Making connections with network ribbon cables
SPECIAL REPORTS: Fiber & Cable
Tomorrow's networks are here today in the form of all-optical switches and routers. Network ribbon cables are making the connection between these complex components and are key to making the optical transport of information free from electronic conversion. Network ribbon cables are more then just another fiber-optic jumper.
Network ribbon cables are used with parallel optics, allowing for massive cumulative data-rate transfer per cable. Current component technology transmits from 1 to 2.5 Gbits/sec per fiber on a typical 12-fiber ribbon, keeping pace with OC-192 (10-Gbit/sec) networks.
Emerging technology allows four or more channels per fiber, which will keep pace with OC-768 (40-Gbit/sec) networks. By using low-cost, readily available vertical-cavity surface-emitting lasers (VCSELs) and multimode fiber, network designers can afford to interconnect all components in a central office (CO), data center, or other point of presence (PoP), while saving space, time, and money.
Ribbons are the core of cable design and form the physical structure through which light travels. In basic terms, the ribbon is made from two to 12 standard 250-micron optical fibers held together in parallel with an acrylate matrix. Beyond this basic description, it is necessary to characterize the geometry and the speed of light traveling down each fiber.
Ribbon attributes and values in the Table are taken from Telcordia GR-20-Core, Issue 2, and Insulated Cable Engineers Association (ICEA) S-83-596-2001, Third Edition, which are the main specifications used for ribbon testing and compliance. These values represent the maximum dimensions found in most ribbon products.
Planarity is the distance any individual fiber varies from the center line of the ribbon. It is highly dependent on the individual manufacturer's ability to control its manufacturing process. Ribbon dimensions, especially outer dimensions, vary with the amount of matrix material applied but are typically less than the stated values. Extreme fiber width is measured from the center of the first fiber to the center of the last fiber. This dimension is critical, since the ribbon is usually mated in an MT ferrule with extremely tight tolerances.
The precision with which a manufacturer can make a ribbon cable is directly reflected in the skew values measured. Skew is the difference in time it takes for light to travel down one fiber in a ribbon as compared to another fiber. The skew value is expressed in picoseconds per meter (psec/m) and is specified as a maximum value. A typical value is <6 psec/m. A picosecond is equal to 0.000000000001 sec, or one-trillionth of a second.
Separation of individual fibers from the ribbon is also a key characteristic of a well-balanced manufacturing process. Though the advantage to ribbon cables is mass termination of fibers using MT ferrules and MTP-, MTX-, and MPO-style connectors, many assemblies are hybrids, with one end connectorized with legacy connectors.
Legacy connectors require that each fiber be separated from the ribbon, then connectorized. Since many ribbon-cable assemblies are made to mate up with existing systems and thus legacy connectors, ribbon separability is a key design characteristic of any network ribbon-cable product.
Separability is defined in Telcordia GR-20, Issue 2, Sec. 5.2.2, and ICEA S-83-596, Third Edition, Sec. 7.13, as the ability to remove individual fibers from the ribbon matrix material. By using furcation tubing on the individual fiber separated from the ribbon, legacy connectors like SC, ST, FC, Escon, and D4 can be attached to the assembly. Additionally, new small-form-factor (SFF) connectors (LC, MT-RJ, VF-45, LX.5, OptiJack, etc.) for systems requiring increased configurability, but with less space requirements than legacy systems, can also be used with network ribbon assemblies.
For installations requiring repeated handling and reconfiguration, it's recommended to use a ribbon interconnect cable for the assembly. Ribbon interconnect cables are quickly becoming the jumper cable of choice when connecting hardware in a CO for parallel optics and anywhere large data transfers at gigabit speeds are required.
The cabled version of the ribbon adds strength members (aramid) and a protective jacket. Also, the cable is National Electrical Code (NEC)-rated to meet indoor fire codes. Ribbon interconnect cables are typically preconnectorized and come in standard lengths of 3, 10, 30, 50, and 100 m with custom lengths available. The average link length between components is about 35 m with more than 80% of assemblies <100 m. These short runs are sometimes referred to as very-short-reach (VSR) installations and are common in most COs, data centers, and PoPs.
Although the addition of strength members and a protective jacket seems simple enough, variables related to this process need to be taken into consideration. The jacket material used is a major determinant in the NEC flame rating given to a particular cable. The jacket also imparts certain characteristics that allow for ease of handling and, more important, higher performance and ease of connectorization, termed "craft-friendliness."
An NEC plenum rating allows the product to be used in any building space, including areas used for air-handling. The NEC riser-rated cables are restricted to non-plenum applications and limited in use. Another NEC rating used extensively in areas with high equipment expense is low-smoke zero-halogen (LSZH) cables.
LSZH cables are typically riser-rated and do not contain halogen elements such as chlorine and fluorine. These elements, when combined with oxygen and in the presence of moisture, produce acids that can etch sensitive optical components. It's easy to see the value in having cables that do not produce acid gases when burned in the high-cost network environment. By carefully choosing jacketing materials, which offer excellent flexibility while maintaining the desired NEC rating performance, manufacturers can provide superior performance in a craft-friendly package.
Aramid adds pulling strength and allows for connectorization of the cables. The aramid is crimped to the connector and relieves the fibers from incurring any undue stress from mating and unmating to equipment. The amount and texture of the aramid used can be a determinant in the performance of the cable and ease of the connectorization process.
Additional considerations need to be taken into account when designing a system using network ribbon cables. One option that should be considered is the cost related to using singlemode versus multimode lasers for VSR applications. Although cable is a significant component of any installation, the majority of costs are in the equipment used.
Therefore, selection of equipment for the near- and long-term goals of each installation is critical. Multimode lasers in the form of VCSELs operating at 850-nm wavelength are currently far less expensive than singlemode lasers operating at 1310- or 1550-nm wavelength.
VCSELs produce a precise cone of light that is coupled efficiently into an optical fiber (see Figure 1), as compared to the older edge-emitting lasers. VCSELs are also based on tried-and-true wafer fabrication processes. So with the economics of VCSELs, the use of multimode fiber is a simple decision for most network designs. This decision is also backed by several parallel optics standards bodies.
Another family of network ribbon cables used in COs, data centers, and PoPs are ribbon distribution cables. These cables form the access point or backbone component of the network installation. As access cables, they provide connection from the outside plant (OSP) long-haul network to the switching and routing equipment inside. In the form of stubbed-out assemblies (see Figure 2), these cables provide an easy transition from OSP to network connectivity.
Additionally, within the premises space, when large amounts of information are to be transported from one piece of equipment to another in a limited amount of space, these cables provide a craft-friendly, compact solution. These NEC-rated riser or plenum cables offer greater flexibility and reach than the limited indoor capabilities of OSP designs. In fact, by using ribbon distribution cables from the cable vault, costly splices associated with transitioning from outdoor to interior spaces can be eliminated. Ribbon distribution cables also offer greater fiber density than traditional tight buffer designs, freeing space for additional capacity and equipment needs.
Roger Vaughn is a product manager with Pirelli Communications Cables and Systems USA LLC (Lexington, SC). He can be reached via the company's Website, www.na.pirelli.com/fiberopticcables.