A Horizontal Fiber to the Node (HFTTN) architecture can relieve cable pathway congestion. It can also provide a reliable and cost-effective means to connect users.
December 1, 2000
Traditional structured wiring has required a proliferation of cabling in the horizontal pathway. As a result, the horizontal pathway has become congested and difficult to manage over time.
There is an alternative. A standards-compliant hybrid fiber/copper architecture can eliminate these pathway problems, as well as provide a reliable, cost-effective means to connect users in the enterprise. As the industry moves toward Voice over IP and other higher speed data networks like Gigabit Ethernet, it is necessary to reexamine how cabling systems are designed and installed.
TRADITIONAL STRUCTURED CABLING
In a traditional structured cabling environment, as defined in ANSI/EIA/TIA-568-A, a minimum of two 4-pair cables must be run to each outlet. In most environments, the number is actually much higher. For most medium- to large-sized companies, the number of four pairs that are run to each work area is three or four. In select industries, such as the financial sector, the number is even greater.
For an average-sized floor that houses 100 users, as many as 400 cables must be pulled. Given an average outer diameter of .24 inches per cable, a 400-cable bundle would have an approximate diameter of almost one-foot. However, the problem is even worse when conduit and cable tray fill ratios are applied. In different regions, fill ratios can range anywhere from 30 to 50 per cent. The number of four-inch conduits required for 400 cables in the worst case would be ten. Similarly, a cable tray would also need to be quite large. These issues apply to all forms of horizontal pathways, whether under floor, raised floor or perimeter systems.
Congestion at the Closet
Since most telecom closets are in the riser or core of the building, they traditionally have fire rated walls. This means all penetrations of these walls must be sealed with a listed firestop system. In this scenario, either ten four-inch conduit sleeves (or one large cable tray) would have to be installed. In either case, the firestop system used makes it difficult to install new cables or make other changes.
The most common pathway used in commercial buildings is within the drop ceiling. Cables are suspended in numerous ways, including straps, J-hooks or, in some cases, with no support at all. Recently, we have seen the increased use of wire mesh or “light” cable tray systems that can be bent and formed to fit. These systems are less costly than traditional ones, but still require significant labour costs to install. Additionally, in many installations there is little room for such a system.
With 400 or more cables in the ceiling, cables will likely be added and removed over the years by a variety of contractors and users. This typically contributes to significant clutter and soon makes it very difficult to change the system. For these reasons, using large numbers of horizontal copper cables may not be the best way to distribute data throughout the enterprise.
Pathways in Modular Furniture
A large percentage of offices utilize systems furniture. Some of these systems provide a pathway within the furniture for low voltage cabling. Recent industry rulings have stated, for safety reasons, that cables may not share the same pathway with modular power harness systems. In order to resolve this issue, some form of external pathway system must be utilized. The external pathway must combine ease of installation with manageability of the cable and the ability to be easily reconfigured with the furniture.
There are now three additional cabling methods documented in the standards as alternatives to the traditional home-run approach: MUTOA (Multi User Telecommunications Outlet Assembly), Consolidation Point and Centralized Fiber Architecture.
Promoted by the modular furniture industry, ANSI/TIA/EIA 568- A TSB-75 defines cabling guidelines for open office (systems furniture) environments and specifies two different methods for distributing cabling — MUTOA and Consolidation Point.
The MUTOA architecture places a multi user outlet at each work group and services the users using long station cords. This approach eliminates the numerous outlets in each cluster of cubicles and makes it easier to reconfigure them. However, this does little to alleviate the clutter in the pathways above the ceiling.
2) Consolidation Point
Consolidation Point architecture maintains that all cable can be terminated (in effect, spliced) at a point close to the work areas. The benefit of this is that only a short length of cable has to be replaced during a reconfiguration of furniture, rather than the entire home run back to the closet. This also does little to alleviate pathway congestion.
3) Centralized Fiber Architecture
Standards have also defined a centralized fiber-based architecture that takes advantage of the longer distances fiber is able to realize.
Since fiber cable can run longer distances than copper, all users in the building are connected back to the main distribution frame directly. This reduces the number of closets needed to serve a building, improves administration, and helps with the pathway problem, as the fiber cables are smaller than the copper cables. Additionally, in a converged voice and data environment, only one cable per work area has to be installed. Today however, Voice over IP is rather immature, and legacy voice systems are still being supported. Therefore, at least one 4-pair cable must be installed along with the fiber in order to support the legacy voice traffic. Based on the earlier scenario of 100 users, the pathways would still have to support 100 fiber cables and 100 4-pair cables.
Of the three alternatives outlined in the standards, the centralized fiber approach cuts the number of cables in the horizontal distribution path in half. Yet, there will still be problems over time as moves and changes are performed. The cost of this solution is also significant, since fiber optic NIC cards are still considerably more costly than those using a copper twisted pair interface.
HORIZONTAL FIBER TO THE NODE (HFTTN)
Let’s take a look at a hybrid fiber/copper approach. All of the proposed solutions above are based on the premise that the network electronics remain centrally located in a main distribution location or a Telecom Closet (TC). A hybrid approach, in which the backbone is extended to network electronics distributed around the enterprise, offers advantages to the pathway and improves other aspects of the cabling system. HFTTN is also fully standards-compliant.
HFFTN and Legacy Voice
In a converged voice and data environment, the HFTTN approach replaces 400 cables in the horizontal pathway with twelve smaller fiber optic cables. In the near term however, we must address the legacy voice systems. Current standards do not make a distinction between voice and data traffic. The goal of the standards is to have one media that can support all services. However, now that convergence is imminent, it is time to separate voice requirements again. The hybrid architecture supports legacy voice by running larger cables to the nodes (extending the backbone) and distributing the voice from these points to the workstations.
By reducing the number of cables from several hundred to approximately 12, the ability to manage them becomes simple. Each node would be serviced by one pair of fibers (or two if redundancy is desired) and one 25 or 50 multi-pair Category 3 twisted pair cable. Once convergence is realized in the facility, the copper cables can easily be removed, leaving only 12 fibers in the enclosed portion of the horizontal pathway.
With the addition of service loops to the horizontal fiber, the nodes can be moved. This will allow the horizontal fiber to be reused when moves, adds or changes take place in the office. In addition, each node acts like a MUTOA, and longer station cords can simply be plugged into the switch and the
n directly into the computer or IP telephone. These cords are also reusable during moves. Some hard-wall offices will be cabled in the home-run fashion from the node. This short segment of cable (approximately 20 metres) would be the only portion of the cabling system that could not be considered completely reusable. Since hard walls move less often than systems furniture, this becomes even less of an issue.
There are many other advantages to the HFTTN approach, including improvements in scalability, flexibility, installation efficiency, troubleshooting, fault tolerance and cable channel performance. Most of these benefits are a result of fewer cables and connections, and of using fiber in the horizontal distribution path. The architecture also easily supports wireless implementations. However, cost will be the major advantage, as expensive fiber NIC cards will not have to be purchased and intermediate closets will be eliminated.
HORIZONTAL FIBER TODAY AND TOMORROW
The HFTTN concept is not new. The concept of consolidating traffic over larger bandwidth links and distributing traffic over less costly (shorter distance) media has proven to be effective architecture for the local loop as well. The HFTTN approach is in practice today. A new class of smaller enclosures, intended to house active electronics as well as passive connectivity, is now available on the market. These intelligent enclosures are designed with cooling, power and connectivity in mind. Smaller scale network switches are also readily available.
In the future, a new generation of switches can take full advantage of this architecture. Managed switches will get smaller with greater port size flexibility. These switches, or wireless hubs, could also be configured into mesh networks within the enterprise to provide true fault tolerance — or even alternate data paths. The switches might also be directly powered with DC voltages using a hybrid fiber/power cable.
The proliferation of cables in the horizontal pathway has become difficult to manage. By extending the network fiber backbone to the work area and implementing an HFTTN architecture, the horizontal pathway congestion problem is eliminated and the benefits of reliability and low cost are achieved. CS
Jorg Lorscheider, RCDD is VP of Product Development for Holocom Networks in Carlsbad, California. He has been in the telecommunications and cabling industry for 15 years in a variety of sales, sales management, marketing and product development capacities. Mr. Lorscheider is also the inventor of several patented products in the telecommunications connectivity area.