18.2 Choosing a Time Source

     

As we have discussed, some applications are highly time dependent and consequently requiring a high degree of accuracy in measuring time. The degree of accuracy we require influences which time source we use. In essence, we have three choices in choosing a source of time:

  • A radio or GPS receiver : We have mentioned that some government and scientific installations broadcast accurate time services available to anyone with the appropriate receiver. These devices can broadcast time either via terrestrial or satellite receivers. GPS satellite receivers are not susceptible to the interference and reception problems of terrestrial receivers. If you read the HP documentation (http://docs.hp.com/hpux/onlinedocs/B2355-90685/B2355-90685.html) on which radio/GPS receivers receive their support, the list is quite short. However, in the NTP configuration file ( /etc/ntp.conf ), there are numerous examples of how to configure other devices such as modems that can dial up organizations like NIST that provide modem access to a reliable time source. If you are worried about the supportability of certain devices, contact HP for clarification . HP-UX supports the use of an HP/Agilent 58503A or Trimble Palisade (now called Acutime 2000 Synchronization Kit; see http://www.trimble.com/acutime2000sync.html) GPS receiver, which receives time from the GPS satellites controlled by the U.S. Naval Observatory. HP-UX also supports the use of a Spectracom Netclock/2 WWVB terrestrial radio receiver (http://www.spectracomcorp.com/netclock2.html). Unfortunately, this covers only North America. In Europe and beyond, there are other terrestrial receivers, e.g., DCF77 (AM and FM) transmissions from Frankfurt, Germany. You need to check with HP as to whether they are officially supported by HP-UX (even though they do work). In the past, these devices would have cost many thousands of dollars. I have just searched the Web for the Trimble Acutime 2000 Synchronization Kit, which Trimble is selling online for $995. With a level of accuracy of within 50 nanoseconds of UTC, this level of accuracy has become very affordable. Once configured, we can synchronize our own clock with a highly reliable source, but in conjunction with the NTP software, we can provide accurate time data to other machines in our network.

  • Another server on your network acting as a timeserver : In this instance, we cannot justify or afford to set up a server connected directly to a GPS/radio receiver. Many machines connected to an atomic clock/GPS receiver are accessible over the Internet. With the NTP software, our server can be a client of a more reliable time source. In some instances, this is not possible, for example, if your corporate network is behind a firewall and you cannot access the Internet for communication over port number 123. If our organization does allow Internet access, then we can access a number of local timeservers; the closer the timeserver, the better. When I talk about a local timeserver, I am referring to the mechanisms NTP uses to choose a time server as a good source of time. There is a value that NTP uses in choosing a timeserver. The value is known as dispersion . The lower the value, the more attractive that particular timeserver is. Dispersion is calculated via timeserver quality + network quality and is measured in milliseconds. In reality, network quality is the overriding factor. If this value is too high, i.e., above 1000 milliseconds , you won't be able to synchronize to that timeserver anyway (dispersion values of +100 will be problematic for NTP daemons). This is possibly one time when just being able to ping a server is not good enough. We need to consider the roundtrip times for a ping , as well as the reliability of individual servers. The list of current publicly accessible timeservers can be found at the University of Delaware (http://www.eecis.udel.edu/~mills/ntp/servers.html).

  • Our own server's inaccurate but reliable, internal clock : This is known as being a local clock impersonator . While it's the least reliable of options, it does give us the possibility of using some time as being the reference for the entire network. In this way, we can say that all machines involved agree on what the time is, even if that time is not entirely accurate. Where we do not have external Internet access, and we cannot justify a GPS/radio receiver, this is one possible solution of providing a time that all machines can at least agree on.



HP-UX CSE(c) Official Study Guide and Desk Reference
HP-UX CSE(c) Official Study Guide and Desk Reference
ISBN: N/A
EAN: N/A
Year: 2006
Pages: 434

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