwidth). By running it a little faster then 1.544Mbs your hoping that any slight line imparment will cause the T1 to stay at or abve 1.544Mbs.

OK, I don't do T1's, and have no idea what the specs say for frequency tolerance, but I just did the math, and if you really mean 4 to 5 Hertz fast, then it is .00032% fast at 5Hz or an accuracy of 99.9997% (better than "five nines") - at that rate, I'd question if the meter is even that accurate.

edit - the above comments just reminded me of my high school science class just after calculators came out - we were dealing with the concept of significant digits, and the teacher could always tell who had slide rules, and who was using a calculators.
People were copying all the digits from the display giving 8 decimal places, when only 2 or 3 places were valid from data given.

Since your clinic is probably a level four, 4-5 Htz drift is wonderful. In case anyone cares, I believe the master (Level 1) is now the NBS cesium-atom clock in Cheyenne Mountain, CO. In the dozen or so T1's and PRI's I've supported over the years, the only clocking issues I ever had were when there wasn't one! As I recall, all my T1 issues were at voice level. Remember guys and gals, that T1 may break down to audio 1/2 a dozen or more times end to end! I remember an SWB guy in TX who swore that my new T1 was coming over 3 different paths. The way he put it, "They grabbed a few channels here and a few.." etc. Then they all broke down to audio about a mile down the road and were 'assembled' into 1 T1 and 'shipped out' to my site. Which pretty well explained why some channels could call L/D and not LCL, Some LCL and not L/D, some both, AND SOME NEITHER, when they handed it off to me. That's the day the direct trunk access on a Mitel REALLY saved the day. It works on T1, too! I was able to tell the test board EXACTLY what could and could not be dialed on each channel/trunk. Over the next few days the guy that showed up at the site for final testing called me from all around Austin.He had a pack of circuit cards that could choke an elephant!

Thanks all. I am grabbing at straws right now, thus the post.

I have rebuilt and replaced most of this system, as well as 2 not just 1 vendor. Processor, T1 card, CSU, power supply, moved slots around, everything. The problem persists. Even reprogrammed a new processor by hand, not using the backup. No go.

Just going to find a sharp corner to bang head on.

And yes, the testers are that accurate, the telco and a 3rd vendor had the exact same readings.
Drifted from +4 to +5 Hz fast.

The Telco tech the CO clocks off GPS?

Oh, best part, the CO is a old Stromberg Carlson!

dOES THE nORTEL HAVE A SEPERATELY REPLACEABLE CLOCK CARD AND HAS IT BEEN REPLACED? (Sorry, dang Caps Lock!:()

I've seen a similar problem and never did find a good solution - only thing that seemed to work was to leave the clock in free run on the Opt11, although doing so seemed to impair faxes and modems. We had replaced all the components including the PRI daughter card that provides timing.

Yes, we did replace the clock board as well.

We have a quote for a new system, I will have the vendor go out after hours, put the system on the floor and plug into the T1, if it runs clean then they can install it.

:-)

i would inquire with Adtran tech support to see if they have a solution. They are very knowledgeable people and may have some ideas without tossing the Opt11.