Historically, tight-buffered cable was used best for indoor applications while loose-tube cable was considered best for outdoor applications. And they were typically not to be used in the other’s place due to their material features. But is it still the case after so many years?
Tight-buffered cable and loose-tube cable are both fiber optic cables that consist of multiple fiber counts inside a single line of fiber cable, for the sake of better protection and cabling. There are some similarities in their designed purpose, but differences are obviously more than similarities between them.
The Tight-Buffered Cables With Versatility
Tight-buffered construction uses 900μm buffered fibers. The core is protected by two-layer coating. The first is plastic and the second is waterproof acrylate. The core of the cable is never at risk of exposure, unlike the loose-buffered cable which can escape its confines. Fibers are surrounded by dielectric strength members (FRP) and are protected by a rugged polyurethane outer jacket and provides superior environmental and mechanical protection. The fiber count for tight-buffered fiber cable varies from 1 to 144 fibers, but generally cables with 2, 6, 12, 24 fibers are the most commonly used. Larger fiber counts such as 48 fibers, 96 fibers and 144 fibers are also available for specific applications.
Figure 1: Construction and sub construction of 48 fibers tight-buffered fiber optic cable.
Tight-buffered cable can be unitized or non-unitized. The only difference between them is that the former is unitized with individual jacket. The unitized design makes the bundled fibers easier to install and manage than non-unitized. Conversely, non-unitized cable is designed just with a single outer jacket, but it is more flexible in cabling and has much smaller packages and cost advantages.
Figure 2: Non-unitized cable and unitized cable.
The Gel-Filled Loose-Tube With Large Fiber Capacity
In loose-tube gel-filled cable construction, 250μm coated fibers are contained in small, rigid tubes, inside which the space is filled, not otherwise occupied by fiber, with water-blocking gel or tapes. There are seven elements in a loose tube cable. Each element plays an important role in the cable according to its own feature.
Figure 3: Construction of loose-tube fiber optic cable.
1. Multiple 250µm bare fibers (in loose tube) 2. One or more loose tubes holding 250µm bare fibers. Loose tubes strand around the central strength member. 3. Moisture blocking gel in each loose tube for water blocking and protection of 250µm fibers. 4. Central strength member (in the center of the cable and is stranded around by loose tubes). 5. Aramid Yarn as the primary tensile strength member. 6. Ripcord (for easy removal of outer jacket). 7. Outer jacket (the common material is the polyethylene).
In general, a loose-tube cable consists of 12 strands of fiber, but can range anywhere as low as 6, all the way up to 244 strands. Because the fiber is not connected to the buffer tubes, the cable can be pulled and stretched without causing any fractures from bending or tension.
According to the different materials or the processing technology for different applications, there are many types of loose-tube fiber optic cables.
Table: Different types of loose-tube fiber optic cables.
Aerial & Duct, Single, Armor, Waterproof
Aerial, Strength Steel Wire, Central Loose Tube
Aerial, FRP Strength Member, Lightning-Proof
Aerial & Duct, Non-Metallic Stranded Loose Tube
Aerial & Duct, Single Armor, Stranded Loose Tube
Direct Burial & Duct, Single Armor, Double Jacket
Direct Burial & Duct, Double Armor, Double Jacket
Aerial, Central Loose Tube, Figure 8 Self-Supporting
Aerial, Single Armor, Figure 8 Self-Supporting
Tight-Buffered Cable vs. Loose-Tube Cable
Both tight-buffered cable and loose-tube gel-filled cable have color coded fibers for easy identification when used as distribution cables. And either armored or non-armored is optional. When the cables are deployed in where rodents are a problem or cables are direct buried and crush loads can be a problem, cables featured with armor is recommended. Popular armored types include interlocking armored and corrugated steel tape.
Tight-buffered is more suitable for indoor use since it’s sturdier than loose-tube cable. However, tight-buffered cables are much more expensive than loose-tube cables, and hold less fibers versus loose-tube cables using a similar diameter due to the difference of the 900μm fiber and the 250μm fiber. Loose-tube gel-filled cable is the best for outdoor use, but since it’s inconvenient to be spliced and terminated, it’s not the first choice for indoor/outdoor use.
Tight-Buffered Cable for Indoor and Outdoor Use
In fact, properly designed and manufactured tight-buffered cable has been sufficiently proved to be suitable for both indoor and outdoor applications in the past two decades. Tight-buffered fiber optic cables offer the flexibility, direct connectability and design versatility necessary to satisfy the diverse requirements existing in high performance fiber optic applications. Such applications include medium distance transmission for telco local loop, LANs, SANs, WANs, and point-to-point links in cities, buildings, factories, office parks and campuses.
Tight-buffered cable has many advantages over traditional outdoor cable spliced to indoor cable. The first advantage is an improvement in reliability. Link reliability will be reduced anytime there is a splice transition. Tight-buffered indoor/outdoor cable comes in the building directly as far as needed for final termination. Thus connectors are assembled onto the fiber cable at the final patching center, not at an inappropriate, hard to reach transition zone. Second advantage is that tight-buffered indoor/outdoor cable is generally easier to terminate than traditional outdoor rated cables. There is no injected gel to clear off prior to the termination process. Overall, we can say tight-buffered cables provide fast, easy, economical termination with no chemical cleaning required.
Loose-Tube Cable for Outdoor Use
Loose-tube cable protects the fiber from stresses caused by the environment, namely moisture and temperature. The gel within the loose-tube construction stops the penetration of water and keeps it away from the fiber. Since fibers can move within the tube, the expansion and contraction that temperature variations produce in other materials does not transmit stress to the fibers. In this sense, loose-tube gel-filled cables offer the best protection in outdoor environment. They are suitable for outdoor applications, including aerial, duct, CATV, long-distance, computer network systems, and LAN communication.
Although loose-tube fiber optical cables are used for high-fiber-count, long distance telco applications, they are an inferior design for LAN applications where reliability, attenuation stability over a wide temperature range and low installation cost are priorities. Since this cable is generally incompatible with indoor flammability codes, when it needs to be installed indoor, it must be normally terminated or spliced close to the cable entryway of a building. Due to the fragile bare fibers and gel filling, which must be cleaned prior to termination, loose-tube gel-filled cable is the most difficult to splice and terminate and also has the highest termination material costs. In the final termination process, all fibers must be either spliced or fed through the tubes of a breakout kit. These processes all add the complexity in termination and the splices inevitably decrease the reliability of the link. All above reasons make loose-tube gel-filled cable an inappropriate choice for indoor/outdoor deployment. But it’s still good as an outdoor cable for its features.
Tight-buffered cable and loose-tube cable are both excellent rugged fiber optic cables, the former of which is usually used for moderate length indoor and indoor/outdoor applications, while the latter is for long-distance outdoor applications.
Key acronyms: LAN = Local area network SAN = Storage area network WAN = Wide area network FRP = Fiber-reinforced plastic, a composite material made of a polymer matrix reinforced with fibers. PE = Polyethylene