Composite cable carries mixed media
ARLYN S. POWELL, JR.
Combining both fiber and copper in one cable seems optimal, but service needs mandate concern
Take a combination of cabling media, such as singlemode and multimode fibers and shielded and unshielded twisted-pair copper wires, and manufacture them all into one cable; the result is known as composite cable. The advantages appear impressive upon initial installation, but when maintenance and modification are needed, the shortcomings surface.
"Composite cable has been around for a while," says Ken Pimental, vice president for sales and marketing at Remee Products Corp. in Florida, NY, "but a lot of users don`t know about it. It`s growing in popularity, though, because it lets you gear up for fiber-optic technology. Even though you might not be ready for fiber, you can put composite cable in the closet. Then when you want it, you won`t have to go back and pull another cable."
Most network providers pull separate fiber and copper cables. But composite cable offers installation advantages over this method. According to Pimental, pulling several separate cables instead of just one composite cable is more time-consuming and expensive. He comments, "Imagine your task is to route individual cables into a new floor of offices. You have to supply telephones, fiber interfacing, computers and color monitors at, say, 250 different desks. Super-VGA color monitors alone require three coaxial cables for the red, green and blue signals."
Meeting all these needs with a single jacketed cable can still prove challenging, but an alternative multiple cable method is available. A skip binder is a web that holds multiple cables together in a bundle. At the closet or workstation, the skip binder is slit open, and the cables are fanned out for easy termination.
Whether the jacketed or the skip-binder approach is implemented, a single cable is easier to pull and takes less time than using multiple cables. "With all the individual media contained inside a single length of cable," notes Pimental, "there is no movement or twisting inside the cable as it is pulled through a conduit, shaft, floor or ceiling." In addition, fewer cable reels and less installation equipment are required.
Presently, though, composite cable is a specialty item. To purchase a composite cable, users have to determine their present and future cabling needs and order a single cable that is custom-manufactured for their network applications.
Charles Amann, product manager at CommScope Inc. in Claremont, NC, sees an increased demand for composite cable stemming from the convergence of voice and data communications. Viewing composite cable as containing singlemode and multimode fibers, Amann remarks, "Users are looking at multimode for their local area network backbones, but they are also looking at some voice applications requiring singlemode."
Amann also claims that some network planners are looking ahead to extremely high data rates or long-distance communications links that may be beyond the capability of multimode. According to Amann, there are no particular manufacturing problems with combining fiber media in the same jacket. Singlemode and multimode fiber cables are similar in construction, and their designs have been thoroughly tested and standardized.
"The only problem it presents," declares Amann, "is that users want different kinds of cable. One user wants six singlemode and 12 multimode fibers, and the next user wants four singlemode and 18 multimode fibers."
Care is needed, however, when dealing with composite cables containing both fiber and copper media. Primarily, the cable must be designed so that the fiber is not damaged by the copper, which would lead to detrimental fiber microbending and signal loss. To prevent internal abuse, crush resistance is usually added to the fiber strands during the manufacturing process.
Moreover, composite cables must be cautiously installed. "In combination," Amann declares, "the cables are a little bit bigger, and they`re harder to pull. You have to be more careful during the installation." According to Amann, the bend radiuses of the fiber and copper conductors are similar, but fiber has a higher pulling tension.
Amann claims that multiple-fiber composite cable is competitively priced with other fiber cables of similar size. He cautions, however, that because composite cable is a specialty item, a minimum cable length has to be ordered to justify a production run. (Typically, this length is approximately one kilometer.)
Composite fiber/coaxial cable, on the other hand, is more expensive than separate cables that have similar composition. The added cost results from the scrap rate, which is higher at the beginning and end (and sometimes in the middle) of production runs, and because the copper-fiber mix requires tighter manufacturing tolerances.
There are other issues that installers and network managers should be aware of when they choose composite cable, says Terry Coffman, senior product planner at AT&T Network Cable Systems in Norcross, GA. In practice, he has found that detailed documentation of composite cable installation and modification is needed.
Says Coffman, "The initial pull is fun, but you have to go back and do the administration and record what cables and fibers went where. That`s where the foul-ups seem to occur." For example, consider a cable move, addition or change six months after a composite-cable installation. The user must be able to distinguish the singlemode fibers from the multimode ones, as well as the buffer colors.
Consequently, the network records must be correct, up-to-date and readily available. Because composite cables are specialty items, industry-standard formats are nonexistent for specification information.
Adding a composite cable at a later date to expand the network or to repair a break mandates the reordering of a specialty item. A reorder can take four to six weeks, depending on the manufacturer`s schedule. In contrast, distributors often provide same-day service for standard items. Specialty reorders are more expensive than standard types because of the costly customized setup and production run.
Coffman advises that hybrid fiber/coaxial cables dictate cautious design. "Copper expands when it`s heated," he says, "but glass fiber doesn`t. Copper also contracts when it`s chilled, but glass fiber doesn`t."
These characteristics control the makeup of the jacketing of both the fiber and the copper to guard against fiber microbending and copper twisting, all of which affect the layout of the composite cable.
From composite cable experiences, Coffman describes a common late-Friday-afternoon scenario. "We get a typical panic call," he says, "that the communications cable has been accidentally cut. All the computers are down. A cabling installer is working overtime to repair the break. However, it`s a special composite cable without traceable documentation.
"Eventually, we send them to a distributor for standard singlemode and multimode fiber. They splice these in rather than wait for replacement composite cable, because they have to be up and running on Monday morning."
Coffman thinks this scenario would go away if a standards body would address composite cable. "It`s different each time, and with each installation," he says. "That`s where we get into trouble. With standardization, the cable administration would flow. The whole thing would work out."
Until standards are approved, composite cable remains a nonstandard item, with all the advantages and disadvantages that custom manufacture implies. q
According to its vendors, composite cable reduces installation time, occupies less space in the cable run, eliminates backtwisting, enables unlimited construction and accommodates future technologies.
Composite or Hybrid?
A troubling issue in many high-technology industries is nomenclature, and the cabling industry is no exception. Take the question of composite versus hybrid. A respected textbook in the field defines composite cable as any cable that mixes media, whether it is shielded and unshielded twisted-pair, copper and fiber, or singlemode and multimode fiber. The definition that appears to be coming to the fore, however, is that composite cable contains two fiber media types within the same jacket. In this case, the term hybrid is reserved for cable that contains both copper and fiber media.
Ken Pimental, Remee Products Corp.
Backtwisting occurs naturally in all types of cable because of the manufacturing process, and it can create operational difficulties. Specialized equipment is available, however, to prevent this phenomenon in composite cabling: A microprocessor and other equipment installed in the cabling line neutralize and synchronize the effect 18 times per minute.
When cable is manufactured, usually one machine--the payoff reel that houses the cable components--remains stationary, while the cabler, which combines the components and puts the finished product onto the takeup reel, is in motion. This built-in tension causes cable to backtwist. If left unchecked, backtwisting can damage the individual cable components--or even cause the cable to break.
A simple example of backtwisting sits on your desk. Like cabling machinery, your telephone has a stationary part--the dialing pad--and a moving part--the headset. When the headset is picked up and moved, the phone cord can become tangled, or backtwisted. That is because the cord is connected to an immobile object, and tension is built up.
Ken Pimental is vice president for sales and marketing at Remee Products Corp. in Florida, NY.
Choosing a Cable Format
Scott Stevens, CommScope Inc.
Typically, there are two options when mixing media types. The first is to put all the cable elements under a single jacket to make a round cable; the second is to extrude the media as separate legs attached by a thin web or binder. When using this second, or multileg, format the cables lie side-by-side in Siamese (or triamese) style.
Each option has its pros and cons. Round cables are easier to pull. They can create cable management problems at the closet, however, because a significant length of jacket may have to be stripped to route the different media to their respective termination points. Multilegged cables are flexible in only one plane and have an increased surface area versus round cable, both of which make cable pulls more difficult. At the closet, though, each leg can be zipped apart, making it easy to route cable elements without affecting their integrity until you reach the termination point.
The cost for each cable type can vary, depending on the type of production run. Multilegged cables typically use less material, giving them a cost advantage if the combination of media is standard. With a custom design, round cable may have a cost advantage because the tooling cost for multilegged cable is higher.
Scott Stevens is an application engineer at CommScope Inc.`s Network Cable Division, Claremont, NC.
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