There are several aspects of fiber optic cable that need to be considered when choosing the correct type of construction for your project. Not only do you have to think of indoor versus outdoor applications, you also need to think of the physical characteristics that make up the cable. When looking at both of these, there are applications for each that will have benefits as well as negatives. When you look at these, you will see that some of them are unavoidable and you will just have to deal with them. A lot of the decision about what cable construction to use is based on what is spec’d in for a job or what is available at the time, as long as it will work for what you are doing. We will try to help to narrow down the choices and provide some insight to help you most effectively pick the correct way to go. There are some other types of cable constructions, but the main two are Tight Buffered and Loose Tube, and they will be discussed below.



Tight Buffered


Tight buffered cable is considered to be easy to use and it does not need a lot of prep time to have to build up the fibers in order to terminate them. Tight buffered fiber optic cable is also referred to as distribution style fiber optic cable. This construction has Kevlar aramid yarn strength members and sometimes it has fiberglass rods to assist with strengthening the cable. Tight buffered style cable is when you have a multi fiber count, such as twelve fibers, that are already built up with a 900um PVC coating on them. This type of optical fiber cable design allows a contractor to be able to place a connector directly on the inner fiber without having to build it up. This is available in indoor as well as Indoor Outdoor constructions. They can be used for back bone applications indoors. There are a couple other names that I have heard for these back bone applications. Mic Cables and Trunk Cables are two of the names that are frequently used. On our end it is important that we ask questions as customers do not always use the names above for the same meaning. Trunk cables can also be MTP/MPO assemblies in which a tight buffered fiber cannot be used and they require a different cable construction in order to terminate this style of connector. One disadvantage to using a tight buffered cable is that the overall size of the cable will be a little larger than the loose tube fiber cable.



Loose Tube


When talking about loose tube fiber optic cables most people automatically think of outdoor cable applications. Many applications do deal with outdoor environments, but we will discuss where there are indoor loose tube applications as well. Loose tube optical cable typically is the term used when you have any type of multi fiber cable that has 250um coated or bare fiber inside it. Let’s start with the outdoor type first, and then we will get into some benefits for indoor applications as well.

So when looking at outdoor loose tube cable, there are a couple different features that are added to assist with keeping water away from your optical fibers. The first one is, that in order to be a true outside plant cable, it will have a water-blocking gel in the inner tubes. The inner tubes, or buffers, are individual tubes inside the cable that typically hold twelve fibers each. So with higher fiber counts, you will have more of these inner tubes. Outside plant cable, also referred to as OSP cable, is the suggested cable type for long haul, as well as short cable runs, where you will be putting the cable in the ground via conduit or an armored cable for direct burial applications.

Another type of application that is loose tube is called an Indoor/Outdoor Dry Loose Tube cable. This type of cable has Kevlar with water blocking solvent or tape that assists with preventing moisture and water from getting in and causing problems with the individual fibers. This type of cable is also allowed indoors because it does not have the petroleum gel or Icky Pik inside, which is flammable and toxic. OSP cable is only allowed 50 feet inside a building by codes. This again is due to the flammability of the jacket and the gel inside. One big disadvantage to using this style has to do with putting connectors on. In order to put a connector on this type of fiber cable you have to build up the bare fibers to at least 900um. This can be accomplished by using a fan-out kit. It just adds to the labor when this needs to be done.

Now that we touched on the outdoor side, we will talk a little about the indoor loose tube cable that is sometimes referred to as micro distribution style cable. Micro distribution cable has 250um fiber inside which is also known as bare fiber. By having this type of design you are able to have a smaller outer jacket diameter which will take up less space in a conduit or raceway. Another reason for this cable construction is the concept of MTP or MPO connectors. This style of connectors has multiple fibers, typically 12, and all of them are in one ferrule. MPO’s need ribbonized fiber when they are being assembled. With micro distribution fiber, since they are bare fibers, you can place these together in ribbon form in order to install these connectors.

Overall, you can see that these two types of cable constructions, loose tube and tight buffered, have some key features that need to be taken into consideration for your application. Whether it is the size of the outer jacket, the type of connectors that will be used, or even where the fiber will be installed, there are advantages to both, and there are disadvantages. Sometimes it will have both, and you will have to go with the kind that has the least amount of disadvantages. Not only does the weather outside play into your application, so does the geographical texture of the ground. This can assist in helping with your decision. When getting ready to install, make sure you check if there is existing architecture in place or will you need to figure out the most cost effective way. This may include putting up new poles or digging and placing the fiber underground.