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News
Resistance Unbalanced:
Hidden Faults
in the LAN
No signal
Magnetic
flux
Unbalanced
Data pair
Balanced
Ethernet transceiver
power
sourcing
equipment
050.7395
041.0515
No saturation signal "OK"
Sales of PoE devices are increasing by 20 % per year. These devices are inevitably
connected to the existing LAN cabling. Users rarely install new PoE-compatible cabling.
This gives rise to significant risks.
Unrecognized weaknesses are often hidden
in low-quality LAN environments. However,
they are relentless when users deploy more
PoE-powered devices. The background is
easy to understand: Thanks to Ethernet, data
signals usually find their way to their destination.
In the event of transmission problems,
the system simply switches back to a slower,
less demanding transmission. The sources
Security through IDC
R&M recommends the use of patch cords
with IDC wiring (insulation displacement
contact) especially for PoE applications.
With IDC, insulation displacement contacts
ensure consistently stable and lowresistance
contacting of the conductor. As
a rule, no resistance unbalance occurs over
the lifespan of the patch cord. No one has
to look for hidden causes of transmission
interference years after installation.
All R&M patch cords with the PowerSafe
quality seal are wired using IDC and are
suitable for Power over Ethernet. When
PowerSafe components are used, R&M
provides a lifetime application warranty
for PoE.
www.rdm.com/
overcoming-resistanceunbalance/
of interference – e.g. excessive resistance in
contacts – remain hidden at first.
A little-noticed parameter can quickly reveal
problems with PoE support: resistance
unbalance within wire pairs (DC Resistance
Unbalance, DCRU). With the same currents
on both conductors, the magnetic flux in
the ferrite core of the input transformers of
the transmitter/receiver unit of the active
devices cancels out with PoE. The data is
transferred without hindrance.
If the two conductors have different resistances,
the current is distributed unevenly
between them. The magnetic flux in the
transformer core will then no longer cancel
out. The DC current can lead to a magnetic
saturation of the transformer, which interrupts
the data transmission. According to
IEEE, even DCRU values of 3 % can lead to
problems with data transmission.
Piercing as a risk factor
Patch cords with piercing wiring often
exhibit this resistance unbalance, something
demonstrated by research carried out by
R&M. The reason is that piercing contacts
age rapidly due to environmental factors or
mechanical loads. The contact resistance of
the two conductors increases over time and
usually increases differently.
This is how the resistance unbalance develops
and grows. It can lead to seemingly
mysterious interruptions in the LAN – a
nightmare for service technicians when it
comes to fault tracking.
Depending on the quality of the piercing
contacts, DCRU values which prevent PoE
transmission may also occur with new patch
cords. That’s why it’s always worth measuring
the entire channel, including the patch cords
used.
In a new white paper, R&M explains more
background on the problem of resistance
unbalance. It is available free of charge on
the website.
050.7067
Matthias Gerber
Market Manager LAN Cabling
matthias.gerber@rdm.com
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