- Optical Time Domain Reflectometers
- (OTDRs)
When To Use an OTDR
When Do You Use An OTDR?
We have brought in various OTDR
manufacturers to teach their use to our students. Since most
of our attendees are contractors, we try to give them a realistic
overview of what jobs they will see and what tools they will
need. It is very important to understand when you need an OTDR
and when it is not appropriate.
If you are installing an outside plant network such as a long
distance network or a long campus LAN with splices between cables,
you will want an OTDR to check if the fibres and splices are
good. The OTDR can see the splice after it is made and confirm
it's performance. It can also find stress problems in the cables
caused by improper handling during installation. If you are doing
restoration after a cable cut, the OTDR will help find the location
of cut and help confirm the quality of temporary and permanent
splices to restore operation. On singlemode fibres where connector
reflections are a concern, the OTDR will pinpoint bad connectors
easily.
OTDRs should not be used to measure cable plant loss. That is
the job of the source and power meter, which duplicates the actual
fibre optic link, as we described in the first part of this article
and is documented by every standard ever written for cable plant
loss. The loss measured will not correlate between the two methods;
the OTDR cannot show the actual cable plant loss that the system
will see.
The limited distance resolution of the OTDR makes it very hard
to use in a LAN or building environment, where cables are usually
only a few hundred feet long. The OTDR has a great deal of difficulty
resolving features in the short cables of a LAN and is more often
than not simply confusing to the user.
And one OTDR manufacturer once told a class of students that
they could justify the cost of an OTDR simply by using it to
test the length of the fibre on a reel when they get it to make
sure they got what they paid for. The class laughed at the instructor
and pointed out the cable manufacturers mark length on the cable
jacket and a £2 calculator would do as well!
Since OTDRs are very expensive and have only specific uses, the
decision to buy one must be made carefully. For that reason,
most instrument rental companies will rent one for a few days
or weeks when you need them. However, if you are not familiar
with their operation or cannot understand the results of OTDR
tests, it would be much better to hire a specialist to do the
testing for you.
