1、 INTERNATIONAL TELECOMMUNICATION UNION ITU-T O.201TELECOMMUNICATION STANDARDIZATION SECTOR OF ITU (07/2003) SERIES O: SPECIFICATIONS OF MEASURING EQUIPMENT Equipment for the measurement of optical channel parameters Q-factor test equipment to estimate the transmission performance of optical channels
2、 ITU-T Recommendation O.201 ITU-T O-SERIES RECOMMENDATIONS SPECIFICATIONS OF MEASURING EQUIPMENT General O.1O.9 Maintenance access O.10O.19 Automatic and semi-automatic measuring systems O.20O.39 Equipment for the measurement of analogue parameters O.40O.129 Equipment for the measurement of digital
3、and analogue/digital parameters O.130O.199 Equipment for the measurement of optical channel parameters O.200O.209 For further details, please refer to the list of ITU-T Recommendations. ITU-T Rec. O.201 (07/2003) i ITU-T Recommendation O.201 Q-factor test equipment to estimate the transmission perfo
4、rmance of optical channels Summary This Recommendation describes the requirements of Q-factor measurement equipment (QFME) currently based on the level shifting method used for estimating the digital transmission performance of an optical channel. Source ITU-T Recommendation O.201 was approved by IT
5、U-T Study Group 4 (2001-2004) under the ITU-T Recommendation A.8 procedure on 22 July 2003. Keywords BER, Bit Error Ratio, decision threshold, Optical Transport Network, Q-factor, Q-factor Measurement Equipment, Signal-to-Noise Ratio. ii ITU-T Rec. O.201 (07/2003) FOREWORD The International Telecomm
6、unication Union (ITU) is the United Nations specialized agency in the field of telecommunications. The ITU Telecommunication Standardization Sector (ITU-T) is a permanent organ of ITU. ITU-T is responsible for studying technical, operating and tariff questions and issuing Recommendations on them wit
7、h a view to standardizing telecommunications on a worldwide basis. The World Telecommunication Standardization Assembly (WTSA), which meets every four years, establishes the topics for study by the ITU-T study groups which, in turn, produce Recommendations on these topics. The approval of ITU-T Reco
8、mmendations is covered by the procedure laid down in WTSA Resolution 1. In some areas of information technology which fall within ITU-Ts purview, the necessary standards are prepared on a collaborative basis with ISO and IEC. NOTE In this Recommendation, the expression “Administration“ is used for c
9、onciseness to indicate both a telecommunication administration and a recognized operating agency. Compliance with this Recommendation is voluntary. However, the Recommendation may contain certain mandatory provisions (to ensure e.g. interoperability or applicability) and compliance with the Recommen
10、dation is achieved when all of these mandatory provisions are met. The words “shall“ or some other obligatory language such as “must“ and the negative equivalents are used to express requirements. The use of such words does not suggest that compliance with the Recommendation is required of any party
11、. INTELLECTUAL PROPERTY RIGHTS ITU draws attention to the possibility that the practice or implementation of this Recommendation may involve the use of a claimed Intellectual Property Right. ITU takes no position concerning the evidence, validity or applicability of claimed Intellectual Property Rig
12、hts, whether asserted by ITU members or others outside of the Recommendation development process. As of the date of approval of this Recommendation, ITU had not received notice of intellectual property, protected by patents, which may be required to implement this Recommendation. However, implemento
13、rs are cautioned that this may not represent the latest information and are therefore strongly urged to consult the TSB patent database. ITU 2003 All rights reserved. No part of this publication may be reproduced, by any means whatsoever, without the prior written permission of ITU. ITU-T Rec. O.201
14、 (07/2003) iii CONTENTS Page 1 Scope 1 2 References. 1 3 Definitions 2 4 Abbreviations and acronyms 2 5 Introduction to Q-factor 3 6 Requirements of Q-factor measurement equipment. 3 6.1 Physical interfaces and bit rates . 3 6.2 Accuracy requirements and acceptance tests . 7 6.3 Presentation of resu
15、lts. 9 7 Miscellaneous functions . 9 7.1 Remote control port 9 7.2 TMN interface 9 8 Operating conditions. 9 8.1 Environmental conditions. 9 8.2 Behaviour in case of power failure. 9 9 Bibliography . 9 10 Background reading 10 Annex A Mathematical procedure for the Q-factor evaluation with the decis
16、ion level shifting method . 10 A.1 Preconditions 10 A.2 Theoretical dependence of the BER on the threshold 10 A.3 Separation of BER(0) and BER(1). 10 A.4 Calculation of the results 11 Appendix I Q-factor theory. 12 I.1 Q-factor theory. 12 I.2 Approximation of the erfc function . 12 I.3 Inverse erfc(
17、x), erfc1(x) . 13 Appendix II Optical channel performance and characteristics . 14 II.1 Optical channel performance 14 II.2 Optical channel characteristics. 14 Appendix III Imperfections to be considered under conditions found in practice 15 III.1 Analogue impairments 15 III.2 Pattern dependencies 1
18、6 III.3 Receiver characteristics 16 III.4 Sample phase position 16 III.5 Effects on the Q-factor . 16 iv ITU-T Rec. O.201 (07/2003) Page Appendix IV Implementation suggestions for QFME 17 IV.1 Block diagrams. 17 IV.2 Functional description 17 IV.3 Measurement modes. 20 Appendix V Additional verifica
19、tion tests . 21 V.1 Receiver pulse response . 21 ITU-T Rec. O.201 (07/2003) 1 ITU-T Recommendation O.201 Q-factor test equipment to estimate the transmission performance of optical channels 1 Scope Q-factor measurement is an established method for characterization of optical channels (see e.g., ITU-
20、T Recs G.972 7 and G.976 8). Particularly at low bit error rates the method has the advantage of taking less time than a traditional BER measurement, where bit errors need to be counted over a statistically significant time period. The Q-factor is defined as the (electrical) signal-to-noise ratio at
21、 the decision circuit of a digital signal receiver (see Annex A, Appendix I and B1). There are various methods of implementing Q-factor measurements, which can be mathematically related to the bit error ratio (see also the methods referred to in ITU-T Recs G.972 7 and G.976 8 for submarine systems).
22、 This Recommendation deals with the level shifting method and applies to QFME using it. The objectives of this Recommendation with regard to Q-factor measuring equipment (QFME) are: Compatibility between measurement equipment produced by different manufacturers: Optical channel transmission performa
23、nce measurements made by QFME compliant to this Recommendation shall provide results to within the specified accuracy limits defined in this Recommendation. Estimate of the actual system performance: The objective of an estimate by QFME of the bit error ratio (BER) achievable over a given optical ch
24、annel is to provide the minimum BER that can be achieved with an optimally designed piece of network equipment. This Recommendation does not intend to define the particular applications of the Q-factor measurement method. Some possible fields of applications are described in the appendices of this R
25、ecommendation. While requirements are given for the QFME, the realization of the equipment configuration is not covered and should be given careful consideration by the designer and user. 2 References The following ITU-T Recommendations and other references contain provisions which, through referenc
26、e in this text, constitute provisions of this Recommendation. At the time of publication, the editions indicated were valid. All Recommendations and other references are subject to revision; users of this Recommendation are therefore encouraged to investigate the possibility of applying the most rec
27、ent edition of the Recommendations and other references listed below. A list of the currently valid ITU-T Recommendations is regularly published. The reference to a document within this Recommendation does not give it, as a stand-alone document, the status of a Recommendation. 1 ITU-T Recommendation
28、 G.691 (2000), Optical interfaces for single channel STM-64, STM-256 and other SDH systems with optical amplifiers. 2 ITU-T Recommendation G.707/Y.1322 (2000), Network node interface for the synchronous digital hierarchy (SDH). 2 ITU-T Rec. O.201 (07/2003) 3 ITU-T Recommendation G.709/Y.1331 (2001),
29、 Interfaces for the Optical Transport Network (OTN). 4 ITU-T Recommendation G.825 (2000), The control of jitter and wander within digital networks which are based on the synchronous digital hierarchy (SDH). 5 ITU-T Recommendation G.8251 (2001), The control of jitter and wander within the optical tra
30、nsport network (OTN). 6 ITU-T Recommendation G.957 (1999), Optical interfaces for equipment and systems relating to the synchronous digital hierarchy. 7 ITU-T Recommendation G.972 (2000), Definition of terms relevant to optical fibre submarine cable systems. 8 ITU-T Recommendation G.976 (2000), Test
31、 methods applicable to optical fibre submarine cable systems. 9 ITU-T Recommendation O.3 (1992), Climatic conditions and relevant tests for measuring equipment. 10 ITU-T Recommendation O.181 (2002), Equipment to assess error performance on STM-N interfaces. 11 IEC 61300-3-29, Measurement techniques
32、for characterising the amplitude of the spectral transfer functions of DWDM components. 3 Definitions This Recommendation defines the following term: 3.1 Q-factor: Electrical Signal-to-Noise Ratio (ESNR) at the input of a receivers decision circuit (see Annex A and Appendix I). 4 Abbreviations and a
33、cronyms This Recommendation uses the following abbreviations: AGC Automatic Gain Control APD Avalanche Photo Diode BER Bit Error Ratio BERT Bit Error Ratio Tester DFB Distributed Feed-Back DWDM Dense Wavelength Division Multiplex EDFA Erbium Doped Fibre Amplifier ER Extinction Ratio ESNR Electrical
34、Signal-to-Noise Ratio FEC Forward Error Correction IEC International Electrotechnical Commission ISI Inter-Symbol Interference ISO International Organization for Standardization OADM Optical Add-Drop Multiplexer ITU-T Rec. O.201 (07/2003) 3 OCh Optical Channel OSA Optical Spectrum Analyzer OSNR Opti
35、cal Signal-to-Noise Ratio OTM Optical Termination Multiplexer OTN Optical Transport Network OTUk Optical Transport Unit-k OXC Optical Cross Connect QFME Q-factor Measurement Equipment PRBS Pseudo-Random Binary Sequence RX Receiver SNR Signal-to-Noise Ratio SPM Self Phase Modulation STM-N Synchronous
36、 Transport Module-N TIA Telecommunications Industry Association WDM Wavelength Division Multiplex 5 Introduction to Q-factor In Annex A and Appendix I, the theory of the Q-factor evaluation is described. Under ideal conditions, it is assumed that the Q-factor is given by the logic levels 0and 1and G
37、aussian noise distributions around the logic levels, which are described by the standard deviations 0and 1. The overlap region of the distribution tails represents the probability for the occurrence of errored decisions. In Appendix IV, some methods for the measurement of the noise distributions and
38、 the determination of the Q-factor are described. In practice, there are a number of influences causing distortions with the effect that the shape of distributions no longer is Gaussian (see list of influences in Appendix III). However, it can be shown, that these distortions mainly affect the top r
39、egions of the distribution, while the tails can still be very accurately approximated by Gaussian distribution. The applicability of a Gaussian tail approximation shall be confirmed by calculating the correlation coefficient according to Annex A. For every Q-factor measurement that is required to me
40、et the accuracy limits of clause 6, the correlation coefficient shall be in the range 0.95 to 1.0. While the Gaussian fit is the basic proven method for “tail extrapolation“ there may exist more sophisticated distribution models, which also meet the accuracy requirements defined in clause 6. Such mo
41、dels are for further study. 6 Requirements of Q-factor measurement equipment 6.1 Physical interfaces and bit rates 6.1.1 Interfacing to the transmission systems The Q-factor measurement equipment shall be capable of operating at optical amplifier monitoring points in the case of in-service measureme
42、nts and, additionally, as a replacement for the system receivers in the case of out-of-service measurements. In the first case, care has to be taken that the signal at the monitoring point is properly dispersion compensated. 4 ITU-T Rec. O.201 (07/2003) Further, in the case of DWDM systems, an optic
43、al channel filter is necessary to select the desired channel for the Q-factor measurement and, in addition, an optical amplification prior to being able to use the filter. Some concern was raised that in-service measurements at optical monitoring points may not reflect the optical channel performanc
44、e due to the implementation of various optimization devices, such as chromatic dispersion compensation or line equalization, in order to achieve appropriate end-to-end transmission performance. The applicability for these types of in-service measurements and which provisions have to be considered ne
45、eds further study. 6.1.2 Bit rates and jitter tolerance The QFME shall operate at one or more bit rates defined in ITU-T Rec. G.707/Y.1322 2 for STM-N signals or in ITU-T Rec. G.709/Y.1331 (OTN) for OTUk signals. The maximum tolerable jitter at the specified bit rates supported by the QFME shall con
46、form to the appropriate ITU-T Recs G.825 4 or G.8251 5. 6.1.3 QFME receiver response The overall amplitude versus frequency response of the receiver (RX), including the O/E converter and, if applicable, low pass filtering, automatic gain control (AGC) and decision or sampling circuitry, shall be cho
47、sen for overshoot-free and minimum inter-symbol interference (ISI) pulse response. As a general design goal, the 4th order Bessel-Thompson low pass characteristics (or equivalent), according to the reference receiver definitions of ITU-T Recs G.957 6 and G.691 1 should be taken. The nominal electric
48、al noise bandwidth Beof the RX shall be: 0.75 fclkwith fclkbeing the clock frequency expressed in Hertz (see 6.1.7 for the details of the bandwidth correction procedure). Instead of defining a tolerance mask for the frequency response, it is preferable to use a pulse mask for appropriate characteriz
49、ation of the receiver pulse response. The definition and verification of the pulse mask is for further study. Possible alternative methods for characterization of the receiver pulse response can be found in Appendix V. 6.1.4 Optical channel filter If an optical channel filter is required for the Q-factor measurement it shall meet the following requirements. The optical 3 dB bandwidth Bchshall be greater than 2 fclkwith a flat top having a 1 dB width of at least 1 fclk(fclkbeing the clock frequency expressed in Hert
