Troubleshooting: Factors That Affect Network Performance
One of the technical questions we received this month
became an extensive conversation about network performance, testing and
the fiber optic cable plant. The conversation focused on a campus
network with multiple links among buildings connected to a central data
center. Part of the network was suffering from high BER (bit error rate)
and it seemed the fiber tech and the network tech were not in agreement
on the cause(s).
The links were less than 1.6km long and had 2 to 4 connections at patch
panels. The fiber was OS2 singlemode. OTDR traces showed connection
losses were all reasonable and the total link loss was less than 2 dB.
The reflectance at all connections was not bad for PC connectors, but
the OTDR measured ORL (optical return loss, the summation of the
reflectance from all connections added to fiber backscatter) as only 37
dB.
What could be causing high BER? Three things are the most obvious; 1) Is
the networking equipment overloaded when operating on a singlemode link
with ONLY 2 dB loss or are the transceivers causing problems. 2) Is
there are problem with the electronics? 3) Reflectance from multiple
connections - is the ORL of only 37 dB causing noise or multipath
interference on the link?
Troubleshooting a link like this
should always begin with checking power in the link. Every
piece of communications equipment has an operating range of receiver
optical power as shown in the graph below. Having too much power at the
receiver can be a big problem on short fiber optic links over singlemode
fiber, opposite of the problem with multimode where not enough power is
the more common problem.
If the power level is too high, the receiver overloads, signals are
distorted and the BER will be high. If the power is too high, the
solution is easy, an attenuator at the receiver can be used to reduce the power to an acceptable level. If
the receiver power is too low, noise is the problem and the BER will be
high. Check the tested transmitter power to see if the output power is
within the transceiver specifications. Even if the transceiver
power levels are OK, it's possible that the transceiver has a digital
problem, so swapping transceivers is may still be needed.
If the transceiver is OK, the loss in the network needs troubleshooting.
If the power is OK, the next thing to check is the fiber optic
transceivers. Most transceivers today are pluggable modules, so swapping
out the current modules for new or known good ones will allow
determining if they are the problem.
If the power and the transceivers check out, the problem may be with
reflectance from the multiple connections in the short network. The
problem with ORL or connection reflectance is more complex. We can begin
the explanation with the phenomenon of "ghosts" on an OTDR.
Ghosts are caused by high reflectance events in the fiber being tested
by the OTDR. Light reflected from the event is sent back toward the
source (the OTDR) where it can be reflected back to the far end again.
Depending on the reflectance, light can be bounced back and forth
several times.
The OTDR trace can show you what happens with reflectance in a link, but
the same thing happens in a communications link. Light reflected back
to the source can be a problem with some laser sources, causing noise or
modulation problems. Light reflected twice goes back toward the
receiver and adds to the incoming signal making it noisy, It's
called multipath interference and can be a big problem in short high
speed links.
Multipath interference is
rarely a problem in long links because there are generally few
connections that are reflective (mostly fusion splices) and the fiber
attenuates the reflected signals before they reach the ends. But it can
be a problem in short singlemode links like a campus network such as the
one we describe above. Short links with multiple connections can have
many reflections back and forth in the link, causing noise in the links
which can affect BER.
Similar problems exist in PONs, either FTTH or optical LANs. That is why
these shorter singlemode networks generally use APC connectors. APC
connectors have an angle-polished physical contact ferrule (thus APC)
that all but eliminates reflectance problems. You can identify APC connectors because they will be green.
If OTDR traces show reflective connections in a network, reflectance may
be causing multipath interference. The best solution is to replace
connectors with APC connectors, preferably fusion splice-on connectors.
Make the changes to a bad link to see if that solves the problem, and if
it does, you know your best solution is to replace all of them.
If you are designing or building a short singlemode fiber link with
multiple connections, using APC connectors is an easy way to ensure you
will not have problems caused by reflectance. And if you have the
option, minimizing the number of connections is another good idea. It
may be possible to eliminate some connections using fusion splices if
the fiber routing is not likely to be changed. If patch panels are
needed, making direct connections between fibers rather than using
patchcords (creating two connections) can also help reduce ORL problems.
Note: Remember that singlemode APC (green) connectors cannot be
mated to regular PC or UPC (blue) connectors - don't mix your colors!
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