Posts Tagged ‘Server’

I have set up at home ISa server and was trying to set it for VPN behind my cable modem service provider so I am able to do VoIP call from my computer from any where….

A NTP Server is a network device that provides network servers and other network infrastructure with an accurate timing reference. A NTP Server obtains precise time from an external time source, such as radio time and frequency broadcasts or GPS. This article describes the external time references that are available to synchronise NTP servers and discusses the merits of each.

There are a number of radio time and frequency broadcasts that are available in various locations. The WWVB signal is a US based reference, DCF-77 in Germany and MSF in the UK. These radio time and frequency broadcasts provide a stable and precise source of time. Additionally, an indoor located radio antenna can be generally be used to receive these signals – lowering installation costs. However, radio signals can be affected by local topography – such as hills and valleys. There is no guarantee of radio signal reception even well inside the transmission range of the signal.

The MSF radio signal broadcast in the UK by the National Physics Laboratory recently moved from Rugby in the East Midlands to Anthorn in Cumbria. Since the move, it has been widely acknowledged that signal reception in the South of England has suffered – even though it is well within the physical reception range for the signal.

Local environments can also affect radio signals. Radio antennas placed inside metal cages or metal clad buildings can experience poor signal reception. Also, antennas located in a basement or underground may fail.

GPS utilised the Global Positioning System intended for worldwide navigation. Each GPS satellite has an onboard highly precise synchronised atomic clock – an ideal reference for NTP server timing.

The GPS system is a constellation of 24 orbiting satellites each broadcasting continuous time and positioning information. This information can be received anywhere on the face of the planet, providing an antenna can be provided with line of sight to the satellites.

GPS has a number of advantages over radio-based time and frequency broadcasts. Firstly, GPS can be received anywhere in the world and is not restricted by transmitter power and radio signal range. Secondly, the GPS signal is a much more accurate reference than radio solutions. Thirdly, GPS timing information can be received by anyone who can provide an antenna with a good view of the sky. However, GPS antennas do need to have line of sight with GPS satellites. Therefore, the ideal location for a GPS antenna is on a rooftop with a full 360-degree view of the sky. Additionally, if buildings or trees obscure the horizon in anyway, it can affect reception integrity. Roof-mounted antennas can significantly add to the installation costs of a NTP server system.

To summarise, radio and GPS based time and frequency references for NTP server systems both have certain advantages and disadvantages. However, the accuracy of GPS and the guaranteed reception, albeit with roof-mounted antennas, make it the reference of choice for NTP server systems.

The NTP GPS server is a dedicated device that uses the time signal from the GPS (Global Positioning System) network. GPS is now a common tool for motorists with satellite navigation devices fitted to most new cars. But GPS is far more than just an aid for positioning, at the very heart of the GPS network is the atomic clocks that are inside each GPS satellite.

The GPS system works by transmitting the time from these clocks along with the position and velocity of the satellite. A satellite navigation receiver will work out when it receives this time how long it took to arrive and therefore how far the signal travelled. Using three or more of these signals the satellite navigation device can work out exactly where it is.

GPS can only do this because of the atomic clocks that it uses to transmit the time signals. These time signals travel, like all radio signals, at the speed of light so an inaccuracy of just 1 millisecond (1/1000 of a second) could result in the satellite navigation being nearly 300 kilometres out.

Because these clocks have to be so accurate, they make an ideal source of time for a NTP time server. NTP (Network Time Protocol) is the software that distributes the time from the time server to the network. GPS time and UTC (Coordinated Universal Time) the civil timescale is not quite the same thing but are base don the same timescale so NTP has no trouble converting it. Using a dedicated NTP GPS server a network can be realistically synchronised to within a few milliseconds of UTC

The GPS clock is another term often given to a GPS time server. The GPS network consists of 21 active satellites (and a few spare) 10,000 miles in orbit above the Earth and each satellite circles the Earth twice a day. Designed for satellite navigation, A GPS receiver needs at least three satellites to maintain a position. However, in the case of a GPS clock just one satellite is required making it far easier to obtain a reliable signal.

Each satellite continuously transmits its own position and a time code. The time code is generated by an onboard atomic clock and is highly accurate, it has to be as this information is used by the GPS receiver to triangulate a position and if it was just half a second out the Sat Nav. unit would be inaccurate by thousands of miles.

I have an iPod touch and I made it like an iphone but there is only one problem. I cannot find a sip or voip server that let’s you have at lease 100 free minutes a week to the USA. Please help me fast!

The Global positioning System (GPS) is often used by computer equipment, such as NTP Server systems, to provide an accurate timing reference for time critical applications. This article provides an overview of GPS for timing applications and describes the equipment used to install a GPS antenna in a static location.

Overview – Using GPS for Accurate Time

The Global Positioning System is a US military system for worldwide navigation. The system consists of 24 orbiting satellites, each satellite has a highly accurate atomic clock on-board synchronised to UTC time. The satellites continuously broadcast time and position information. The time and position information can be obtained worldwide with a GPS receiver and antenna. GPS works continuously in any whether conditions, anywhere in the world. Additionally, there is no set up fee or subscription charges to utilise the GPS systems. Many computer timing systems and NTP Server systems utilise GPS as an accurate external timing reference.

The Accuracy of GPS Timing Systems

GPS receivers provide highly accurate position and timing information. Typically, a GPS receiver can provide positioning information to an accuracy of 15m. NTP Server systems can obtain timing information from GPS to a resolution of a few nanoseconds.

The GPS Signal

The transmitted GPS signal is very weak low-power radio signal, designated L1 and L2. L1 is the civilian GPS frequency transmitted at 1575.42 MHz. The signals travel by line of sight and can pass through clouds, glass and plastics but are blocked by objects such as metal and brickwork. Therefore, the ideal location for a GPS antenna is on rooftop with a full 360-degree view of the sky. However, often installation on the side of a building or in a window can provide adequate results. As a rule of thumb, the better the view of the sky, the greater the likelihood of a good consistent signal lock.

GPS Timing Antenna Types

The GPS antenna acts as an amplifier to boost the GPS signal for transmission along a cable, usually coax, to the GPS receiver. GPS Timing antenna’s provided with NTP server systems utilise a pole-mounting system. The antenna screws to a threaded pole for installation on rooftops. This arrangement provides the GPS antenna with a rigid mount easily able to withstand high winds without damage. Typically the GPS antenna is fairly small in size, measuring less than 90cm in diameter. Low-cost patch type antennas are also available, but these are generally better suited to vehicle applications.

GPS Antenna Cable Types and Cabling Distances

The cable distance that can be utilised by a GPS antenna depends mainly on the amplification of the GPS antenna and the quality of coax used in the installation. A typical GPS timing antenna may have a gain of 35 db. Relatively low-quality coax such as RG58 has an attenuation of 0.64 db/m at 1575 MHz. Therefore, a cable run of 55m can be obtained using RG58 cable. With very high quality coax cable, such as LMR400, an unaided cable run of 200m can be achieved. However, very high quality coax can be expensive. A good price-performance compromise is LMR200 cable, which can be run unaided to 80m.

Extending Cabling Distance with In-Line GPS Amplifiers

In-line GPS amplifiers provide further amplification of the GPS signal to increase the cable distance between the GPS antenna and receiver. GPS amplifiers are fitted in-line with the antenna cable and obtain power from the receiver via the coax cable. No external power-supplies are required. Typically, a GPS amplifier may add a further 20 dB of gain, adding 30m of low-quality RG58 coax, 40m of LMR200 coax or 100m of high quality LMR400 coax. Additionally, multiple in-line amplifiers may be utilised to further increase cable distance.

Sharing a Single Antenna Between Multiple Receivers – GPS Splitters

GPS splitters allow a single GPS antenna to be utilised by two or more NTP server systems. The GPS splitter splits the signal received from the GPS antenna into multiple outputs for synchronizing multiple NTP servers. GPS splitters are generally available with 2, 4 or 8 outputs.

Protecting GPS Systems – GPS Surge Suppressors

Surge suppressors protect expensive NTP server equipment from electro-static discharges, such as lightning, that may be picked up by an externally mounted GPS antenna. Surge suppressors are installed in-line on the coax cable between the antenna and receiver, ideally where the cable enters the building. Surge suppressors require a low-impedance ground, to discharge any received surge. The surge suppressor requires no power-supply or additional cabling.