CONNECTIONS_65-e
Specialist magazine CONNECTIONS no. 65
Specialist magazine CONNECTIONS no. 65
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News<br />
Resistance Unbalanced:<br />
Hidden Faults<br />
in the LAN<br />
No signal<br />
Magnetic<br />
flux<br />
Unbalanced<br />
Data pair<br />
Balanced<br />
Ethernet transceiver<br />
power<br />
sourcing<br />
equipment<br />
050.7395<br />
041.0515<br />
No saturation signal "OK"<br />
Sales of PoE devices are increasing by 20 % per year. These devices are inevitably<br />
connected to the existing LAN cabling. Users rarely install new PoE-compatible cabling.<br />
This gives rise to significant risks.<br />
Unrecognized weaknesses are often hidden<br />
in low-quality LAN environments. However,<br />
they are relentless when users deploy more<br />
PoE-powered devices. The background is<br />
easy to understand: Thanks to Ethernet, data<br />
signals usually find their way to their destination.<br />
In the event of transmission problems,<br />
the system simply switches back to a slower,<br />
less demanding transmission. The sources<br />
Security through IDC<br />
R&M recommends the use of patch cords<br />
with IDC wiring (insulation displacement<br />
contact) especially for PoE applications.<br />
With IDC, insulation displacement contacts<br />
ensure consistently stable and lowresistance<br />
contacting of the conductor. As<br />
a rule, no resistance unbalance occurs over<br />
the lifespan of the patch cord. No one has<br />
to look for hidden causes of transmission<br />
interference years after installation.<br />
All R&M patch cords with the PowerSafe<br />
quality seal are wired using IDC and are<br />
suitable for Power over Ethernet. When<br />
PowerSafe components are used, R&M<br />
provides a lifetime application warranty<br />
for PoE.<br />
www.rdm.com/<br />
overcoming-resistanceunbalance/<br />
of interference – e.g. excessive resistance in<br />
contacts – remain hidden at first.<br />
A little-noticed parameter can quickly reveal<br />
problems with PoE support: resistance<br />
unbalance within wire pairs (DC Resistance<br />
Unbalance, DCRU). With the same currents<br />
on both conductors, the magnetic flux in<br />
the ferrite core of the input transformers of<br />
the transmitter/receiver unit of the active<br />
devices cancels out with PoE. The data is<br />
transferred without hindrance.<br />
If the two conductors have different resistances,<br />
the current is distributed unevenly<br />
between them. The magnetic flux in the<br />
transformer core will then no longer cancel<br />
out. The DC current can lead to a magnetic<br />
saturation of the transformer, which interrupts<br />
the data transmission. According to<br />
IEEE, even DCRU values of 3 % can lead to<br />
problems with data transmission.<br />
Piercing as a risk factor<br />
Patch cords with piercing wiring often<br />
exhibit this resistance unbalance, something<br />
demonstrated by research carried out by<br />
R&M. The reason is that piercing contacts<br />
age rapidly due to environmental factors or<br />
mechanical loads. The contact resistance of<br />
the two conductors increases over time and<br />
usually increases differently.<br />
This is how the resistance unbalance develops<br />
and grows. It can lead to seemingly<br />
mysterious interruptions in the LAN – a<br />
nightmare for service technicians when it<br />
comes to fault tracking.<br />
Depending on the quality of the piercing<br />
contacts, DCRU values which prevent PoE<br />
transmission may also occur with new patch<br />
cords. That’s why it’s always worth measuring<br />
the entire channel, including the patch cords<br />
used.<br />
In a new white paper, R&M explains more<br />
background on the problem of resistance<br />
unbalance. It is available free of charge on<br />
the website.<br />
050.7067<br />
Matthias Gerber<br />
Market Manager LAN Cabling<br />
matthias.gerber@rdm.com<br />
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