Fiber-optic premises network achieves cost-effective redundancy and flexibility

Oct. 1, 1996

Fiber-optic premises network achieves cost-effective redundancy and flexibility

A premises network architecture called fiber-optic riser for redundancy and cost-efficiency allows the construc tion of a fully protected fiber-optic riser system using fewer discrete runs of fiber cable while achieving more flexibility at less cost than available riser system methods

seth levine and patrick alesi

dvi communications inc.

The fiber-optic riser for redundancy and cost-efficiency (forrce) architecture, a new fiber-optic installation technique developed by DVI Communications Inc., solves the common riser system problems of numerous cables, inefficient allocation of spares and large amounts of expen-

sive conduit installed on every floor of a premises network installation. Developed in response to user requirements for a fully redundant, yet cost-effective, fiber-optic riser system, it has been successfully installed in both large and small buildings. In fact, after studying the forrce concept, two major manufacturers of fiber-optic cable, Lucent Technologies and Siecor Corp., have both approved and warranted the forrce design for use with their products.

The forrce architecture takes advantage of unitized cable, which is made up of individual bundles of fiber enclosed in a main jacket, and borrows techniques from the installation of 10Base-5, "thick" Ethernet network cabling and old copper telephone riser systems. A unitized cable contains a relatively high fiber-strand count. The strands, however, are packaged in bundles, producing a set of cables within a cable (see Fig. 1). This type of cable is available from major cabling manufacturers.

During installation, one unitized cable consisting of several subunits is run up the entire main riser shaft, leaving a "service loop" at each floor. At the top floor of the run, the cable is brought across to a secondary riser shaft and dropped straight down to the floor housing the fiber-optic main distribution frame. Both ends of this cable are terminated at the distribution frame.

At each floor, one or more subunits are extracted from the cable and cut in half. The end of each half is terminated, effectively establishing two individual fiber runs for that floor (see Fig. 2). Communications equipment on the floor is then patched to the distribution frame via any of the fiber-run routes, as is done in traditional redundant fiber-optic riser system designs.

At first glance, the forrce concept appears to be a ring architecture. This is true only during the cable pull. Once termination is completed, the topology is transformed into a star architecture. When the bundles are split out of the main cable and terminated, a star network is formed between the distribution frame and the closets. What`s more, the topology can easily be transformed into a ring architecture, when necessary, with little effort. Though the requirement would be rare, it is possible to install both a star-wired backbone for a Fast Ethernet or an Asynchronous Transfer Mode network and a ring backbone for a Fiber Distributed Data Interface network coexisting in the same cable. Indeed, the forrce design is both a physical star and a physical ring. Logically, it is a star, but it could be reconfigured into a logical ring as well.

Of course, the forrce design is not explicitly mentioned in the eia/tia-568 cabling standard. However, Section 5.2 of the standard allows for the accommodation of both star and ring topologies. In this manner, the forrce approach adheres to all the backbone standards under eia/tia-568, including having no more than three crossconnects between closets and no more than two levels of closet hierarchy.

Traditional risers

To clarify the forrce architecture, the design of a typical redundant, star-wired fiber-optic riser system is described. In most buildings with multiple floors, vertically stacked telecommunications closets are connected to a vertical riser system. On a typical floor, a large computer room or data center generally contains a myriad of switches, routers and other backbone-network equipment. Furthermore, this room also houses the fiber-optic main distribution frames for the building`s backbone communications cabling plant.

To connect each telecommunications closet to the distribution frame, a fiber-optic cable must be run up the riser from the frame to the closet. Each cable end is routinely terminated and tested, thereby fashioning a link between the closets. This straightforward process is repeated for every floor in the building. However, it does not protect against cable failures or a loss in a telecommunications closet.

If the main riser system is lost due to a cable cut, fire or other disaster, then all the communications systems in the entire building become inoperative. To prevent such a network outage, an alternate riser system is usually identified, located some distance from the main riser to minimize the chance of both risers being taken out of commission at the same time. For this setup, a second fiber-optic cable is run from the telecommunications closet, across a ceiling or under a floor, to the secondary riser and down to the distribution frame. Accordingly, this fiber cable is terminated and tested. The alternate riser cable process is repeated for each floor (see Fig. 3).

Upon scrutiny, though, some key disadvantages become apparent in the traditional alternate riser system. First, of course, is the high number of discrete cables that must be run. Each cable pull must be set up and done independently--an inefficient and costly procedure.

The second disadvantage concerns the distance the redundant cable must traverse to get to the secondary riser shaft. Often, this route passes through a ceiling and demands a long cable length, depending upon the floorplate of the building. The route must be carefully surveyed for each floor due to the vagaries of interior building design.

Fiber protection

Third, the fiber probably represents the communications infrastructure of the company as well as the backbone network. Consequently, the fiber must be fully protected. Protection typically involves placing rigid conduit wherever the fiber runs in the ceiling, and in each floor. Rigid conduit is expensive, whether it is traditional electrometallic tubing or fiber-optic corrugated conduit (also known as innerduct, or interduct) for virtually all fiber-optic cable interior installations.

Lastly, testing is a vital, expensive and time-consuming part of a fiber-optic installation. In traditional premises networks, each strand must be terminated and tested over its course from the closet to the distribution frame. This testing generally takes two technicians, one positioned at each end of the cable. In contrast, forrce risers require only one technician for installation and testing.

To demonstrate the neat installation possible with a forrce riser, consider as an example the layout of a six-story building. Premises cabling plant requirements specify 12 fiber strands on each floor. Moreover, riser redundancy is mandated, thereby calling for 24 fiber strands terminated in each telecommunications closet.

Based on these premises needs, the network pro vider orders a unitized cable consisting of 6 bundles of 12 fibers each, for a total of 72 strands. Note that this total represents a standard product offering from most major fiber-optic cable manufacturers.

During installation, the fiber-optic cable is run up from the distribution frame and stops at each floor. There, a service loop of about 50 feet of cable is pulled out from the riser. (This method was commonly used for many years to install 10Base-5 thick Ethernet networks; it exposed "tick marks" on the cable where taps could be installed.) The pull continues from floor to floor until the top floor is reached. Here, the cable is pulled across the route, either through the floor or ceiling, to the secondary riser and then dropped straight down to the distribution frame.

At this point, the ends of every fiber strand in the cable are routed and dressed into the distribution frame. Next, the strands are both terminated and tested by one technician, because all the strands are available at one place. The entire premises fiber cabling plant is, therefore, cost-effectively checked.

Subsequently, at each telecommunications closet, the main jacket of the cable is slit open. One cable bundle subunit is isolated and cut. Both fiber cable ends are terminated at the closet equipment rack. This procedure results in two cables. One cable runs from the closet down to the distribution frame through the primary riser; the second cable travels up through the primary riser, across to the secondary riser, and back down to the distribution frame. These new fiber terminations are tested again by one technician by jumpering the strands in the distribution frame, re-creating one cable and testing both ends in the closet.

More flexibility

In addition to installation merits, the forrce design also increases the flexibility of the premises fiber-optic cabling plant. For example, assume that an installed six-floor premises cabling plant contains 12 redundant fiber strands in a traditional riser design. Then, consider the case where the network user wants to increase communications capacity by adding two more fiber strands on a particular floor. In one option, two of the redundant fibers could be used, but this allotment translates into fewer disaster recovery capabilities. Another op tion would be to pull more fiber cable, but this ap proach would be costly and time-consuming.

Suppose a forrce installation is in place. In this architecture, every fiber strand runs through every floor. User demand for more network capacity merely involves the identification of an unused pair of fibers on another floor and the reterminating of the fibers on the floor where they are needed. The unused fiber pair is identified and jumpered in the original telecommunications closet.

To obtain this extra capacity, the appropriate subunit is slit on the floor location where the extra strands are needed. Then, the fiber strands are isolated and terminated. In this manner, the fiber pair has been "borrowed" from one floor and redistributed on the new floor. This approach is not possible with a traditional premises design. Granted, installation skill and care are needed to cut the proper strands. However, the "borrower" approach is less expensive than pulling more cable and does not sacrifice redundancy.

Another way to solve the capacity problem is to use a cable that contains one more subunit than there are floors in the building being wired. This extra subunit is only terminated and tested in the distribution frame, and runs the entire length of the premises network through all the floors. Used as a "floater" cable, it can be isolated and terminated on any floor at any time. This method obviates the need to pull extra capacity to every floor and then possibly not use it. The extra subunit means having potential additional capacity everywhere.

Disaster strikes

In a premises network disaster such as a flood or fire that renders the distribution frame unusable, the telecommunications equipment in most installations is relocated to or replaced on another floor. In a traditional premises network, new fiber cable is generally installed. There is no practical way to link the equipment in the telecommunications closets to the data center or main computer room.

In a forrce installation, any telecommunications closet can be used as a backup distribution frame. This universal replacement capability is possible because every fiber passes through every floor. Once a backup location for the distribution frame has been identified, the unitized cable is slit open, and the bundles feeding the other floors are isolated and terminated only as necessary. The distribution frame can be made operational again in just the time it takes to perform the terminations.

New installation methods allow secure, low-loss fiber terminations without the need for oven curing, polishing and extensive training. For most disasters, it would take longer to move the electronics equipment to a new location than it would to get the substitute distribution frame online. A pre-stored "crash kit" containing all the necessary connectors and equipment helps speed disaster recovery. When the original distribution frame is rebuilt, the technician only has to jumper or splice the strands in the backup location to restore the premises links to their original connections.

Sheath footage

In premises networks, the sheath footage of the installed fiber-optic cable is an important cost. Sheath footage is the length of fiber-optic cable pulled, regardless of the size of the cable. It is directly related to the cost in labor of pulling the cable. The less sheath footage used, the lower the installation cost.

Compared to traditional riser installations, the forrce architecture results in lower sheath footage used (see Table 1). Note that one assumption made is that the floors are contiguous. Where the floors are not contiguous--for example, where the distributed frame is emplaced several floors below the telecommunications closets--sheath footage in a traditional premises network design increases greatly because of the extra cable length needed to reach the serviced floors.

The forrce architecture results in an order of magnitude decrease (based on the number of floors) in sheath footage. As the number of floors increases, the sheath footage increases (due to increased labor costs) at a much slower rate than in traditional designs.

Strand footage

Another important premises network cost is strand footage, defined as the overall length of the fiber strands if they were all fused end-to-end. In most network installations, fiber-optic cable materials are priced according to strand footage.

Here, the forrce architecture is at a slight disadvantage because every strand must pass through every floor. The cost trade-off is that having every strand available on every floor increases the flexibility of the premises network design (see Table 2).

Concerning strand footage, an interesting effect occurs where spare capacity is added. For example, if four extra strands were added to the riser system, they would protect against future changes in technology or in new communications applications. Such a cable in a forrce installation means the addition of a four-strand subunit that would run through the entire riser system. In a traditional premises network design, an individual four-strand cable would have to be run to every floor. The forrce design increase in strand footage is therefore linear, whereas the traditional premises design increases strand footage nonlinearly.

Conduit footage

In wiring a premises network, the installation of horizontal cabling and, therefore, conduit, whether rigid or flexible, is costly in terms of both labor and materials (see Table 3). Note that the forrce design uses only one horizontal run across the top floor of the installation. Therefore, the horizontal conduit requirement is independent of the number of floors in the installation. In a traditional premises design, the horizontal conduit requirement is linearly related to the number of floors in the installation. u

Seth Levine, director of infrastructure systems at DVI Communications Inc., New York, NY, developed the concept of the forrce design. Patrick Alesi, senior technical staff member at DVI, provided technical assistance in the production of this article, and Ben Occhiogrosso, president of the company, contributed to its writing.

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