Electrical computers and digital processing systems: support – Synchronization of clock or timing signals – data – or pulses
Patent
1997-06-23
1999-12-28
Butler, Dennis M.
Electrical computers and digital processing systems: support
Synchronization of clock or timing signals, data, or pulses
713502, 713503, G06F 112
Patent
active
06009530&
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
This invention relates to a method of determining the accuracy of the real time clocks which are provided within each element of a distributed telecommunications network.
Managed Network Elements (NEs), for example a shelf of electronic switches each contain a Real Time Clock (RTC) which is used for time stamping event reports sourced from the respective element. Such reports may, for example, be the start or finish times of a traffic connection. It is clearly important that all the RTCs are synchronised to as great an accuracy as can be obtained. The known method of setting the RTC within each element relies on sending a "time set" message from the Network Management Centre (NMC) to the element. These messages suffer varying delays during transmission around the network and in practice have been found to provide a setting accuracy of only between 3 and 5 seconds, depending on the network size and loading of the network. The known setting method is "open looped"--the NMC has no way of knowing to what accuracy a respective RTC has been set after the time set command has been sent.
SUMMARY OF THE INVENTION
According to the present invention a method of operating a synchronous digital hierarchy telecommunication network, includes the steps of determining the accuracy of the real time clocks associated with each element of the network comprising; sending a marker signal at a reference time along with the traffic signals to one or more elements in the network such that the element records the time of arrival of the marker signal using its clock, causing the element to send back to a network management centre data indicative of the time of arrival of the marker signal, and comparing the recorded time to the reference time to determine the accuracy.
The invention makes use of the fact that the delays in traffic signals in a telecommunications network are low and relatively invariant and thus the marker signal can be transmitted to an element in the network substantially instantaneously, whereas the message signals previously described can take several seconds.
In a preferred embodiment a marker signal is generated by corrupting a traffic signal, each network element then being arranged to time stamp and record within its respective event log the onset (or finish) of the corrupted signal. The network management centre is preferably arranged to interrogate the event log of each network element so as to initiate the transmission back to the network management centre of the time of arrival data.
Once the inaccuracies have been determined there still remains the problem of setting the NE clock to an improved accuracy. In the past this was done by sending a message signal indicating the correct time to the network element. However, as previously described, due to delays in transmission there is no guarantee that each real time clock will be set to the correct time. Preferably therefore the method comprises the further step of setting an inaccurate clock by sending a message signal so as to cause the clock to be adjusted by the difference between the reference and its recorded time.
Because only a difference signal is being transmitted, it does not matter how long that signal takes to arrive at the respective network element.
BRIEF DESCRIPTION OF THE DRAWINGS
An example of the invention will now be described by way of example with reference to the accompanying diagrammatic drawings, in which:
FIG. 1 shows schematically a telecommunications network;
FIG. 2 shows schematically a portion of traffic signal incorporating a marker signal; and
FIG. 3 shows a flow chart of the operation of the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring to FIG. 1 an SDH (Synchronous Digital Hierachy) telecommunications network comprises a plurality of Network Elements (NE) 1 which may be widely distributed and which are connected to each other and to a Network Management Centre (NMC) 2 by communications links 3, embedded within the traffic paths. Each Network Element 1 inc
REFERENCES:
patent: 4473889 (1984-09-01), Ross
Butler Dennis M.
GPT Limited
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