1-3 Structured Wiring Systems for LANs
CHARLES E. CARNEY
LAURA E. DENARDISE
A LAN cannot be a reliable and valuableorganizational resource unless it is supported by a well-designedand properly constructed wiring infrastructure. Considerable effortshould be placed on planning and developing a structured wiringsystem that supports an organizations current and future LANrequirements.
LAN REQUIREMENTS
An efficiently functioning LAN should exhibit the followingcharacteristics:
- Reliability.
- Ubiquity.
- Flexibility.
- Cost effectiveness.
- Compliance with standards.
- Manageability.
Reliability
What happens behind the wall plate and in the wiring closetshould be invisible to the end user. Careful design of the wiringplan and construction of the system will produce a reliable,transparent system. Reliability considerations include designingthe cable plant to avoid electromagnetic interference (EMI) in thecable runs, carefully designing and selecting equipment for wiringclosets, and building levels of redundancy into the design.
Ubiquity
The wiring plan should not be targeted to the needs of specialtyor niche applications or departments. An integrated design approachshould be taken to accommodate the entire organization. Theinfrastructure should reach all floors, departments, and rooms. Ofcourse budgetary, political, and other constraints arise inimplementing an organizationwide wiring infrastructure. Theplanning of the wiring infrastructure, however, should encompassan entire organization while incorporating where possible thewiring that is already in place.
Flexibility
It must be possible to change the wiring plan to meet neworganizational requirements. The plan has to accommodate moves,adds, and changes, and be adaptable to new networked devices andhigher LAN speeds as technology evolves and organizational needsincrease.
In some industries, particularly financial services, there canbe as many as three moves per end user per year. In such a highlydynamic environment, the LAN wiring system must be highlystructured. A system that is both flexible and highly structuredsounds a bit contradictory, but these two qualities are obtainablegiven careful planning and construction.
To allow end users geographic flexibility, the wiring systemmust be well structured because wiring changes have to be trackedand implemented. For bandwidth flexibility, the wiring plan shouldbe prepared using some foresight into organizational growth andtechnology advances. The level and type of cable installed should be adaptable to greatly increased bandwidth needs. Imaging andmultimedia are applications that demand selfish amounts ofbandwidth. Distributed data bases, electronic document flow, andthe increasing number of users also make necessary higher bandwidthand quicker response times. The wiring systems should be able toaccommodate a reasonable amount of growth without a major overhaulto the system.
Cost Effectiveness
The cost components of a wiring system are labor and materials.The wiring plan should reflect the fact that labor is a major costwhen a wiring system is implemented from the ground up, and is themajor cost item for changes made to an already installed wiringsystem. A cable system should last an absolute minimum of fiveyears in order to be cost effective. In planning and designing thewiring plan, there are tradeoffs between the cost associated withdifferent grades or types of cable and an organizations requirements several years down the road.
Compliance with Standards
A new comprehensive wiring standard, Electronic IndustriesAssociation/Telecommunications Industries Association-568(EIA/TIA-568), is gaining rapid industry acceptance. Before wiringstandards existed, organizations were limited to standards set bymajor vendors (e.g., IBMs Cable System, AT&Ts Systimax StructuredCabling System, or DECs OpenConnect). The EIA/TIA-568specifications and the vendor-based solutions share the sameconcepts and many of the wiring specifications. The EIA/TIAprovides a complete open standard that is usable by third partywiring equipment vendors, wiring system planners, and designers.
Manageability
After the wiring system is installed it needs ongoing attentionto run smoothly. This involves implementing the proper policies andprocedures (e.g., move, add, and change procedures and wiringcloset access policies), an organizational support structure, andacquiring the appropriate hardware and software tools to helpmanage the system. The primary objective of this chapter is toprovide an overview of structured wiring systems and how theyrelate to implementing a LAN architecture.
TRANSMISSION MEDIA
Just as pavement is the most basic element of a highway, thephysical media are the fundamental building blocks of a structuredwiring system. They are not the most glamorous part of a wiringsystem, but it is important to understand the alternatives and knowwhen to use them.
The major LAN transmission media are coaxial cable, unshieldedtwisted pair, shielded twisted pair, and fiber-optic cable. Thedifferences among cable types include speed, ease of installation,and cost. Wide area communications can also be sent by satelliteand microwave, but this chapter focuses on the major LANtransmission media.
Coaxial Cable
Coaxial cable, or coax, has a single, solid copper core, whichis wrapped in a dielectric, a copper outer conductor, and a PVCcovering. The outer conductor can be woven mesh or solid copper.Coaxial cable is a high-bandwidth medium and is a common transmission medium for LANs.
Two types of coax are thicknet and thinnet. Thicknet, alsoreferred to as attachment unit interface (AUI) cable, is thick andinflexible and is installed in a physical bus configuration, eitherfor end station connections or in building risers to connect LANson different floors. A 15-pin amphenol type connector is used withthicknet.
Thinnet is more flexible than thicknet and the terminator usedin the BNC. Thinnet is often used to daisy chain workstations ornodes to form either physical buses or rings.
Coaxial cables inflexibility and relatively high cost aremaking it a less desirable choice than twisted pair wiring. Newgrades of twisted pair can handle very high data rates.
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