RNP > Technologies > NTP > About NTP > NTP Terminology

NTP Terminology

dispersion (clock dispersion/jitter)

In a repetitive process of reading the time in a clock, the differences can vary randomly. The difference among these differences is called jitter.

drift (clock drift)

Measure (in hertz per second) which determines how fast the skew of a clock varies.

stability (clock stability

It determines how well a clock can keep a frequency constant.

accuracy (clock accuracy)

It determines how close a clock is to a standard time reference such as UTC.

GMT – Greenwich Mean Time

Standard reference of time, preceding the UTC standard. It gets the time from astronomical events, such as the solar day.

GPS – Global Positioning System

A constellation of 24 satellites revolving around the Earth which, by the transmission of signals, permits determining the current time with a very high degree of precision.

NTP – Network Time Protocol

Internet Protocol used to synchronize the computer clocks (and some kinds of network equipment) with the standard reference of time UTC.

offset (clock offset)

Difference in time between two clocks, usually expressed in milliseconds.

NTP packet (NTP packet)

Message sent by the network according to the format of the NTP protocol.

precision (clock precision)

Smallest possible time increase which can be read by a program.

reference clock

Device providing the current time with very high precision. Typically, these devices can be atomic clocks, GPS receivers or radio receivers.

resolution (clock resolution)

Smallest possible time increase allowed by a clock.

roundtrip delay

Time that a host takes to send an NTP packet to another host and receive another NTP packet in response to it.

primary server

Another name given to an NTP stratum 1 server.

secondary server

Another name given to an NTP stratum 2 server.

skew (clock skew)

Difference (in hertz) between the real frequency of a clock and the frequency that it should have to reach the perfect time.

stratum

Level of NTP hierarchy. It classifies the serves in levels (strata), thus indicating the distance between this server and a reference clock. Level 1 indicates a server directly connected to a reference clock, while the highest level (stratum 16) usually indicates that the clock is not working or unaccessible. In general, a stratum "n" server is (n-1) hops away from the stratum 1 of the NTP hierarchy to which it belongs.

UTC – Universal Time Coordinated

Standard reference of time accepted worldwide. It gets the time from the magnetic resonance of the atom of cesium.