Thomas,
I get this question a lot when it comes to T.38 fax relay and Cisco modem relay regarding T1/E1 circuit timing and NTP.
In most cases the T.38 or modem relay issues are related to T1 or E1 syncronized clock timing between multiple circuits within the same chassis. For example, a single 2600 router with two T1 or E1 circuits must use a single clock source. You CANNOT have each T1 or E1 have its own clock source. If you only have a single T1 or E1 connected to the central office and you are using their clock source (clock source line) and you are not getting any errors or slips, then circuit timing is not your problem.
Basically, NTP (Network Time Protocol) and central office T1 or E1 clock sources have absolutely nothing to do with each other. NTP could never get even close to the timing accuracy of the central office T1 or E1 clock crystals. Even though they both refer to 'timing', they are are used for entirely separate functions. You need both technologies in most WANs and they are not mutually exclusive.
Here are two key points:
1) T1 or E1 central office synchronous clocks are used to keep both ends of a T1 or E1 span in nearly perfect sync. These 'clocks' cost over US$100,000 each and provide circuit timing for thousands of spans. When referring to a T1 or E1 clock, it means a bit error clock that keep 2 million bits per second syncronized between both ends within 1 in 1E10 error rate.
2) NTP, on the other hand, is for keeping disparate time clocks (not layer 1 connected) in sync to within a 10th of a millisecond by exchanging only 2 packets per 30 second interval. While this is plenty accurate to keep time between network devices close enough to not make centralized management an issue, this is still about one millionth the accuracy of the central office T1 or E1 clock source above.
Hope this helps...
/Rick
