R&M Data Center Handbook

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3.10.4 Short Links
The standard ISO/IEC 11801 Amendment 2 specifies that Class E A can only be fulfilled with Cat. 6 A components
when the minimum length of 15 m for the permanent link (PL) is observed. The use of shorter PLs is allowed when
the manufacturer guarantees that the PL requirements are met.
The wish for shorter links and the question of headroom
Today, many users tend to shorten the links in order to save material, energy and costs. But what effect does this
have on the performance and transmission reliability of the network
Shortening the distance between the two modules in a 2-connector permanent link results in a lower attenuation of
the cable between the modules, which in turn increases the interfering effect of the further module.
The standard, however, lists only one module in its calculation, which is problematic: Particularly, the limit values
for NEXT and RL (return loss) can not be maintained any longer. This is why the standard defines a minimum link
length of 15 m.
What happens if a high quality module is paired with a high-quality cable The result is a PL with much headroom
in terms of length in the range between 15 m to 90 m. Using the R&M Advanced System for example, ensures a
typical NEXT headroom of 6 dB with 15 m and 11 dB with 90 m (see Figure below).
NEXT headroom of permanent links, measured in an installation using the new Cat. 6 A module of R&M.
The zero line shows the limit value in acc. with ISO/IEC 11801 Amendment 2.
In the R&M freenet warranty program, R&M guarantees a NEXT headroom of 4 dB for a permanent link of above
15 m, built with components from the R&M Advanced System. Alternatively, the high headroom provides customers
with the possibility to set up very short PLs, which fulfill all the requirements of the standard ISO/IEC 11801
Amendment 2.
The advantage of additional headroom
Basically, it makes sense to lay permanent links that are exactly as long as is needed. However, that would mean
that the links would often be shorter than the minimum length of 15 m. When components are used that only meet
the standard requirements, the permanent links must be artificially extended with loops to 15 m to make sure that
the limit values are reached. This not only entails additional costs but these cable loops also take up space in the
cable conduits and interfere with the ventilation, which results in an increase in infrastructure costs and energy
consumption.
R&M Data Center Handbook V2.0 © 08/2011 Reichle & De-Massari AG Page 139 of 156