Choosing a NTP Server - Selecting the Best Timing Source For You

The NTP server is an integral part of the modern computer network. Without Network Time Protocol and NTP time servers many of the modern functionality of computers that we take for granted such as online reservation, Internet trading and satellite communication would be impossible.

Synchronisation in computers is dealt with by NTP. NTP and NTP servers use a single time reference to synchronise all machines on a network to that time. This time reference could in fact be anything such as the time on a wrist watch perhaps. However, synchronisation is pointless unless a UTC (coordinated universal time) time source is used as UTC has been developed to allow the whole world to synchronise to the same time, allowing truly global synchronisation.

UTC is based on the time told by atomic clocks although compensation measures such as Leap Seconds are added to UTC to keep it inline with Greenwich Meantime (GMT).

Atomic clocks are very expensive and extremely delicate pieces of equipment and not the sort of thing that can be housed in the office server room. Fortunately a NTP server can receive a UTC time source from several different locations.

The Internet is perhaps the most widely used source of time references. Unfortunately however, there are draw backs in using the Internet for a timing source. Firstly the Internet timing sources can't be authenticated. Authentication is a security measure used by NTP to check that timing source is genuine. Secondly, to use an Internet timing reference means a hole has to be left open in the network's firewall, again compromising security. Thirdly, Internet timing sources are notoriously inaccurate and those that aren't can often be too far away from a client to provide any useful precision.

However, if security and high level of accuracy to UTC time is not required then the Internet can provide a simple and affordable solution.

A far more secure method of receiving a UTC timing reference is to use the specialist national time and frequency transmission broadcast by several countries. The UK (MSF), USA (WWVB), Germany (DCF) and Japan (JJY) all boast a long wave timing signal. While these signals are limited in range and strength, where available they make an ideal timing source as the radio receiver can pick these signals up from inside a building. These transmissions can also be authenticated providing a high level of security.

The third and perhaps simplest solution is to use a GPS NTP server. These use the signals sent from the Global Positioning System which contains timing information. This is ideal as the GPS signal can be received literally anywhere in the world so if there is no radio transmission your area then the GPS network will provide a secure and authenticated solution.

The only downside to GPS is that an antenna has to have a good view of the sky and therefore need to be positioned on the roof. This obviously has logistical drawbacks if the server room is in the basement of a sky-scraper.

In selecting a timing source, the most important thing to remember is where the NTP server is going to be situated. If it is indoors and there is no opportunity to run and antenna to the roof then the radio transmissions would be the best alternative. If there are no radio transmission in your country/area or the signals are blocked by local topography then the GPS is an ideal solution.

Installing a NTP Server

NTP or Network Time Protocol servers are network devices that are designed to distribute accurate time to network time clients and other network infrastructure. This article describes how to configure and install NTP server systems on a network and discusses the advantages and disadvantages of various reference clock options.

NTP servers are generally supplied as 1U high rack mountable network devices. They obtain an accurate time from an external time reference, such as GPS or radio, and provide an accurate timing resource for a computer network. NTP or Network Time Protocol is a protocol designed for distributing time to client computers over an IP network. The protocol is UDP based and as such requires the TCP/IP network infrastructure to be installed.

Hardware Installation

Stratum 1 NTP time servers rely on an external timing reference to obtain accurate time. Various external timing references are available. Options may vary with the installations regional location.

GPS (Global Positioning System) is a popular timing reference. The advantages of a GPS reference are that it is highly accurate and can be utilised anywhere in the world. A typical GPS NTP server installation can synchronise to within a few microseconds of UTC time. The disadvantage of GPS is that ideally a roof-mounted external antenna is required with a good view of the sky. The maximum cabling distance between an NTP server and GPS antenna is governed by the quality of coax utilised. Relatively low-quality coax, such as RG58 can be used to around 50m. Higher quality coax, such as LMR200 can be utilised to around 80m. Very high quality coax, such as LMR400 can be utilised to around 200m. Additionally, GPS amplifiers can be used to amplify the GPS signal and extend cable runs. It is also good practise to install a surge suppressor to externally mounted GPS antennas, to protect against the possibility of damage caused by lightning strikes.

Local radio time references are available in many countries. Radio time references tend to be local to the country of origin and maybe neighbouring countries. The advantage of radio is that generally a good signal can be obtained indoors, close to the NTP server installation. However, radio time services are less accurate than GPS and reception areas are regional. A typical radio NTP server installation can synchronise to within a few milliseconds of UTC time. A number of factors can affect radio reception, including: locating the radio antenna underground or in a basement; locating the antenna inside a metal cage (including metal cladding) and locating the antenna close to electrically noisy equipment.

A number of regional radio time references are available including: WWVB, Colorado, US; DCF-77, Frankfurt, Germany and MSF-60, Rugby, UK. The DCF-77 time transmission is available throughout Central and Western Europe. The MSF-60 time signal is available throughout the British Isles and much of North-West Europe.

NTP Server Configuration

For a minimal installation, NTP servers are extremely easy to install. They ideally need to be provided with a static IP address. DHCP is not a good option since the IP address is leased and can change periodically. The device then needs to synchronise its internal reference time with the selected external timing reference. Synchronisation can take as long as 30 minutes depending on the drift of the local battery-backed clock when the device was powered down. Once synchronised, the NTP server can provide precise time to network time clients and other network devices and infrastructure.

Many other configuration options are available on an NTP server: authentication options are available for secure installations; IP address restrictions can be specified; status reporting functions such as 'syslog' as well as precision and status information.

NTP Client Configuration

Many operating systems such as Microsoft Windows 2000/XP/2003 and Vista have built in SNTP client functionality. The standard NTP distribution is available for LINUX, Free BSD and UNIX operating systems; Novell also has a NTP compliant NLM available. Many network infrastructure devices, such as Cisco routers and switches can also be synchronised using NTP. Essentially, a network time client only needs to be pointed to the IP address or DNS name of a NTP server in order to synchronise time.

NTP and SNTP

SNTP or Simple Network Time Protocol is a simplified version of NTP that is generally used by small computers and micro-controllers. SNTP provides a subset of NTP functionality for computers that do not require the precise synchronisation ability of NTP. SNTP and NTP are however completely interchangeable. SNTP clients can synchronise to NTP servers and vice-versa. The client software supplied by Microsoft with Windows 2000 and XP is a SNTP rather than an NTP implementation.

Summary

NTP servers are extremely easy to install and configure. However, a little thought needs to be given to selecting the correct external timing reference for your region and to antenna installation.