GB2374758A

GB2374758A - Network performance monitoring system

Info

Publication number: GB2374758A
Application number: GB0202420A
Authority: GB
Inventors: Kenneth Guild; Godfrey Hill; Bruno Cotta; Anna Tzanakaki; John Ellison
Original assignee: Ditech Networks Inc
Current assignee: Ditech Networks Inc
Priority date: 2001-02-08
Filing date: 2002-02-01
Publication date: 2002-10-23
Also published as: GB0103161D0; GB0202420D0

Abstract

A channel performance monitoring system for an optical network is provided which comprises measuring means for measuring the Signal to Noise Ratio (SNR), bit error rate (BER) and power level of a signal at selected points in the network; and a network management system, including comparing means to compare data from the measuring means with known data relating to the network, and processing means to process data from the comparing means to determine channel performance information. The invention provides intelligence within a network management system (NMS) to enable an optical channel to be set up automatically across an existing all-optical network infrastructure and ensure that it conforms to an agreed performance specification. Where non-compliance is identified corrective action can be applied either automatically and problem reports issued or, alternatively, the circuit is flagged as non routable, again with appropriate reports. Corrective action may be in the form of changes to the circuit configuration of operating conditions (within accepted profiles), by invoking protection switching or by other means.

Description

A NETWORK PERFORMANCE MONITORING SYSTEM Field of the Invention The present invention relates to the field of all-optical communications networks.

Background to the Invention The ability to configure and test all-optical networks and to localise problems when they occur is a critical requirement for any carrier class network. In all-optical networks this problem is particularly difficult because optical networks are typically less articulated than electronic networks, as less frequent regeneration is required, and because of the analogue nature of the transmission path and the way impairments add to a signal as it traverses the connection. Gross changes in optical channel performance can be identified by'normal'network management methods. However it is much more difficult to locate the source of a problem when a series of minor impairments add together to cause a channel failure.

Summary of the Invention According to a first aspect of the present invention a channel performance monitoring system for an optical network comprises: measuring means for measuring the Signal to Noise Ratio (SNR), bit error rate (BER) and power level of a signal at selected points in the network ; and a network management system, including comparing means to compare data from the measuring means with known data relating to the network, and processing means to process data from the comparing means to determine channel performance information.

According to a second aspect of the present invention a method of monitoring the performance of a channel path within an optical network comprises the steps of: measuring the Signal to Noise Ratio (SNR), bit error rate (BER) and power level of a signal at selected points on the channel ; comparing the measured SNR, BER and power level at each point with known information about the network; and, processing the results of the comparison to obtain information about the performance of the channel.

Preferably the signal has a digital wrapper.

Preferably, the optical network comprises a number of nodes and SNR, BER and power level are measured at the input and output of some or all of the nodes. The results of a measurement of SNR, BER and power level provide a characteristic error profile for a channel. Preferably, the measurement of SNR, BER and power level is carried out at intervals, however it may be carried out continuously.

Preferably, the system further includes data storage means for storing the measurements of SNR, BER and power level and the processing means is adapted to analyse the measurements to identify historical trends and patterns.

Preferably, small perturbations can be made to the signal and measurements of SNR, BER and power level can be compared to those made without the perturbation to provide further information on any faults in the channel.

The present invention provides intelligence within a network management system (NMS) to enable an optical channel to be set up automatically across an existing all-optical network infrastructure and ensure that it conforms to an agreed performance specification. Where non-compliance is identified corrective action can be applied either automatically and problem reports issued or alternatively the circuit is flagged as non rouable, again with appropriate reports. Corrective action may be in the form of changes to the circuit configuration of operating conditions (within accepted profiles), by invoking protection switching or by other means.

Brief Description of the Drawings Figure 1 is a schematic representation of an example of an optical channel and system according to the present invention; and, Figure 2 is a schematic diagram showing an example of the management structure of an optical network in accordance with the present invention.

Detailed Description Two phases are considered in the management process for an optical network.

The first is a connection set up phase to determine channel routing and to test and optimise the connection performance prior to it's acceptance. The second is an operational phase to guard against long-term changes and ensure satisfactory operation during it's operational lifetime. According to the present invention, in both phases, a combination of traditional management system information is used together

with data from special measurements made at selected points in the network. Measurements may be those made at system or circuit set up, or at intervals since circuit set up or results of continuous measurements (made for example when diagnostic checks are necessary). A range of special data processing methods, including examination of historical trends and pattern recognition methods, may be used to identify and distinguish between different types of analogue optical impairments.

In the set up phase, a constraint based routing algorithm is used to determine a preferred route for the Optical Channel (OCh) across the network. This is carried out at a central point, such as in a network management system where there is ready access to the required information, such as the availability of network resources and the intelligence to compute target profiles for the required connection. Then either of two broad approaches can be used to set up an OCh connection. Firstly, under central control from the network management system (NMS) in which the NMS communicates with each of the network elements to enable intelligent decisions based on measurement and routing information to be made with regard to circuit performance and regenerator placement. Secondly, through distributed control in which routing and resource information are sent step by step through the planned connection and used in conjunction with measurement data to determine performance and enable regenerator decisions. The automatic set up process must not mask existing network faults.

Once an OCh has been set up it is performance tested. Figure 1 shows an optical channel which is part of an optical network. Three nodes are shown and a signal with a digital wrapper is input at the lefthand node. A set of measurements of SNR, BER and power level are made at circuit set up, and at intervals from set up, at the input and output of each node. Each set of measurements is stored in a historical log and is also compared with data already in the historical log to identify degradation over time. The result of the comparison is analysed together with"normal" management data to determine what errors are present in each inter-node section. As each type of error has a characteristic signature in terms of the measurements that are made, the error can be diagnosed by reference to a look up table, as is described in more detail later on.

The present invention is also useful for determining the optimum position for regenerators on a particular connection through the optical network. An analogue

circuit extends from the source transmitter (which converts electrical signals to optical signals) to the destination receiver and regenerator (which carries out the converse conversion process). In long circuits, one or more intermediate regenerators may be needed. In this case performance monitoring must be applied separately to each section. However it may be advantageous to compare the performance of each of the separate regenerator sections and adjust the position of the intermediate regenerators so that approximately equal power budget margins apply to each section.

In this way best overall performance of the connection can be ensured.

The means to measure power level at the input and output of each node is present in many optical network solutions. Measurement of SNR are BER at the input and output of each node can be achieved with the use of commercially available modules.

An example of a channel set up and monitoring process in accordance with the present invention will now be described with reference to Figure 2. The optical network has several layers, including a optical multiplex section (OMS), an optical transport section (OTS) and an optical channel section (OCS). At network commissioning parameters associated with the OMS are preset and recorded. The settings used for ensuring satisfactory operation of each OMS are not modified as a means of improving the operation of an individual channel. Initially the network manager computes a preferred connection path using a constraint based routing algorithm, based on a combination of performance and cost criteria, taking into account network planning information such as constraints set by distance, link bandwidth, available wavelengths, available resources, protection requirements, load balancing, required performance and equipment configuration.

To test the connection a signal having a digital wrapper is input at an end node. The use of a digital wrapper preserves client transparency. The connection for each section between regenerators is tested using Trail Trace Identifier (TTI) technology which is part of the digital wrapper. Measurements of SNR, BER and power are then made at various points and compared with predetermined thresholds by the element manager. Firstly, the channel power is checked to determine the loss across a section. The power level is checked against OMS commission data and if it is OK it is reported to the NMS. If it is not OK then an alarm is activated and/or the signal is rerouted. The SNR is checked at the input and output of each node and compared with a look up table. Once the end of a regenerator section is reached if it

satisfies the connection requirements then it is optimised and an SNR threshold is set.

If it is not OK the regenerator section failure is reported to the NMS. If the degradation is gradual then the regenerator spacing is recalculated to achieve adequate performance. Alternatively a local node may take a decision to insert a regenerator.

If it is not gradual the major source of the degradation is identified and the signal is rerouted around it. The new route is checked as described above.

There area number of effects that can contribute to a loss in channel performance. Linear effects include : noise, wavelength misalignment, loss variations (due to micro-bending, fibre damage, connector damage, component aging, component malfunction and polarisation dependent gain and loss), optical crosstalk, polarisation mode dispersion and chromatic dispersion. Non-linear effects include selfphase modulation, cross-phase modulation, modulation instability and four wave mixing.

Routine checks for these faults are for gross change-coarse delta margins made by the NMS, checking settings for finer delta margins, SNR and BER measurements, application of a perturbation onto the signal to localise the faults, wavelength misalignment, crosstalk-time record of fluctuations and compare percentage of errored 1 s and Os, direct detection methods-PMD, and spectral broadening indicating SPM. Table 1 shows various problems and what measurements can be used, either alone or in combination, to diagnose the problems.

BER SNR Power OSA PMD Erroredl's & O's Perturbation History? Noise Wavelength misalignment Loss variations Optical crosstalk PMD # # Chromatic dispersion SPM XPM MI FWM Table 1

Claims

CLAIMS 1. A channel performance monitoring system for an optical network comprising: measuring means for measuring the Signal to Noise Ratio (SNR), bit error rate (BER) and power level of a signal at selected points in the network; and a network management system, including comparing means to compare data from the measuring means with known data relating to the network, and processing means to process data from the comparing means to determine channel performance information.
2. A method of monitoring the performance of a channel path within an optical network comprising the steps of: measuring the Signal to Noise Ratio (SNR), bit error rate (BER) and power level of a signal at selected points on the channel ; comparing the measured SNR, BER and power level at each point with known information about the network; and, processing the results of the comparison to obtain information about the performance of the channel.
3. A channel performance monitoring system for an optical network substantially as described herein with reference to the accompanying drawings.
4. A method of monitoring the performance of a channel path within an optical network substantially as described herein with reference to the accompanying drawings.