[189277] in North American Network Operators' Group
Re: NIST NTP servers
daemon@ATHENA.MIT.EDU (Mel Beckman)
Fri May 13 18:12:38 2016
X-Original-To: nanog@nanog.org
From: Mel Beckman <mel@beckman.org>
To: Lamar Owen <lowen@pari.edu>
Date: Fri, 13 May 2016 22:12:31 +0000
In-Reply-To: <E75D69EF-0D3E-442F-A032-CF440B7D1BBE@beckman.org>
Cc: NANOG <nanog@nanog.org>
Errors-To: nanog-bounces@nanog.org
"Either method needs the specs" should read "Either method meets the specs.=
"
-mel beckman
> On May 13, 2016, at 1:39 PM, Mel Beckman <mel@beckman.org> wrote:
>=20
> Lamar,
>=20
> Because you need microsecond-level time accuracy (which is beyond NTP's c=
apabilities) you'll requires an adjunct protocol, such as PPS, to get that.=
For continued NTP delivery despite periodic GPS signal loss, then you nee=
d an OCXO internal clock.=20
>=20
> But anyone satisfied with NTP's millisecond time accuracy at worst needs =
a $1 temperature-compensated internal clock. Either method needs the specs =
for a Stratum 1 time source on a local network.=20
>=20
> As you point out, the hobbyist SBCs can't deliver even basic clock accura=
cy. =20
>=20
> But another key consideration beyond accuracy is the reliability of a ser=
ver's GPS constellation view. If you can lose GPS sync for an hour or more =
(not uncommon in terrain-locked locations), the NTP time will go free-runni=
ng and could drift quite a bit. You need an OCXO to minimize that drift to =
acceptable levels.=20
>=20
> But I see that the price premium for an OCXO clock is only $100 to $200 o=
n low-cost (I.e., ~$500) commercial NTP servers. So buyers need only make a=
minor cost adjustment to have very good, and inexpensive, COTS NTP perform=
ance and reliability.=20
>=20
> -mel beckman
>=20
>>> On May 13, 2016, at 9:30 AM, Lamar Owen <lowen@pari.edu> wrote:
>>>=20
>>> On 05/13/2016 10:38 AM, Mel Beckman wrote:
>>> You make it sound like TXCOs are rare, but they're actually quite commo=
n in most single board computers. True, you're probably not gonna find them=
in the $35 cellular-based SBCs, but since these temperature compensated os=
cillators cost less than a dollar each in quantity, they're quite common in=
most industrial species for well under $100.
>>=20
>> Correct, they're not rare in the industrial line (for that matter you ca=
n get TCXO-equipped RTL-SDR dongles, but that's not NTP-related). Somethin=
g like a Transko TFC or TX-P or similar is enough for reasonable timing for=
basic purposes, and they're not expensive. They're also not a stock item =
on the consumer-level SBC's either. I looked at one of our half-dozen ODro=
id C2's, and the main processor clock, X3, is under the heatsink, so I can'=
t see what part is being used. X1 and X2 are outside, and it doesn't appea=
r that they are TCXO modules, although I didn't use a magnifier to check th=
e part number and might have made an error.
>>=20
>> The Nicegear DS3231 RTC has a TCXO, and might be the best low-cost choic=
e at $12 (need to have an RPi, ODroid, or similar on which to mount it). I=
t's not that TCXO's are rare or expensive, it's that they're not often cons=
idered to be important to accuracy in many circles.
>>=20
>>> An Ovenized XCO is absolutely not required for IT-grade NTP servers.
>>=20
>> No, but it is for my purposes here. But, as I said in my post:
>>=20
>>=20
>>> You really have to have at least a temperature compensated quartz cryst=
al oscillator (TCXO) to even begin to think about an NTP server, for anythi=
ng but the most rudimentary of timing. Ovenized quartz oscillators (OCXO) =
and rubidium standards are the next step up, ...
>>=20
>> I was just saying that OCXO and Rb are just the next step up if you woul=
d like more stability, that's all. For 'within a second' on a GPS-discipli=
ned clock (even without the 1PPS signal) you wouldn't necessarily need TXCO=
. But that's what I meant by 'anything but the most rudimentary of timing.=
' Rudimentary is down to the millisecond in my environment. PTP takes you=
to the next level, and beyond that you don't use network timing but put a =
dedicated time distribution network running IRIG-B or similar. But that is=
beyond the scope of a typical IT NTP server's needs.....
>>=20
