Application of SNTP clock synchronization card (network clock server) in telecommunication system


Application of SNTP clock synchronization card (network clock server) in telecommunication system

Application of SNTP clock synchronization card (network clock server) in telecommunication system

Anhui Jingzhun electronic technology official wechat – ahjzsz

This paper describes the application and development trend of time synchronization system in telecom network by introducing and analyzing the application example of time synchronization system in a telecom switch.

There are various functional subnets in the telecommunication network of a province, such as program-controlled telephone network, in intelligent network, PHS wireless local telephone network, data communication network, 160 / 168 audio system, multimedia communication network and other supporting networks and management networks. The time used by most of the equipment in these networks is provided by the internal time of the equipment.

The billing, maintenance, management and other functions carried in these networks have high requirements for time equipment, so the system requires that the information transmitted between networks can be highly consistent in time and accurately track Beijing standard time. When the internal time of the equipment is corrected manually regularly or irregularly, the human error and time delay introduced and the time deviation caused by the quality difference of the internal time source of the equipment, It will lead to inconsistent time of devices in the network.

Three problems must be solved in UTC (absolute time) time synchronization for various time devices on each subnet of the telecommunication network: first, select high-precision time source as far as possible; 2、 Transfer the high-precision time to the time equipment to ensure that the error in the transmission process is as small as possible; 3、 Use absolute time synchronization time equipment, make full use of their own time calibration mechanism to automatically realize time synchronization, and eliminate manual factors as far as possible.

1、 Time synchronization concept

1. Time source

Atomic clocks of all countries in the world are compared with each other according to the specified method, and their data are processed by special international institutions to calculate the unified atomic time of the world, which is called international atomic time, or IAT for short. UTC (coordinated universal time) is essentially an atomic time, because its second length is equal to that of atomic time, but it is made as close to universal time as possible through manual intervention.

Global positioning system (GPS) is the second generation of satellite navigation system in the United States. GPS satellite has the widest coverage, the most convenient use and high accuracy. The time accuracy can reach 1 ~ 10 relative to UTC μ s. All kinds of time source equipment can meet the requirements of time synchronization in the telecommunication network.

After obtaining the absolute time, it is also necessary to maintain the absolute time accuracy, which is related to the quality of the frequency oscillator used by the time source server. In general, the accuracy of constant temperature crystal oscillator can meet the time needs of telecom enterprise time source server. For very few occasions requiring high precision, various atomic clocks with high stability can be used.

2. Time transmission technology

The devices requiring time synchronization are often not near the time source server and cannot directly use the time information. Therefore, it is necessary to accurately transmit the absolute time information to each time device. The transmission of time information usually includes wireless transmission mode (high-frequency or low-frequency radio signal) and wired transmission mode (DCN, DDN, etc.). Generally speaking, the telecommunication network adopts the wired transmission mode with high precision to transmit time information.

In wired transmission mode, DCN is a computer TCP / IP network, and network time protocol (NTP) or simple network time protocol (SNTP) belongs to TCP / IP protocol family. The most important technical point of this protocol is to obtain the transmission time delay and compensate the delay. The calculation method of transmission delay and time deviation of the protocol is a real-time dynamic mechanism. Therefore, NTP network time protocol has nothing to do with the specific networking scheme and can adapt to a variety of computer networks. The TCP / IP protocol family has rfc867 and rfc868 protocols, which can also transmit time through DCN. However, due to the lack of delay compensation mechanism and low accuracy, rfc867 and rfc868 can only be used as reference methods.

Higher time accuracy can be obtained by transmitting time in the form of DDN dedicated line, including hardware signal coding protocols such as IRIG-B (AC), IRIG-B (DC), DCLS and so on. This method is not affected by network load. After fixed compensation, the error is less than 1 ~ 10 μ s. This method is suitable for microsecond precision applications such as signaling analysis. A dedicated line needs to be established. The time receiver needs complex equipment to analyze various hardware signal coding protocols, which costs a lot of investment.

3. Synchronize the internal time of the device

After transmitting the absolute time to each designated equipment in various ways, the key work is to calibrate the equipment synchronously. According to the time synchronization calibration method, each designated equipment can be divided into three types:

Equipment with standard time interface (including RS-232, NTP, igrg interface, etc.);

Equipment with command line interface;

There is no command interface, which can only be adjusted manually.

There are great differences in the frequency stability of clock crystal oscillator of various time equipment, and the frequency and timing of time calibration are also different. Due to the problems of sense of responsibility, understanding difference, reaction speed and so on, there are also huge uncertain factors in manual operation. Therefore, it is necessary to explore the specific scheme of time synchronization of each time equipment according to the situation. The general principle of time synchronization is to establish a unified mechanism that eliminates human factors as much as possible, automatically executes various synchronization operations and synchronization commands, and grasps the execution time and frequency.


2、 Application Research on time synchronization of switch

At present, there are S1240, EWSD, C & C08, ZXJ10, SP30, e10b, F150 and other switches running on a provincial telecom network. They all generate and manage a system time value respectively. The switch uses it to generate various alarms, timed traffic lists and network control reports to provide real-time fault location and analysis and judgment for maintenance personnel; At the same time, the switch generates a detailed bill according to the specific value of time, so as to realize the reconciliation processing and billing and settlement functions of long-distance call service, local intelligent network service, inter network settlement service and so on.

The billing bills provided by the front desk to users come from long-distance telephone bureaus, gateway office switches, SSP switches, etc. due to the quality difference of internal time, these devices cause the deviation between internal time and standard time, and nested bills and repeated bills may appear, which is easy to cause users’ billing disputes. In order to avoid this situation, at present, the maintenance personnel manually correct the system time of the switch, which can not ensure that the time of each equipment of the whole telecommunication network can be accurately consistent with the standard time during each adjustment. This deviation has affected the operation quality of the telecommunication network of a province over time. Therefore, it is imperative to establish a “time synchronization system of Telecom exchange in a province” in the telecom network of a province and provide a unified time standard for all systems of the telecom network.

1. Networking structure

The project adopts a two-level structure, that is, two provincial NTP (Internet Time Protocol) time servers are established in the provincial center respectively. It obtains the standard TOD (time of day) signal through the external GPS receiver and transmits the TOD signal to the NTP server to form a primary time server; At the same time, establish their own NTP secondary time servers in various cities. The provincial central primary time server transmits the absolute time to the NTP secondary time server in various cities through the wan to form a unified time synchronized across the province; Each local network receives the modification command of the secondary time server through the front-end computer of the local network management system to adjust the time, realize the centralized processing of the switch operation parameters and unify the standard time.

Since the DCN network of a province operates well, the project can transmit time signals through the DCN network.

2. Selection of networking scheme

After the time synchronization of the existing switch, in order to meet the needs of centralized management, the switch time synchronization system of this project mainly has the following two networking schemes.

Scheme 1: build a new time synchronization system platform.

Ethernet switches and switch front-end computers are configured in provincial centers and cities, and the primary time server and secondary time server are connected to the provincial DCN network through Ethernet; The secondary time servers in various cities send the time command to the front-end computer of the switch through the Ethernet switch.

In terms of the connection mode between the front-end machine of the switch and the switch, there are mainly the following two connection modes.

(1) Separate occupation of serial interface

This method is mainly aimed at the surplus resources of the serial port of the switch. The front-end computer of the switch is directly connected with the switch through the RS-232 serial interface. In this way, it does not need to consider whether the switch port is idle, and it can directly occupy the operation, but it needs to occupy a serial interface of the switch.

(2) Ethernet mode

This method is aimed at the situation of switches that can provide Ethernet interface. Through the connection mode based on TCP / IP or UDP / IP, a command transmission channel is established between a variety of switches and the front-end machine of the switch through LAN. In this way, in addition to requiring the switch to have redundant Ethernet interface, it is also necessary to configure an additional LAN switch to connect the switch with the front-end computer.

Scheme 2: use the system platform connected to DCN network.

At present, several information systems in a province, including the local network management monitoring system, have been connected to the DCN network, and the local network management monitoring system has realized the centralized monitoring of the switches in the province. Therefore, the port multiplexing mode can be used to transmit the time correction command through the local network management monitoring system, which mainly solves the problem of insufficient switch ports. In 0, the secondary time server can send a time correction command to the switch through the local network management front-end machine when the port is idle, which saves the switch port resources without changing the networking mode between the front-end machine and the switch.

Scheme comparison: at present, there are many types of switches in the local network in the province, and some of them are imported models, so the port resources are tight. Adopting scheme 1 will bring additional engineering investment to the system and occupy switch port resources. Adopting scheme 2 not only saves the investment of network equipment and switch port resources, but also will not have a great impact on the networking structure of the project. Therefore, according to the actual production situation and the safety and centralized management of the system, scheme 2 is adopted for the project, Use the local network management monitoring system platform to build the switch time synchronization system.


3. Architecture

The switch time synchronization system mainly solves the following three problems:

(1) Obtain UTC time and obtain absolute standard time; (2) The transmission of UTC time distributes the obtained absolute time to the operation terminals of each switch through the time synchronization network; (3) Correction of switch time.

According to the actual situation of switch network and network management network in a province, the architecture of time synchronization system of Telecom switch in a province is also mainly divided into the above three parts.

Firstly, the GPS time signal of the satellite is obtained by the GPS receiver, which is extracted and transformed into absolute time, and then the absolute time is used to synchronize the provincial central level time server; The provincial central level time server obtains the absolute time and transmits it to the centralized network management monitoring system of each local network through the existing computer wide area network; After the front-end computer (time synchronization terminal) of the monitoring system obtains the absolute time, it synchronizes the internal time of the switch through the command interface. Among them, the local network centralized network management monitoring system provides command interface, as shown in Figure 1.

In addition to these three parts, the switch time synchronization system also includes the background control subsystem, which formulates the switch time synchronization strategy and coordinates the work of other parts, such as accepting the execution time of GPS absolute time and forcibly refreshing the hardware time of the switch, so as to ensure that each local network monitoring system interface receives the centralized dispatching command of the provincial center.

4. Correction of switch time

The correction of switch time shall consider: (1) whether the switch time needs to be corrected; (2) Timing of correcting switch time; (3) How should the switch time be corrected.

If the error between the switch time and absolute time is within the allowable range, the switch time will not be corrected. When the switch time and absolute time exceed the error range, the switch time will be corrected, and the correction time should avoid the reported time and traffic peak in the whole period. When the time synchronization interface of the switch network management system does not receive the switch time correction command during the switch time correction, the front-end computer of the switch network management system sends all commands of the original switch centralized operation and maintenance system and obtains all reports as usual.

When it is necessary to refresh the switch time, the time synchronization server sends a series of correction commands to the front-end computer (time synchronization terminal) of the network management system. After receiving the correction command, the time synchronization terminal directly operates the switch by using the system interface and receives the report returned by the command. In order to ensure the accuracy and security of the time synchronization correction operation of the switch, it is necessary to calculate and verify the delay and compensation from the time synchronization terminal to the switch.

Before the formal time modification command is issued, the system uses the method of preliminary command to detect the load and command delay of the switch, in order to obtain the most effective command delay, and compare the detection results with the normal standard delay. If the delay is too large, it means that the switch is not suitable for clock adjustment at this time, and the real time correction command will not be issued. It will be executed when there is an appropriate load in the future, so as to ensure the safety of the system and the smooth execution of the command.

The switch has two ways to correct the time: one is to adjust the difference, and the other is to set the absolute time. If the difference is adjusted (such as S1240, EWSD, axe10, CC08), no compensation is required; If the method of setting absolute time (such as NEC) is adopted, when the real time correction command is issued, it has been pre adjusted according to the execution time of the above test command: the accurate network synchronization time of the machine plus a time difference. When it is sent to the switch for execution, the gap between it and the standard network synchronization time will be minimized, so as to ensure the accuracy of the switch time as much as possible.

3、 Research on Application of time synchronization in other systems

After the time synchronization system is established, each level of time server is equivalent to a time platform to provide standard time to the equipment requiring accurate time. The time server not only provides the standard time to the switch to adjust the switch time, but also transmits the standard time to other equipment and systems, and then modifies the time in different ways according to the synchronized equipment or system.

1. Time synchronization mode of IP network

IP network is an open computer Internet, which supports TCP / IP protocol and NTP protocol. As long as a port is opened on the IP network switch or hub for interconnection with the Ethernet port of the time server, set the IP address of the time server to be accessed by all workstations and servers in the network, and then load and start the NTP process for the workstations and servers to automatically realize the synchronization between servers. The reliability of time synchronization is guaranteed by NTP time protocol, and the accuracy is very high (10 ~ 100ms in 2Mbit / s bandwidth TCP / IP network).


2. Time synchronization mode of intelligent network

The time synchronization of each application can be realized by simply switching the network route between the application system server and workstation and the designated upper level reference time server to ensure mutual access by using TCP / IP protocol and starting the NTP client process in the server and workstation.

3. Time synchronization mode of audio network

The existing voice network application system of each local network has no interface with the provincial integrated DCN network according to the networking organization. It only needs to connect the LAN to the Ethernet switch of the integrated DCN, adjust and set the appropriate IP address, and maintain the TCP / IP protocol connection with the secondary time server.

4. Basic implementation methods of time synchronization of various application systems

According to statistics, there are dozens of computer application systems built and under construction on the internal support network of telecom enterprises in the province. Although the client and server may adopt the same time reference system within an application system, there is no common standard time between many application systems. In addition, the accuracy of time crystal oscillator in each application system server is different, which will also produce time error.

With the development of new telecom services, the interaction between computer application systems is becoming more and more frequent, and time is an absolute necessity of data interaction. The time deviation caused by different time standards may not only delay the data, but even lead to data inversion in serious cases. Therefore, each computer must be connected to the time synchronization system with the system server, which is also in line with the general trend of optimization and integration of provincial computer systems.

Time synchronization network is different from general telecom service network. It does not directly face customers. It should belong to a part of telecom support network and serve the whole telecom network. At the same time, telecom enterprises can also use the time synchronization network to develop value-added services, provide accurate time resources to the society, and expand the scope of services, such as finance, securities, ticketing, commerce and so on.

In short, time synchronization network belongs to a new network. Many networks and network applications need to do further research and exploration from the perspective of the whole network, which can not be achieved overnight.