Abstract:
A method of measuring temporal drift of an electronic apparatus linked to a network and capable of providing a time-stamp including capturing at least two messages including a gauge reference time circulating around the network, and determining precision of the time-stamping function of the apparatus as a function of the gauge reference time and of a time-stamp provided by the apparatus. The apparatus can be a network analyzer and each message captured includes at least one Ethernet frame including a gauge reference time in accordance with NTP (Network Time Protocol).

Description:
BACKGROUND 
     (1) Field 
     This invention relates to a method and a device for measurement of the time deviation of an electronic equipment item connected to a network. It applies, in particular, to providing a measurement of the time deviation of the time-stamping system of network analyzers. 
     (2) Description of the Related Art 
     Time-stamping systems for network analyzers lack precision. As it happens, these time-stamping systems have the task of dating the network frames captured from the analyzed link. A problem that this invention is intended to resolve thus consists in evaluating the time deviation of the time-stamping function of a network analyzer in relation to a reference time. 
     Unlike a measurement equipment item such as a multimeter or an oscilloscope, network analyzers do not undergo periodic calibration because the metrology companies are not equipped, and the network analyzers are not designed, for that. 
     Until then the performances of the time-stamping function of network analyzers were not called into question. The inventors therefore saw a need for standardization of network analyzers. 
     BRIEF SUMMARY 
     To this end, according to a first aspect, this invention applies to a method for measurement of the time deviation of an electronic equipment item connected to a network and able to provide a time-stamping, characterized in that it comprises:
         a step of capturing at least two messages including a standard reference time circulating on the said network and   a step of determining the precision of the time-stamping function of the equipment item according to the said standard reference time and a time-stamping provided by the said equipment item.       

     By virtue of these arrangements, the time deviation is measured without dismantling or specific defining of the equipment item, since a nominal and operational use is made of this equipment item. In addition, the implementation of the method that is the object of this invention makes it possible to achieve a level of precision for the standardization that is more or less high according to the need of the user. This invention also has the advantage of making it possible to standardize several equipment items simultaneously. 
     According to specific characteristics, the said equipment item is a network analyzer. 
     According to specific characteristics, during the capturing step, each captured message comprises an Ethernet frame circulating on the said network, which frame includes the standard reference time. 
     According to specific characteristics, each message including a standard reference time is in accordance with the NTP protocol (acronym for “Network Time Protocol” for network time protocol). 
     According to specific characteristics, during the capturing step, a plurality of messages including a standard reference time is captured over a period in excess of ten minutes. 
     Preferentially, this period is on the order of one hour. In fact, the longer the capture period is, the better the precision of the standardization. 
     According to specific characteristics, during the step of determining the precision of the time-stamping function of the equipment item, a time deviation is calculated by comparing the period elapsed between the dates of capture of at least two messages including a standard reference time provided by the time-stamping function of the equipment item with the difference in the standard reference times contained in these messages. 
     According to specific characteristics, the method that is the object of this invention, such as briefly explained above, comprises a step of generating the standard reference time from signals provided by a constellation of satellites equipped with atomic clocks. 
     These signals in fact originate from a precise time reference source. 
     According to specific characteristics, during the capturing step, the said equipment item applies a capture filter that lets in only the said messages including a standard reference time. 
     According to a second aspect, this invention applies to a device for measurement of the time deviation of an electronic equipment item connected to a network and able to provide a time-stamping, characterized in that it comprises:
         a means for capturing at least two messages including a standard reference time circulating on the said network and   a means for determining the precision of the time-stamping function of the equipment item according to the said standard reference time and a time-stamping provided by the said equipment item.       

     According to specific characteristics, the said equipment item is a network analyzer. 
     According to specific characteristics, the capture means is adapted for capturing at least one Ethernet frame circulating on the said network, which frame includes the standard reference time. 
     According to specific characteristics, the capture means is adapted for capturing at least one message including a standard reference time in accordance with the NTP protocol (acronym for “Network Time Protocol” for network time protocol). 
     According to specific characteristics, the capture means is adapted for capturing a plurality of messages including a standard reference time over a period in excess of ten minutes. 
     According to specific characteristics, the means for determining the precision of the time-stamping function of the equipment item is adapted for calculating a time deviation by comparing the period elapsed between the dates of capture of at least two messages including a standard reference time provided by the time-stamping function of the equipment item with the difference in standard reference times contained in these messages. 
     According to specific characteristics, the capture means is adapted for applying a capture filter that lets in only the said messages including a standard reference time. 
     According to a third aspect, this invention applies to a computer program that can be loaded into a computer system, the said program containing instructions enabling the implementation of the measurement method such as briefly explained above. 
     According to a fourth aspect, this invention applies to a non-transitory computer readable medium for information items readable by a computer or a microprocessor, removable or otherwise, storing instructions of a computer program, characterized in that it enables the implementation of the measurement method, such as briefly explained above. 
     Since the advantages, purposes and characteristics of this measurement device, this computer program and this medium for information items are similar to those of the measurement method that is the object of this invention, such as briefly explained above, they are not repeated here. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other advantages, purposes and characteristics of this invention will become apparent from the description that is going to follow, presented with an explanatory but in no way limitative intent, with reference to the attached drawings, in which: 
         FIG. 1  schematically shows a specific embodiment of the device that is the object of this invention and a network conveying frames including a reference time, and 
         FIG. 2  shows, in the form of a logic diagram, steps implemented in a specific embodiment of the method that is the object of this invention. 
     
    
    
     DETAILED DESCRIPTION 
     As seen on  FIG. 1 , in a specific embodiment of the device that is the object of this invention, an equipment item consists of a network analyzer to be standardized  105  comprising a microcontroller  110  and a program memory  115  storing a program  120  implementing the method that is the object of this invention. Network analyzer  105  uses a port  125  for connection to a network  150 , the said port being associated with a filter  130  and a time-stamping means  135 , in a manner known in itself. In fact, network analyzer  105  is provided with the capacity to date with precision the network frames that it receives. 
     In this way, computer program  120  that is the object of this invention may be loaded into a computer system and contains instructions enabling the implementation of the measurement method such as briefly explained above. Program memory  115  may be a memory component, a memory integrated into microcontroller  110 , a removable memory, for example a memory card or a CD-ROM. This medium stores the instructions of computer program  120 . 
     Network  150  conveys Ethernet frames and is of NTP (acronym for “Network Time Protocol”) type. It is connected to a reference time server  155 , itself connected to a reception system  160  for signals sent out by satellites  165  provided with atomic clocks  170 . In this way the NTP network uses a very precise time reference (for example originating from the constellation of “GPS” satellites, acronym for “Global Positioning System”, the satellites of which are equipped with an atomic clock). 
     The implementation of the invention concerns three subsets:
         the test environment, which is able to provide a precise time reference carried in messages in accordance with the NTP protocol;   the capture, during a test period, of specific messages comprising a control reference time; as explained below, the greater the test period is, the better the precision and   the calculation of the deviation by comparison of the dates of capture of the first and last messages comprising a reference time, during the test, with the reference times which these messages comprise.       

     As is seen on  FIG. 2 , in order to implement a specific embodiment of the method that is the object of this invention, a step  205  for startup of the period of the test is performed. During this step  205 , the user, or a stored value, determines the period of the test, in excess of ten minutes and preferentially on the order of an hour. It is seen that the startup may be manual, that is to say performed by the user, or automatic, for example at a fixed daily time or at each time network analyzer  105  is turned on. During step  205 , the equipment item to be standardized is connected and is configured to record the messages comprising an NTP reference time. 
     At the same time, during a step not shown, server  155  performs generation of the standard reference time from signals provided by a constellation of satellites equipped with atomic clocks. Server  155  sends out, on network  150 , NTP frames comprising the standard reference time periodically, for example every 64 seconds. 
     Then, during a step  210 , the capture and time-stamping of a message circulating on the network is performed, in a manner known in itself. During a step  215 , it is determined whether this message comprises a reference time. If not, one returns to step  210 . If the message comprises a reference time, a reading of the reference time is performed during a step  220 . 
     Thus, to facilitate implementation of this invention, it is recommended to apply a capture filter, if the equipment item to be tested so permits, in order to capture only the NTP traffic. Preferentially, during step  215 , the presence of NTP messages in the capture performed is verified regularly. It is noted that the sending period for these messages is dependent on the environment (it is, for example, 64 seconds). 
     Then, during a step  225 , it is determined whether the period of the test is completed. If not, one returns to step  210 . If the period of the test is completed, in step  230  the difference between the time-stampings of the first and the last messages received during the period of the test is determined. This time-stamping information also is called “timestamp” on numerous equipment items. The result T analyzer  indicates the elapsed period measured by the equipment item.
 
 T   analyzer =Timestamp last NTP message −Timestamp first NTP message .
 
     During a step  235 , the difference between the reference times of the first and last messages received during the period of the test is determined. In this way, the difference between the reference time indicated in the last NTP message captured during the test and the reference time indicated in the first NTP message captured during the test is calculated. The result indicates the elapsed reference period. It is noted that the reference time corresponds to the “Transmit Timestamp” field of the NTP message.
 
 T   reference =Transmit Timestamp last NTP message −Transmit Timestamp first NTP message .
 
     During a step  240 , the difference between the differences measured during steps  230  to  235  is determined, in order to determine the time-stamping deviation of the equipment item in relation to the reference time. The time deviation of the equipment item during the period of the test thus is obtained by calculating the difference between the two periods obtained previously:
 
Deviation analyzer   =T   reference   −T   analyzer .
 
     For example, this deviation is given as an absolute value or, after division by the period of the test, as a relative value, for example as ppm (acronym for “part per million”). 
     During a step  245 , the maximal precision of the deviation measured during step  240  is determined. The maximal precision of the measurement is linked directly to the precision of the time reference (P ref , without unity), the jitter (or “jitter”) of the NTP network frames (J ntp , seconds), the period of the test (T, seconds). This maximal precision (P max , without unity) is given by the following formula:
 
 P   max   =P   ref   +J   NTP   /T  
 
     It is noted that, with setting up of NTP V4 synchronized on the time propagated by the GPS constellation, a precision of 1 ppm is achieved in 1 hour of testing. 
     As is understood upon reading the above description, the implementation of this invention has the advantage that it does not require any dismantling or specific defining of the equipment item since it is based on a nominal and operational use of the equipment item. In addition, this solution makes it possible to achieve a level of precision of the standardization that is more or less high according to the need of the user. This invention also makes it possible to standardize several equipment items simultaneously.