Time protocols are the tools you use to transport time information across large distances.
Pulse per second (PPS) – PPS, the simplest form of synchronization, is a signal that outputs a high logic level once a second. It does not contain information about the specific time of day or year; it outputs a pulse only once a second. The pulse width is generally 100 ms, but many receivers allow the user to specify the pulse width, as long as it is less than one second. Figure 5 shows a PPS signal.
Figure 5. Pulse per Second
IRIG-B – This protocol is used to transmit time data. The signal is similar to PPS, but instead of a single pulse once a second, IRIG-B sends coded bits that make up a data frame that is one second long. This data frame presents time information in seconds, minutes, and days and provides a status byte. IRIG-B has a synchronization precision of tens of nanoseconds.
Figure 6 shows how time is sent using IRIG-B. The entire frame is only one second long. It is based on pulse-width modulation (PWM), where a 25 percent duty cycle represents a 0, a 50 percent duty cycle is a 1, and a 75 percent duty cycle is a Pause (P) to separate the pulses for seconds, minutes, days, and the status. Two pause cycles (R in this diagram) signify the end of a timestamp. Figure 6 shows an IRIG-B signal, which is read from right to left.
Figure 6. IRIG-B
IEEE 1588 – IEEE 1588 is a packet-based protocol that you can use over Ethernet. It defines a standard set of clock characteristics and value ranges for each characteristic. By running a distributed algorithm, called the best master clock (BMC) algorithm, each clock in the network identifies the highest-quality clock; that is the clock with the best set of characteristics.
The highest-ranking clock, called the “grandmaster” clock, synchronizes all other “slave” clocks. If the grandmaster clock is removed from the network, or if its characteristics change in such a way that it is no longer the “best” clock, the BMC algorithm helps participating clocks automatically determine the current best clock, which becomes the new grandmaster. This algorithm offers a fault-tolerant and administrative-free way of determining the clock used as the time source for the entire network.
The grandmaster clock periodically issues a “sync” packet containing a timestamp of the time when the packet left the grandmaster clock. The grandmaster may also issue a follow-up packet containing the timestamp for the sync packet. The use of a separate follow-up packet allows the grandmaster to accurately timestamp the sync packet on networks where the departure time of a packet cannot be known accurately beforehand.
Global positioning system (GPS) – GPS is a radio frequency encoded signal that provides time, position, and velocity information by means of a network of triangulation satellites. This time-reference signal, which is globally available, delivers synchronization precision between tens and hundreds of nanoseconds. To use this signal, you need a GPS antenna with a clear view of the sky.