Time Server

NTS-3000, NTS-4000, NTS-5000, NTS-dialup

User Manual

 

Select chapter: 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 , 10 , 11 , 12 , 12.1, 12.2, 12.3

 

Page 7/12

 

 

7. NTP-Network Time Protocol

 

 

NTP (Network Time Protocol RFC 1305) is a common method for time synchronization over networks. The NTP is much different from any of known other communication protocols. It is because NTP does not base on the principles of synchronizing machines to each other. It is based on the principles of having all machines get as close as possible to the UTC time provided by NTS–4000. How it works?

NTS–xxxx forms a statistic of delays and other data necessary to calculate local client RTC time offset. Knowing time difference the adjustment of the own RTC clock can be preceded individually by each NTP client.

NTP works on a hierarchical model in which a small number of servers gives time to a large number of clients. The clients on each level, or stratum, are in turn, potential servers to an even larger number of clients on a higher numbered stratum. Stratum numbers increase from the primary (stratum 1) servers to the lowest numbered strata at the bottom of the tree (stratum 15). Clients can use time information from multiple servers to determine automatically the best source of time and prevent wrong time sources from corrupting their own time.

In most cases it will take several minutes (or even hours) to adjust a system time to the ultimate degree of accuracy. There are several reasons for this. The most important one is that NTP averages the results of several time exchanges in order to reduce the effects of variable latency. This may take several minutes for NTP to even reach consensus on what the average latency is. Generally it happens in about 5-10 minutes. In addition, it often takes several adjustments for NTP to reach a synchronization. The result is that users should not expect NTP to immediately synchronize two clocks. The ntpdate command can be used if an instant synchronization is needed.

The peers command can be used in ntpq to determine whenever the synchronization has been achieved. When a client has synchronized, the synchronization server is listed with an asterisk in front of it.

To allow clocks to quickly achieve the high accuracy, yet avoid overshooting the time with large time adjustments, NTP uses a system where large adjustments occur quickly and small adjustments occur over time. For small time differences (less than 128 ms), NTP uses a gradual adjustment. This is called slewing. For larger (but still less than 17 minutes) time differences, the adjustment is immediate. This is called stepping.

If the accuracy of a clock becomes too insufficient (by more than about 17 minutes), NTP aborts the NTP daemon, with the assumption that something has gone wrong with either the client or server. In order to synchronize well with a server, the client needs to avoid step adjustments.

Due to NTP specification NTS–4000 is visible over network as a peer. Single peer can contain more than single timeservers in order of a hierarchy called stratum. The top of the stratum tree is preserved for radio-controlled timeservers such as NTS–4000. Therefore NTS–4000 will always be your stratum1 timeserver. Other connected computers can also work in timeserver mode but they will be set down to stratum2 or even much below.

The NTS–xxxx time server supports multiple source of time (depends of configuration). Each source is Stratum 0 (except RTC working on Stratum 5 level). The NTS–xxxxx includes following time sources:

 

•2x 1PPS GPS (pulse per second) signal PLL/FLL locked STRATUM-0

 

•2x GPS (NMEA) 1.5GHz radio signal (worldwide) STRATUM-0

 

•2x DCF77 55.7kHz radio signal (Central Europe - Germany only)* STRATUM-0

 

•1x IRIG-B signal (external connector) STRATUM-0

 

•1x PPS_IN signal (external connector e.g. to cesium 5051A) STRATUM-0

 

•1x OCXO (1PPS) internal quartz oscillator for GPS failure STRATUM-0

 

•1x RTC internal quartz clock systems for backup. STRATUM-5

 

 

 

and extra /NTS-dialup only !/

 

 

•1x Dial-up PSTN or ISDN driver (NTP) STRATUM-0

 

•1x Dial-up GSM/GPRS/EDGE driver (NTP) STRATUM-0

 

 

* - special feature option (required to be purchased separately)

A high precision synchronization is chosen by NTP automatically. The NTP always selects best available source of time. Selection is based on several time references like: stratum level, availability of timeserver, network delay, time difference, internal jitter factor, etc.

NTP clients of NTS–xxxx are referred to be a Stratum 2 clients. If they serve time to other clients, they are also referred as Stratum 2 servers. The maximum NTP stratum number for a client is 15.

NTP uses the UDP protocol on port 123 to communicate between clients and servers. Attempts are performed at designated intervals until the server responds. All NTS-xxxx antennas works redundantly and they are NTP visible as Stratum 0.

The interval depends on a number of factors and ranges from about once a minute to once every 17 minutes. Using UDP prevents retries from using up network bandwidth if a time server with a large number of clients goes down. The bandwidth requirements for NTP are also minimal.

Unencrypted NTP Ethernet packets are 90 bytes long (76 bytes long at the IP layer). A broadcast server sends out a packet about every 64 seconds. A nonbroadcast client/server requires 2 packets per transaction. When the first starts, transactions occur about once per minute, increasing gradually to once per 17 minutes under normal conditions. Poorly synchronized clients will tend to poll more often than those well synchronized clients. Starting from NTP version 4 implementations, the minimum and maximum intervals can be extended beyond these limits, if necessary.

Find more information on NTP protocol at: www.ntp.org and Elproma NTP site.

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