1、 International Telecommunication Union ITU-T L.75TELECOMMUNICATION STANDARDIZATION SECTOR OF ITU (05/2008) SERIES L: CONSTRUCTION, INSTALLATION AND PROTECTION OF CABLES AND OTHER ELEMENTS OF OUTSIDE PLANT Test, acceptance and maintenance methods of copper subscriber pairs Recommendation ITU-T L.75 R
2、ec. ITU-T L.75 (05/2008) i Recommendation ITU-T L.75 Test, acceptance and maintenance methods of copper subscriber pairs Summary The new generation of digital subscriber line (xDSL) demands definition of new requirements and test and maintenance methods. The test method, the object of Recommendation
3、 ITU-T L.75, aims at simplifying the task of measuring metallic cables and broadband access networks and at ensuring the integrity of services carried over them. Source Recommendation ITU-T L.75 was approved on 29 May 2008 by ITU-T Study Group 6 (2005-2008) under Recommendation ITU-T A.8 procedure.
4、Keywords Copper cable requirements for broadband transmission, indoor and outside broadband networks qualification, xDSL. ii Rec. ITU-T L.75 (05/2008) FOREWORD The International Telecommunication Union (ITU) is the United Nations specialized agency in the field of telecommunications, information and
5、 communication technologies (ICTs). 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 with a view to standardizing telecommunications on a worldwide bas
6、is. 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 Recommendations is covered by the procedure laid down in WTSA Resol
7、ution 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 conciseness to indicate both a telecommunication administration
8、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 Recommendation is achieved when all of these mandatory provisions are m
9、et. 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. INTELLECTUAL PROPERTY RIGHTS ITU draws attention to the possi
10、bility 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 Rights, whether asserted by ITU members or others outside of the R
11、ecommendation 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, implementers are cautioned that this may not represent the latest informa
12、tion and are therefore strongly urged to consult the TSB patent database at http:/www.itu.int/ITU-T/ipr/. ITU 2009 All rights reserved. No part of this publication may be reproduced, by any means whatsoever, without the prior written permission of ITU. Rec. ITU-T L.75 (05/2008) iii CONTENTS Page 1 S
13、cope 1 2 References. 1 3 Definitions 1 4 Abbreviations and acronyms 1 5 Conventions 2 6 Metallic cables and access network test, acceptance and maintenance methods, intended for service quality assurance 2 6.1 Spectral emulation method (SEM) foundation. 2 6.2 Test and acceptance procedures . 2 6.3 M
14、aintenance . 3 Appendix I Traditional test methods and SEM comparison. 4 I.1 Introduction 4 I.2 Spectral emulation method (SEM) . 7 I.3 Two methods comparison. 9 I.4 Method validation. 10 Appendix II Brazilian experience . 11 II.1 Network architecture 11 II.2 Test and acceptance procedure. 15 Biblio
15、graphy. 16 Rec. ITU-T L.75 (05/2008) 1 Recommendation ITU-T L.75 Test, acceptance and maintenance methods of copper subscriber pairs 1 Scope This Recommendation describes a methodology to qualify metallic cables for broadband use, before and after their installation on access networks. The qualifica
16、tion process is based on measurements of transmission rate (TR) carried out on the worst possible use condition of the cables, i.e., when all pairs are transmitting at the same time. This methodology is known as the spectral emulation method (SEM) and can be applied to qualify modern broadband netwo
17、rks and old telephone networks used to carry xDSL signals. 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 publication, the editions indicated were valid. All
18、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 the currently valid ITU-T Recommendations is regu
19、larly 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.992.5 Recommendation ITU-T G.992.5 (2005), Asymmetric digital subscriber line (ADSL) transceivers Extended bandwidth ADSL2 (ADSL2plus). ITU-T G
20、.993.2 Recommendation ITU-T G.993.2 (2006), Very high speed digital subscriber line transceivers 2 (VDSL2). 3 Definitions None. 4 Abbreviations and acronyms This Recommendation uses the following abbreviations and acronyms: ADSL Asymmetric Digital Subscriber Line B Bandwidth DSLAM Digital Subscriber
21、 Line Access Multiplexer FEXT Far-End CrossTalk IL Insertion Loss IPTV Internet Protocol TeleVision NEXT Near-End CrossTalk N-PSD Noise Power Spectral Density PSD Power Spectrum Density PSELFEXT Power Sum Equal Level Far-End CrossTalk PSNEXT Power Sum Near-End CrossTalk SEM Spectral Emulation Method
22、 2 Rec. ITU-T L.75 (05/2008) S/N Signal-to-Noise ratio TR Transmission Rate VP Victim Pair xDSL General representation for Digital Subscriber Line 5 Conventions None. 6 Metallic cables and access network test, acceptance and maintenance methods, intended for service quality assurance 6.1 Spectral em
23、ulation method (SEM) foundation Crosstalk is a critical factor in channel capacity evaluation. The summation of all crosstalk (unwanted) signals coupling in a determinate cable pair from all other pairs is the main factor that increases noise. In order to evaluate this parameter, power sum calculati
24、ons are performed using near- and far-end crosstalk measurements. Annex A shows a drawback associated with these measurements and calculations and the advantage of the SEM. SEM is based on Nyquist and Shannon theories, which state that the channel capacity is related to bandwidth, signal-to-noise ra
25、tio, and the number of symbols used to modulate each carrier (tone). Nyquist: C = 2 B log2n Shannon: C = B log2 1+ S/N B: Bandwidth S/N: Signal-to-noise ratio n: number of symbols SEM consists of feeding all cable pairs, except the one selected as a victim pair (VP), with controlled xDSL signals. Tr
26、ansmission rate (TR), noise power spectral density (N-PSD) and signal-to-noise ratio (S/N) on the VP are then measured, instead of evaluating cross-talk, power-sums, impedance, return loss, insertion loss, capacitance, resistance, resistance unbalance, capacitance unbalance, etc. The impact of all t
27、hese parameters will be reflected in the TR measurements. This process is repeated, switching the VP, until all cable pairs have been measured. 6.2 Test and acceptance procedures Acceptance of all access networks used with xDSL systems is recommended before providing any wideband service. To do that
28、, test performance must be executed on groups of up to 600 pairs. Also, before installing any new cable on a new network, all pairs, in groups of up to 100 pairs, are recommended to be measured to avoid detecting problems when they are already installed. A 100-pair test procedure is based on the fol
29、lowing algorithm: a) Feed pairs number 2 to 100, at the near-end side, with non-coherent signals, with power spectrum density (PSD) limited according to the xDSL technology being evaluated (e.g., Figure B.1 of ITU-T G.992.5 defines the PSD mask limit for an ADSL2plus operation; see also ITU-T G.993.
30、2 for VDSL PSD definition). At the far-end side, these pairs must be properly terminated on a resistive load defined as the xDSL impedance level (e.g., 100 for ADSL2plus). Those pairs are called disturbing pairs. b) Connect one DSLAM port at the near-end side and a modem at the other side of the pai
31、r number 1, selected as the VP. Pair 1 is, then, the pair under test. Rec. ITU-T L.75 (05/2008) 3 c) Measure the VP TR and S/N. Save these measurements on a database. d) Disable DSLAM port and switch the VP far-end side from the modem to a spectrum analyser. Measure and save the N-PSD on the databas
32、e. e) Select another VP and switch it to the set DSLAM/modem. It becomes the new VP. Feed the former VP with the full bandwidth spectrum signal, as in the first step a. f) Repeat steps c and d until all pairs have been measured. g) The minimum test report must include: i) Cable identification. ii) C
33、able length. iii) Local temperature. When testing cables on laboratories, room temperature must be controlled and stabilized in the range of 20 5C. Care must be taken to ensure that the cable internal temperature has reached room temperature. iv) A graphic and a list of all pairs and their respectiv
34、e TRs and S/Rs, ordered from the one with the highest TR (fastest pair) to the one with the lowest TR (slowest pair). v) N-PSD must be kept saved on the database for maintenance purposes, addressed in step d above. The test procedure for networks is similar, except that the far-end side pairs can be
35、 located in distinct ending boxes or cabinets. Remote test modules, capable of being remotely controlled, must be used to switch the far-end side of the pairs to modems, terminating loads and spectrum analysers. Automatic test equipment is recommended to: speed up the test process; save test reports
36、 in a database for traceability and statistics purposes; and to guarantee the necessary reliability. In the case of cable tests, it is mandatory to measure both cable sides in order to evaluate the upstreams and downstreams for xDSL applications. For network tests, once cables have been previously m
37、easured, downstream measurement is enough. The requirements shall be expressed as the minimum TR value, and agreed between the supplier and customer. See Appendix I for a comparison of this method with other traditional methods. 6.3 Maintenance After testing, comparing results with requirements and
38、accepting the network, broadband service can then be provided and, then, a reliable service quality maintenance strategy must be carried out. A proper form to implement it is to enquire periodically the TR of each working port of all DSLAMs, compare them with values obtained during the acceptance pr
39、ocess, and alarm in cases of performance degradation and faults. Inspection points, capable of switching any pair to a test bus, are recommended to be installed on terminal boxes and cabinets. They will provide easy access to cables (or wires) connected to both sides of the boxes or cabinets when fi
40、eld maintenance becomes necessary. TR, S/N and, especially N-PSD, can be measured and compared to the previous measurements collected during the network acceptance. Additional to the above-mentioned measurements, impulsive noise is also recommended to be measured and saved on the database, especiall
41、y due to its importance to Internet protocol television (IPTV) service quality. 4 Rec. ITU-T L.75 (05/2008) Appendix I Traditional test methods and SEM comparison I.1 Introduction Depending on the type of multiplex used on a determined DSL technology, frequency or time, it is necessary to measure, i
42、n cables and access networks, far-end crosstalk (FEXT), near-end crosstalk (NEXT) or both. NEXT is a measure of the unwanted signal coupling from a transmitter at the near-end into neighbouring pairs measured at the near-end side. NEXT is measured for all pair combinations in a cable group. Power su
43、m NEXT (PSNEXT) calculation takes into account the combined crosstalk on a receive pair from all near-end disturber pairs operating simultaneously. Similarly, equal level far-end crosstalk (ELFEXT) is the measure of the unwanted signal coupling from a transmitter at the far-end into neighbouring pai
44、rs measured at the near-end. Power sum ELFEXT (PSELFEXT) is calculated based on the combined crosstalk on a receive pair from all far-end disturbers operating simultaneously. It is important to notice that ELFEXT is actually the far-end crosstalk (FEXT) minus disturbing pair insertion loss (IL). To
45、calculate the PSNEXT of a 100-pair cable, it is necessary to measure the cross-talk of the 2-combination of those 100 pairs. The number of k-combinations (each of size k) from a set S with n elements (size n) can be determined with the binomial coefficient: )!(!knknkn=Calculating this binomial for n
46、=100 and k=2, results in 4950 combinations. Besides that, the total number of frequency points within the specified bandwidth shall be a minimum of 100 times the number of decades covered by the frequency range. As an example, for a 0.1-8 MHz xDSL cable, it will be necessary to measure for each of t
47、he 4950 pair combinations, a minimum of 200 frequency points, resulting in 990 000 measurements. For PSELFEXT, the problem is worse. As ELFEXT between, for example, pair n and pair p ELFEXT(n, p) is different from ELFEXT(p, n),the latter should also be measured, doubling the total number of crosstal
48、k measurements. It is also necessary to measure the 100-pair attenuations. Taking all that into account, 2 000 000 frequency measurements will have to be done just to calculate ELFEXT and PSELFEXT. Considering this fact, even with automatic test systems, it is impractical to evaluate PSELFEXT and PS
49、NEXT on all cables that come out of a production line. Normally, cable manufacturers test only samples of groups on samples of cables. In other words, cables are poorly evaluated when they need, for the purpose of data transmission, to be fully evaluated. Figures I.1, I.2 and I.3 illustrate the plots of 100-pair PSNEXT, IL and PSELFEXT of an 8 MHz xDSL cable. Rec. ITU-T L.75 (05/2008) 5 Figure I.1 Insertion Loss of 8 MHz xDSL cable 6 Rec. ITU-T L.75 (05/2008) Figure I.2 PSNEXT Figure I.3 PSELFEXT Rec. ITU-T L.75 (05/2008) 7 I.2 Spectral
