Reference Clock Support
The xntpd daemon includes support for a number of types of reference
clocks. A reference clock is generally (though not always) a radio
timecode receiver that is synchronized to a source of standard time, such
as the services offered by the NRC in Canada and NIST in the U.S. The
interface between the computer and the timecode receiver is device depen-
dent and will vary, but is often a serial port.
For configuration purposes, xntpd treats reference clocks like normal NTP
peers. However, unlike normal peers, reference clocks are referred to by
an invalid IP address.
Reference clock addresses are of the form 127.127.t.u, where t is an
integer denoting the clock type and u indicates the type-specific unit
number, in the range 0-3, that is used to identify multiple instances of
clocks of the same type. Most of these clocks require support in the form
of a serial port or special bus peripheral. The particular device is nor-
mally specified by adding a soft link /dev/device%d to the particular
hardware device involved. The device is compiled in xntpd according to the
clock type.
The following table lists the supported reference clock types, device
names, clock names, and descriptions:
________________________________________________________________
Type Device Name Description
________________________________________________________________
1 (none) LOCAL Undisciplined Local Clock
3 pst WWV_PST PSTI/Traconex WWV/WWVH Receiver
4 wwvb WWVB_SPEC Spectracom WWVB Receiver
5 true TRUETIME TrueTime GPS/GOES/OMEGA Receivers
15 true TRUETIME TrueTime Generic Receivers (alias)
18 acts NIST_ACTS NIST Automated Computer Time Service
25 true TRUETIME TrueTime Generic Receivers (alias)
________________________________________________________________
Note
Although clock types 15 and 25 still exist as aliases for True-
Time clocks, it is best to use type 5, because the aliases might
change in the future.
Reference clocks are configured using a server statement in the configura-
tion file. Typically, this is the only command necessary to configure a
reference clock. The following is the format for this command:
server 127.127.t.u [prefer] [mode m] [minpoll #] [maxpoll #]
In the preceding command:
t Specifies the clock type number.
u Specifies the unit number. This is typically 1, but can range from 0-
3.
prefer
Modifies the clock selection algorithm.
mode m
Specifies a clock mode for those clock drivers that support multiple
modes of operation.
minpoll #
maxpoll #
Specifies the minimum and maximum polling interval for reference clock
messages, in seconds to the power of two. For most directly connected
reference clocks, both minpoll and maxpoll default to 6 (64 seconds).
For modem reference clocks, minpoll defaults to 10 (147.1 minutes) and
maxpoll defaults to 14 (4.5 hours). The allowable range is 4 (16
seconds) to 17 (36.4 hours) inclusive.
The clock drivers, and the addresses used to configure them, are described
as follows:
127.127.1.u - Undisciplined Local Clock
This driver can have the following applications:
+ Allow a machine to use its own system clock as a reference clock,
using no outside clock discipline source. This is useful if you want
to use NTP in an isolated environment with no radio clock or NIST
modem available. Choose a machine that has a good clock oscillator
and configure it with this support. Set the clock using the best
means available. Then, point all the other machines at this one or use
broadcast (not multicast) mode to distribute time.
+ You want to use a particular server's clock as the clock of last
resort when all other normal synchronization sources have gone away.
This is useful if that server has an ovenized oscillator. For this you
would configure this clock at a higher stratum (3 or 4) to prevent the
server's stratum from falling below that.
+ An external discipline source is available, such as the NIST
"lockclock" program, which synchronizes the local clock using a tele-
phone modem and the NIST Automated Computer Time Service (ACTS), or
the Digital Time Synchronization Service (DTSS), which runs on DCE
machines. In this case, set the stratum to zero, indicating a bona
fide stratum-1 source. Use this with caution since there is no easy
way to telegraph through NTP that something might be wrong in the dis-
cipline source itself. In the case of DTSS, the local clock can have
a rather large jitter, depending on the interval between corrections
and the intrinsic frequency error of the clock oscillator. In extreme
cases, this can cause clients to exceed the 128-ms slew window and
drop off the NTP subnet.
In the default mode, the behavior of the clock selection algorithm is modi-
fied when this support is in use. The algorithm is designed so that the
local clock support is not selected unless no other discipline source is
available. This can be overridden with the prefer keyword of the server
configuration command, in which case only this support is selected for syn-
chronization and all other discipline sources are ignored. This behavior
is intended for use when an external discipline source controls the system
clock.
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