On 05/13/2016 03:39 PM, Eric S. Raymond wrote:
Traditionally dedicated time-source hardware like rubidium-oscillator GPSDOs is sold on accuracy, but for WAN time service their real draw is long holdover time with lower frequency drift that you get from the cheap, non-temperature-compensated quartz crystals in your PC. There is room for debate about how much holdover you should pay for, but you'll at least be thinking more clearly about the problem if you recognize that you *should not* buy expensive hardware for accuracy. For WAN time service, in that price range, you're wither buying holdover and knowing you're doing so or wasting your money.
Eric, Thanks for the pointers; nice information. A cheap way to get a WAN frequency standard is to use a WAN that is delivered over something derived from the telco's synchronous network; a POS on an OC3 with the clock set to network has an exceptionally stable frequency standard available. Less expensive, get a voice T1 trunk delivered (robbed-bit signaled will typically be less expensive than PRI) and grab clock from that; tarriffs for RBS T1/fractional T1 around here at least are less than an analog POTS line). Very stable. The plesiochronous digital hierarchy on copper or synchronous digital hierarchy/SONET on fiber have cesium clocks behind them, and you can get that stability by doing clock recovery on those WAN circuits. Back when this was the most common WAN technology frequency standards were there for the taking; Ethernet, on the other hand, not so much. But a nice catch on using the isochronous nature of USB. Cheap webcams also take advantage of the isochronous transfer mode. Do note that isochronous is often not supported in USB-passthrough for virtualization, though. But you shouldn't use a VM to do timing, either. :-) Now I'm looking for myself one of those Navisys devices you mentioned..... do any of them have external antenna inputs, say on an SMA connector (MCX is in my experience just too fragile) with a bias tee to drive phantom to an active antenna? The quick search I did seemed to indicate that the three you mentioned are self-contained with their own smart antenna. External antenna input would be required here, where we use timing-grade GPS antennas to feed our Z3816's. But for straight 1PPS and GPS timecode, dealing with the Z3816's complexity is overkill. Thanks again for the info; looking forward to seeing how NTPsec develops.