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250um vs. 900um

Posted by: on September 18, 2014

Ever wonder what the difference is between loose-tube (mini distribution) 250um fiber and tight-buffered 900um fiber?

Loose-tube 250um and tight-buffered 900um fiber cables actually start with the same 250um bare fibers that feature the same size fiber core (i.e., 50um for multimode and 9um for singlemode), 125um cladding and soft 250um coating.

The difference between the two is all in the cable construction.

An Added Layer

Tight-buffered 900um fiber includes an additional 900um layer of hard plastic over the 250um fibers for protection. Within the cable, several of these color-coded 900um tight buffered fibers are situated around a central strength member and then covered with Kevlar or aramid yarn for protection, a rip cord and then the jacket.

250um vs 950um

Tight-buffered 900um fiber cable comes in various fiber counts that typically range from 2 to 144 fibers, with larger fiber counts featuring fiber subunits of 6 or 12 fibers within the cable. For example, a 144-fiber cable usually has twelve 12-fiber subunits while a 36-fiber cable could have six 6-fiber subunits or three 12-fiber subunits.

Down the Tubes

Loose-tube 250um fiber places up to 12 bare 250um fibers inside a flexible plastic tube, which are also color coded and situated around a central strength member with Kevlar or aramid yarn for protection. Buffered loose-tube cables feature an outer waterblocking tape around the tubes, beneath the outer jacket.

The tubes themselves are gel-filled to prevent water migration, or they are available with a dry waterblocking technology—sometimes referred to as gel-free cable. Both of these materials are vital to prevent water from migrating into the tubes and potentially freezing, expanding and breaking the fiber. Dry waterblocking technology significantly reduces installation time by eliminating the need to clean off the gel prior to termination.

Loose-tube 250um fiber cable comes in various fiber counts that typically range from 6 to 144, with some manufacturers like Belden offering higher counts of up to 216 fibers. With the exception of a 6-fiber cable, the fibers are grouped into sets of 12 for maximum density.

Speaking of density, without the 900um plastic coating, loose-tube 250um fiber cables are less than half the size of 900um fiber cables—1.4 in (35.6 mm) for a 144-fiber tight buffer cable and only .67 in(17 mm) for an outdoor 144-fiber loose-tube cable.

From Outdoor to In

Generally speaking, tight-buffered 900um fiber cables are used for indoor applications, including intra-building riser and plenum applications and in the data center. Loose-tube 250um fiber cables are typically used in outside plant (OSP) applications, such as inter-building duct, aerial and direct buried installations.

While indoor/outdoor cables are popular for eliminating the need for service entrance splicing to in-building cable, OSP loose-tube 250um cabling must be terminated within 50 feet of entering a facility. To accomplish this, breakout kits are used to build the 250um cable up for protection and termination to 900um connector boots.

The problem with breakout kits is that they add additional material costs and a significant amount of labor. One option is to terminate the 250um fiber directly to 250um connector boots. This can speed network deployment in the data center and fiber-to-the-home applications.

250 A?µm Boot - Transparent - ImageBelden now offers an optional Brilliance Universal 250um connector boot for direct termination, eliminating the need for the breakout kit. Just remember that the smaller diameter, less rugged 250um terminations and fiber need to be properly protected inside splice cassettes.

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  1. July 28, 2015 at 09:10

    Timothy, I’m glad you enjoy reading! In response to your question, tight buffered 900um is used primarily indoors, although, occasionally, it is used in campus environments with an indoor/outdoor rating. The challenge with 900um tight buffer in an outdoor, uncontrolled environment is that, as the temperature cycles between hot and cold, the tight buffer expands and contracts at different rates than the glass which causes stress on the glass and ultimately increased IL and PDL (polarization dependant losses) because this "tight buffer" is mechanically bonded to the glass during the extrusion process. Loose tube 250um, on the other hand, has no extruded coating bonded to the glass so it does not suffer from the same performance degradation during temperature cycling. Hopefully this answers your question!

  2. Timothy Morrissey
    July 26, 2015 at 01:58

    Hello. I really enjoy reading these tidbits of info so I keep up to speed with the latest in tech. I believe, however, that there is an error in the above text in the first paragraph of the last section. I had to read it several times, but isn't it the tight-buffered 900um fiber cables are used for outdoor applications and the loose-tube 250um fiber cables are used inside, instead of the reverse?


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