Building Cabling Page Topics

Introduction Standards Topologies Telecommunications Closet Categories of Wire Connectors Wall Plates Distances Use of Fiber Optic Cable Compromise Considerations Request for Proposal (RFP) Cost Estimation Further Reading References

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Building Cabling

This document describes the standards applicable to a cable system needed to support a Local Area Network (LAN) in a building. Included is cost estimation information.

Introduction

The hardware components, wires and other elements that compose the computer network within your building, and the software, protocols and data that travel along the wires are jointly called a Local Area Network or LAN. The creators of a LAN have several things to consider during the design and installation of the LAN. The following is a list and brief explanation of the standards and cabling components of LANs.

Standards

The EIA/TIA (Electronics Industries Association/Telecommunications Industries Association) wiring standards provide guidelines for the network wiring of buildings. The standards define a generic telecommunications wiring plan for buildings that will support many different vendors' products and equipment. It also provides direction for the design of telecommunications products and specifies a standard of performance for those products.

Adherence to standards will enhance reliability, flexibility and increase efficiency in the use of computers and telecommunications. Equipment (computers, printers, servers, etc.) can be moved quickly and easily and re-attached to the network by plugging into a telecommunications outlet in another room. A multitude of equipment, including voice (telephone, intercom, etc.), video and computers can use the same low-voltage, building-wide, standards-based, wire-and-patch panel installation.

Topologies

The standards define a structured wiring scheme in which all telecommunications outlets throughout the building are connected in a star topology to a central cross-connect or distribution point in a telecommunications closet.

Common Network Topologies:

The star network topology connects computers, printers, servers and other elements at one central location called a cross-connect or hub.

The buss network topology connects computers, printers, servers and other elements in a continuous line or buss. The one advantage to this topology is that it requires no hub. But consider a situation where a printer is connected at one end of the buss and computers are at the other points. If the connection to a computer is broken, all computers on the line beyond the broken computer connection also lose their connection to the printer. This topology is used mostly for small networks (2-5 devices).

The ring topology is a modified star topology, except that there is no central cross-connect, but rather a ring-connect that retains most of the benefits of the star topology. Ring topology is possible on a standard star-wired building. The network software and wiring concentrator (hub) will determine whether the network will be star or ring topology.

Telecommunications Closet

The telecommunications closet should ideally be located near the center of the building. However, if more outlets are used at one end of the building, it might be more efficient to locate the telecommunications closet near that end of the building. The telecommunications closet can be a closet, such as a converted janitor closet or a small (preferably enclosed) area in the corner of a centrally located classroom. All of the network wires throughout the building will be run to this location. It is desirable, but not necessary, that a separate electrical circuit be available to the telecommunications closet.

It is usual practice to go overhead to distribute the wire. Access to all parts of the building through the ceiling should be a consideration. It is much more convenient, of course, if there is a suspended ceiling. In some instances it might be necessary to drill through the ceiling. The standards define certain accessibility requirements for ceiling distribution systems. In older buildings, it might be necessary to use surface mount plastic or metal channel as raceway to distribute the wire throughout the building. This can be more desirable and cost effective than it might seem. Consider surface mount raceway that has the appearance of typical building components such as baseboards or ceiling moldings that can be used to distribute both electrical and network outlets.

It is recommended that the wires coming into the telecommunications closet be attached to the back of a patch panel. A patch panel is a flat plate with rows of RJ-45 jacks set side-by-side. Each is numbered to correspond to the wall outlets distributed throughout the building. Terminating all wires to a patch panel provides simplicity and flexibility in the management of the network.

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Categories of Wire

There are several categories of wire recognized in the EIA/TIA standards. Category 5 wire - four-pair, unshielded twisted pair (UTP) - is the cable of choice because of its 100 Mbps capability. The network software and hardware other than the cables will determine the actual network speed.

You will encounter the terms plenum or nonplenum. Plenum or nonplenum wire refers to the outside insulation used on the wire. Nonplenum wire uses a standard PVC plastic for the insulation; plenum wire has special plastic insulation around it which, in case of a fire, has special non-toxic fume characteristics. Plenum wire typically is double the cost of nonplenum wire. Safety considerations, as well as building and fire codes, will determine the use of plenum or nonplenum wire.

Existing installations already might have other types of cables such as thin or thick ethernet, arcnet or shielded twisted pair. These can be continued to be used. There are media changing devices available to join many different network cable systems.

Connectors

All wire cable connectors (jacks and plugs) in an installation should be RJ-45 Category 5 connectors. RJ-45 connectors are similar in appearance to standard RJ-11 telephone connectors, which usually have 4 pins, but RJ-45 connectors are wider and have eight pins.

When Category 5 cable is terminated to a connector (jack or plug), there are eight color-coded wires (four twisted pairs). The standards define two different pin/pair assignment combinations. They are T568A and T568B. The main difference between the two standards is the juxtaposition of the orange and green pairs. A cable that is wired T568A at one end and T568B at the other end will not work. Most commercial and school installations are going with the pin/pair assignment designated T568B.

Standard EIA/TIA T568B (also called the AT&T specification, previously called 258A):

Wiring installers memorize this configuration and then wire a building without seeing all the other parts of the network. Installers assume that the other cables and devices are wired to the same standard. An installer can arrive months later and successfully add or change cables without seeing any cable but the one being configured.

Wall Plates

The standards specify that two jacks should be placed at each outlet location. The jacks can be on the same wall plate. A separate Category 5 wire should be run from the telecommunications closet to each jack. If one jack and wire system fails at the location, there is a second as a reserve. This avoids the need for immediate repair to maintain service at that location. It is recommended that a minimum of two wall outlet locations be installed in each classroom. One telecommunications outlet can be associated with voice and/or video; the other can be associated with data.

Distances

The standards specify that the maximum distance of any cable run shall be 90 meters (295 feet) as measured from the termination of the cable in the telecommunications closet to the wall outlet. An allowance is made for an additional maximum length of 3 meters (9.8 ft.) of cable from the wall outlet to the work station.

If other wire types exist within the building, they can be joined to the new installation by means of media changing devices. Other types of cables and mixed cable systems will have different distance standards. Media changing devices should provide distance guidelines for their use. The general specifications for a LAN using Ethernet protocols:

1. 10Base2 is 10MHz Ethernet running over thin, 50 Ohm baseband coaxial cable. 10Base2 also is referred to as Thin Ethernet or Thin-net. 10Base2 is limited to 185 meters (607 ft.) per unrepeated cable segment. The minimum distance between T-connectors is 1.6 feet. Terminators must be used at both ends of a segment. One of the ends must be grounded.
2. 10Base5 is 10MHz Ethernet running over standard (thick) 50 Ohm baseband coaxial cabling. 10Base5 is commonly call Thick Ethernet. 10Base5 is limited to 500 meters (1,640 ft.) per unrepeated cable segment.
3. 10BaseF is 10MHz Ethernet running over fiber-optic cabling. 10BaseF depends on the signaling technology and medium used but can go up to 2 kilometers. 10BaseF networks are star-wired, so there is no minimum distance requirement between devices because devices cannot be connected serially.
4. 10BaseT is 10MHz Ethernet running over unshielded, twisted-pair cabling. 10BaseT is accepted to have a maximum run of 100-115 meters, but is based on signal loss in Dbs (11.5db maximum loss source to destination). 10BaseT networks are star-wired, so there is no minimum distance requirement between devices as described in this document because devices cannot be connected serially.

Repeaters can make cable runs longer, but there are rules for repeaters. For an Ethernet LAN, there can be no more than five repeated segments, nor more than four repeaters between any two Ethernet stations. Of the five cable segments, only three can be populated. This is referred to as the "5-4-3" rule (5 segments, 4 repeaters, 3 populated segments). The "5-4-3" rule is independent of the type of cable used on any one segment. If a type of cable is consistent throughout a network then theoretical distance limits equal five times the single segment length. If a network contains mixed cable, then distances are estimates and depend heavily on the bridging devices between segments.

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Using Fiber Optic Cable

The use of fiber only needs to be considered for use in the backbone (the connect between telecommunications closets) to retain electrical isolation between buildings or additions. Its use as a cable distribution media throughout a building, in place of category 5 wire, is an unnecessary expense.

Compromise Considerations

Though compromises to the standards should be avoided, there might be circumstances that make it necessary. A few suggestions that could help lower costs but minimize compromises to the standards include such things as: pull two wires to two locations in every classroom but connect just one jack per classroom; or have professionals involved with the design and termination of connectors but have internal staff pull the wire and prepare the telecommunications closet.

Request for Proposal (RFP)

The network wiring of a school building is not a trivial matter and should involve professionals. Unfortunately, it is sometimes difficult to determine the "professionals." The intent of this document is to help make educators wise "shoppers" of professional networking expertise, however a well written RFP makes the difference.

The three main goals of the network wiring RFP are:

1. To establish that the installation must adhere to EIA/TIA standards. The standards should help ensure the quality of the installation by establishing a certain level of expertise on the part of those who respond to the RFP. References still should be checked on the work of each bidder. The performance and reliability of the installation only will be as good as the knowledge and skill of the contractor, regardless of claims or warranties.
2. To clearly describe the work to be done. A well-written and clearly understood RFP will result in bids within a consistent price range. A bid that is within the pattern also helps show that the bidder knows what they are doing.
3. To define the terms and conditions of the agreement. A contractor who knows that there are guidelines to the process and that work done will be monitored carefully is likely to do a better job.

Cost Estimation

Smaller installations (40-60 jacks) will be higher per jack while larger installations (100+ jacks) will begin moving the per jack costs lower. In any installation there are certain fixed costs for a contractor, such as drafting the design, materials ordering, travel expenses for the installation crew and others. This is why smaller installations tend to be higher per jack than larger installations.

Complications within a building will affect the overall cost of an installation. Obviously, an install during new construction will be the least expensive. If the building is relatively new and has good crawl spaces and suspended ceilings, the per jack cost likely will be low. If the building is older and/or has limited internal access spaces, costs per jack could be higher, however surface-mount equipment might help contain these costs. Another factor that could affect the overall cost of a network wiring installation, particularly in older buildings, might be the need for more air conditioning electrical service within the building.

When fiber optic cable is buried between buildings, the conditions along the route of a buried fiber will affect its installation cost. One interesting note is that a concrete parking lot or street need not be much of an obstruction, nor create a hassle because of the need to tear it up. Boring machines can make relatively long horizontal runs under surface barriers.

Further Reading

World Wide Web FAQs about cabling

http://www.faqs.org/faqs/LANs/cabling-faq/

Detailed document about Ethernet:
http://www.techfest.com/networking/lan/ethernet.htm

References

Ordering Information for Global Engineering Documents

Global Engineering Documents
15 Inverness Way
East Englewood, CO
80112-5704
Phone: (800) 854-7179
FAX: (303) 792-2192

The entire set of EIA/TIA Wiring Standards Documents can be purchased for $299. It comes in one three-ring notebook.

The individual sections of the standards include:

• EIA/TIA-568
  "Commercial Building Telecommunications Wiring Standard"
  February 1995
  $122
• TSB-36
  "Technical Systems Bulletin Additional Cable Specifications for Unshielded Twisted Pair Cables"
  $29
• TSB-40
  "Additional Transmission Specifications for Unshielded Twisted-Pair Connecting Hardware"
  $36
• EIA/TIA-569
  "Commercial Building Standard for Telecommunications and Spaces"
  October 1990
  $94
• EIA/TIA-606
  "The Administration Standard for the Telecommunications Infrastructure of Commercial Buildings"
  $82
• EIA/TIA-607
  "Commercial Building Grounding and Bonding Requirements for Telecommunications"
  $44.00

The EIA/TIA standards are a living document. The criteria contained in them are subject to revision and updating as warranted by advances in network terminal equipment or wiring technology.

A Two-Volume Set Addressing the EIA/TIA Standards Documents
A how-to handbook, The Telecommunications Distribution Methods Manual (TDMM), interpreting the EIA/TIA Telecommunications Wiring Standards also is available from Global Engineering for $329. It is described as a reference for the seasoned professional and the newcomer to the telecommunications industry.