WORKSHOP

Growing The Right Cabling Plant

by John E. Barton

As a network manager, you may eventually be confronted with planning a newcable plant for your network. There are a lot of available choices, andmany factors to consider. Most network managers find that a mix of fiberin the riser or backbone, and

either fiber or Category 5 (Cat 5) unshielded twisted pair (UTP) in thehorizontal or station cable plant does the trick. This approach providesa good balance to satisfy immediate bandwidth needs, potential for growthand cost considerations.

In the network world, standards are everything, and no less so for yourcable plant. In this case, the relevant standards are EIA/TIA Standards568a: Commercial Building Telecommunications Wiring Standard; and 569: CommercialBuilding Standards for T elecommunications Pathways and Spaces. Planningaround standards will ensure that you have the flexibility to meet bothtoday's and tomorrow's needs, and a clear growth path to future technology.

Choosing the Right Structure You should plan your cable plant aroundtwo levels of distribution. This will accommodate changes in office layout,occupants and locations of equipment. The main distribution facility (MDF)will probably be co-located with primary network services, bridging androuting, and possibly network management. The MDF will be connected by risercabling to wiring closets, or intermediate distribution facilities (IDFs),which support workstation locations in clearly defined geographic portionsof the building. Hopefully, the IDFs will be stacked above each other onthe various floors, to make it easy to run riser cabling connecting theIDFs to your MDF.

You will want to wire each workstation location supported by an IDF directlyback to a patch panel that supports a hub (data) and to a punch block (voice)in that IDF. This "star" wiring configuration lends itself toany network topology. And by designing the voice cabling to run parallelwith the data cabling, and including spare cable runs to each workstation,it's possible to run any service on any wire from any IDF built into yourdesign.

When planning your cable plant layout, resist the temptation to exceed the90-meter lobe length specified by the EIA/TIA standard (total electricallength of the station cable, from IDF patch panel to wall connector) foryour station cabling, even if your choice of hubs will drive the signalfarther than that on Cat 5 UTP. Your next generation of hubbing equipment,designed to that standard, may not.

Choosing Your Media Types There is a wide range of media choicesavailable, but a modern network is typically driven by bandwidth and constrainedby cost. This narrows to a choice, for most networks, between multimodefiber and Cat 5 UTP. While nearly everyone would like to have fiber to thedesktop, most of us settle for fiber only in the backbone. The extra costof FO hubs and network interface cards (NICs) is often unjustifiable.

Let's look for a minute at the reality of network bandwidth demands. Mostrun at either 10 Mbps or 16 Mbps. If you have 30 nodes on a segment, that10 or 16 Mbps is shared among all 30 nodes. Each node receives 1/30 of 10Mbps or, in reality, 1/30th of 25 percent of 10 Mbps-about the maximum trafficfor an Ethernet segment. That's about 83 Kbps per node. But trends are movingrapidly toward switched rather than shared media. Replace the hub with aswitch and each node gets a full 10 Mbps. Replace the 10-Mbps cards with10/100-Mbps c ards and each node gets a full 100 Mbps.

If the switch supporting an IDF has 90 workstations attached, the riserbetween that IDF and your MDF will have to handle the network traffic forall 90. If your planning factor for station traffic is 100 Mbps to eachworkstation, then your riser needs 100 Mbps or better. If your future mayrequire some standard above 100 Mbps, say 155 Mbps or 622 Mbps, then fiberis the right choice for your riser. If you foresee bandwidth requirementsin the riser beyond 155 Mbps, you may want to install some single-mode fiberin the riser as a contingency. However, if you have installed spare floorsleeves to pull new riser cabling through, you can always add the additionalriser capacity when you need it, when fiber costs will probably be lower.Note that station cabling is another matter altogether. The last thing youwant to do is pull more station cabling midway through the life of yourcable plant.

One interesting option for the riser is Air Blown Fiber (ABF). AlthoughABF is a proprietary system, the fiber strands will support a standards-basedinstallation. The fiber bundles are blown through pre-installed hard rubbertubes with compressed air or nitrogen. You won't need to pull a lot of excessfiber in the initial installation because it is quite simple to blow additionalfiber through the pre-installe d tubes as your bandwidth requirements increase.

Now that we have a very cost-effective solution for data, what about voiceservice? You'll need a cable plant for voice that parallels your data cableplant. The voice MDF will be located near the data MDF and feed the sameriser path. While the data riser will contain relatively few cables to eachIDF, the opposite is true for voice service. The voice riser will need eithertwo pair or four pair for each telephone instrument, all the way from theMDF to the desktop. Since each wire pair will need to support only 56 Kbpsto 128 Kbps, there is little rationale for using any media higher than Cat3 UTP.

Voice station cabling, howev er, is another matter. One highly effectivedesign strategy is to run three or four Cat 5 cables from the IDF closetto each workstation and terminate them all in the same information outletbox with RJ-45 jacks. That would allow you to use any of the four cablesfor voice, data, printer, modem or fax. The difference between cables ata workstation outlet depends entirely upon whether they terminate at theIDF on the data patch panel, or the voice punch blocks.

Other Media Choices Multimode fiber and Cat 5 typically is the bestcombination of bandwidth and cost. Yet there may be other reasons for choosingfiber as a medium. Because the photons conducted by fiber neither emit anelectromagnetic field nor are effected by such a field, data carried byfiber is free from Radio Frequency Interference (RFI) and ElectromagneticInterference (EMI), and it is secure from most attempts to steal your data.However, the EMI/RFI over copper issue is easily dealt with by observingminimum distances from sources of RFI/EMI (specified in the EIA/TIA standard)and by running UTP in shielded conduit past particularly "dirty"emitters.

Another property of fiber is its ability to carry data long distances. Standard62.5/125 micron multimode fiber (FDDI grade) is generally used for distancesof up to 2 kilometers. Single mode fiber, us ed by long distance carriers,can transmit data up to 80 kilometers without signal regeneration, and canhandle data rates of many gigabits per second (OC-48 is 2.488 Gbps). It'sunlikely that electro-optics for data rates in the gigabit range will becomecost effective for LAN implementation in the next few years.

Coaxial cable offers longer transmission distances and better freedom fromEMI/RFI than does UTP, and the implementation is less expensive than fiber,but coaxial does not lend itself to the "star wired" physicaltopology which is basic to a structured cable plant. Coaxial's only validuse is to link a stack of remote workgroup hubs with an IDF at some distancewhen cos t of implementation is the driving issue. Still, fiber is a farbetter choice, and the price of the implementing electronics is rapidlybecoming competitive with those required for coaxial.

Cat 3 or Cat 4 UTP is another option. Cat 3 will serve well for today's10-Mbps LANs, and Cat 4 is fine for a 16-Mbps installati on, but neitheris suited for higher-speed networks. Given the slight price difference betweenCat 3 and Cat 5, you should never plan to install less than Cat 5 for anynew wiring plant unless you can be certain that it will only be used forvoice during the life of the installation.

Other Important Issues You must plan for a grounding backbone thatruns parallel to your communications riser and is tied into building steeland a buried earthing electrode. This is so important that the EIA/TIA hasestablished a separate standard (Standard 607) for the TelecommunicationsBonding Backbone.

The path from IDF to workstation should keep the cable length as short asreasonably possible. Large bundles of cable should be supported by cabletray or conduit, the cable should maintain adequate separation from powerlines and other potential sources of EMI (see EIA/TIA Standard 569, Table10.4-1), and all cabling must be properly marked by the installer to showwhere each end of each cable terminates (EIA/TIA Standard 606 establishesprocedures for this important aspect of your cable plant).

Testing is a critical element of proper installation. The latest revisionof EIA/TIA Standard 568A devotes Annex E to test procedures and criteriafor UTP, and Annex H focuses on optical fiber testing. Insist that the properprocedures are followed.

John E. Barton is vice president of operations at Grevco, a developerof interactive multimedia training software and the high-speed networksthat support that software. He can be reached at ops@grevco.com.

May 15, 1996