The loose-tube advantage for data center applications

The traditional round cable goes head-to-head with ribbon fiber in terms of value and performance.

A data center is at the heart of many successful enterprise business processes. As such, it needs to operate at 100% efficiency. Dubbed the “factory floor of the Information Age,” the data center can house one company’s data storage or host multiple Web sites and co-locations for many companies’ networks.

Utmost consideration must be given to the detailed planning and designing of an efficient data center for reliability and longevity, whether the scenario is large or small. The TIA-942 Telecommunications Infrastructure Standard for Data Centers has recognized nine core elements of a data center, seven of which are defined spaces (computer room, telecom room, entrance room, main distribution area, horizontal distribution area, zone distribution area, and equipment distribution area) and two of which are the backbone and horizontal cabling subsystems that tie the spaces together. From outside services in, the backbone cabling subsystem provides the interconnection between the entrance room, computer room, and telecom room out to the main distribution area. Backbone cabling also connects the switches in the main distribution area to the local area network (LAN) switches in the horizontal distribution areas.

The cabling infrastructure must be capable of servicing present and future applications in the data center. Fiber-optic cable is the preferred medium for the data center backbone due to channel capacity and scalability, total immunity to electromagnetic interference (EMI) and radio frequency interference (RFI), elimination of the crosstalk inherent in copper cabling, smaller diameter, and ease of installation, particularly when using pre-terminated modular trunk cables. Furthermore, plenum-rated cables are commonly specified for maximized flame and smoke protection within the data center.

Selecting the type of fiber-optic cable boils down to the applications and the installation. Cable construction choices are ribbon, loose tube, or tight buffer. Where ribbon cable has found its niche because of its high-density configuration, specifiers are now taking a look at other constructions to fit unique and stringent data center requirements as well as industry standards. These additional considerations address issues that include higher mechanical robustness, improved cooling efficiency, fire loading, as well as flexibility and modularity for easy moves, adds, and changes (MACs).

Making connections

Everything begins and ends with connectors. Installers agree that when it comes to fiber terminations, the modular MPO/MTP pre-terminated multifiber has become popular in the data center because of its ease of installation and its reliability. The MPO or MTP (“multiple terminations push-pull” or “mechanical transfer pull”) is a linear multi-array connector containing 12 fibers in a ferrule with an easy push-on and pull-off design.

The most common single point of cabling-system failure is the connector interface. Pre-terminated assemblies are the most reliable cable and connector combination to address this issue. The cable is pre-terminated in the factory to allow quick deployments, and is pre-tested to ensure reliability. Pre-calculated lengths of pre-terminated, multi-fiber MTP/MPO trunk cable assemblies have been proven to substantially reduce installation times and provide better reliability. The factory-terminated ends arrive on site with protective encasements and pulling eyes.

The MPO connector originally was designed specifically to accept a ribbon array of fibers, as the fibers must line up with the flat connector housing; however, termination technologies have evolved to the point where it is no longer necessary to have a flat ribbon to terminate to MPO connectors. A popular option is to use ribbonized optical fibers from a loose-tube cable. This ribbonizing process is performed by the cable assembly manufacturer, and the result is an assembly with greatly enhanced functionality.

The ribbon cable is aptly named for its flat, ribbon-like structure, which houses fibers side-by-side. Twelve fibers in a row are bonded and held together by an ultraviolet acrylate matrix. Because the fibers are first bonded in rows of 12 and then stacked in layers of ribbons (for counts greater than 12), the ribbon-making and cable-making process becomes expensive. Depending on the fiber type and total fiber count, a loose-tube construction can save up to 40% on the initial cable price.

Furthermore, the flat nature of the ribbons can become an obstacle, with some installers reporting that high-count ribbon cables become stiff. Because of the stacked-matrix makeup, the cable has an inherent preferential bend, which means it will bend in one preferred axis. If twisted or pulled in the opposite direction of the axis, the fibers could become damaged. For this reason, manufacturers are working on alternative connecting methods that do not use ribbon-type cables.

The alternative to a ribbon cable is a loose-tube cable, which allows the fibers to lie freely within dry buffered tubes inside the jacket. Because the fibers are loose and not bonded in a ribbon matrix, they do not exhibit any preferential bend and can be flexed in any direction with significantly reduced risk of damage relative to the ribbon cables.

During the factory termination process, the loose fibers are first ribbonized and then terminated with an MPO-style connector. Another benefit of loose-tube trunk cables becomes evident when MPO-to-LC terminations are required. In this case, the loose-tube cables are already in loose form and do not require a “de-ribbonizing” process whereby ribbon fibers must be separated from the ribbon matrix before they can be terminated with LC-type connectors. The separation of 12 fibers from a ribbon matrix can further damage the fibers if this process is not performed correctly.

Loose-tube cables prove to be much more flexible over the tight bends in the concentrated pathways found in the data center environment. For example, trying to bend a 48-fiber ribbon cable around tight corners, such as from overhead baskets into racks or cabinets, may be difficult as the typical bend radius is 7.8 inches. The 6.9-inch bend radius of a more compact loose-tube construction can be twisted and turned without affecting the fiber, resulting in a more “forgiving” cable.

The tighter bend radius of loose-tube cables has additional benefits for slack loops. Slack loops for future applications are wound in fiber distribution panels. Ribbon cables have larger slack-loop diameters.

Size and toughness

Ribbon cables were designed for high-density applications, but their overall diameter is actually larger than a loose-tube cable with the same fiber counts. The loose-tube cable contains subsets of 12 fibers totaling a maximum of 18 subsets-or 216 fibers total-under one jacket. Compared to a ribbon cable, the loose-tube cable is less than half the weight and approximately 25% smaller in diameter.

Smaller optical cables can help prevent pathway and cabinet congestion, while enhancing airflow and facilitating more efficient cooling. Thicker cables that impede airflow can also introduce a fire-loading problem in data centers.

Ribbon cable is specifically designed for the controlled environment of indoor applications. When data center cabling runs underneath an access floor, it becomes susceptible to crushing and subsequent cable degradation. The Insulated Cable Engineers Association’s (www.icea.net) ICEA S-83-596 Standard for Optical Fiber Premises Distribution Cable defines a crush- and tensile-performance rating for an indoor distribution style cable. One way to mitigate the crush factor is through armoring. Interlocking armor is usually a spirally wrapped aluminum tape wound around the cable under the outer jacket. Armored cables are heavier, thicker, and more costly than those without armor.

An indoor/outdoor-rated cable offers the mechanical robustness of an outdoor cable with the flexible, smaller diameter of an indoor cable. It has an outdoor crush rating typically twice that of an indoor distribution cable, along with a robust tensile strength rating of 600 lbs. In installations in which the backbone cable also services the enterprise network outside of the data center spaces-such as between facilities-an indoor/outdoor cable typically has a far better operating temperature range, including a dramatically lower operating-range floor (~ -40º C) than a ribbon cable (~ 0º C). Because the cable can be installed both indoors and outdoors, there is no need for a splice termination point within 50 feet of building entrance.

The data center is evolving as new applications or blade servers are added, particularly in shared co-locations. Upgrading a ribbon-style cable or changing out connectivity is not a simple procedure. To remove the ribbon cable from the MPO connector includes “de-ribbonizing,” which sometimes can result in fiber damage. With a loose-tube construction, the MPO can be cut off and the cable can be re-terminated by fusion splicing a pigtail or via traditional field termination.

CAROL EVERETT OLIVER, RCDD, is the marketing analyst for Berk-Tek, a Nexans Company (www.berktek.com) and author of numerous articles on cabling topics. BENI BLELL,RCDD, is the fiber-optic product business manager for Berk-Tek with more than 13 years’ experience in fiber optics.