236 lines
9.6 KiB
HTML
236 lines
9.6 KiB
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<HTML>
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<HEAD>
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<META HTTP-EQUIV="Content-Type" CONTENT="text/html; charset=iso-8859-1">
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<META NAME="GENERATOR" CONTENT="Mozilla/4.01 [en] (Win95; I) [Netscape]">
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<TITLE>NIST Modem Time Service
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</TITLE>
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</HEAD>
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<BODY>
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<H3>
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NIST Modem Time Service</H3>
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<HR>
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<H4>
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Synopsis</H4>
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Address: 127.127.18.<I>u</I>
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<BR>Reference ID: <TT>NIST</TT>
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<BR>Driver ID: <TT>ACTS_NIST</TT>
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<BR>Serial Port: <TT>/dev/acts<I>u</I></TT>; 1200 baud, 8-bits, no parity
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<BR>Features: <TT>tty_clk</TT>
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<BR>Requires: <TT>/usr/include/sys/termios.h</TT> header file with modem
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control
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<H4>
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Description</H4>
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This driver supports the NIST Automated Computer Time Service (ACTS). It
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periodically dials a prespecified telephone number, receives the NIST timecode
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data and calculates the local clock correction. It designed primarily for
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use when neither a radio clock nor connectivity to Internet time servers
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is available. For the best accuracy, the individual telephone line/modem
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delay needs to be calibrated using outside sources.
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<P>The ACTS is located at NIST Boulder, CO, telephone 303 494 4774. A toll
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call from Newark, DE, costs between three and four cents, although it is
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not clear what carrier and time of day discounts apply. The modem dial
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string will differ depending on local telephone configuration, etc., and
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is specified by the phone command in the configuration file. The argument
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to this command is an AT command for a Hayes compatible modem.
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<P>The driver can operate in either of two modes, as determined by the
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mode parameter in the server configuration command. In mode 0 the driver
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operates continuously at intervals determined by the fudge time1 parameter,
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as described above. In mode 1 the driver is enabled only when no other
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sources of synchronization are available and when we have gone more than
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MAXOUTAGE (3600 s) since last synchronized by other sources of synchronization.
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<P>The accuracy produced by this driver should be in the range of a millisecond
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or two, but may need correction due to the delay characteristics of the
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individual modem involved. For undetermined reasons, some modems work with
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the ACTS echo-delay measurement scheme and some don't. This driver tries
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to do the best it can with what it gets. Initial experiments with a Practical
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Peripherals 9600SA modem here in Delaware suggest an accuracy of a millisecond
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or two can be achieved without the scheme by using a fudge time1 value
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of 65.0 ms. In either case, the dispersion for a single call involving
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ten samples is about 1.3 ms.
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<P>For reliable call management, this driver requires a 1200-bps modem
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with a Hayes-compatible command set and control over the modem data terminal
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ready (DTR) control line. Present restrictions require the use of a POSIX-compatible
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programming interface, although other interfaces may work as well. The
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ACTS telephone number and modem setup string are hard-coded in the driver
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and may require changes for nonstandard modems or special circumstances.
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<P>The fudge time1 parameter represents a propagation-delay correction
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factor which is added to the value computed by ACTS when the echo-delay
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scheme is used. This scheme does not work with all modems; for those that
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don't, fudge flag2 should be set to disable the feature. In this case the
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fudge time1 parameter represents the total propagation delay due to all
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causes and must be determined by external calibration.
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<P>The ACTS call interval is determined by a counter initially set to the
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fudge time2 parameter. At each poll interval, minpoll (usually 64 s) is
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subtracted from the counter. When the counter is equal to or less than
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zero, the fudge flag1 is set, which causes up to three call attempts to
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be made to ACTS. The fudge flag1 is reset after a valid clock update has
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been determined or by a device fault, timeout or manually using <TT>ntpdc</TT>.
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After a valid clock update, the counter is reset for the next interval.
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Setting the <TT>fudge time2</TT> parameter to zero disables automatic call
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attempts. Manual call attempts can be made at any time by setting <TT>fudge
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flag1</TT> using ntpdc.
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<P>The NIST timecode message is transmitted at 1200 bps in the following
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format:
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<PRE>
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jjjjj yy-mm-dd hh:mm:ss tt l uuu mmmmm UTC(NIST) *
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jjjjj = modified Julian day
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yy-mm-dd = year, month, day
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hh:mm:ss = hours, minutes, seconds
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tt = DST indicator (see driver listing)
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l = leap-second warning (see driver listing)
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uuu = DUT1 correction (see driver listing)
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mmmmm = modem calibration (see driver listing)
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on-time = '*'</PRE>
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The timecode message is transmitted continuously after a signon banner,
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which this driver ignores. The driver also ignores all but the yy-mm-dd,
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hh:mm:ss and on-time character '*' fields, although it checks the format
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of all fields of the message. A timestamp is captured at the '*' character,
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as required by the ACTS specification, and used as the reference time of
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the timecode. If a message with an on-time character of '#' is received,
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the driver updates the propagation delay. The driver disconnects when (a)
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ten valid messages have been received, (b) no message has been received
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for 15 s, (c) an on-time character of '#' is received. These messages are
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processed by a trimmed-mean filter to reduce timing noise and then by the
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usual NTP algorithms to develop the clock correction.
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<P>Since the accumulated error grows with the interval between calls, it
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is important that the intrinsic frequency error be minimized. This can
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be done by observing difference in offsets between two calls placed some
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hours apart and calculating the uncorrected frequency error. This error,
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as a fixed-point value in parts-per-million, should be installed in the
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ntp.drift file before the daemon is started. Some experimentation may be
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necessary in order to reduce the intrinsic frequency error to the order
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of 1 ppm.
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<P>The behavior of the clock selection algorithm is modified when this
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driver is in use. The algorithm is designed so that this driver will never
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be selected unless no other discipline source is available. This can be
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overridden with the prefer keyword of the server configuration command,
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in which case only this driver will be selected for synchronization and
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all other discipline sources will be ignored.
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<P>Unlike other drivers, each ACTS call generates one clock correction
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and that correction is processed immediately. There is no wait to allow
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the clock filter to accumulate samples. In addition, the watchdog timeout
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of the local clock algorithm is disabled, so that a correction received
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from this driver that exceeds CLOCK_MAX (128 ms) causes an immediate step/slew.
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<P>Since the interval between updates can be much longer than used with
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ordinary NTP peers, the local clock procedure has been modified to operate
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in either of two modes, depending on whether the interval between updates
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is less than or greater than CLOCK_MAXSEC (1200 s). If less than this value,
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the local clock procedure operates using the standard NTP phase-lock loop
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as with other NTP peers. If greater than this value, the procedure operates
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using a modified frequency-lock loop suggested by Judah Levine in his lockclock
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algorithm designed specifically for ACTS.
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<H4>
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Call Management</H4>
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Since ACTS will be a toll call in most areas of the country, it is necessary
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to carefully manage the call frequency. This can be done in two ways, by
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specifying the interval between calls, or by setting a flag bit manually
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or via a cron job. The call interval is determined by a counter initially
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set to the fudge time2 parameter. At each poll interval, minpoll (usually
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64 s) is subtracted from the counter. When the counter is equal to or less
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than zero, the fudge flag1 is set, which causes up to three call attempts
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to be made. The fudge flag1 is reset after ten offset samples have been
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determined in a single call or by a device fault, timeout or manually using
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ntpdc. Upon successful completion of a call, the eight samples have been
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shifted into the clock filter, the local clock updated and the counter
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reset for the next interval. Setting the fudge time2 parameter to zero
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disables automatic call attempts.
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<P>Manual call attempts can be made at any time by setting fudge flag1
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using ntpdc. For example, the ntpdc command
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<PRE>
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fudge 127.127.18.1 flags 1</PRE>
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will ask for a key identifier and password and, if authenticated by the
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server, will set flag1. There may be a short delay until the expiration
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of the current poll timeout.
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<P>The flag1 can be set from a cron job in the following way. Construct
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a file with contents
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<PRE>keyid 11
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passwd dialup
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fudge 127.127.18.1 flags 1
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quit</PRE>
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Then, run the following program at specified times as required.
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<PRE>/usr/local/bin/ntpdc <file</PRE>
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<H4>
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Monitor Data</H4>
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When enabled by the <TT>flag4</TT> fudge flag, every received timecode
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is written as-is to the <TT>clockstats</TT> file.
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<H4>
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Fudge Factors</H4>
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<DL>
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<DT>
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<TT>time1 <I>time</I></TT></DT>
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<DD>
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Specifies the time offset calibration factor, in seconds and fraction,
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with default 0.0.</DD>
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<DT>
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<TT>time2 <I>time</I></TT></DT>
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<DD>
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Not used by this driver.</DD>
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<DT>
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<TT>stratum <I>number</I></TT></DT>
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<DD>
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Specifies the driver stratum, in decimal from 0 to 15, with default 0.</DD>
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<DT>
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<TT>refid <I>string</I></TT></DT>
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<DD>
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Specifies the driver reference identifier, an ASCII string from one to
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four characters, with default <TT>NIST</TT>.</DD>
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<DT>
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<TT>flag1 0 | 1</TT></DT>
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<DD>
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Not used by this driver.</DD>
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<DT>
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<TT>flag2 0 | 1</TT></DT>
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<DD>
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Not used by this driver.</DD>
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<DT>
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<TT>flag3 0 | 1</TT></DT>
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<DD>
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Not used by this driver.</DD>
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<DT>
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<TT>flag4 0 | 1</TT></DT>
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<DD>
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Not used by this driver.</DD>
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</DL>
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Additional Information
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<P><A HREF="refclock.htm">Reference Clock Drivers</A>
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<HR>
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<ADDRESS>
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David L. Mills (mills@udel.edu)</ADDRESS>
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</BODY>
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</HTML>
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