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.

In one aspect the invention provides, in a telecommunication network, a method 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.

An example of the invention will now be described by way of example with reference

to the accompanying diagrammatic drawings, in which:

Figure 1 shows schematically a telecommunications network; and

Figure 2 shows schematically a portion of traffic signal incorporating a marker signal.

Referring to Figure 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 includes

a Real Time Clock (RTC) 4.

When it is desired to check the accuracy of one or more of the RTCs 4, a test set 5 is arranged to inject a marker signal into an NE traffic port 6 along with the traffic signals and at a time determined by an accurate reference clock 7 such that the marker signal will be relayed with low delay to one or more NE's 1 within the network.

The arrival of the marker signal is time stamped by the respective NE's RTC 4 and

recorded within it's event log (not shown). The Network Management Centre 2 then interrogates the NEs' event log and compares the arrival of the marker signal as indicated by the respective

RTC 4 with the time of transmission as determined by the reference clock 7.

In the SDH system described above the traffic signal may be an STM-1 signal

(Synchronous Transmission Module, the '1' indicating the first level in SDH notation)

containing a TU-12 signal (Tributary Unit, the '12' indicating a 2 Mbit s signal). Referring to Fisure 2 at a time determined, e.e.. from the ^1" :ιbv time standard the test set 5 is arranged

at 00 seconds to transmit a TUAIS (Tπbutory Unit Alarm Indication Signal) of 10 seconds duration An AIS signal within the traffic signals indicates to other Network Elements 1 that a

fault has been found but that no action need be taken The arrival of the AIS signal is recorded

within the event log of each or a particular NE 1 as previously described

If the NMC 2 determines that a particular RTC 4 is in error a message is sent indicating

that the RTC is to be adjusted by the difference in time, typically a few seconds, between the

time of transmission of the marker signal as indicated by the reference clock 7 and the time of

arrival as indicated bv the RTC 4

Although as indicated it is the beginning of the marker signal against which the RTC 4 is calibrated, it is of course possible that the end of the signal can be used instead Although the invention has been described with reference to an SDH system the invention is equally applicable to a PDH system (Plesiochronous Digital Hierarchy), or other type of similar network, such as the north American SONET network, and which is used to provide the time

distribution