Typical causes of failure in fiber inspection tests

Typical causes of failure in fiber inspection tests

It has been several years since the initial introduction of the current BBU (Baseband Unit) and RRH (Remote Radio Head) architecture of today’s wireless cellular systems, this article will focus on typical causes of failure in systems.

Typical causes of failure in test systems include following:

1. Mismatched modules used

Meaning either mismatched line rate, mismatched optical wavelength, or mismatched fiber type (SM vs MM). The most common optical wavelengths used with single mode fiber are 1550 nm and 1310 nm, and the most common wavelengths used with multimode fiber are 850 nm and 1300 nm. As one gains more experience and knowledge, they should easily be able to recognize whether the fiber type needed is single mode or multimode simply by knowing the optical wavelength being used. Although the receiver section (Rx) for the SFP+ module is rather broadband (1260 nm to 1600 nm) meaning this receiver may function with either 1550 nm or 1310 nm wavelengths. It is always best practice to use matched pairs of SFP modules between the BBU and RRH for best results.

2. Incorrect fiber type used

The SFP+ requires single mode (SM) fiber, terminated in duplex LC connectors. Generally the jacket color for SM fiber is yellow. The core of SM fiber is 9 μm in diameter, and the ferrule (the opaque white part you see) is 1.25 mm in diameter for an LC connector. Compare that with a multimode (MM) fiber terminated in duplex LC connectors. The ferrule of the MM fiber is also 1.25 mm, however the core is either 50 μm or 62.5 μm, and the jacket color for MM fiber is usually orange. If a MM fiber is used in a SM system, or vice-versa, there will be problems and the system will be inoperable. Therefore, it is extremely important to always use the correct fiber type which is matched to the SFP(+) type being used in that particular system. To the naked eye, you cannot see the difference between SM and MM fiber simply by looking at the LC connector and the visible 1.25 mm ferrule (typically white color for both SM and MM types of fiber).

3. Distance between BBU/RRH too long for module type and fiber used

An experienced installer or site technician may have gone through and verified that the SFP modules are matching and the correct fiber type for the SFP modules was used, and they may have even ensured that the fibers are clean and damage free yet the system still doesn’t work. Most likely the cause would be that the system was configured using MM type SFP modules and MM fiber, but the distance between the BBU and RRH is too long for the required line rate. Multimode fiber is simply not capable of high speed data transmission (>1 Gb/s) over very long distances. Therefore whenever the distance between the BBU and RRH is long, SM type SFP modules and SM fiber should always be used. SM fiber was significantly more expensive in the past, and the line rates were lower, so MM fiber was used for the cost savings. Luckily today SM fiber and even the SM modules have come down in cost significantly, and with data rates constantly increasing, the majority of systems being deployed now use only SM equipment.

4. Dirty fiber

 This accounts for the majority of problems found in these types of systems.

5. Damaged fiber

The fiber can be easily damaged, and surface scratches and/or chips and pits can be impossible to see with the naked eye yet they can easily cause a significant loss of optical transmit power. Other types of damage to fibers include pinched fibers or fibers which have been excessively bent (often referred to as macro bends). Optical fiber will behave like an attenuator if it is excessively bent or pinched. Excessive bending can occur easily if a person is pulling fiber through a maze of tubes and must pass through some 90+ degree corners, even something as simple as over tightening a cable tie can cause sharp bends, or when someone tries to roll up the excess fiber in an attempt to clean up the installation. Pinched fibers can easily occur when clamping the assembly to a tower or other structure. A user must take care to ensure that the fiber does not get pinched by the clamps used to secure it.

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