>> Our PRI between our two switches regularly goes down for anywhere from
>>> 30 seconds to 5 minutes. This seems to occur most between 8:30AM and
>>> 9:00AM and then again between 2:00PM and 2:30PM. Verizon claims they
>>> can't figure it out.
>> Clock slip?
> That's my gut feeling on it. I've tried explaining that to Verizon but
> it just falls on deaf ears.
Can you be more precise about what PRI "goes down" means ?
I guess that could be interpreted a lot of different ways.
When the trouble happens, are calls in progress affected, or do they stay up while "new" calls in one direction or another are blocked ?
Have you monitored the PRI line at your end with a suitable analyzer so you can see what is happening at the various layers (or does your switch provide any detailed debugging trace) ?
For example, does something strange happen down at Layer 1 (physical layer) when the problem occurs ... loss of pulses, loss of framing, excessive bit errors, T-1 alarm conditions, whatever ? Do the ESF Performance Monitoring history data show anything (I assume it is running over a B8ZS ESF T-1 with some sort of PM functionality at each end)? Is there a smart jack involved, and/or any LEDs to take a look at?
Or is some trouble occurring at Layer 2 (link layer) with the LAPD protocol causing the D-channel to appear 'down' ?
How about Layer 3 (network layer) ... are you seeing any incriminating result codes during the failure ?
While the problem exists, does the Verizon switch busy out your lines to incoming calls ? Surely their 5E or DMS or SC or GTD or whatever must maintain (or they can selectively enable) some sort of event log for your line.
Although I suppose a timing/sync problem with clock slippage **might** be the culprit, it's funny that it only manifests itself twice a day rather than constantly [for severe uncontrolled slips] or at evenly-spaced intervals throughout the day & night [for less frequent slips].
Of course, the toughest part is getting through to talk to someone at Verizon who even has a clue what you're talking about. If you're lucky enough to get to talk one-on-one with one of the switch techs or craft or engineers who can log into a maintenance console and knows what a PRI ISDN line really is, then usually you'll make some progress getting it resolved in fairly short order.
Finally it may be worth checking with the folks in comp.dcom.telecom.tech to see what they think/suggest about your problem.