Synchronization technology of NTP network time server in large LAN

This chapter uses a dual-machine mutual backup ntp network time server to implement a local area network time synchronization solution.

1. Configuration of the time synchronization system
The whole network is equipped with a time synchronization system, which consists of two GPS time servers (ntp network time servers) (supporting NTP protocol), switches, networks and clients. The two GPS time servers (ntp network time servers) are placed in the switch cabinet of the computer room and provide time to the clients of the whole network in the NTP protocol. The two GPS time servers (ntp network time servers) work in redundant hot standby mode, which improves the reliability of the system.
NTP (Network Time Protocol) is a TCP/IP protocol used to publish accurate time in the entire network. Its own transmission is based on UDP. It can synchronize computers with their servers or clock sources (such as quartz clocks, GPS, etc.), provide high-precision time correction (the difference between the standard on LAN is less than 1 millisecond, and tens of milliseconds on WAN), and can use encryption confirmation to prevent malicious protocol attacks.
Supported protocols: NTP, SNTP, ARP, UDP, TCP, Telent, ICMP, SNMP, DHCP and TFTP;
Network interface: RJ-45 Ethernet interface;

2 Performance of the time synchronization system
2.1 The GPS time server (ntp network time server) receives GPS (Global Positioning System) satellite signals or receives external IRIG-B (DC) time reference signals.
2.2 The time signal receiving unit of the GPS time server (ntp network time server) can receive the Coordinated Universal Time (UTC) signal sent by the GPS satellite as an external time reference signal. Under normal circumstances, the time signal receiving unit of the GPS time server independently receives the time reference signal sent by the GPS satellite. When the time signal receiving unit of a GPS time server fails, the GPS time server can automatically switch to the time reference signal received by the time signal receiving unit of another GPS time server, so that the time reference signals can serve as backup for each other. When the time signal receiving unit of the GPS time server returns to normal, the GPS time server automatically switches back to normal working state, and the switching time is less than 0.5 seconds. During the switching, the time synchronization signal output by the GPS time server will not be wrong.
2.3 The GPS time server can output a special IRIG-B (DC) code signal for interconnection between GPS time servers (ntp network time servers). This signal is used as the "backup" external time reference of the interconnected GPS time servers (ntp network time servers). When the "main" external time reference of the GPS time server fails, the signal stops outputting. This eliminates the uncertainty of the working state caused by the failure of the "main" external time reference when the GPS time servers are interconnected.
2.4 The GPS time server has an internal timekeeping function. When receiving an external time reference signal, it is synchronized by the external time reference signal; when not receiving an external time reference signal, it switches to internal timekeeping, so that the time synchronization signal output by the GPS time server can still maintain a certain accuracy. When the external time reference signal is received, it automatically switches to normal operation, and the switching time is less than 0.5S. The time synchronization signal output by the clock will not be wrong during the switching.
2.5 All output signals of the clock device are isolated and output, with strong anti-interference ability, and each output is electrically isolated from each other.
2.6 The clock device has a self-reset capability, and can automatically resume normal operation when the device CPU fails momentarily due to interference.
2.7 A certain output signal of the clock device is allowed to be short-circuited for more than 5 minutes without causing permanent damage to the output circuit.
2.8 A certain output signal of the clock device is short-circuited without affecting other output signals.
2.9 The clock device adopts a modular structure design, and the CPU board, B code generation board, and NTP signal output board are hot-swappable.
2.10 The clock device has the functions of working status indication, alarm display and alarm signal output. The electrical interface type of the alarm signal is a relay open contact, and the contact withstand voltage is >250V DC.
2.11 Each clock device provides a TTL pulse signal (programmable output 1PPS, 1PPM, 1PPH, event) for clock accuracy index testing.
2.12 The clock device has a time display function and can display year, month, day, hour, minute, and second.

3 Structure of the time synchronization system
The system structure consists of two GPS time servers (ntp network time servers) screens, switches, networks and clients.
The two GPS time servers (ntp network time servers) are hot standby for each other, with GPS time server 1 as the "normal" time server and GPS time server 2 as the "standby" time server. When the GPS signal receiving unit of the "normal" time server (GPS time server 1) fails, it automatically switches to the time reference signal received by the GPS signal receiving unit of the "standby" time server (GPS time server 2), thereby ensuring the reliability of the system. When the GPS signal receiving unit of the "normal" time server (GPS time server 1) returns to normal, the GPS time server automatically switches back to normal working state.
When the GPS time server (ntp network time server) cannot receive two external time reference signals, it switches to internal timekeeping to maintain a certain time accuracy; when the reception of the external time reference signal is restored, it automatically switches back to the working state of receiving the external time reference signal.
The RJ45 ports of the two GPS time servers are connected to the switch and two IP addresses are assigned. For example, the IP address of GPS time server 1 is IP1 and the IP address of GPS time server 2 is IP2. The two IP addresses are used as standard time servers in the network. The client PC can use the NTP service provided by the operating system or the client software provided by our company. This software can input the IP addresses of the two GPS time servers, with IP1 address as the primary address and IP2 address as the backup address. When the client cannot obtain time information from the primary address for some reason, it will automatically obtain time information from the backup address.
The embedded video recorder (DVR) does not have the NTP time synchronization function. The management software provided by the DVR manufacturer needs to be installed on the PC to transmit the time information to the DVR.

4 System Features
4.1 Professional Manufacturer, Professional Quality
Our company is dedicated to the research and development and application of GPS/Beidou timing products. Based on years of professional accumulation, we give full play to our technical advantages and application experience in the field of timing products. Relying on relevant scientific research institutes, combined with the US GPS global positioning system, China Beidou star system, B code benchmark decoding and receiving technology, intelligent taming constant temperature crystal oscillator (or atomic clock) timekeeping technology, we independently developed a domestic leading timing product. This product is a timing system for unified time reference in various industries. It is fully automatic and intelligent, maintenance-free, and suitable for unattended operation.
4.2 High Precision
Using the original high-precision GPS timing module from the United States, the clock synchronization accuracy can reach 30nS.
4.3 More comprehensive reliability and security design
4.3.1 Apply GPS timing technology/B code benchmark decoding and receiving technology/high-stability crystal oscillator timekeeping technology to achieve multi-reference redundant timing, which can intelligently judge the stability and advantages of GPS signals and external B code time reference signals, and provide a variety of time reference configuration methods.
4.3.2 Accurate frequency measurement and "intelligent learning algorithm" are used to keep the output signal of the timekeeping circuit precisely synchronized with the GPS satellite/IRIG-B time reference, eliminating the influence of frequency deviation caused by the aging of the crystal oscillator.
4.3.3 Since the 1PPS and other time signals output by the device are the frequency-divided second signals output by the built-in oscillator, they are synchronized with the GPS system but are not affected by the jump of the GPS second pulse signal, which is equivalent to the reproduction of the UTC time reference.
4.3.4 The GPS receiving antenna focuses on lightning protection design, stability design, and anti-interference design. The signal reception reliability is high and is not limited by the geographical conditions and environment of the power plant/substation.
4.3.5 High-performance, wide-range switching power supply is selected, which is stable and reliable, the power supply of the device is adaptive, and the system power supply is processed by special filtering and common mode interference elimination circuit.
4.3.6 Within the lightning protection range of the on-site lightning rod, the GPS antenna is equipped with a special antenna lightning arrester to make the clock device have a double lightning protection effect.
4.3.7 The clock device chassis is anti-magnetic and has strong anti-interference ability.
4.3.8 The device has self-reset capability and can automatically resume normal operation when a program error occurs due to interference.
4.3.9 All input and output signals of the device are electrically isolated.

4.4 Online operation monitoring of the time synchronization system can be realized
4.4.1 The front panel of the GPS time server (ntp network time server) has multiple status indicators, including "power indicator" light, "second pulse indicator" light, "GPS signal input" light, "B code signal input" light, "GPS signal input abnormal" light, and "B code signal input abnormal" light, which are convenient for daily inspections by operation duty personnel.
4.4.2 The GPS time server (ntp network time server) has power interruption alarm, GPS loss of synchronization alarm, and external "B code input" (backup time reference) disappearance alarm. These alarm signals are relay empty contact signal outputs, which can be connected to the monitoring system to monitor the operation status of the time synchronization system at any time.
4.4.3 Complete performance monitoring of Beidou and GPS signals, automatic or manual selection of the main satellite signal. Support local and remote network management, remote management of the system through WEB, and management of the system's satellite reception status, equipment working status, parameter settings and other information.
4.5 The system is easy to operate and maintain, with rich signal interfaces, flexible configuration and easy expansion
. 4.5.1 The clock device uses buttons to set up, and the pulse and serial port signal outputs are programmable and easy to operate.
4.5.2 The clock device adopts a fully modular plug-and-play structure design, supports hot swapping of boards, flexible configuration and easy maintenance, which provides convenience for the access of other signal reference sources (Galileo satellite signals, DCLS signals of upstream ground links, NTP time reference signals, etc.) in the future.
The 4.5.3 system not only realizes full compatibility between the GPS time server (ntp network time server) and the signal expansion box board, but also provides rich signal interface resources and an open special interface design platform with excellent compatibility. It can provide multi-channel pulse signals (1PPS, 1PPM, 1PPH, events, open contacts, differential, TTL, 24V/110V/220V active), IRIB-B signals (TTL, 422, 232, AC), DCF77 signals (active, passive), time messages (RS232, RS422/485), and NTP network time signals, which can meet the time synchronization interface requirements of different equipment on site.
4.6 System installation and debugging are safe and convenient.
4.6.1 The clock device is a rack-mounted structure with a 2U, 19” standard chassis, which is easy to install.
4.6.2 The clock device adopts a modular structure design, supports hot-swappable boards, and has flexible configuration and easy debugging.
4.6.3 Each device provides a programmable TTL pulse signal (1PPS/1PPM/1PPH) for clock accuracy index testing.
4.6.4 The GPS time server (ntp network time server) can display the number of valid satellites tracked through a digital tube, intuitively reflecting the status of the device receiving GPS satellites.

5 System technical indicators
5.1 Operating conditions
5.1.1 Environmental conditions
Working temperature: -200C～+700C;
Storage temperature: -450C～+850C;
Humidity: <95%.
5.1.2 Power
supply AC power supply: 220V±20% or 110V±20%, 47Hz～63Hz;
DC power supply: 220V±20% or 110V±20%.
The power supply of the device is adaptive.
5.1.3 Anti-interference
The device does not malfunction under the influence of lightning overvoltage, primary circuit operation, switch field failure, secondary circuit operation and other strong interference.
The fast transient interference test, high-frequency interference test, radiated electromagnetic field interference test, damped shock wave interference test, impulse voltage test and insulation test of the device meet the standard GB/T15153.1-1998 (Telecontrol equipment and system Part 2: Working conditions Part 1: Power supply and electromagnetic compatibility), and reach level III and above.

5.2 Technical Parameters
5.2.1 GPS receiving antenna
Antenna environment requirements:
Working temperature: -45℃～+85℃;
Storage temperature: -50℃～+90℃;
Humidity: 100%, condensation;
Volume: φ96mm×126mm.
5.2.2 GPS receiver
Receiving frequency: 1575.42MHz (L1 signal).
Receiving sensitivity: Capture <-160dBW, Tracking <-163dBW.
Simultaneous tracking: Under normal conditions, 8 to 12 GPS satellites can be tracked simultaneously;
not less than 4 satellites when the device is cold started;
not less than 1 satellite when the device is hot started.
Capture time: When the device is cold started, <10min; when the device is hot started, <1min.
Internal battery: Battery type: lithium battery;
Battery life: ≮25000h.
5.2.3 Timekeeping accuracy of the timekeeping unit inside the clock device: The crystal oscillator of the timekeeping unit uses OCXO, and the timekeeping accuracy is better than 7*10-8.
5.2.4 Power consumption of clock device: ≤20W.
5.2.5 Insulation resistance of clock device: ≮20MΩ.
5.2.6 System mean time between failures (MTBF): ≥70,000 hours; system mean time to repair (MTTR): generally no more than 30 minutes, service life no less than 20 years. No maintenance is required under normal use conditions.
5.2.7 Output time and Coordinated Universal Time (UTC) time synchronization accuracy: ≤0.5μS.
5.2.8 NTP interface:
NTP request volume per second Time mark accuracy User terminal request volume that can be processed
0～100 1～10ms 48,000
100～200 10～100ms 96,000
5.2.9 Weight: 3kg.