1、INTERNATIONAL TELECOMMUNICATION UNION ITU-T TELECOMMUNICATION STAN DARD IZATION SECTOR OF ITU CONSTRUCTION, INSTALLATION AND PROTECTION OF CABLES AND OTHER ELEMENTS OF OUTSIDE PLANTS L.15 (03/93) OPTICAL LOCAL DISTRIBUTION CONSIDERED FOR THEIR CONSTRUCTION NETWORKS - FACTORS TO BE ITU-T Recommendati
2、on L.15 (Previously “CCITT Recommendation”) COPYRIGHT International Telecommunications Union/ITU TelecommunicationsLicensed by Information Handling ServicesITU-T RECMN+L.LS 93 W 4862593 0593524 527 FOREWORD The ITU Telecommunication Standardization Sector (ITU-T) is a permanent organ of the Internat
3、ional Telecom- munication Union. The 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 basis. The World Telecommunication Standardization Conference (WTSC), which meets every
4、four years, established the topics for study by the ITU-T Study Groups which, in their turn, produce Recommendations on these topics. ITU-T Recommendation L.15 was prepared by the ITU-T Study Group VI (1988-1993) and was approved by the WTSC (Helsinki, March 1-12, 1993). NOTES 1 As a consequence of
5、a reform process within the International Telecommunication Union (ITU), the CCITT ceased to exist as of 28 February 1993. In its place, the ITU Telecommunication Standardization Sector (ITU-T) was created as of 1 March 1993. Similarly, in this reform process, the CCIR and the IFRB have been replace
6、d by the Radiocommunication Sector. In order not to delay publication of this Recommendation, no change has been made in the text to references containing the acronyms “CCITI, CCIR or IFRB” or their associated entities such as Plenary Assembly, Secretariat, etc. Future editions of this Recommendatio
7、n will contain the proper terminology related to the new ITU structure. 2 telecommunication administration and a recognized operating agency. In this Recommendation, the expression “Administration” is used for conciseness to indicate both a O ITU 1994 All rights reserved. No part of this publication
8、 may be reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying and microfilm, without permission in writing from the ITU. COPYRIGHT International Telecommunications Union/ITU TelecommunicationsLicensed by Information Handling ServicesITU-T RECMH*L.L5 93
9、m 4862591 0591525 463 CONTENTS Scope Abbreviations . Concept of layers within a network architecture 3.1 Layer classification . Optical fibre cable Passive optical components Network installation and maintenance . Electrical power supply Optical safety . Other local distribution network references .
10、 References . Annex A A . 1 Introduction A.2 Network architectures . Recommendation L.15 (03/93) Page 1 1 1 1 2 2 3 4 4 4 4 4 4 4 1 COPYRIGHT International Telecommunications Union/ITU TelecommunicationsLicensed by Information Handling ServicesITU-T RECMN*L.35 93 M 4862591 0593526 3TT INTRODUCTION T
11、he existing local distribution network for customer access consists almost entirely of copper pairs for all countries. However, recent progress in the application of optical plant technology to local distribution networks has provided substantial experimental experience in several countries. This ha
12、s been taken into account to determine some of the factors to be considered for construction of optical local distribution networks. Firstly, it may not be appropriate simply to replace copper conductor cables by optical fibre cables since the characteristics of optical fibre cables differ very sign
13、ificantly from those of copper conductor cables. Secondly, advances in technology provide potential for a greatly increased range of services that can include wideband direct to the customer. Network architectures therefore require to be determined taking into consideration both operational and main
14、tenance system requirements. This Recommendation sets out practical advice on choice and use of optical plant for the constniction of optical distribution networks. Several different architectures are described in the annex to this Recommendation to assist in understanding the selection of hardware.
15、 The advice given is based on the experience gained mainly from early applications and the user should be aware that rapid progress will lead to early revision of this Recommendation. 11 Recommendation L.15 (03/93) COPYRIGHT International Telecommunications Union/ITU TelecommunicationsLicensed by In
16、formation Handling ServicesITU-T RECMN*L-35 93 4862593 0593527 236 Recommendation L.15 OPTICAL LOCAL DISTRIBUTION NETWORKS - FACTORS TO BE CONSIDERED FOR THEIR CONSTRUCTION (Helsinki, 1993) 1 Scope This Recommendation: - deals with optical distribution networks constructed from single-mode fibre cab
17、les; - sets out physical network architectures that have been used to meet differing system objectives; - takes account of environmental conditions in customer serving areas; - makes reference to system related requirements relevant to the work of Study Group XV; - refers to passive optical componen
18、ts used for construction of the network; - deals with installation and maintenance issues; - considers electrical power supply; and - considers optical safety requirements. 2 Abbreviations Abbreviations are used in this Recommendation as set out below: - FTTC Fibre to the Curb - FTTH Fibre totheHome
19、 - OFA Optical Fibre Amplifier - OPS Optical Power Splitter - ONT Optical Network Termination - PON Passive Optical Network - WDM Wavelength Division Multiplex device. 3 Concept of layers within a network architecture 3.1 Layer classification It is helpful to consider functions of a network conceptu
20、ally classified in terms of layers, each dealing with different aspects of its construction and operation. These aspects may first be divided into a Transport layer, containing further layers of physical plant and circuit configurations, and an Intelligent layer, consisting of the computer controlle
21、d software programs that are used to run the network. The physical plant layer is the subject of this Recommendation and may be further divided into more layers considered as comprising an: - optical fibre layer; - optical fibre cable layer; - structural facility layer. Recommendation L.15 (03/93) 1
22、 COPYRIGHT International Telecommunications Union/ITU TelecommunicationsLicensed by Information Handling Services ITU-T RECMNUL-15 93 4862591 0591528 172 3.1.1 Optical fibre layer A suitable choice of fibre and splicing technology should be made. Single-mode fibre, normally to Recommendation G.652 i
23、s the most appropriate for a wide range of telecommunications services in the local distribution network since this fibre benefits from an economy of scale and has long-term potential for future services. With current usage of single-mode fibre, splicing techniques will allow average splice losses o
24、f less than 0.5 dB to be achieved. Passive components may be used in conjunction with optical fibre and thus are considered as part of the optical fibre layer. 3.1.2 Optical fibre cable layer This requires cable design and fibre sizes to be determined appropriate to the network architecture adopted,
25、 see clause 4. This layer will include optical plant hardware such as splice closures, cabinets and housings for OPSs, WDMs, OFAs, connectors and patch panels, all of which have specific environmental constraints. 3.1.3 Structural facility layer Where this layer was evolved for copper networks it is
26、 usually in the form of a star radiating from the exchange. Therefore, it will allow the introduction of optical spines to generate the network architectures discussed in the Annex A. Hence, the changes required for optical networks concern mainly the manner in which the optical cables and optical f
27、ibres are deployed within the outside plant. However, use of optical plant may lead to development of new structures and plant layout. For example, a duct installed for the copper network may be sub-divided, compartmented, using appropriately sized plastic tubes, each to accommodate one of the small
28、er diameter optical cables. Also, special consideration shall be given to aerial support structures for optical cables. 4 Optical fibre cable Cables of single-mode optical fibre shall be used which comply with Recommendation L.10 for the cable and normally RecommendationG.652 for the fibre. They sha
29、ll be constructed and installed in accordance with the manual “Construction, installation, jointing and protection of optical fibre cables” i. It is best to avoid the need for a large number of different designs of optical cable that may involve different skills for their use. Experience has shown t
30、hat the local distribution network may use the same design as for other networks. Ribbon cable may be attractive for high fibre count cables, with small and lightweight low fibre count cables being required for customer access. 5 Passive optical components Guidance on passive optical components is i
31、n preparation for a new chapter to be published in the revision of the manual “Construction, installation, jointing and protection of optical fibre cables” 11. In this Recommendation, a passive optical component is considered to be an optical component with no active electronics. That is, there is n
32、o electrical to optical conversion within the device. Although actively powered, OFAs are classified as passive optical components since they operate through stimulated emission of a rare earth doped fibre, which is in itself passive when considered in relation to the above definition. Examples of p
33、assive optical components which may be required in a local distribution network are: - fusion and mechanical splices; - optical connectors; - OPSs; 2 Recommendation L.15 (03/93) COPYRIGHT International Telecommunications Union/ITU TelecommunicationsLicensed by Information Handling ServicesITU-T FIEC
34、flN*L.LS 93 4862593 O593529 O09 m - optical attenuators; - optical filters; - OFAS: - optical isolators. Further aspects to consider for use of passive optical components listed in the above examples are given below: - Fusion and mechanical splices are considered in detail in Recommendation L.12. Th
35、e choice of the splicing technique used in an optical local distribution network depends upon a number of factors such as performance, cost, physical location, specific system requirements, and maintenance considerations. Splice points are often required to provide a point of flexibility. - Optical
36、connectors provide a means of flexibility for rapid re-configuration within a network. Mated connectors can provide a test access point for maintenance or test access may be gained by a connector through a spare OPS leg. - OPSs split the optical power from the input fibre to two or more output fibre
37、s. The split may be equal or asymmetric between the output fibres, it can be designed to split at a specific ratio for a specific wavelength, or it can be made insensitive across a wavelength range. For the local distribution network splits of between 2 and 32 output fibres are generally appropriate
38、. - WDMs are necessary to multiplex and demultiplex various services carried by different wavelengths on a single fibre. They may be either simple two wavelength region devices or high resolution devices for resolving many tens of closely spaced wavelength regions. - Optical attenuators with either
39、fixed or variable attenuation are necessary to adjust optical power budgets to the required ranges. NOTE - Temporary attenuators, such as clip-on attenuators, can be applied to live fibre networks by macro- bend techniques. These are useful for reducing optical power to a safe level when working on
40、a system and for reducing reflected power returned into a system from a bare fibre end. These techniques are particularly applicable to live working. - Optical filters are necessary to pass the required wavelength region of a service and to reject other service wavelength regions or optical test wav
41、elengths within a network. The spectral response of the filter may select very narrow or very broad wavelength regions depending upon the application. - OFAs provide compensation for optical fibre and other passive component losses. - Optical isolators are of particular benefit for a complex network
42、 since they provide immunity from the many sources of optical reflection intrinsic to the design of local distribution networks. They may also be used to isolate sections of a network for the purpose of live working during maintenance. 6 Network installation and maintenance Installation and maintena
43、nce procedures are extremely important matters which should be taken into account when considering costs for the future deployment of optical fibre in local distribution networks. Whenever possible, installation procedures should require only a low skill level from personnel. All stages of network c
44、onstruction may be monitored optically and sheath joints closed after testing has been completed. Splicing methods with splice loss prediction can be beneficial. Maintenance of the network should be carried out whenever possible while the system is live and faults detected, diagnosed and repaired wi
45、thout service interruption. Optical network maintenance using, for example, OTDRs and clip-on techniques can be used to supplement point-to-point transmission monitoring processes. Recommendation L.15 (03/93) 3 COPYRIGHT International Telecommunications Union/ITU TelecommunicationsLicensed by Inform
46、ation Handling ServicesDiverse routing providing a standby network will allow planned rather than reactive maintenance techniques to be used for repair of faults within the network. Diverse routing is particularly important for the high revenue earning or high-risk sections of a network. This subjec
47、t requires further study based on experience with optical local distribution networks to increase the detail of the advice given in this clause. 7 Electrical power supply Optical local distribution networks can provide a problem for the supply of power to the ONT. All experiments to date rely on com
48、mercial power or an auxiliary copper power supply network. Loss of commercial power presents a high risk of losing service to these systems. A solution to this problem is required to enable high deployment of optical local distribution networks to take place. 8 Optical safety This subject needs furt
49、her study. International standards shall be referred to as appropriate, e.g. IEC Publication 825. 9 Particular attention is drawn to the Local Network Guide being produced by Working Party XV/4 2. Other local distribution network references References 11 2 CCITT manual Construction, installation, jointing and protection of optical fibre cables, ITU, Geneva 1988. CCITT Study Group XV, Reports R 60 and R 65, Working Party XV/4 (Local Network Guide). Annex A (This annex forms an integral part of this Recommendation) A.l Introduction The contents of this annex are a result of the close
