R&M Data Center Handbook

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The permanent link model of ISO/IEC 11801 Amendment 2
ISO/IEC distinguishes between four configurations
of the permanent link (Figure on right), relevant for
network planning. The determining parameters for
transmission quality are NEXT (near-end crosstalk)
and RL (return loss).
ISO/IEC calculates the limit values of NEXT of all
four configurations [in dB] for the frequency range
1 = f = 300 [MHz] with this formula:
⎛
− 20lg⎜
10
⎝
74,3−15lg(
f )
−20
+ 10
94−20 lg( f )
−20
⎞
⎟
⎠
[1]
The two terms in the bracket refer to the effect of
the module at the near end and the effect of the
cable. ISO/IEC specifically states that they should
not be taken as individual limit values. This means
that the overall limit value can be reached with
very good cables and lower quality modules or with
very good modules and lower quality cables.
Different formulas are applicable to the frequency
range 300 = f = 500 [MHz]. The ISO/IEC calculates
the NEXT limit values for configuration PL2 [in dB]
like this:
87,46
21,57lg( f )
− [2] (blue line in Figure)
Permanent link configurations acc. to ISO/IEC 11801 Amendment 2
36
A moderate requirement applies to configuration
PL3 with additional consolidation point and
frequency range 300 = f = 500 [MHz]; it is
calculated with this formula:
102,22
27,54lg( f )
− [3] (violet line in Figure)
NEXT [dB]
34
32
30
PL2
PL3
Short Link
Less stringent requirements are applicable to the
configurations PL1, PL2 and CP2 in the frequency
range above 450 MHz: If the insertion loss (IL) at
450 MHz is lower than 12 dB, the required NEXT
curve can be lowered, calculated with the following
formula [2]:
1,4((
− 450) /50)
f [4] (yellow line in Figure).
28
26
250 300 350 400 450 500
Frequency [MHz]
NEXT limit values for the permanent link configurations 2 and 3
and for short links according to ISO/IEC 11801 Amendment 2.
Conclusion
Short links in structured cabling are not only possible but – combined with high-quality Cat. 6A components – they
are even recommended. Cat. 6 A modules and installation cables from R&M allow the setting up of permanent links
with advantages that impact positively on the cost factor:
• easier installation • no unnecessary cable loops • material savings of up to two thirds
• improved ventilation, lower energy consumption
• reduced costs of installation and operations
In 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 / 2.
Page 142 of 156 © 08/2011 Reichle & De-Massari AG R&M Data Center Handbook V2.0