BITS are the key nodes of synchronous network. It gets synchronized signals by tracking satellites or from cesium oscillator or from benchmark signals of higher-level time/frequency equipment, takes a series of process like selecting, filtering, tracking and maintaining, and then outputs stable and high-precision time/clock signals for downstream service devices such as GSM, WCDMA, CDMA, TD-SCDMA, LTE, SDH, Accounting system, BOSS and NMS. Therefore, the performance of BITS has great impact in the normal and stable operation of operators’ service. How to measure and evaluate the synchronization performance of BITS efficiently and accurately will be an important task that the equipment manufacturers and operators have to face.
With the emergence of synchronization requirements in packet networks, ITU-T has released the profile of Primary Reference Time Clocks (PRTC), which includes the time and phase transmission requirements in packet network. This profile is called G.8272. The original G.8272 recommendations released in 2012 defines such a clock that provides 100ns phase and time performance for packet networks.
In 5G networks, the time synchronization requires high precision and low latency. To meet the requirement of new access technology and network security against GNSS vulnerabilities, it needs to improve the performance of present phase and time. ITU-T released the new profile of Enhanced Primary Reference Clocks, also called G.8272.1, in November, 2016. The performance level and reliability recommended in the new profile has laid foundation for the time, phase and frequency synchronization of networks in the coming years.
Constructed with IP technology by operators, IP Carrier Network is a special network which is used to carry services with high transmission quality (such as soft switch, video, VPN for key customers, etc.). Different from the synchronization technology in traditional SDH and MSTP networks, in IP Carrier Network, the frequency synchronization adopts synchronous Ethernet (SyncE) technology, and time synchronization adopts IEEE1588 (PTP) technology. As the entire synchronization hierarchy has been mature and perfect, the synchronization measurements in IP Carrier Network include time measurement and frequency measurement. In addition, the performance verification job of IP Carrier Network can be achieved through the coordination of NMS and instrument.
With the popularization of 4G network in recent years, wireless services have experienced explosive growth. Compared to huge cost that PTN and IP RAN access solutions take, the multi-service convergence capability of GPON has great advantage and can also perform backhaul of base station. It is expected to become the dominant mode in small base station backhaul in future. The requirement of constructing backhaul network with PON technology in base station becomes a trend as such technology is widely used today. In such case, mobile communication system raises high precision requirement of time and frequency synchronization to meet the roaming and switching of mobile services. Therefore, PON devices should support time synchronization of high precision and stable frequency performance in order to meet the requirements of constructing backhaul network in base station.
With the development of 3G and LTE mobile communication systems, the mobile networks have entered the era of broadband. And the requirement for high-precision time/phase synchronization becomes more and more important. In systems of CDMA2000, TD-SCDMA, TD-LTE, LTE-A, it needs to synchronize precisely between each base station, otherwise it will cause severe interference problem in transmit and receive ends. To synchronize, the traditional solution is to configure GPS/BDS synchronous module in each base station. However, it brings a lot of problems, such as high cost of GPS/BDS module, difficult maintenance, limited indoor deployment, high fault rate, and security risks as well. When the satellite system is not available, a large number of base stations will not be able to use. Another solution is to build up time synchronization network with timing information provided by transmission network according to synchronous protocols. China Mobile adopts the ground transmission of PTP signal over PTN network, provides time synchronization for LTE base station, establishes PTP network in more than 300 cities, and provides time precision of ±1.5μs for TD-SCDMA and TD-LTE. By the end of June 2016, PTP functions are supported by 670 time servers, 1.5 million PTN nodes, 44,000 OTN nodes, 1.3 million TD-LTE base stations and 560,000 TD-SCDMA base stations. Our instrument provides solution for time and clock synchronization performance testing of LTE base station equipment.
In 5G network, the requirement for performance improves dramatically. At the same time, it puts forward more and more precision in time/frequency synchronization. TDD modulation is us in 5G base station. In addition to traditional mobile services, it probably has applications in other fields. In some special services, it requires very high precision of time synchronization, possibly up to several hundred nanoseconds, or even dozens of seconds. For example, in EPC network of 5G it uses base station to provide location services with around dozens of nanoseconds for time synchronization. The key technology of LTE-Advanced, known as Coordinated Multiple Points Joint Processing (CoMP-JP), requires the relative time accuracy within 100 nanoseconds between adjacent base stations. Therefore, performance tests of time/frequency synchronization on 5G base station devices are carried out in the R&D and production, middle-phase, network access selection, engineering opening and acceptance, and daily maintenance process. We provide time and clock synchronization performance test solution of 5G base station devices.
The packet network synchronous system is composed of time/frequency synchronous devices and packet transport devices/access devices, etc. In the manufacturing and joint-debugging sections of such devices, it needs to check the synchronization consistency. At the same time it needs to improve the test efficiency and lower the cost as well, especially in the manufacturing process of time/frequency synchronization devices, which have hundreds of thousands of time port and, frequency ports to be verified. In the past, one instrument can test one channel at a time, while for multiple channels it takes very long time and heavy cost. In such cases, the manufacturers have to take sample tests instead. However, without 100% inspection, it would bring potential quality hazards for the out-going devices.
The devices in IP Carrier Network mainly include PTN, OTN, IP RAN, which are covering applications in backbone layer, convergence layer and access layer. To meet the requirement of different operations in IP carrier network, the telecom manufacturers has successively launched a series of products like PTN, OTN, IP RAN and relevant time/frequency synchronization test solutions. It has effectively promoted the market development for IP carrier network devices.
Ethernet Switches are widely used in the Internet as the core devices of Ethernet network. In recent years, PTP and SyncE technologies are applied more and more in the Ethernet switches. Many manufacturers at home and abroad launched a series of such devices with synchronization functions for the industries such as telecom, electricity and railways. Therefore, how to verify the synchronous performance of multiple ports in such devices effectively becomes a difficult task that the manufacturers have to face. The synchronization performance measurement of Ethernet Switches includes time synchronization and frequency synchronization verification. For different scenario, networking structure and clock modes, the test items, content, and requirements are quite different. In the manufacturing section of Ethernet Switches it needs to verify PTP synchronization consistency of multiple ports, and to concern on how to improve the inspection efficiency.
With the large-scale deployment and application of 3G/4G network and full IP development of network and services, the packet transmission technology will replace SDH/MSTP network and become the mainstream transport network. At this time, on one hand the new transport network technology will request higher requirements for synchronization performance; on the other hand, in the evolution from circuit-switched network to packet-switched network, the compatibility of traditional TDM services and the inter-connection between traditional circuit-based networks require that the packet networks should provide high-quality synchronization and timing performance. In such case, it needs to achieve frequency synchronization and interoperability between different devices. Frequency synchronization is closely related to the type of clock devices. In the environment of packet network, the types of devices involved in frequency synchronization include PTN, POTN, PON devices, as well as SDH, MSTP OTN devices that need to interconnect with packet network. The type of clock devices may include EEC, SEC, PEC and hybrid clock types.