| A NTP Server connects to a computer network with | | | | such as NIST (National Institute for Standards and |
| the purpose of synchronising all computers, routers and | | | | Technology) in the US (WWVB signal) and NPL |
| other devices to the exact same time. NTP servers | | | | (National Physical Laboratory) in the UK (MSF signal). |
| use Network Time Protocol to adjust the drift of | | | | These signals are broadcast in long wave and can be |
| different machines to match the reference time. | | | | picked up within the broadcast area although the |
| NTP servers rely on using a reference clock; most | | | | signals can be blocked by local geographical features. |
| networks that use a NTP server will use a UTC | | | | Another method to receive a UTC timing reference is |
| (Coordinated Universal Time) time source. UTC is | | | | to use the onboard atomic clocks on GPS (Global |
| based on the time told by the incredibly accurate and | | | | Positioning System) network. While GPS is most |
| expensive atomic clocks. | | | | commonly known as a positioning system the satellite |
| Atomic clocks work on the principle that a single atom | | | | actually relays timing information which is used by GPS |
| (in most cases the caesium -133) will resonate at an | | | | receivers to calculate the time it has travelled and |
| exact rate at certain energy levels. The accuracy of | | | | therefore the distance. |
| atomic clocks is so proficient that UTC was developed | | | | While the GPS signals are not broadcast in UTC |
| to allow international Atomic Time (TAI) and | | | | format they are highly accurate and NTP has no |
| Greenwich Meantime (GMT) to be combined, allowing | | | | problem in converting them. |
| for the slowing of the Earth's rotation by adding leap | | | | The NTP server checks the time stamp from the |
| seconds and therefore keeping the Sun at the Earth's | | | | UTC source and uses the information to calculate if |
| meridian at noon. | | | | the network clocks are drifting and adds or subtracts |
| Failure to account for this slowing in the Earth's spin | | | | a second to match the reference clock. The NTP |
| would result in the eventual drift of day and night (albeit | | | | server will do this at set intervals, normally every |
| in many millennia). | | | | fifteen minutes to ensure perfect accuracy. |
| A NTP server can be set to receive a UTC time | | | | NTP is accurate to within 1/100th of a second (10 |
| signal from across the Internet although these can | | | | milliseconds) over the public Internet and can perform |
| vary tremendously in accuracy and are reliant on | | | | even better over LANs and WANS with accuracies |
| reasonably close distances from client and server. | | | | of 1/5000th of a second (200 microseconds) not |
| Relying on an Internet based timing references can | | | | unheard of. |
| also leave a network open to malicious users as they | | | | To ensure further accuracy the NTP service (or |
| can not utilise NTP authentication which is a security | | | | daemon on Linux) runs in the background and does not |
| measure used to ensure a timing reference is what it | | | | believe the time it is told until after several exchanges |
| says it is. | | | | and each one has passed a protocol specification (a |
| Many dedicated NTP servers are designed to receive | | | | test), the server is then considered. It usually takes |
| a more accurate and authenticated timing reference. | | | | about five good samples) until a NTP server is |
| One method utilises radio transmissions that are | | | | accepted as a timing source. |
| broadcast by several national physics laboratories | | | | |