1、 International Telecommunication Union ITU-T L.66TELECOMMUNICATION STANDARDIZATION SECTOR OF ITU (05/2007) SERIES L: CONSTRUCTION, INSTALLATION AND PROTECTION OF CABLES AND OTHER ELEMENTS OF OUTSIDE PLANT Optical fibre cable maintenance criteria for in-service fibre testing in access networks ITU-T
2、Recommendation L.66 ITU-T Rec. L.66 (05/2007) i ITU-T Recommendation L.66 Optical fibre cable maintenance criteria for in-service fibre testing in access networks Summary In the FTTx era, we must provide effective and efficient maintenance for optical cable networks. With a view to realizing a highl
3、y reliable optical cable network that transports WDM signals with a wide spectral bandwidth, we need to establish maintenance criteria for testing in-service fibre lines without interfering with optical communication signals in the access network. This Recommendation deals with important considerati
4、ons with respect to the maintenance band and maintenance test light filtering requirements for testing in-service fibre lines without interfering with optical communication signals in access networks. Source ITU-T Recommendation L.66 was approved on 18 May 2007 by ITU-T Study Group 6 (2005-2008) und
5、er the ITU-T Recommendation A.8 procedure. ii ITU-T Rec. L.66 (05/2007) FOREWORD The International Telecommunication 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
6、 responsible for studying technical, operating and tariff questions and issuing Recommendations on them with 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
7、 the ITU-T study groups which, in turn, produce Recommendations on these topics. The approval of ITU-T Recommendations 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 colla
8、borative basis with ISO and IEC. NOTE In this Recommendation, the expression “Administration“ is used for conciseness to indicate both a telecommunication administration and a recognized operating agency. Compliance with this Recommendation is voluntary. However, the Recommendation may contain certa
9、in mandatory provisions (to ensure e.g. interoperability or applicability) and compliance with the Recommendation 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 requiremen
10、ts. The use of such words does not suggest that compliance with the Recommendation is required of any party. 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. IT
11、U takes no position concerning the evidence, validity or applicability of claimed Intellectual Property Rights, 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
12、property, protected by patents, which may be required to implement this Recommendation. However, implementers are cautioned that this may not represent the latest information and are therefore strongly urged to consult the TSB patent database at http:/www.itu.int/ITU-T/ipr/. ITU 2008 All rights rese
13、rved. No part of this publication may be reproduced, by any means whatsoever, without the prior written permission of ITU. ITU-T Rec. L.66 (05/2007) iii CONTENTS Page 1 Scope 1 2 References. 1 3 Definitions 2 4 Abbreviations 2 5 Fundamental requirements for in-service fibre line testing 2 6 Testing
14、and measurement methods. 2 7 Requirements for in-service fibre line testing 2 7.1 Test light wavelength allocation. 2 7.2 Wavelength bandwidth of test light 3 7.3 Requirements for test light cut-off filter. 4 7.4 Requirements for measurement equipment 5 Appendix I Japanese experience: Fibre Bragg gr
15、ating filter-embedded in an optical connector. 6 I.1 Introduction 6 I.2 Structure of filter-embedded connectors 6 I.3 Design and fabrication of chirped FBG filter. 7 I.4 Performance of filter-embedded connector 7 Appendix II Japanese experience: Spectral filtering criteria for U-band test light for
16、in-service line monitoring . 9 II.1 Introduction 9 II.2 Filtering characteristic of sideband noise of test light 9 II.3 References 11 Appendix III In-service fibre line testing for PONs . 12 III.1 Fundamental requirements for in-service fibre line testing for PONs . 12 ITU-T Rec. L.66 (05/2007) 1 IT
17、U-T Recommendation L.66 Optical fibre cable maintenance criteria for in-service fibre testing in access networks 1 Scope This Recommendation mainly describes the maintenance band and requirements for maintenance test light filtering (such as cut-off bandwidth, isolation and other optical characteris
18、tics) for testing in-service fibre lines without interfering with optical communication signals in access networks. This Recommendation provides guidance on the use of an out-of-band remote test system. An alternative approach is to monitor key parameters of the transmission equipment, such as the O
19、LT transmitted power and the ONU received power, but this approach is not examined in this Recommendation. 2 References The following ITU-T Recommendations and other references contain provisions which, through reference in this text, constitute provisions of this Recommendation. At the time of publ
20、ication, 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 recent edition of the Recommendations and other references listed below. A list of t
21、he 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. ITU-T G.694.1 ITU-T Recommendation G.694.1 (2002), Spectral grids for WDM applications: DWDM frequency
22、grid. ITU-T G.694.2 ITU-T Recommendation G.694.2 (2003), Spectral grids for WDM applications: CWDM wavelength grid. ITU-T G.695 ITU-T Recommendation G.695 (2006), Optical interfaces for coarse wavelength division multiplexing applications. ITU-T G.698.1 ITU-T Recommendation G.698.1 (2006), Multichan
23、nel DWDM applications with single-channel optical interfaces. ITU-T G.983.1 ITU-T Recommendation G.983.1 (2005), Broadband optical access systems based on Passive Optical Networks (PON). ITU-T G.983.3 ITU-T Recommendation G.983.3 (2001), A broadband optical access system with increased service capab
24、ility by wavelength allocation. ITU-T G.984.1 ITU-T Recommendation G.984.1 (2003), Gigabit-capable Passive Optical Networks (G-PON): General characteristics. ITU-T L.25 ITU-T Recommendation L.25 (1996), Optical fibre cable network maintenance. ITU-T L.40 ITU-T Recommendation L.40 (2000), Optical fib
25、re outside plant maintenance support, monitoring and testing system. ITU-T L.41 ITU-T Recommendation L.41 (2000), Maintenance wavelength on fibres carrying signals. ITU-T L.42 ITU-T Recommendation L.42 (2003), Extending optical fibre solutions into the access network. 2 ITU-T Rec. L.66 (05/2007) ITU
26、-T L.53 ITU-T Recommendation L.53 (2003), Optical fibre maintenance criteria for access networks. IEC 61746 IEC 61746 (2005), Calibration of optical time-domain reflectometers (OTDR). 3 Definitions For the purpose of this Recommendation, the definitions given in ITU-T Recommendations G.698.1, G.983.
27、1 to G.983.8, G.984.1, L.25, L.40, L.41 and L.53 apply. 4 Abbreviations This Recommendation uses the following abbreviations: BER Bit Error Rate CO Central Office FTTx “Fibre to the x“, where “x“ indicates the final location on the user side of any one of a variety of optical fibre architectures, e.
28、g., FTTH, FTTB, FTTC. OLT Optical Line Terminal ONT Optical Network Terminal OPM Optical Power Meter OTDR Optical Time Domain Reflectometer PON Passive Optical Network WDM Wavelength Division Multiplex 5 Fundamental requirements for in-service fibre line testing With a view to realizing a highly rel
29、iable optical access network that transports WDM signals with a wide spectral bandwidth, in-service fibre line monitoring techniques are important in terms of providing effective and efficient maintenance of optical cable networks. The fundamental requirements of in-service fibre line testing are as
30、 follows: It should be carried out without degrading optical communication signals in the optical access network. It must be capable of evaluating optical fibre characteristics even if there is interference with the communication light. 6 Testing and measurement methods There are several ways to imp
31、lement maintenance functions. OTDR testing, loss testing, monitoring a proportion of the signal power (power monitoring) and identification light detection are commonly used. These testing methods for point-to-point optical networks are stipulated in ITU-T L.25 and ITU-T L.40. As regards point-to-mu
32、ltipoint and ring optical networks, their testing methods are also described in ITU-T L.53. 7 Requirements for in-service fibre line testing 7.1 Test light wavelength allocation The wavelength allocation of a broadband optical access system and spectral grids for PON and WDM applications are defined
33、 by ITU-T G.983.3, ITU-T G.694.1, ITU-T G.694.2 and ITU-T G.698.1, respectively. Communication wavelength bands extend to a long wavelength ITU-T Rec. L.66 (05/2007) 3 band (L-band: 1565-1625 nm) and the minimum channel spacing is defined as 12.5 GHz. To eliminate interference with the test light, t
34、he maintenance test light wavelength must not be a wavelength that is used for communication signals. The maintenance wavelength for in-service testing is defined by ITU-T L.41. There are several recommended maintenance wavelength bands depending on the communication light wavelength used by a given
35、 transmission system. When the communication wavelength band extends to the L-band, the ultra long wavelength (U-band) of 1650 nm is used for maintenance testing as shown in Figure 1. L.66(07)_F01L.41 and G.Sup39G.983.3 (PON)G.694.2 (CWDM)1300 1400 1500 1550 1600 1650 nmU-band 1625-1675 nm(maintenan
36、ce band)C-band1530-1565 nmS-band1460-1530 nmL-band1565-1625 nmE-band1360-1460 nmO-band1260-1360 nmUP1260 13601300 1400 1480 1500 1550 1600 1650 nmEXTENSIONDOWNVIDEO (1550-1560)DIGITAL (1539-1565)EXTENSION18 grid1300 1400 1500 1550 1600 1650 nm1271 1291 1311 1331 1351 1371 1391 1411 1431 1451 1471 14
37、91 1511 1531 1551 1571 1591 1611Figure 1 Maintenance wavelength allocation 7.2 Wavelength bandwidth of test light The wavelength bandwidth of the test light source should be designed taking into consideration the cut-off bandwidth of the optical filter. Figure 2 shows the wavelength spectra for a te
38、st light source and a test light cut-off filter when the L-band is used as the communication light. The diagram in Figure 2 shows the wavelength allocations for the light source and the cut-off filter. The low edge of A and the high edge of C for the test light source and the low edge of B and the h
39、igh edge of D for the cut-off filter should satisfy the relationship in Equation 7-1. nm DCABtest dB PtPcdSXLt + (7-2) Here, the fibre losses and connection losses in the optical fibre line are regarded as being independent of the wavelength. L.66(07)_F03Optical coupler Test light cut-off filterPcdP
40、tPcuOLT ONTMeasurementequipmentCommunication light cut-off filterFigure 3 Test set-up for in-service line Test light cut-off filters should eliminate the maintenance test light without having any detrimental effect on transmission quality. Therefore, wavelength B should be longer than the transmissi
41、on wavelength and shorter than wavelength A, and wavelength D should be longer than wavelength C as shown in Figure 2. ITU-T Rec. L.66 (05/2007) 5 The required crosstalk needed to avoid any deterioration in the transmission quality by the test light is considered as a requirement of the test light c
42、ut-off filter. To avoid any effects of other transmission lights, PON and WDM transmission systems, which are stipulated in ITU-T G.983.3 and ITU-T G.698.1, respectively, have specified tolerances to the reflected optical power and maximum optical crosstalk, respectively. In these systems, the param
43、eters of the tolerance to the reflected optical power and the maximum optical crosstalk correspond to the S/X ratio. By attenuating the test light power less than the tolerance to other communication lights of the transmission system, in-service testing can be carried out without any affect on trans
44、mission quality. High total optical power systems that exhibit non-linear effects are not considered here because transmission systems for the optical access network have low output optical power. 7.4 Requirements for measurement equipment The requirements for measurement equipment for in-service te
45、sting should be also considered. In-service testing must be carried out on the premise that light with a wavelength other than the communication light wavelength is input into the detector of the measurement equipment. Therefore, in order to accurately measure the characteristics of the optical fibr
46、e line in such a situation, the measurement equipment (such as an OTDR and an OPM) should have a tolerance to the communication light power. In Figure 3, the communication light passes through the optical coupler for in-service testing and is launched into the measurement equipment. To suppress the
47、effect of the communication light, it is useful to install an optical filter adjacent to the measurement equipment that allows test light to pass but not communication light. In terms of OTDR measurement, the introduced communication light (Pcu dBm) must be much lower than the Rayleigh backscattered
48、 signal of the test light. The Rayleigh backscattered coefficient (Crb dB) is expressed in Equation 7-3 by using Rayleigh backscattered parameter (K) and OTDR pulse width Tp as defined in IEC 61746. () dB 10log10TpkCrb = (7-3) Therefore, the optical filter requirement as regards suppressing the fluc
49、tuation of the OTDR trace is given by the following equation. ()CrbLdmPpLcPcu + (7-4) In Equation 7.4, Pp is the peak power of the test light source, Ldm is the optical loss corresponding to the OTDR dynamic range and Lc is the cut-off of the optical filter installed adjacent to the measurement equipment. Here, if the communication signal power is regarded as a sort of noise in the OTDR trace, in order to reduce the fluctuation caused by the communication signal in the OTDR trace to less than 0.2 dB, the att
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