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Guide to selection of Permanent Links & Channels
This guide will provide some support on how to find the optimal permanent links and channel. It is the fundamental principle for generic cabling that a combination of any manufacture of patch cable, horisontal cable and connecting hardware of a specific component category will always produce cabling of the corresponding cabling class. In this way open systems are supported and no supplier specific cabling should be necessary. In the vast majority of cases this is also true. Any combination of, for instance, Cat. 5+ cables and connecting hardware will normally give Class D+ permanent links and channels. As a side remark it should be noted that the word "Category" is always used for components in ISO/IEC and CENELEC, while they use the word "Class" for permanent links and channels. This is a potential source of confusion since the word "Category" is used for both components and (basic) links and channels by TIA/EIA. Now, when the intention of standardisation is to support open systems why is then permanent link and channel testing widely applied? The answer lies in four factors: 1. Many people do not realize the intended benefits of standardisation. 2. In a few cases combination of components of a specific Category does not assure the corresponding "Class" performance. 3. People want to know the actual performance level of the specific cabling, which might be significantly better than the minimum limits guaranteed by the "Class" in question. 4. A manufacturer of cable or connecting hardware wants to document that his component works well together with other common components on the market. The first and last of the above points do not need any discussion, but a comment should be given to points 2 and 3. The rapidly developing cabling requirements cause changes in existing performance requirements as well as introduction of new cabling parameters. In this ever changing environment it may happen that component performance requirements are not fully assuring the resulting permanent link and channel performance. Two typical examples are: Example 1: Combination of horisontal and patch cables of a specific category does not assure the corresponding performance of the channel class, because the mean impedance of the cables is not specified. This will have to be done in the future to solve this inconsistency of standardisation. Example 2: The 1999 addendums to ISO/IEC 11801 and CENELEC EN 50173 do not contain component requirements although new and more strict permanent link and channel parameters are specified. This potential conflict will be solved in the future 2nd editions of both standards. It must again be stressed, however, that only in a few cases the components of a specific category do not result in the corresponding performance of the class in question. The most significant reason for adopting permanent link and channel testing is probably to demonstrate actual performance of the permanent link and channel and to show that this "significantly" exceeds the minimum requirements of the permanent link and channel class in question by focusing on one or more of the most superior parameters. The end user should, of course, always focus on all parameters as the minimum performance is always defined by the worst performing parameter. Considerations for selection of cabling type for a LAN are not included in the present discussion, but are found in the similar sections for cables. Discussion of EMC performance is found in the sections for cables and connecting hardware, where the very important and often violated requirements to correct termination of cable screens to connecting hardware are also discussed. Here the only remaining EMC remark should be that the lowest performing component always defines the min. EMC performance of the full installation. Qualification Status of Cables and Connecting Hardware The most important point for selection of permanent links and channels is to be sure that there is a constant performance level of the cables and connecting hardware of the regular production. The performance of connecting hardware is most often extremely stable. This performance is mainly determined by the applied connector jack construction and design of PC board, which usually do not vary except for intentional design changes. Contrary to this the cable performance may vary depending on production fluctuations. Responsible cable producers control such fluctuations, but alternatively the performance of the permanent links and channels could vary to unacceptable low levels. Consequently the qualification status of the cables and connecting hardware in question should have major attention by the end user. If a 3P verification of the components applies, a regular 3P control of the production in question is carried out. If a manufacturer verification applies, you rely on the confidence to the producer that he maintains a constant quality. Configuration of Permanent Links and Channels A large number of permanent link and channel test reports are being circulated in the market and unfortunately a large variation in configurations are applied. This concerns presence or non- presence of cross-connect and consolidation/transition point, and distance between wall outlet and consolidation/transition point. The nature of the worst case configuration is being discussed by standardisation groups, but generally this will depend on the parameter one considers. It is an ongoing discussion, but generally 3P would recommend:
Standardisation of Permanent Links & Channels
3P certification testing covers all standards, and draft standards which are concluded to be mature. Class D (1995): This is the maximum rating of the presently only published ISO/IEC and CENELEC standard. Please note that the permanent link and channel requirements are not published in these standards, but only the very impractical and soon outdated "link" requirements. Class D (1999): This is the maximum rating of the amendments to ISO/IEC and CENELEC standards which are expected for publication in September 1999. These documents define permanent link and channel configurations and performances, and include both new parameters and more tight limits. Class D (2000): This is the 100 MHz rating of the 2nd editions of ISO/IEC and CENELEC standards. 2000 could become 2001 depending on the release date of the standards. These documents are expected by 3P to define more strict limits to a number of Class D (1999) parameters, corresponding to the American Cat. 5e requirements (with supplementary ISO/IEC and CENELEC requirements). Class E: This is the 200 MHz rating of the 2nd edition of ISO/IEC and CENELEC standards. Class F: This is the 600 MHz rating of the 2nd editions of ISO/IEC and CENELEC standards. Furthermore, the TIA/EIA standardisation operates with basic link and channel standardisation, which unfortunately will be different than the ISO/IEC and CENELEC requirements. A fundamental difference will be that TIA/EIA basic links includes the hand-held field tester measurement cords in the permanent link performance. This is unacceptable to ISO/IEC and CENELEC as the tester cords will never be part of the installation and therefore it will possibly give mis-information about the permanent link performance. Except for this major difference and a number of smaller conflict points the following correlation between ISO/IEC / CENELEC and TIA/EIA cabling standardisation applies:
ISO/IEC & CENELEC TIA/EIA Verification programmes for Permanent Links & Channels
3P programmes Certification and listing of permanent links and channels is based on testing of samples assembled using the normal worst-case configuration. Both long and short samples of a permanent link or channel are included in the testing. This means for permanent links:
It should be noted that a permanent link may also include a consolidation/transition point and that this, as well as cable length, will affect the permanent link performance. However, the permanent link performance limits vary and always correspond with the actually applied configuration which will allow the application of a 3P certified permanent link to other configurations than actually tested. For channels the situation is different. The customer specifies his worst case configuration, which is tested against the general channel requirements without considering the selected configurations. 3P's approval of a specific channel therefore cannot be extended to cover more components than those included in the tested configuration. The following types of channel configurations have so far been used:
The testing includes verification of all permanent link and channel parameters specified in the amendments to CENELEC EN 50173 and ISO/IEC 11801, which also covers all parameters specified by TIA/EIA 568-A amendments. Furthermore the minimum EMC performance of permanent links and channels is measured if requested by the supplier. Results of any EMC performance measurements are presented in tables 22 through 25. As discussed in the guide to selection of cabling from permanent link and channel testing the qualification status of the cables and connecting hardware gives significant information to help to identify any risk of fluctuating component quality, which may cause degradation of permanent link and channel performance. Therefore the qualification status of components is specified for each permanent link and channel. Likewise the latest date of approval is specified for each permanent link and channel. The risk of change in component performance will of course increase with time. 3P has therefore decided that a certification will only be valid approx. one year from granting of certification unless documentation from component producers is received by 3P and demonstrates that component performance is unchanged. The date of certification will then be amended in accordance with the latest date of documentation. The following electrical performance standards apply for each of the four performance ratings Class D, Class D+, Class E and Class F: Class D:
Class D+: All Class D standards referenced above
Class E:
Class F:
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