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:

• The permanent link requirements must always reflect the configuration actually tested (length of cables, two or three samples of connecting hardware in the permanent link). This will allow extension of the permanent link with specified cables and connecting hardware to form a channel.
• The distance between any consolidation/transition point and wall outlet may be critical for return loss. The shorter the distance the worse return loss performance. No minimum distance is specified, but 7 or 10 metres would be a convenient average distance.
• A tested channel configuration cannot be extended by more cable or connecting hardware as performance may then no longer be in compliance with the channel requirements.
• Channel performance requirements may be easier to pass than permanent link requirements for high performance cabling. This is because application of a very superior patch cord will improve for instance near end crosstalk performance by several dB's. The effect of a less than perfect connecting hardware performance is then camouflaged by the up to 5 metres patch cord. The applied patch cable therefore forms an important part of the tested channel configuration