Premises network planning to tia standards requires careful consideration.
Dion King
Siecor Corp.
One of the most challenging decisions a telecommunications manager can make is choosing the proper design for an optical-fiber cabling plant. Optical-fiber cable, which has extremely high bandwidth, is a powerful telecommunications medium that supports voice, data, video, and telemetry/sensor applications. However, the effectiveness of the media is greatly diminished if proper connectivity is not designed into the system. Such connectivity allows for flexibility, manageability, and versatility of the cable plan.
The traditional practice of installing cables dedicated to each new application is all but obsolete. Instead, designers and users alike are learning that proper planning of a structured cabling system can save time and money. For example, a cable that is installed for point-to-point links also should meet specifications for later upgrades as it becomes part of a much larger network. Thus, anticipating future applications and providing additional fibers for unforeseen applications and for interfacing with local service providers can avoid duplication of cable, connecting hardware, and installation labor.
A structured cabling design that uses a physical hierarchical star topology offers a communications transport system that can efficiently support all logical topologies (star, bus, point-to-point, and ring). The guidelines presented here follow the general recommendations set forth by the tia/eia-568a Commercial Building Telecommunications Cabling Standard. The term "logical topology" refers to the method by which different nodes in a network communicate with one another. It involves protocols, access, and standards-based applications at the electronic circuit level. "Physical topology" simply refers to the physical arrangement of the cabling or cabling system by which nodes are attached.
To be universal for all applications, a structured cabling plan must support all logical topologies. These topologies define the electronic connection of the system`s nodes. Fiber applications can support point-to-point, star, ring, and bus logical topologies.
All of the logical topologies are easily implemented with a physical star cabling scheme as recommended by the TIA/EIA-568A standard. Implementing logical point-to-point and star topologies on a physical star is straightforward.
While data networks that use bus or ring topology dominate the market--for example, Ethernet, Token Ring, and Fiber Distributed Data Interface (FDDI)--the benefits of physical star cabling have led electronics vendors and standards bodies to develop electronic solutions designed to interface with a star network. These applications are typically implemented with an "intelligent hub" or concentrator. This device establishes the bus or ring in the backplane of the device, and the connections are made from one or more central locations. Therefore, from the standpoint of a physical connection, these networks appear to be a star topology and are best supported by a physical star cabling system.
There are some situations where a physical ring topology would seem appropriate. But an optical-fiber physical star topology best supports the varied requirements of a structured cabling system.
Regardless of the topology, however, the network must be made reliable. The ultimate protection against system downtime is achieved through redundant routing. Redundant fibers are placed in a second route to immediately take over in the event a cable is damaged. Redundant routing should be considered when zero downtime for the physical cabling plant is required.
Physical star implementation
The more important recommendations of the TIA/EIA-568a standard as they relate to optical fiber and topologies are based on a hierarchical star for the backbone and a single star for horizontal distribution.
The rules for backbone cabling include a maximum distance between the main crossconnect (MC) and the horizontal crossconnect (HC) of 2000 meters for 62.5/125-micron multimode fiber and 3000 meters for singlemode fiber. A maximum of one intermediate crossconnect (IC) should be positioned between an HC and MC. The MC is allowed to provide connectivity to any number of ICs or HCs, and the ICs are allowed to provide connectivity to any number of HCs.
The standard does not distinguish between campus (outside) and building (inside) backbones because these are determined by the facility size and campus layout. But, in most applications, the MC-to-IC connection can be a campus backbone link and the IC-to-HC a building backbone link.
The horizontal cabling is specified to be a single star topology linking the horizontal crossconnect closet to the work-area telecommunications outlet with a distance limitation of 90 meters. This distance limitation is not based on fiber capabilities but instead on copper distance limitations to support data requirements. The Telecommunications Industry Association (TIA--Arlington, VA) has published telecommunications systems bulletins TSB-75 and TSB-72, which allow the better use of optical fiber`s superior performance characteristics in horizontal applications. These publications cover the multiuser outlet and centralized optical-fiber cabling, respectively.
Campus backbone
The campus backbone cabling is the segment of the network that typically presents the designer and user with the most options, especially in large networks such as those found at universities, large industrial parks, or military bases. The campus backbone also is typically the most constrained by such physical considerations as duct availability, right-of-way, and physical barriers.
In smaller networks (both in number of buildings and geographical area), the best design involves linking all the buildings requiring optical fibers to the MC. The crossconnect in each building then becomes the IC linking the HC in each building to the MC. The location of the MC should be in close proximity to (if not collocated with) the predominant equipment room, data center, or private branch exchange (PBX). Ideally, the MC is centrally located among the buildings being served, has adequate space for the crossconnect hardware and equipment, and has suitable pathways linking it with the other buildings. This network design would be compliant with the TIA/EIA-568A standard. A single hierarchical star for the campus backbone includes these advantages:
- Provides a single point of control for system administration.
- Allows testing and reconfiguration of the system`s topology and applications from the MC.
- Enables easy maintenance for security against unauthorized access.
- Allows graceful change.
- Easily accommodates the addition of future campus backbone links.
Larger campus networks (both in number of buildings and geographical area) may require a two-level hierarchical star. This design provides a campus backbone that does not link all the buildings to the MC. Instead, it uses selected ICs to serve a number of buildings. The ICs are then linked to the MC. This option may be considered when the available pathways do not allow for all cables to be routed to an MC or when it is desirable to segment the network because of geographical or functional communication requirement groupings. In large networks, this arrangement often translates to a more effective use of electronics such as multiplexers, routers, or switches to better utilize the bandwidth capabilities of the fiber or to segment the network.
No more than five ICs should be used unless unusual circumstances exist. If the number of interbuilding ICs is kept to a minimum, the user can experience the benefits of segmenting the network without significantly sacrificing control, flexibility, or manageability. When such a hierarchical star for the campus backbone is used, it`s important that it be implemented via a physical star in all segments to ensure flexibility, versatility, and manageability.
But the two main conditions under which the user may consider a physical ring for linking the interbuilding ICs and MC are when the existing conduit supports it or when the primary (almost sole) purpose of the network is fddi or Token Ring. Connecting the outlying buildings in a physical ring is seldom recommended. The ideal design for a conduit system that provides a physical ring routing would dedicate x number of fibers of the cable to a ring and y number of fibers to a star by expressing (not terminating) fibers through the ICs directly back to the MC. This arrangement allows the end-user the best of both ring and star topologies, although the design requires a more exacting knowledge of current and future communications requirements.
Building backbone
The building backbone design between the building crossconnect (MC or IC) and the HC usually is straightforward, though various options sometimes exist. Siecor recommends a single-hierarchical star design between the building crossconnect and the HCs. The only possible exception occurs in an extremely large building, such as a high rise, where a two-level hierarchical star may be considered. The same options and decision processes apply here. Sometimes, building pathways link HC to HC in addition to IC to HC, especially in buildings with multiple HCs per floor. Only in rare applications should a user design an HC-to-HC link. For example, this pathway might be of value in providing a redundant path between HC and IC, although a direct connection between HCs should always be avoided.
The benefits of fiber optics (high bandwidth, low attenuation, and system operating margin) allow greater flexibility in cabling design. Because of the distance limitations of copper and its small operating margin, passing data over unshielded twisted-pair (UTP) cable requires compliance with stringent design rules. In addition to the traditional horizontal cabling architecture, optical fiber supports designs that specifically address the use of open systems furniture and/or centralized management.
The traditional network design consists of an individual outlet for each user within 300 feet of the telecommunications closet (TC). This network typically has data electronics equipment (hub, concentrator, or switch) located in each TC within the building. Because electronics are located in each closet, this network is normally implemented with a small-fiber-count backbone cable (12- or 24-fiber count). This network uses a single cable per user in a physical star from the HC to the telecommunications outlet.
For open-systems furniture applications, a multiuser outlet network is best. The multiuser outlet calls for a high-fiber-count cable to be placed from the closet to an area in the open office where there is a fairly permanent structure, such as a wiring column or cabinet, in a grid-type wiring scheme. At this structure, a multiuser outlet, capable of supporting six to 12 offices, is installed instead of individual outlets. Fiber-optic patch cords are then installed through the furniture raceways from the multiuser outlet to the office area, allowing the user to rearrange his furniture without disrupting or relocating the horizontal cabling.
Centralized optical-fiber cabling is intended as a cost-effective alternative to the optical horizontal crossconnect when deploying 62.5-micron optical-fiber cable in the horizontal in support of centralized electronics. While Category 5 utp systems are limited to 100 meters of total length and require electronics in each TC for high-speed data systems, fiber-optic systems do not require the use of electronics in closets on each floor and, therefore, support a centralized cabling network. Fiber-optic network management therefore is greatly simplified and enables more efficient use of ports on electronic hubs and easy establishment of work-group networks. Centralized cabling provides direct connections from the work areas to the centralized crossconnect by allowing the use of pull-through cables, or a splice or interconnect in the TC instead of a horizontal crossconnect. While each of these options has its benefits, splicing a low-fiber-count horizontal cable to a higher-fiber-count building backbone cable in the TC often is the best choice. This type of network is supported by the tia, with details published in TSB-72, "Centralized Optical Fiber Cabling."
Equipment rooms
The MC is ideally collocated in the equipment room with the pbx, data center, security monitoring equipment, or other active equipment being served. But physical constraints sometimes make this impossible, such as when multiple equipment rooms are being served and are not centralized. The MC does not have to be with any equipment room, and its location may be based entirely on such geographic and physical constraints as duct space and termination space. There should be only one primary MC. Connection to the equipment room(s) then can be provided either with fibers in separate sheaths or combined with the backbone fiber cables.
For ultimate flexibility, manageability, and versatility of the fiber network, all backbone cables and links to equipment rooms should be terminated at the MC. Fibers can be crossconnected to the required equipment room as needed with the simple installation of a jumper. If there are link-loss concerns, security concerns, or cost issues, selected fibers can be spliced through the MC to specific equipment rooms. This arrangement may forfeit a degree of flexibility, but as long as all of the cables come to the MC, the routing can be changed by altering the splice plan.
This article is adapted from an article first published in the May 1998 issue of Lightwave, another PennWell publication.
Point-to-point logical topologies are still common in today`s customer premises installations. Two nodes that require direct communications are directly linked by the fibers, normally a fiber pair (one to transmit, one to receive). Common point-to-point applications include Fibre Channel, terminal multiplexing, satellite up/down links, Escon, and telemetry/sensor applications.
A star logical topology is an extension of the point-to-point topology. It`s a collection of point-to-point topology links, all of which have a common node that is in control of the communications system. Common star applications include the following:
- A switch, such as a private branch exchange, Asynchronous Transfer Mode, or data switch,
- A security video system with a central monitoring station,
- An interactive videoconference system serving more than two locations.
The ring logical topology, which is very prevalent in the data-communications area, is supported by two primary standards: Token Ring (ieee 802.5) and Fiber Distributed Data Interface (fddi--ansi x3t9). In this topology, each node is connected to its adjacent nodes in a ring. The nodes can be connected in single or dual (counter-rotating) rings. With counter-rotating rings (which are the most common), two rings transmit in opposite directions. If one node fails, one ring will loop back on the other automatically, allowing the remaining nodes to function normally. This loopback requires two fiber pairs per node instead of the one pair used in a simple ring. fddi networks typically use a counter-rotating ring topology for the backbone and a single ring for the horizontal.
The bus logical topology also is used for data communications and is supported by the ieee 802.3 standard. All nodes share a common line. Transmission occurs in both directions on the common line rather than in one direction, as on a ring. When one node transmits, all the other nodes receive the trans- mission at approximately the same time. The most popular systems requiring a bus topology are Ethernet, the Manufacturing Automation Protocol, and Token Bus.
The traditional network design consists of an individual outlet for each user within 300 feet of the telecommunications closet. Each closet within the building typically contains data-electronics equipment.
The multiuser outlet design calls for a high-fiber-count cable to be run from the telecommunications closet to an area in the open office where there is a fairly permanent structure. Here, a multiuser outlet capable of supporting six to 12 offices is installed instead of individual outlets.
Centralized cabling provides direct connections from the work areas to the centralized electronics. Splicing a low-fiber-count horizontal cable to a higher-fiber-count building backbone cable in the telecommunications closet is often the best choice for such connections.
Dion King, registered communications distribution designer (rcdd), is a senior systems engineer at Siecor Corp. (Hickory, NC). He is a member of bicsi and the Institute of Electrical and Electronics Engineers and a contributing author of the Handbook of Optoelectronics for Data Communication.