Abstract

Pirelli Communications Cables and Systems recently developed a high fibre count ribbon distribution cable called COLink™ for premises applications. The cable utilises 12 fibre ribbon stacks inside gel-free buffer tubes stranded around a central strength member, and is either OFNR or OFNP rated. The plenum (OFNP) version may be used for riser applications as well, as it exceeds GR-409-CORE and ICEA 596 riser mechanical and environmental specifications. Fibre counts of up to 576 fibres are available in either riser or plenum versions of the cable. Stranded design for ribbon distribution cables offers several advantages over a central tube design. The advantage of individually protected sub-units translates into improved handling and minimises accidental damage to ribbons. Multiple ribbon sub-units in the cable also reduce cost of installation and complexity by limiting the need for furcation tubes within a rack. Buffer tubes can simply be routed from the entry point of the rack to individual panels before exposing the ribbons for individual termination. The stranded nature of the design allows for greater flexibility and kink-resistance, as well as the capability of handling greater tension; and the multiple sub-unit design results in greater compression and impact resistance than traditional central tube designs. This increased overall cable robustness is advantageous when protecting vital revenue generating data streams.

The biggest design challenge in the development of the cables that utilises gel-free buffer tubes for premises applications is GR-409-CORE compliance to impact and compression resistance, as well as environmental performance. The gel-free nature of the buffer tubes presented various processing challenges as well, particularly with respect to manufacturing repeatability. Careful consideration to minute design and processing details need to be made in order to fully maximise quality without compromising manufacturability and cost.

Introduction

Ribbon distribution cables are utilised in various network locations. One of the most common applications is in transitions from OSP environment to the premises where NEC regulations required flame ratings and clean, quick terminations are desired. In this function many of the cables are preconnectorised to panels in stub assemblies where the other end of the cable is simply fusion spliced to the OSP cable. Other applications include use as back-plane jumpers, network backbone, and parallel optics transmission. These cables must have some of both the OSP and premises cables. This means that in addition to exceeding Telcordia GR-409-CORE and ICEA-569, cables pre-tested to tensile strengths of 600lbs per Telcordia GR-20-CORE. The high fibre count density attained using ribbon, the limited duct space available in many institutions, CO's, POP's, etc., and the increasing acceptance of OSP ribbon cable all contribute to an increasing use of ribbon cables in the premises. Space is at a premium in many CO's and the more you occupy the more it costs. One way to limit that cost is to increase the port density in the racks. Traditional tight buffered cables are predominate in legacy systems, but are much too large to support this capacity increase. Ribbon distribution cables, especially stranded design, are specially suited for the application. Utilising the individual buffer tubes to route inside the racks and panels the ribbon is continuously protected until final termination within the panel.

As ribbon distribution cable gained popularity the fibre counts increase to meet the demand for new applications. Ribbon distribution cables originally designed as central tube product has increased in fibre count to such a degree that a stranded design must be used for maximum benefit. Central tube design cable is cost-effective and extremely functional for fibre counts of 12 to 216 fibres and when running from point to point. Once the fibre counts exceed 216 fibres or more complex point to multi-point connections is required stranded design ribbon distribution cables become advantageous. One reason for this is the port density of patch panels currently limited to 216 fibres and many times 72, 96 or 144 count. One can immediately see the benefit in using a standard ribbon distribution design with limited duct or back-plane access. For example; four 144 port patch panels can be preterminated with one 576 fibre ribbon distribution cable using the individual buffer tubes to route to each panel where as four separate 144 fibre central tube cables would have to be used.

Design

When first tasked with developing stranded ribbon distribution cables several technical specifications including Telcordia GR-409-CORE, Telcordia GR-20-CORE, ICEA-596, as well as customer specifications were taken into consideration. Much more emphasis was placed on customer requirements, but standard specifications were maintained for wide acceptance of product offering. Tensile strength was of major concern for the customer as they expected the same crews that install the OSP to handle the ribbon distribution as well. Another concern was equality between riser and plenum specifications, so the decision was made for both cable ratings to meet the more stringent riser mechanical and environmental specifications.

Several cable designs were considered, but from the beginning the customers' end use of the product was kept in mind. Divisions of 72, 96, 144 and 216 fibres were of utmost importance in the design process, as they would mate with current panel port densities of the most popular panel manufacturers. Using these fibre counts and realising the space constraints associated with most applications a four-position cable design was decided upon. Both a 288 version with 72 fibre sub-units and a 576 fibre with 144 fibre sub-units were chosen. These fibre counts and tube configurations covered any combination of panel port densities.

Figure 1: Pirelli COLink™, stranded ribbon distribution cable

Capitalising on Pirelli's extensive knowledge of high fibre count OSP ribbon cable development; process technology and equipment was utilised to develop the stranded ribbon distribution cables. Some of the more unique characteristics in contrast to OSP products are the gel-free tubes and flame ratings requiring flame-retardant buffer tube materials.

Development

Buffer tube material selection was the first development consideration and the most important. The tube material is critical in determining the performance of the cable. All material attributes were reviewed and considered. The main characteristics used though were cost, burn characteristics, coefficient of thermal expansion (CTE), and processability. The two basic materials looked at were PVC and PVDF; both have good CTE and process characteristics, but differ substantially in cost and burn characteristics. As shown in Table I, PVC is low cost, but is limited in flame performance for plenum appliances. PVDF is costly but excels in burn characteristics.

Table 1: Comparison of PVC and PVDF

Ultimately the PVDF compound was chosen for the plenum rated cable design and PVC for the riser cable design based on the NEC rating required. Special consideration was made to process conditions during manufacture of the buffer tubes. As with most fibre optic cable designs, the buffering process is the most critical as it the first operation in making the cable. Any adjustments to buffering can and do transfer to later processing steps with significant outcomes. Some of the variables monitored included the ribbon payoff tension, ribbon pitch, tube ovality, and excess fibre length (EFL). Payoff tension is used to provide an adequate amount of back tension such that the ribbon stack maintains a correct orientation and twist.

Figure 2: Twisted ribbon stack prior to entry into buffer tube

The unique feature of these cables is the gel-free tube, which caused some changes to the normal process of producing buffer tubes. In a gel filled tube the gel helps to hold the ribbons in position once placed in the tube. Without gel the tube was collapsed at the beginning and end of the buffering run in order to hold the twist induced in the ribbon stack during manufacturing. These sections are later discarded after jacketing.

Figure 3: Buffering tube protection

Ribbon pitch, tube ovality, and EFL all contribute to the cable performance during environmental testing and were monitored closely to insure consistent product. EFL is the most important and is controlled mainly during the stranding process. Ribbon pitch is the distance between twists placed in the ribbon during buffering. The pitch allows for the cable to be bent in any direction without adversely effecting the ribbon. Stranding and jacketing operations are well understood and are fairly straightforward. It is important to note though that the reverse oscillated lay (ROL) stranding places the ribbon in a configuration such that the amount of excess fibre length is not to much as to adversely affect environmental performance, but is great enough to eliminate stress during tensile loading. As one can see from Figure 4. Below the ROL stranding process wraps the tubes back and forth around a Central Strength Member (CSM). The ROL also, as with OSP cable, allows for mid-span access, which is not capable with taught sheath central tube cable installations.

Figure 4: Reversal point in reverse oscillated lay (ROL) stranding

A cable jacket primarily serves as mechanical protection for the cable components, but in NEC rated cables it must also be highly flame retardant to meet the stringent requirements of burn testing. Again, as with the buffer tube material section, two materials, PVC and PVDF, were evaluated. PVC is flame retardant but not as much as PVDF, however PVDF is very expensive, typically in the range of eight times greater than PVC. Costs of this magnitude are prohibitive for use in larger quantities such as jacketing without a significant benefit. After reviewing the attributes of both, the PVC compound was chosen for the riser design to reduce cost and the PVDF compound was chosen for the plenum version to consistently meet NFPA 262 (a.k.a. UL 910) test requirements.

Testing

Once design and development was completed the cables were prototyped and readied for testing. Prototypes were constructed of both the 288F design and 576F design in riser and plenum. Results from testing are summarised in Table II. From the data shown, one can see that previous experience with ribbon and OSP ribbon cables was quite beneficial in the immediate success of the design.

Table II: Test results

Field installation

Acceptance of design in the market place was quickly confirmed when several orders were placed for 216 and 288 fibre cables by major RBOC and CATV customers only one month after its product release. Installation of the cables ranged from backbone runs from central office to co-location equipment and stub assemblies from the OSP vault rooms to patch panel racks several floors above. One such installation involved cable runs in 1.25" duct from manhole to the equipment rooms with excellent results. Lengths of cable installed were 600; 1,100' and 1,450', which validated the requirement for 600lb pull strength as an initial design requirement. Since the cable design borrowed heavily from traditional OSP products, Installers were able to use existing tools and techniques to pull, access, and terminate the cable.

Conclusion

Continuing trends of higher patch panel port densities, developments in parallel optics, and increased space constraints are driving use of ribbon and ribbon distribution cables further into the network. Regulations and concerns over flame propagation in highly congested vault rooms, CO's, POP's, etc. will make the need for NEC rated cables a necessity. Efficiencies gained in OSP construction from mass fusion are also realised in premises environments. Mass termination connectors, such as MTP, MPO, SMC, etc. are being utilized in the network making ribbon a necessity. All of these elements combined contribute to the increasing demand for high fibre count ribbon distribution cable in the premises. Attention to detail, thoughtful design consideration, and use of tried and true processes made for a smooth product development. The ribbon distribution cable utilising a stranded design was found and proven to incorporate the functionality our customers needed in a package that was easily manufactured. Through application of proven design, processes, and attention to customer specifications a successful; development project was completed exceeding all requirements.

This paper was delivered at the 51st seminar IWCS
Coronado Spring, Florida, USA - November 2002
Printed by courtesy of IWCS - © IWCS 2002

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