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5.5
OTDR Testing
The following procedure describes the steps to perform OTDR analysis with the OneAdvisor.
5.5.1 OTDR Test Overview
An OTDR sends thousands of very short pulses of light at designated wavelengths into the fiber under test.
It detects the amplitude and time delay of both the scattering and the reflection of the light as it returns to
the same interface. Through computations, an OTDR builds a picture of the loss characteristics of the fiber
throughout its path. It then asserts the location and nature of events such as splices, bends, breaks,
connectors and the fiber end. It also provides the aggregate loss of the fiber from start to end.
The Insertion Loss test described below can tell you if a fiber run is performing acceptably. If it fails your
threshold (-3dB), you do not know where the problem or problems are. An OTDR provides location and
magnitude of every contributor to the losses incurred in the fiber system. Thresholds can be set not just for
total loss (dB) but for each contributing element.
Most Operators expect the following limits for events and measurements made by an OTDR
Measurement
Passing Criteria
Reflectance (per event)
More negative than -45 dB
Loss through a Connector
Less than .5 dB
Path loss end to end
Less than 3 dB
USEFUL INFORMATION: It is important to know the fiber distance from the DU connection to the RU
connection. This is typically the length of the hybrid plus any jumpers. These parts are typically purchased
at some standard length. When you run the OTDR test, you want to see the Test Set declare “End of Fiber”
at a distance that correlates to the “as built” information for the system under test. A declaration of “End of
Fiber” before that length indicates a connection is still open or there is a fiber break.
5.5.2 Test Setup
Tower Macro Site - Single Mode Duplex Fiber - Macro sites are usually tested at the base of the tower at
the BBU/DU. A 1310/1550 OTDR with a 10m (min) loopback cable at the top of the tower. The OTDR will
test the duplex fiber from the BBU/DU to the top of the tower and back to the bottom.
