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diff --git a/contrib/ntp/html/driver18.htm b/contrib/ntp/html/driver18.htm new file mode 100644 index 0000000..5410d98 --- /dev/null +++ b/contrib/ntp/html/driver18.htm @@ -0,0 +1,235 @@ +<HTML> +<HEAD> + <META HTTP-EQUIV="Content-Type" CONTENT="text/html; charset=iso-8859-1"> + <META NAME="GENERATOR" CONTENT="Mozilla/4.01 [en] (Win95; I) [Netscape]"> + <TITLE>NIST Modem Time Service +</TITLE> +</HEAD> +<BODY> + +<H3> +NIST Modem Time Service</H3> + +<HR> +<H4> +Synopsis</H4> +Address: 127.127.18.<I>u</I> +<BR>Reference ID: <TT>NIST</TT> +<BR>Driver ID: <TT>ACTS_NIST</TT> +<BR>Serial Port: <TT>/dev/acts<I>u</I></TT>; 1200 baud, 8-bits, no parity +<BR>Features: <TT>tty_clk</TT> +<BR>Requires: <TT>/usr/include/sys/termios.h</TT> header file with modem +control +<H4> +Description</H4> +This driver supports the NIST Automated Computer Time Service (ACTS). It +periodically dials a prespecified telephone number, receives the NIST timecode +data and calculates the local clock correction. It designed primarily for +use when neither a radio clock nor connectivity to Internet time servers +is available. For the best accuracy, the individual telephone line/modem +delay needs to be calibrated using outside sources. + +<P>The ACTS is located at NIST Boulder, CO, telephone 303 494 4774. A toll +call from Newark, DE, costs between three and four cents, although it is +not clear what carrier and time of day discounts apply. The modem dial +string will differ depending on local telephone configuration, etc., and +is specified by the phone command in the configuration file. The argument +to this command is an AT command for a Hayes compatible modem. + +<P>The driver can operate in either of two modes, as determined by the +mode parameter in the server configuration command. In mode 0 the driver +operates continuously at intervals determined by the fudge time1 parameter, +as described above. In mode 1 the driver is enabled only when no other +sources of synchronization are available and when we have gone more than +MAXOUTAGE (3600 s) since last synchronized by other sources of synchronization. + +<P>The accuracy produced by this driver should be in the range of a millisecond +or two, but may need correction due to the delay characteristics of the +individual modem involved. For undetermined reasons, some modems work with +the ACTS echo-delay measurement scheme and some don't. This driver tries +to do the best it can with what it gets. Initial experiments with a Practical +Peripherals 9600SA modem here in Delaware suggest an accuracy of a millisecond +or two can be achieved without the scheme by using a fudge time1 value +of 65.0 ms. In either case, the dispersion for a single call involving +ten samples is about 1.3 ms. + +<P>For reliable call management, this driver requires a 1200-bps modem +with a Hayes-compatible command set and control over the modem data terminal +ready (DTR) control line. Present restrictions require the use of a POSIX-compatible +programming interface, although other interfaces may work as well. The +ACTS telephone number and modem setup string are hard-coded in the driver +and may require changes for nonstandard modems or special circumstances. + +<P>The fudge time1 parameter represents a propagation-delay correction +factor which is added to the value computed by ACTS when the echo-delay +scheme is used. This scheme does not work with all modems; for those that +don't, fudge flag2 should be set to disable the feature. In this case the +fudge time1 parameter represents the total propagation delay due to all +causes and must be determined by external calibration. + +<P>The ACTS call interval is determined by a counter initially set to the +fudge time2 parameter. At each poll interval, minpoll (usually 64 s) is +subtracted from the counter. When the counter is equal to or less than +zero, the fudge flag1 is set, which causes up to three call attempts to +be made to ACTS. The fudge flag1 is reset after a valid clock update has +been determined or by a device fault, timeout or manually using <TT>ntpdc</TT>. +After a valid clock update, the counter is reset for the next interval. +Setting the <TT>fudge time2</TT> parameter to zero disables automatic call +attempts. Manual call attempts can be made at any time by setting <TT>fudge +flag1</TT> using ntpdc. + +<P>The NIST timecode message is transmitted at 1200 bps in the following +format: +<PRE> +jjjjj yy-mm-dd hh:mm:ss tt l uuu mmmmm UTC(NIST) * + +jjjjj = modified Julian day +yy-mm-dd = year, month, day +hh:mm:ss = hours, minutes, seconds +tt = DST indicator (see driver listing) +l = leap-second warning (see driver listing) +uuu = DUT1 correction (see driver listing) +mmmmm = modem calibration (see driver listing) +on-time = '*'</PRE> +The timecode message is transmitted continuously after a signon banner, +which this driver ignores. The driver also ignores all but the yy-mm-dd, +hh:mm:ss and on-time character '*' fields, although it checks the format +of all fields of the message. A timestamp is captured at the '*' character, +as required by the ACTS specification, and used as the reference time of +the timecode. If a message with an on-time character of '#' is received, +the driver updates the propagation delay. The driver disconnects when (a) +ten valid messages have been received, (b) no message has been received +for 15 s, (c) an on-time character of '#' is received. These messages are +processed by a trimmed-mean filter to reduce timing noise and then by the +usual NTP algorithms to develop the clock correction. + +<P>Since the accumulated error grows with the interval between calls, it +is important that the intrinsic frequency error be minimized. This can +be done by observing difference in offsets between two calls placed some +hours apart and calculating the uncorrected frequency error. This error, +as a fixed-point value in parts-per-million, should be installed in the +ntp.drift file before the daemon is started. Some experimentation may be +necessary in order to reduce the intrinsic frequency error to the order +of 1 ppm. + +<P>The behavior of the clock selection algorithm is modified when this +driver is in use. The algorithm is designed so that this driver will never +be 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 driver will be selected for synchronization and +all other discipline sources will be ignored. + +<P>Unlike other drivers, each ACTS call generates one clock correction +and that correction is processed immediately. There is no wait to allow +the clock filter to accumulate samples. In addition, the watchdog timeout +of the local clock algorithm is disabled, so that a correction received +from this driver that exceeds CLOCK_MAX (128 ms) causes an immediate step/slew. + +<P>Since the interval between updates can be much longer than used with +ordinary NTP peers, the local clock procedure has been modified to operate +in either of two modes, depending on whether the interval between updates +is less than or greater than CLOCK_MAXSEC (1200 s). If less than this value, +the local clock procedure operates using the standard NTP phase-lock loop +as with other NTP peers. If greater than this value, the procedure operates +using a modified frequency-lock loop suggested by Judah Levine in his lockclock +algorithm designed specifically for ACTS. +<H4> +Call Management</H4> +Since ACTS will be a toll call in most areas of the country, it is necessary +to carefully manage the call frequency. This can be done in two ways, by +specifying the interval between calls, or by setting a flag bit manually +or via a cron job. The call interval is determined by a counter initially +set to the fudge time2 parameter. At each poll interval, minpoll (usually +64 s) is subtracted from the counter. When the counter is equal to or less +than zero, the fudge flag1 is set, which causes up to three call attempts +to be made. The fudge flag1 is reset after ten offset samples have been +determined in a single call or by a device fault, timeout or manually using +ntpdc. Upon successful completion of a call, the eight samples have been +shifted into the clock filter, the local clock updated and the counter +reset for the next interval. Setting the fudge time2 parameter to zero +disables automatic call attempts. + +<P>Manual call attempts can be made at any time by setting fudge flag1 +using ntpdc. For example, the ntpdc command +<PRE> +fudge 127.127.18.1 flags 1</PRE> +will ask for a key identifier and password and, if authenticated by the +server, will set flag1. There may be a short delay until the expiration +of the current poll timeout. + +<P>The flag1 can be set from a cron job in the following way. Construct +a file with contents +<PRE>keyid 11 +passwd dialup +fudge 127.127.18.1 flags 1 +quit</PRE> +Then, run the following program at specified times as required. +<PRE>/usr/local/bin/ntpdc <file</PRE> + +<H4> +Monitor Data</H4> +When enabled by the <TT>flag4</TT> fudge flag, every received timecode +is written as-is to the <TT>clockstats</TT> file. +<H4> +Fudge Factors</H4> + +<DL> +<DT> +<TT>time1 <I>time</I></TT></DT> + +<DD> +Specifies the time offset calibration factor, in seconds and fraction, +with default 0.0.</DD> + +<DT> +<TT>time2 <I>time</I></TT></DT> + +<DD> +Not used by this driver.</DD> + +<DT> +<TT>stratum <I>number</I></TT></DT> + +<DD> +Specifies the driver stratum, in decimal from 0 to 15, with default 0.</DD> + +<DT> +<TT>refid <I>string</I></TT></DT> + +<DD> +Specifies the driver reference identifier, an ASCII string from one to +four characters, with default <TT>NIST</TT>.</DD> + +<DT> +<TT>flag1 0 | 1</TT></DT> + +<DD> +Not used by this driver.</DD> + +<DT> +<TT>flag2 0 | 1</TT></DT> + +<DD> +Not used by this driver.</DD> + +<DT> +<TT>flag3 0 | 1</TT></DT> + +<DD> +Not used by this driver.</DD> + +<DT> +<TT>flag4 0 | 1</TT></DT> + +<DD> +Not used by this driver.</DD> +</DL> +Additional Information + +<P><A HREF="refclock.htm">Reference Clock Drivers</A> +<HR> +<ADDRESS> +David L. Mills (mills@udel.edu)</ADDRESS> + +</BODY> +</HTML> |
