Recalling the development of Ethernet synchronization technology, we have used Internet Network Time Protocol NTP (Network TIme Protocol) technology, Simple Network Time Protocol (SNTP) technology, GPS technology or T1 / E1 and Ethernet on Ethernet Form a hybrid network to increase the clock synchronization capability of Ethernet, but due to the limitations of NTP's own technology, its accuracy can only be between 1 ~ 50ms, and cannot achieve the required synchronization accuracy or convergence speed; GPS is widely used in CDMA base stations and many Other applications provide time and frequency synchronization, but GPS receivers need to set up antennas in the air, which is more difficult to implement in the office or operator's computer room; in the T1 / E1 and Ethernet hybrid network, T1 / E1 is used to transfer the clock, Use Ethernet to expand bandwidth, but in terms of network construction costs, this method is not economical.
IEEE1588 standard evolution and main features
In view of this, the "Precision Clock Synchronization Protocol Standard for Network Measurement and Control Systems" drafted by the Network Precision Clock Synchronization Committee was approved by the American Institute of Electrical and Electronics Engineers (IEEE) as the IEEE1588 standard at the end of 2002. The IEEE 1588 standard is particularly suitable for Ethernet, and can achieve microsecond-level high-precision clock synchronization in a geographically dispersed IP network.
The core idea of ​​IEEE1588 is to use the master-slave clock mode to encode time information, and use the network symmetry and delay measurement technology to achieve master-slave time synchronization. IEEE1588 can achieve frequency synchronization and time synchronization at the same time. The accuracy of time transfer depends mainly on two conditions: counter frequency accuracy and link symmetry. The key to IEEE1588 implementation is the delay measurement. Only by ensuring that the Ethernet equipment meets the requirements of the IEEE 1588 standard can the IP network deliver multiple real-time services and data services with multiple broadcast services. At present, the IEEE1588 verification test items for carrier-grade Ethernet equipment mainly include the following.
(1) Correction factor test: test whether the PTP device can accurately calculate the correction factor (CorrecTIon Factor).
(2) PTP equipment scale test: test that the master clock can support the maximum number of slave clocks under different message rates.
(3) BMC test: mainly refers to the best master clock (BMC) selection test and error switching test.
(4) Test of PTP packet priority: test how PTP equipment can ensure the forwarding of PTP packets, combined with L2 and L3 QoS test.
(5) Multi-time domain test: test the scale of multi-time domain and whether there is mutual interaction in multi-time domain.
(6) Loading the control plane: When testing the PTP protocol, by simulating STP and routing protocols, etc., the control plane can be loaded and the unstable conditions of the network can be simulated at the same time.
(7) Abnormal test and load additional stress test.
(8) Test of protocol timer: After sending the Sync message, the interval time for sending Follow UP can be controlled.
(9) Stability test: Test the stability of the PTP device by sending abnormal packets.
Three main modes of clock synchronization test
Now mainly analyze the correction coefficient error test (CorrecTIon Factor Error), PTP large-scale test (PTP Scalability) and the best master clock selection algorithm test (Best Master Clock). Able to complete these test items mainly include instruments such as IXIA's XM12, IXN2X or Spirent TestCenter.
Correction Factor Error Test (Correction Factor Error)
One of the most important functions of the Transparent Clock is to be able to correctly measure the delay (ns level) when the PTP packet passes through it. This delay is also called "dwell time". The transparent transmission clock carries delay information in the PTP message sent downstream, called correction factor. If the CF is not accurate, the downstream slave clock cannot be accurately synchronized with the upstream master clock.
The test instrument can measure the actual latency of each PTP packet through the transparent transmission clock (Actual Latency), and compare the CF value reported in the PTP message to more effectively test whether the CF value calculated by the transparent transmission clock is accurate.
PTP Scalability
Most PTP systems have many slave clocks. As the number of slave clocks increases in the system, it will increase the processing load of the master clock or the boundary clock. Therefore, when designing, arranging, and upgrading PTP equipment, large-scale benchmark tests of the master clock, boundary clock, and transparent clock are very important. Using the test system, a large number of master and slave clocks in multiple time domains can be simulated. The scale that the PTP device can support depends on many factors, for example, the transmission rate of Sync and Delay-Request messages, whether it is in unicast mode or multicast mode.
Best Master Clock Selection Algorithm (Best Master Clock)
The optimal master clock (MBC) selection algorithm is mainly applied to the slave clock ports of the slave clock and the boundary clock, and the master clock with the best quality is selected in this time domain. This algorithm mainly compares different clock quality parameters and selects the best master clock in a specific priority order. The test system can be used to simulate multiple master clocks with different clock quality parameters. If the device under test is a boundary clock, the slave clock simulated by the downstream test system can easily determine whether the system's grandmaster and the device under test are the same.
Due to the high-precision synchronous work, the inherent data transmission time fluctuation of Ethernet technology is reduced to an acceptable range that does not affect the control accuracy. Only a "synchronized" IP network is a true carrier-grade network, and it can provide guarantees for the IP network to deliver multiple real-time services and data services in multiple broadcast services.
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