Many techniques were developed to synchronize and to disseminate time. They are different especially for the accuracy level and for the transmission media used. The synchronization is generally achieved by mean of electronic time signals generated ad hoc by devices directly connected to a primary time standard and transmitted by radio or by cable. Time signals can be classified into two main classes: video or sound signals (called right time signals) useful for low accuracy manual synchronization, digital signals for automatic synchronization with medium-high accuracy that exhibit an accuracy ranging for tens of milliseconds to nanoseconds.
There exist also synchronization techniques that can be defined as passive, in which is it not used a time signal generated ad hoc, instead it is used a signal generated for other pourpose, e. g. a television signal (television passive method), stable enough in time and that can be received both at the time standard site and at the synchronizing clock site. Measuring the instant of time in which happens a particular event in the signal, e. g. the synchronism pulses, with the time scale of the time standard and with the time scale of the clock to be synchronized, the offset between the two time scales can be computed and can be used later to adjust the clock to be synchronized.
In the last years, with the growth of telematic networks, especially Internet, synchronization methods that send time information through Internet were developed.
There are many synchronization systems for Internet, but all have in common the client-server working principle. The system time of a computer that offers the service (server) is generally synchronized with an external reference clock that can be in some cases a primary time standard. The server sends the time information through Internet every time another computer (client) query it.
The servers that offer a right time service generally send the time information as a WWW (World Wide Web) page that can be requested and displayed with an hypertextual browser like Mosaic, Netscape, Internet Explorer, etc.
Network Time Protocol
The Network Time Protocol (NTP) is the most flexible, accurate and automatic synchronization system available for the Internet. It can be used for almost every type of computer.
Time Protocol and Daytime Protocol
Before the coming of NTP, the Time Protocol and the DayTime Protocol have been developed. Both of them allow automatic synchronization like NTP, but on the contrary of NTP they do not support any kind of compensation for the transmission delay between the client and the server. For this reason, they are only accurate enough for computers on the same local network. For more information see document RFC 868 about Time Protocol and document RFC 867 about Daytime Protocol.
The modern navigation systems are based on the reception of radio signals synchronized with atomic clocks. The receivers of these systems, in addition to the geographical position of the receiver site, can give an accurate synchronization signal as by-product.
The telephone synchronization services send the time information in a codified form of character strings, generally one per second, like a normal data transmission over a switched telephone line that can be received by a computer with a modem.
The broadcasted synchronization services were the first to be developed. Often, the time information is at the same time in form of digital signal for automatic synchronization and in form of video or sound right time signal for manual synchronization. There are some radio stations that transmits only time information, while the most part periodically insert the time information inside the normal broadcasted programs.