Various new data services, such as e-commerce, multimedia communications, IP telephony, etc., are new growth points in the development of telecommunications services, while traditional services also have problems with interconnection and inter-network settlement of many enterprises, and communication services involve Security, authentication and billing are all closely related to a common symbol "time". Accurate "time" signs are becoming more and more important to modern communication networks. Therefore, it is absolutely necessary to introduce a new support network—time synchronization network in the communication network.
There are various functional subnets in the telecommunication network, such as program-controlled telephone network, IN intelligent network, PHS wireless local telephone network, data communication network, 160/168 voice communication system, multimedia communication network, soft switch, NGN network and other supporting networks and management Network. The time used by most of the devices in these networks is provided by the internal clock of the device. The billing, maintenance, and management functions carried in these networks have high requirements for time equipment. Therefore, the system requires that the information transmitted between networks be highly consistent in time and accurately track Beijing Standard Time. When the internal clock of the device is calibrated regularly or irregularly manually, the time deviation caused by the introduction of human error and time delay and the quality difference of the internal time source of the device will cause the time of each device in the network to be inconsistent.
Based on the above considerations, a telecommunications company carried out two phases of time synchronization system construction from 2004 to 2007, and constructed a time synchronization system based on the BITS level 1 time service of each local network. The system built 21 local network synchronization subsystems and provinces. Unified network management. At present, the system realizes the time synchronization of switching network elements and data network elements, and has access to business equipment including various local network program-controlled switches PSTN, intelligent network, softswitch, NGN, BIMS, PHS, voice system, etc.
Improvement
of the time synchronization system To synchronize the UTC (absolute time) time of various time devices on each subnet of the telecommunication network, three problems must be solved: selecting a high-precision time source; determining a reasonable synchronization plan; and necessary deviation compensation Measures.
1. Realization of time synchronization of switching network elements
The realization of the time synchronization of the switching equipment (switching network element) of a telecommunications company: the system obtains the time value of the synchronized object through the network management or other equipment interface, compares it with the absolute time source, and automatically calibrates the synchronized object according to the preset error threshold When the time or alarm is activated, the operator confirms whether to correct, and the system automatically completes the adjustment operation after the operator confirms. The time calibration flowchart is shown in Figure 1.
Figure 1 The main process of time calibration There are
three issues to consider in the calibration of the exchange time: whether the exchange time needs to be calibrated; the timing of calibrating the exchange time; how should the exchange time be calibrated.
According to the actual production situation and the system's security and centralized management requirements, the time synchronization system can use the port multiplexing method to transmit time calibration commands through the local network management monitoring system. This method mainly solves the problem of insufficient switch ports. The front-end processor of the time synchronization system can send a time calibration command to the switch through the local network management acquisition machine when the port is idle, which saves switch port resources and does not need to change the original network management monitoring system and the switch networking mode.
2. Realization of time synchronization of
data network elements Data network elements are generally synchronized by NTP and SNTP. SNTP is the same as NTP, but the client software does less processing. NTP is a time transfer protocol based on TCP/IP. The system uses NTP to synchronize data network elements through an Ethernet network, or provides standard time output for data network elements, and the business system synchronizes standard time. A telecommunications company's time synchronization network has access to more than ten sets of data network elements, including BIMS system, UTipas system, voice communication system, intelligent network system, soft switch platform, etc. Different data network elements are based on different operating system platforms, including HP_ux, IBM-UNIX, SUNsolaris and windows2000/xp/2003.
A telecommunications company's time synchronization network uses a unified three-level time server method for timing data network elements. In this method, one or two machines must be selected as the three-level TS within the data network element. The three-level TS is synchronized to the time server of the time synchronization network, and other hosts in the system are synchronized with the third-level TS, as shown in Figure 2. .
NE timing data synchronization network of FIG. 2 time
3. Specific application of time synchronization system
(1) Application in billing The
clear demand for time synchronization in current business networks mainly comes from the need for call billing, including telephone billing, broadband/narrowband Internet billing, and intelligent network services Billing etc. Therefore, PSTN switches, PHS switches, IP network RADIUS, intelligent network service platforms, etc. should be connected to the time synchronization network. With the development of softswitching and future 3G services, more value-added services may be gradually realized, and the billing of these service platforms will also require time synchronization.
For the fixed telephone network, the billing information of each call, including the calling and called number and the start and end time of the call are given by the calling office. The time label of the call start and end time is the time of the calling office switch's own clock. When the call is a local network call, the calling office for charging is the calling local office; when the call is a long distance call, the calling office for charging is the calling toll office. If there is a big difference between the time of the exchange clock of the local office and the time of the exchange clock of the long-distance office, there may be a phone "make two calls at the same time" on the billing bill (simultaneous local and long-distance calls) Contradictory records.
(2) Application in inter-network settlement.
Many operators jointly operate the communication market, and there must be problems of interconnection and inter-network settlement. Moreover, users request to improve the quality of service to make calls clearly, and there must be a detailed billing list for the telephone bill. It is also a trend that the billing unit becomes smaller. These all require high billing accuracy, especially in the settlement between different operators' networks.
(3) Application of
softswitch network In softswitch network, charging information is mainly generated on softswitch equipment. At present, the billing time unit generated by the softswitch equipment reaches 0.1s. Therefore, the accuracy of the time source that provides a time standard for softswitch billing should reach the level below 50ms. Its billing and management will need time synchronization, so the softswitch core network platform needs time synchronization.
(4) Application of PHS network
The actual situation of PHS network synchronization is generally that a certain network management server in the PHS network is selected as the time source, and the time synchronization is realized inside the PHS. There may be deviations from the standard time and the external network, and it must be adjusted manually after a period of time. There are problems such as large deviations and manual maintenance.
(5) Application
of signaling monitoring In order to avoid false information due to incorrect signaling sequence, it is necessary to require that the time information of all signaling streams is accurate. Since the time stamp of the signaling flow is generated by each signaling collection point, each signaling collection point must maintain time synchronization.
(6) Application of Intelligent Network Service Platform
At present, Chengdu Telecom's intelligent network mainly includes equipment from three manufacturers. Each service subnet is relatively independent, and the time label is not unified, which causes inconvenience to billing calculation, fault location, and routine maintenance.
Post-construction application of time synchronization system 1. Application of
IDC network
In addition to providing Internet access services, IDC can provide users with carrier-class rich network resources and comprehensive network management and application services. New hotspots and growth points of data services. IDC hosting, leased hosting, virtual hosting and other business applications and equipment maintenance all have time synchronization requirements.
2. Application
of transmission network At present, the existing time synchronization demand of transmission network is not very urgent, mainly belongs to the demand of network management maintenance. In the service of large customers, the unsynchronized network management time will cause the failure of accurate timing or timeout of the leased circuits of important customers, which affects the business income and reputation of the enterprise, and is not conducive to the maintenance of operators. At the same time, inaccurate time recording also affects fault judgment and post-mortem analysis, resulting in inaccurate statistical data and affecting network quality analysis.
3.3G application
3G new business has an increasingly urgent need for synchronization. With the continuous advancement of telecommunications restructuring, all operators will become full-service operators. As the fastest-growing communication business today, mobile communication services are the core of the user market of all communication operators and the focus of construction investment.
In 3G networks that use cdma2000 and TD-SCDMA two standards, high-precision time synchronization (better than 10μs) between base station systems is required. Because in the soft handover, if there is no time synchronization between the RNC (Radio Network Controller) and NodeB, it may cause a mismatch in the mail command in the selector, so that the call connection cannot be established.
The WCDMA system itself does not need time synchronization during normal operation. However, when providing certain value-added services, such as high-precision location positioning services based on base stations, time synchronization between base station equipment is required, and it is high-precision (μs level) Time synchronization. The reason why WCDMA does not require GPS is that in the WCDMA protocol, a primary synchronization channel and a secondary synchronization channel (SCH) are set. The system obtains system synchronization by decoding the SCH, so there is no need for a common synchronization source between WCDMA base stations. And cdma2000 does not have a synchronization channel similar to WCDMA, so all base stations must be synchronized.
In the 3G network, there will be other new mobile services, such as location-based services. There are several technical options for implementing location-based services in a mobile communication network, one of which is to use a mobile phone to receive wireless signals from multiple nearby base stations for positioning. This requires that the precise location of the base station is predicted and all time synchronized. According to the different positioning accuracy requirements, the accuracy requirements of the base station time synchronization are also different. cdma2000 is synchronized with the world time, and its forward link transmission clocks of all base stations in the world are accurately synchronized to microseconds. The intervention of precise time sources makes "precise positioning" technology possible. Generally speaking, if the time synchronization error between the base stations is 1μs, the position positioning accuracy is about 300m (the propagation speed of the radio wave is equal to the speed of light 3×108m/s, assuming that the clock synchronization error between the base stations is 1μs, if If this 1μs error is not corrected, it will be converted to a distance of 1000ns×3×108m/s=300m).
With the in-depth development of computer network technology and its applications, more and more people carry out activities in the virtual space formed by computer networks. Network systems in many fields, such as finance, radio and television, transportation, e-commerce, positioning, navigation, and large-scale distributed commercial databases, need to maintain time synchronization and time accuracy between computers on a large scale, so telecommunications companies can also use the established The time synchronization network of China Telecom develops value-added services, provides accurate time resources to social demand units, and expands the scope of services. This is also in line with China Telecom’s transformation to a modern integrated information service provider.
The effectiveness and key issues after the system improvement
Before the time synchronization system of a telecommunications company was put into operation, the local network switch clock calibration was mostly manual, and the labor investment was large. There was no unified calibration system, no unified reference time, and the effect was relatively poor. After the system was put into operation, the province's long-distance exchange offices, gateway interface offices, independent tandem offices, and important end offices of the local network have all been connected to the time synchronization network through time servers. Since the time synchronization system was put into operation in 2004, all local network PSTN switches have been connected to the time synchronization system to realize automatic comparison and automatic adjustment, the switch billing time has been unified, the maintenance efficiency has been significantly improved, and the local network exchange professional maintenance Unanimous affirmation of personnel.
The time synchronization network is a brand-new support network, and some problems will inevitably be encountered in the planning, construction, and implementation of the network. At present, there is no unified corporate standard for the accuracy requirements of time transmission in the time synchronization network and the accuracy required by various business applications within the group, and further research is needed to formulate corresponding specifications.
Time synchronization integration brings together many support networks, network management networks, DCN networks, and different business subnets within the telecommunications network. It involves many business departments and business subnets, and has related interfaces with third-party systems. The maintenance interface is clear. In addition, some business networks have special requirements for security, and further research and consideration are needed to ensure network security.
A telecommunications company’s internal support network has dozens of computer application systems that have been built and are under construction. Although, in a certain application system, the client and server may use the same time reference system, but there is no common standard time to comply with among many application systems. In addition, the accuracy of the internal time crystal oscillator of each application system server is different, which will also cause time errors.
With the development of new telecommunications services, the interaction between computer application systems becomes more and more frequent, and time is an absolute necessity for data interaction. The time deviation caused by different time standards may not only delay the data, but even cause data inversion in severe cases. Therefore, each computer system server must be connected to the time synchronization system, which is also in line with the general trend of optimization and integration of computer systems in the province.
The time synchronization network is an important telecommunications support network that serves the entire telecommunications network. It is different from the general telecommunications business network. It does not directly face customers. At present, there is no unified standard for its business applications. In actual applications, operators need to conduct further research and exploration from the perspective of the entire network, verify and test in many aspects, to ensure safety, stability, and steady implementation step by step.