Network Testing Device and Method for Selecting Its Time Source

Abstract

To provide a network testing device that can obtain high-quality measurement results. [Solution] The system comprises a transmitting / receiving unit 5 connected to the network or device under test, which transmits and receives packets to and from the network or device under test according to a predetermined communication standard; a reference oscillator 6 that generates an oscillation signal of a predetermined frequency; and a control unit 9 that selects an NTP server that meets predetermined NTP server conditions connected to the network via the transmitting / receiving unit 5 as a time source, and if there is no NTP server that meets the NTP server conditions, uses the reference oscillator 6 as a time source.

Description

Technical Field

[0006] , , , ,

[0001] The present invention relates to a network test apparatus that measures network performance for a test network or a test apparatus.

Background Art

[0002] As a test network or a test apparatus, for example, the network performance such as the throughput, delay, and synchronization error (Time Error), which is the difference from a reference time such as UTC (Coordinated Universal Time), of a wireless terminal (UE: User Equipment) that performs mobile communication using a mobile communication system is tested by a network test apparatus to determine whether it is the desired performance.

[0003] In such a network test apparatus, in order to obtain high-quality measurement results when measuring delay and the like, a highly accurate time source is required.

[0004] Patent Document 1 describes that based on a GNSS signal received from a GNSS (Global Navigation Satellite System) satellite, the set voltage of a reference oscillator is controlled, and when the slope of the change in the set voltage of the reference oscillator (the amount of change per unit time of the set voltage) is within an allowable range, it is determined that the appropriate timing is to transition to holdover.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0006] However, in environments where network test equipment is used, there was a problem in that it was uncertain whether high-quality measurement results would be obtained, depending on the reception conditions of the GNSS signal, such as the number of satellites acquired and the conditions of the propagation path.

[0007] Therefore, the present invention aims to provide a network testing apparatus that can select a highly accurate time source and obtain high-quality measurement results. [Means for solving the problem]

[0008] The network test apparatus of the present invention comprises a reference oscillator (6) that generates an oscillation signal of a predetermined frequency, and a control unit (9) that controls the frequency of the oscillation signal generated by the reference oscillator, and further comprises a network connection unit (5) that is connected to a network and communicates with devices connected to the network, wherein the control unit selects an NTP server that satisfies predetermined NTP (Network Time Protocol) server conditions connected to the network via the network connection unit and uses it as a time source, and if there is no NTP server that satisfies the NTP server conditions, it uses the reference oscillator as a time source.

[0009] This configuration selects an NTP server that meets the NTP server requirements and uses it as the time source. As a result, a highly accurate time source is selected, and high-quality measurement results can be obtained.

[0010] Furthermore, the network test apparatus of the present invention includes a GNSS receiver (4) that receives GNSS signals from a GNSS (Global Navigation Satellite System) satellite, and the control unit, when there is no NTP server that satisfies the NTP server conditions, selects a GNSS signal that satisfies predetermined GNSS signal conditions from the GNSS signals received by the GNSS receiver and uses it as a time source, and when there is no NTP server that satisfies the NTP server conditions and there is no GNSS signal that satisfies the GNSS signal conditions, the reference oscillator is used as a time source.

[0011] In this configuration, if there is no NTP server that meets the NTP server requirements, a GNSS signal that meets the predetermined GNSS signal requirements is selected and used as the time source. Therefore, a highly accurate time source is selected, and high-quality measurement results can be obtained.

[0012] Furthermore, in the network testing device of the present invention, if there is no NTP server that satisfies the NTP server conditions and no GNSS signal that satisfies the GNSS signal conditions, the control unit searches for an NTP server that satisfies the NTP server conditions or a GNSS signal that satisfies the GNSS signal conditions according to a preset priority order.

[0013] This configuration ensures that if there are no NTP servers that meet the NTP server requirements and no GNSS signals that meet the GNSS signal requirements, an NTP server that meets the NTP server requirements or a GNSS signal that meets the GNSS signal requirements is searched for according to a pre-configured priority order. As a result, a highly accurate time source is selected, and high-quality measurement results can be obtained.

[0014] Furthermore, in the network testing apparatus of the present invention, the control unit switches the time source based on predetermined measurement conditions, predetermined intervals, or user operation.

[0015] This configuration allows for the switching of the time source based on predetermined measurement conditions, predetermined intervals, or user operation. Therefore, by switching the time source in response to changes in measurement conditions, the surrounding environment, or user requests, a highly accurate time source can be selected, resulting in high-quality measurement results.

[0016] Furthermore, in the network testing apparatus of the present invention, the control unit changes the NTP server conditions or the GNSS signal conditions according to the measurement items and measurement scenes in the network measurement.

[0017] This configuration allows the NTP server conditions or GNSS signal conditions to be changed according to the measurement items and measurement scenarios in network measurements. As a result, the appropriate time source is selected according to the measurement items and measurement scenarios, enabling the acquisition of high-quality measurement results.

[0018] Furthermore, in the network testing apparatus of the present invention, the control unit sets the NTP server conditions or the GNSS signal conditions according to the selected time accuracy or measurement accuracy.

[0019] This configuration allows NTP server conditions or GNSS signal conditions to be set according to time accuracy or measurement accuracy. Therefore, an appropriate time source is selected according to time accuracy or measurement accuracy, and high-quality measurement results can be obtained.

[0020] Furthermore, in the network testing device of the present invention, the control unit stores a plurality of candidate NTP servers to be used as time sources, and allows users to add and delete them.

[0021] This configuration allows users to add and remove candidate NTP servers for use as time sources. As a result, users can select the appropriate NTP server for their time source, leading to higher quality measurement results.

[0022] Furthermore, in the network testing device of the present invention, the control unit removes NTP servers that meet predetermined conditions from the list of candidates for NTP servers to be used as a time source.

[0023] This configuration automatically removes NTP servers that do not meet certain conditions from the list of candidates for the time source. As a result, inaccurate NTP servers are automatically removed, allowing for high-quality measurement results.

[0024] Furthermore, in the network testing apparatus of the present invention, the control unit records the information of the selected time source along with the measurement results.

[0025] With this configuration, information on the time source selected at that time is recorded together with the measurement result. Therefore, the correspondence between the measurement result and the time source information can be established, and the quality of the measurement result can be determined from the time source information.

[0026] Further, the time source selection method of the present invention is a time source selection method for a network test device (1) including a reference oscillator (6) that generates an oscillation signal of a predetermined frequency, a network connection unit (5) that is connected to a network and communicates with a device connected to the network, and a control unit (9) that controls the frequency of the oscillation signal generated by the reference oscillator. The method includes a step of selecting an NTP server that satisfies a predetermined NTP server condition and is connected to the network via the network connection unit as a time source, and a step of using the reference oscillator as a time source when there is no NTP server that satisfies the NTP server condition.

[0027] With this configuration, an NTP server that satisfies the NTP server condition is selected as the time source. Therefore, a highly accurate time source is selected, and a high-quality measurement result can be obtained.

Effect of the Invention

[0028] The present invention can provide a network test device capable of obtaining a high-quality measurement result.

Brief Description of the Drawings

[0029] [Figure 1] FIG. 1 is a block diagram of a network test device according to an embodiment of the present invention. [Figure 2] FIG. 2 is a flowchart for explaining the procedure of time source selection processing of a network test device according to an embodiment of the present invention.

Mode for Carrying Out the Invention

[0030] Hereinafter, a network test device according to an embodiment of the present invention will be described in detail with reference to the drawings.

[0031] In Figure 1, the network test apparatus 1 according to one embodiment of the present invention comprises a GNSS antenna 2, a DC power supply 3 for the antenna, a GNSS receiver 4, a transmitting / receiving unit 5 as a network connection unit, a reference oscillator 6, a 1PPS signal generation unit 7, a display operation unit 8, a control unit 9, and a battery 10.

[0032] The GNSS antenna 2 receives GNSS signals transmitted from a GNSS satellite (not shown). The GNSS antenna 2 is detachably connected to a connector 11 provided on the main body of the network test device 1.

[0033] The DC power supply 3 for the antenna supplies the necessary power voltage to the GNSS antenna 2 connected to the connector 11.

[0034] The GNSS receiver 4 receives the GNSS signal from the GNSS antenna 2 via the connector 11 and capacitor 12. Based on the GNSS signal received from the GNSS antenna 2, the GNSS receiver 4 outputs, for example, a reference 1PPS (1 Pulse Per Second) signal to the 1PPS signal generation unit 7 and the control unit 9. The GNSS receiver 4 measures the signal strength and signal quality of the GNSS signal received from the GNSS antenna 2 and transmits this GNSS signal information to the control unit 9.

[0035] The GNSS receiver 4 is capable of receiving GNSS signals such as GPS (Global Positioning System) signals, GLONASS (Global Navigation Satellite System) signals, Galileo signals, and BDS (BeiDou navigation System) signals.

[0036] The transmitting / receiving unit 5 is connected to a test network or device under test (not shown) via a measurement terminal 13, and transmits and receives packets to and from the test network or device under test according to a predetermined communication standard. The transmitting / receiving unit 5 transmits received packets to the control unit 9. The transmitting / receiving unit 5 transmits packets created by the control unit 9 according to a predetermined communication standard. The predetermined communication standard is, for example, Ethernet®.

[0037] The reference oscillator 6 is a frequency oscillator built into the main unit of the device, and its frequency is controlled by a set voltage from the control unit 9. The reference oscillator 6 outputs an oscillation signal with a highly stable frequency, such as a reference frequency of 10 MHz, to the 1 PPS signal generation unit 7. The reference oscillator 6 functions as a time source that supplies time to the network test device 1.

[0038] The 1PPS signal generation unit 7 includes a counter 71 that counts a reference 1PPS signal generated based on the GNSS signal received by the GNSS receiver 4 from the GNSS satellite, and outputs the count value from the counter 71 to the control unit 9. The 1PPS signal generation unit 7 generates a 1PPS signal based on the oscillation signal of the reference oscillator 6 and outputs it from the connector 14.

[0039] In other words, while receiving a GNSS signal from the GNSS receiver 4, the control unit 9 controls the set voltage of the reference oscillator 6 based on the count value from the counter 71, so that an oscillation signal synchronized with the GNSS signal is output from the reference oscillator 6, and the GNSS signal becomes the time source.

[0040] The display operation unit 8 is composed of, for example, a touch panel that serves both as a display and input operation unit. It displays images for inputting measurement conditions and various parameters necessary for performing various measurements, as well as images showing the results when various measurements are performed. It outputs the information input via these images to the control unit 9.

[0041] The control unit 9 is composed of, for example, a computer unit equipped with a CPU (Central Processing Unit) (not shown), RAM (Random Access Memory), ROM (Read Only Memory), a hard disk drive, and input / output ports.

[0042] The ROM and hard disk drive of this computer unit store various control constants and maps, along with a program that allows the computer unit to function as a control unit 9. In other words, the CPU executes the program stored in the ROM and hard disk drive, causing the computer unit to function as a control unit 9. The hard disk drive may be a CF (Compact Flash) card or the like, using flash memory.

[0043] The input / output ports of the control unit 9 are connected to the GNSS receiver 4, the transmitting / receiving unit 5, the 1PPS signal generation unit 7, and the display / operation unit 8, enabling the control unit 9 and each unit to transmit and receive signals.

[0044] The control unit 9 includes a measurement unit 91 that sends and receives packets to and from the network and equipment under test to measure the delay, throughput, and other parameters of the network and equipment under test.

[0045] The battery 10 is detachably and replaceably attached to the main body of the network test device 1 and supplies the power necessary to drive each part when performing various measurements. Note that the battery 10 is not essential as it can be used depending on the environment in which the network test device 1 is used, and a configuration that can use commercial power or other sources is also possible.

[0046] In this embodiment, the measurement unit 91 requires a time source with high time accuracy to supply time to the network test device 1 in order to obtain high-quality measurement results when measuring the delay.

[0047] Therefore, the control unit 9 synchronizes the time using an NTP (Network Time Protocol) server connected to the network via the transmitting / receiving unit 5 as the time source, and inputs the synchronized time to the measurement unit 91. Alternatively, the control unit 9 may be configured to control the set voltage of the reference oscillator 6 based on the time synchronized with the NTP server so that an oscillation signal synchronized with the NTP server is output from the reference oscillator 6, or to generate a 1PPS signal.

[0048] The control unit 9 stores multiple candidate NTP servers for time synchronization as a time source, and selects an NTP server that satisfies all predetermined NTP server conditions, such as delay time and fluctuations, to perform time synchronization.

[0049] Furthermore, the calculation of delay time and fluctuations may be performed using known techniques such as Root Delay, which represents the delay to the NTP server included in the NTP packet, and Root Dispersion, which represents the fluctuations in the delay to the NTP server. In addition, the NTP server may be given priority and weighting based on the hierarchy level from stratum0 to stratumN (where N is a positive integer), which is the hierarchy level in NTP.

[0050] Candidate NTP servers for time synchronization can be added or removed by the user. Furthermore, if a candidate NTP server becomes inaccessible a certain number of times or fails to meet the NTP server criteria a certain number of times, it may be automatically removed from the list of candidates, and a notification of the removal or a suggestion for removal may be provided. Candidate NTP servers may be prioritized, and a determination of whether they meet the NTP server criteria may be made according to a priority order, such as delay time or fluctuation. If an NTP server that meets the criteria is found, time synchronization is initiated with that NTP server, and the determination of the NTP server criteria for subsequent NTP servers with lower priority may not be performed.

[0051] If there is no NTP server that meets the NTP server requirements, the control unit 9 selects a GNSS signal that satisfies all the conditions, such as signal strength and signal quality, based on predetermined GNSS signal conditions and GNSS signal information from the GNSS receiver 4, and performs time synchronization as the time source. The control unit 9 may also assign a priority to the receivable GNSS signals and determine whether the GNSS signal conditions are met according to the priority. If a GNSS signal satisfies the GNSS signal conditions, such as signal strength and signal quality, the control unit 9 synchronizes the time with that GNSS signal and does not perform the GNSS signal condition check for subsequent GNSS signals with lower priority.

[0052] If there is no NTP server that satisfies the NTP server conditions and the control unit 9 cannot receive a GNSS signal that satisfies the GNSS signal conditions, the control unit 9 uses the reference oscillator 6 as the time source.

[0053] If the control unit 9 finds no NTP server that meets the NTP server conditions and cannot receive a GNSS signal that meets the GNSS signal conditions, it uses the time of the reference oscillator 6 as a time source and attempts to search for an NTP server that meets the NTP server conditions or a GNSS signal that meets the GNSS signal conditions in order of priority. In this search for an NTP server that meets the NTP server conditions or a GNSS signal that meets the GNSS signal conditions, if an NTP server that meets the NTP server conditions or a GNSS signal that meets the GNSS signal conditions exists, the control unit 9 may use that NTP server or GNSS signal as a time source.

[0054] The control unit 9 performs time source switching by searching for an NTP server that satisfies the NTP server conditions and selecting it as a new time source, or by searching for a GNSS signal that satisfies the GNSS signal conditions and selecting it as a new time source, based on predetermined measurement conditions, predetermined intervals, or user operation.

[0055] The control unit 9 changes the NTP server conditions and GNSS signal conditions according to the measurement items and measurement scene in the network measurement.

[0056] Measurement items include, for example, network throughput, latency, and jitter, and measurement scenarios include measurements within mobile networks and measurements within local networks.

[0057] The control unit 9 may set NTP server conditions and GNSS signal conditions according to the selected time accuracy and measurement accuracy. For example, the control unit 9 stores NTP server conditions and GNSS signal conditions corresponding to the time accuracy and measurement accuracy, and sets the corresponding NTP server conditions and GNSS signal conditions based on the user's selection of time accuracy and measurement accuracy.

[0058] The control unit 9 records, for example, in a memory unit (not shown), the measurement results along with information such as whether an NTP server, GNSS signal, or reference oscillator 6 was selected as the time source, and information about the selected time source. Information about the time source includes delay time and fluctuations for an NTP server, and signal strength and signal quality for a GNSS signal.

[0059] The time source selection process by the network test device 1 according to this embodiment, configured as described above, will be explained with reference to Figure 2. The time source selection process described below is performed by predetermined measurement conditions, predetermined intervals, or user operations.

[0060] In step S1, the control unit 9 searches for an NTP server that satisfies the NTP server conditions. After executing the process in step S1, the control unit 9 executes the process in step S2.

[0061] In step S2, the control unit 9 determines whether or not there was an NTP server that met the NTP server conditions.

[0062] If the control unit 9 determines that there is an NTP server that meets the NTP server conditions, it executes the process in step S3. If the control unit 9 determines that there is no NTP server that meets the NTP server conditions, it executes the process in step S4.

[0063] In step S3, the control unit 9 selects an NTP server that satisfies the NTP server conditions as the time source. After executing the process in step S3, the control unit 9 terminates the time source selection process.

[0064] In step S4, the control unit 9 searches for a GNSS signal that satisfies the GNSS signal conditions. After performing the processing in step S4, the control unit 9 performs the processing in step S5.

[0065] In step S5, the control unit 9 determines whether or not there was a GNSS signal that met the GNSS signal conditions.

[0066] If the control unit 9 determines that there is a GNSS signal that satisfies the GNSS signal conditions, it executes the process in step S6. If the control unit 9 determines that there is no GNSS signal that satisfies the GNSS signal conditions, it executes the process in step S7.

[0067] In step S6, the control unit 9 selects a GNSS signal that satisfies the GNSS signal conditions as the time source. After executing the process in step S6, the control unit 9 terminates the time source selection process.

[0068] In step S7, the control unit 9 uses the reference oscillator 6, which is a local oscillator, as the time source. After executing the process in step S7, the control unit 9 terminates the time source selection process.

[0069] Thus, in the above-described embodiment, the control unit 9 selects an NTP server that satisfies predetermined NTP server conditions to be used as a time source, and if there is no NTP server that satisfies the NTP server conditions, the reference oscillator 6 is used as a time source.

[0070] This selects an NTP server that meets the NTP server requirements and uses it as the time source. As a result, a highly accurate time source is selected, and high-quality measurement results can be obtained.

[0071] Furthermore, if there is no NTP server that satisfies the NTP server conditions, the control unit 9 selects a GNSS signal that satisfies predetermined GNSS signal conditions from the GNSS signals received by the GNSS receiver 4 and uses it as a time source. If there is no NTP server that satisfies the NTP server conditions and no GNSS signal that satisfies the GNSS signal conditions, the control unit 9 uses the reference oscillator 6 as a time source.

[0072] This means that if no NTP server that meets the NTP server requirements is available, a GNSS signal that meets the predetermined GNSS signal requirements is selected and used as the time source. As a result, a highly accurate time source is selected, and high-quality measurement results can be obtained.

[0073] Furthermore, if there are no NTP servers that meet the NTP server conditions and no GNSS signals that meet the GNSS signal conditions, the control unit 9 searches for an NTP server that meets the NTP server conditions or a GNSS signal that meets the GNSS signal conditions according to a pre-set priority order.

[0074] This means that if there are no NTP servers that meet the NTP server requirements and no GNSS signals that meet the GNSS signal requirements, an NTP server that meets the NTP server requirements or a GNSS signal that meets the GNSS signal requirements will be searched for according to a pre-set priority order. As a result, a highly accurate time source is selected, and high-quality measurement results can be obtained.

[0075] Furthermore, the control unit 9 switches the time source based on predetermined measurement conditions, predetermined intervals, or user operation.

[0076] This allows the time source to be switched based on predetermined measurement conditions, predetermined intervals, or user operation. Therefore, by switching the time source in response to changes in measurement conditions, the surrounding environment, or user requests, a highly accurate time source can be selected, resulting in high-quality measurement results.

[0077] Furthermore, the control unit 9 changes the NTP server conditions or GNSS signal conditions according to the measurement items and measurement scene in the network measurement.

[0078] This allows the NTP server conditions or GNSS signal conditions to be changed according to the measurement items and measurement scenario in network measurements. As a result, the appropriate time source is selected according to the measurement items and measurement scenario, enabling the acquisition of high-quality measurement results.

[0079] Furthermore, the control unit 9 sets NTP server conditions or GNSS signal conditions according to the selected time accuracy or measurement accuracy.

[0080] This sets NTP server conditions or GNSS signal conditions according to time accuracy or measurement accuracy. As a result, an appropriate time source is selected according to time accuracy or measurement accuracy, allowing for high-quality measurement results.

[0081] Furthermore, the control unit 9 stores multiple candidate NTP servers to be used as time sources, and allows users to add and delete them.

[0082] This allows users to add and remove candidate NTP servers to be used as time sources. As a result, users can select the appropriate NTP server to use as the time source, leading to higher quality measurement results.

[0083] Furthermore, the control unit 9 removes NTP servers that meet predetermined conditions from the list of candidates for NTP servers to be used as a time source.

[0084] This process removes NTP servers that do not meet the specified conditions from the list of candidates for the time source. As a result, inaccurate NTP servers are automatically removed, allowing for high-quality measurement results.

[0085] Furthermore, the control unit 9 records the information of the selected time source along with the measurement results. This records the time source information selected at the time, along with the measurement results. Therefore, a correspondence can be established between the measurement results and the time source information, allowing the quality of the measurement results to be determined from the time source information.

[0086] In this embodiment, a configuration is shown in which the transmitting / receiving unit 5 is used as the network connection unit, but a configuration with a dedicated network connection unit may also be used. Furthermore, in this embodiment, a configuration is shown in which the network connection unit is used in combination with the network under test or the device under test and the network for acquiring time for connecting to an NTP server, but a configuration with a dedicated network connection unit for acquiring time (a transmitting / receiving unit for acquiring time) may also be used. In addition, there may be a time acquisition terminal (not shown) different from the measurement terminal 13, and the time acquisition terminal may be connected to the transmitting / receiving unit 5 or a time acquisition transmitting / receiving unit (not shown) to connect to the time acquisition network.

[0087] While embodiments of the present invention have been disclosed, it will be apparent to those skilled in the art that modifications can be made without departing from the scope of the invention. All such modifications and equivalents are intended to be included in the following claims. [Explanation of Symbols]

[0088] 1. Network Test Equipment 4 GNSS receivers 5. Transmitter / Receiver Unit (Network Connection Unit) 6. Reference Oscillator 7 1PPS signal generation section 8 Display operation section 9. Control Unit 71 counter 91 Measuring part

Claims

1. A reference oscillator (6) that generates an oscillation signal of a predetermined frequency, A network test apparatus (1) comprising a control unit (9) that controls the frequency of the oscillation signal generated by the reference oscillator, It includes a network connection unit (5) that is connected to a network and communicates with devices connected to the said network, The control unit selects an NTP server that meets predetermined NTP server conditions connected to the network via the network connection unit and uses it as a time source, and if there is no NTP server that meets the NTP server conditions, the reference oscillator is used as the time source in the network test apparatus.

2. The system includes a GNSS receiver (4) that receives GNSS signals from a GNSS satellite, The network test apparatus according to claim 1, wherein the control unit, when there is no NTP server that satisfies the NTP server conditions, selects a GNSS signal that satisfies predetermined GNSS signal conditions from the GNSS signals received by the GNSS receiver and uses it as a time source, and when there is no NTP server that satisfies the NTP server conditions and there is no GNSS signal that satisfies the GNSS signal conditions, uses the reference oscillator as a time source.

3. The network testing apparatus according to claim 2, wherein the control unit searches for an NTP server that satisfies the NTP server conditions or a GNSS signal that satisfies the GNSS signal conditions, according to a preset priority order, when there is no NTP server that satisfies the NTP server conditions and no GNSS signal that satisfies the GNSS signal conditions.

4. The network test apparatus according to claim 2, wherein the control unit performs switching of the time source by user operation under predetermined measurement conditions or at predetermined intervals.

5. The network testing apparatus according to claim 2, wherein the control unit changes the NTP server conditions or the GNSS signal conditions according to the measurement items and measurement scenes in the network measurement.

6. The network test apparatus according to claim 2, wherein the control unit sets the NTP server conditions or the GNSS signal conditions according to the selected time accuracy or measurement accuracy.

7. The network testing apparatus according to claim 2, wherein the control unit stores a plurality of candidates for NTP servers to be used as a time source and allows users to add and delete them.

8. The network testing apparatus according to claim 7, wherein the control unit removes an NTP server that satisfies predetermined conditions from a list of candidates for an NTP server to be used as a time source.

9. The network testing apparatus according to claim 2, wherein the control unit records information of the selected time source along with the measurement results.

10. A time source selection method for a network test apparatus (1) comprising a reference oscillator (6) that generates an oscillation signal of a predetermined frequency, a network connection unit (5) that is connected to a network and communicates with devices connected to the network, and a control unit (9) that controls the frequency of the oscillation signal generated by the reference oscillator, The steps include selecting an NTP server that meets predetermined NTP server conditions and is connected to the network via the network connection unit to use as a time source, A time source selection method comprising the step of using the reference oscillator as a time source if there is no NTP server that satisfies the aforementioned NTP server conditions.

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