You're surrounded. It's under the floor, above the ceiling, in your walls. It's alive with energy, headed straight for your computer, and you probably don't know a thing about it. Are you ready to wrestle the unseen demon? Relax. It's only premise wiring, and it's nothing to worry about. Or is it?

Cable is cable, so buy the cheapest!
Follow that advice and you're just asking for trouble. That's because all cables are not created equal and many are inferior. In fact, our experts estimate that 70% of network failures are primarily caused by cable-related problems.

So what's a LAN manager to do? Read on. You'll learn the key points of building premise-wiring systems as well as important details about the cables behind your walls. You'll find out what separates good cables from bad, along with some tips for building your network right the first time.

Birth of a notion.
As network technologies developed, so did the need for standardization. Many organizations developed their own standards for wiring, cables, and connectors to ensure that components would work together in the same premise wiring system.

Over time, however, one standard rose to dominate the cable and network industries. Developed jointly by the Telecommunications Industries Association and the Electronic Industries Association, TIA/EIA-568 specifies complete electrical and physical guidelines for industry-standard premise wiring systems.

E pluribus unum - one out of many.
As specified by the TIA/EIA, a fully functional premise wiring system is the result of multiple subsystems working together as a single network.

Backbone wiring encompasses all wiring between telecommunications closets, equipment rooms, and entrance facilities, including all cables, mechanical cable terminations, and intermediate and main cross-connects. Backbone wiring runs between telecommunications closets, equipment rooms and entrance facilities on the same floor, from floor to floor, and even between buildings.

Horizontal wiring encompasses all cable from a work-area wallplate or network connection to the telecommunications closet. The outlets, cable, and cross-connects in the closet are all part of the horizontal wiring, which gets its name because the cable typically runs horizontally above ceilings or along the floor.

The work area includes all cable components between a horizontal-wiring wallplate or LAN outlet and end-user telecomm devices, such as telephones, data terminals, computers, modems, etc. Work-area components can include connectors, cables, adapters, terminators, and more.

The telecommunications (or wiring) closet is a room or cabinet that holds distribution frames, cross-connects, and other hardware needed to connect horizontal wiring to backbone wiring. Each building must have at least one wiring closet.

An equipment room houses building telecommunications systems such as PBXs, servers, and the mechanical terminations of the telecomm wiring system. Considered different than a wiring closet because of the complexity of the components it contains, an equipment room nonetheless may take the place of a building¿s wiring closet or it may be a separate entity.

Unlike the other five components of premise wiring systems, cabling administration isn't a place. It's a thing - a process that includes all aspects of premise wiring related to documenting and managing the system, testing the system, as well as the architectural plans for the system.

Recommended media for backbone cabling.
Unshielded twisted-pair (UTP) cable - Choose 4-pair, 100-ohm, solid-conductor UTP cable for short- to medium-distance backbone cable in voice and data networks. Solid-conductor cables are intended for stable runs and shouldn't be subject to repeated flexing or twisting. We recommend Category 5 or the newly ratified Category 5e cable for new UTP installations to avoid expensive rewiring in the future.

Shielded twisted-pair (STP) cable - Use 2-pair, 150-ohm shielded twisted-pair cable for IBM® Token Ring networks. Type 1A STP cable is constructed of four solid copper strands wrapped in a foil shield. Type 2A STP cable includes an extra 4-pair UTP strand for phone circuits.

Fiber optic cable - Even though fiber is more expensive and requires more careful handing than other cables, it¿s the preferred medium for backbone cable because it offers maximum range, bandwidth, and flexibility.

Compact and lightweight, fiber provides high-speed transmission over a wide bandwidth. Fiber also carries data over much farther distances than copper cable, and it's immune to EMI (electromagnetic interference). Fiber backbone cable is also far less likely to require replacement.

Coaxial cable - Although recognized by the TIA/EIA as a suitable, economical choice for backbone cable in small, Thin Ethernet (10BASE2) applications, our technical experts don't recommend 50-ohm coaxial cable for new installations.

Backbone topology.
The TIA/EIA standard specifies a conventional hierarchical star topology for backbone cabling. In a star topology, all the wiring on a network radiates from a central location called a main cross-connect (usually the wiring closet).

Each telecommunications closet or entrance facility is cabled directly to the main cross-connect or can be cabled to it through intermediate cross-connects (see Figure 1).

Horizontal wiring.
Most cables in a building are part of the horizontal wiring system. This includes all cables dedicated to premise switching equipment and telecommunication services, as well as all LAN and datacomm cables.

These kinds of cables are installed during a building's construction phase - as part of the basic infrastructure - so once a building is completed, horizontal cables are far less accessible than backbone cables.

Future changes require much time, effort, and expense. That's why it's so important to consider your choice of horizontal cable and layout carefully, especially for new construction.

Recommended media for horizontal cabling.
UTP - Low cost, 100-ohm UTP cable supports a range of applications up to 100 MHz, making it a preferred medium for horizontal wiring.

STP - This 150-ohm twisted-pair wire is usually installed as a hybrid system. Called Type 2A, hybrid cable consists of one 150-ohm STP data cable and one 100-ohm UTP Category 3 voice cable, both in the same sheath. This type of cable is generally used for Token Ring applications. But thanks to its extended bandwidth, STP can also be used for broadband video applications up to 300 MHz or for 155-Mbps ATM. TIA/EIA TSB-53 defines STP's extended specifications.

Fiber optic - Because of its increased bandwidth capabilities and the availability of work-area outlet connectibility, fiber optic cable is becoming a popular choice for horizontal connections. The TIA/EIA-568A standard recognizes two fiber types: 9/125 µm for single-mode applications and 62.5/125 µm for multimode applications.

Optical signal wavelengths of 1310/1550 nm for single-mode fiber and 850/1300 nm for multimode fiber are commonly used to transmit data. For testing, an 850-nm signal is recommended for multimode; 1300-nm for single-mode.

Coaxial cable - As with backbone cable, coax isn't recommended for horizontal wiring, because the entire system could collapse if one cable is disrupted.

The work area outlet.
The work area encompasses all cabling from wall outlets to end-user devices such as terminals, workstations, telephones, etc. It includes the wallplate itself, connectors, even the adapters that link cabling to the wall outlet. Work areas are designed to tolerate frequent moves, but they still need careful management.

As noted, two outlet jacks are recommended for each work area, one for voice and one for data. The TIA/EIA defines two standard pinning specifications (T568-A and T568-B, see diagram). All pin/pair assignments must conform to one of these specifications.

T568-A is generally used for analog voice applications requiring two lines. T568-B is more commonly used for data applications. Take care to ensure that all terminating hardware has the same Category and pinning specification as your cable, because mixing the two standards may result in crossed pairs that can bring down your network.

Also be careful to follow standard procedures when installing work area outlets. For example, because of patch-cord length limits, power cables and outlet locations must be properly separated. Ensure the proper amount of twists in each cable, observe bend-radius limits, be careful not to bundle cables too tightly, etc.

The telecommunications closet.
This is the service point housing all equipment associated with telecomm wiring systems. Its primary function is to serve as a termination point for the horizontal cabling system, but all transmission media terminates in this area.

The closet houses cross-connects and all associated electronic equipment, backbone and horizontal cabling, and associated pathways for the cable. Most auxiliary equipment is also usually housed in this area, including security systems, key-entry systems, file servers, etc.

The equipment room.
The equipment room houses all the telecommunications systems and mechanical terminations of a premise-wiring system. It's considered separate from the telecommunications closet based on the complexity of the equipment it houses (such as PBXs, computing equipment, switches, etc.); however, all functions of an equipment room may be incorporated in a wiring closet.

What to know before you buy.
Conductors.
Copper cables have two types of conductors: solid or stranded. Solid-conductor cables are less expensive, have better conductivity, and are easier to terminate than stranded cables.

Additionally, solid-conductor cable is ideal for both backbone and horizontal wiring. Despite these advantages, however, solid-copper cable is comparatively inflexible, and breakage is likely if it's repeatedly bent or twisted. That's why most twisted-pair cables today are made with stranded conductors-smaller-gauge wires twisted together to form a single conductor.

Stranded cable has two main advantages over solid-conductor cable: flexibility and durability. It's flexible enough to bend back and forth, so it's much easier to work with, and it's far more resistant to damage from minor scratches or nicks that may occur during cable stripping.

Because the conductor wires are twisted, very little surface area is exposed to damage. In comparison, one small scratch or nick on a solid-conductor cable may be enough to ruin it, because far greater surface area is exposed to damage from cutting or crushing forces.

Shielding.
Twisted-pair cable shields are metallic covers (usually copper or aluminum) that surround insulated conductors. They¿re constructed of foil, wire strands, or braided metal. Our techs recommend using shielded cable to protect data transmissions from external EMI, which can cause signal distortion or interference.

The quality of a shield depends on its surface- transfer impedance (STI), which indicates its ability to reduce EMI - especially at high frequencies. The greater the reduction factor and the less external voltage that¿s picked up inside the cable, the better the shield. There are several types of cable shields.

Solid metal tubes provide proper shielding at all frequencies, which makes them the best of all. Unfortunately, their rigid construction severely limits cable flexibility, so they're used only in very special applications - industrial environments where EMI, chemicals, and cable crushing pose frequent threats.

Braided shields are made from very thin 40-32 AWG copper wire. The wire is woven into a braid with one flat ribbon of wires winding clockwise around the cable core and a second ribbon running counterclockwise. The result is a stronger, more flexible cable. This type of shield best defends against lower-frequency interference (less than 10 MHz), such as that generated by AC electricity, AM radio signals, and CB radios.

Note that a shield with a single braid can't provide 100% protection because of gaps in the weave, expecially since these gaps may expand the conductors, leaving them bent and flexed.

Braided-shield cables offer 75-85% coverage and provide adequate protection for most lower-frequency applications. Cables subject to higher-frequency interference require 85-95% shielding, which can be achieved using double-braided shields that offer up to 99% protection.

Other types of shielding include spiral wire shields, foil shields, and hybrid shields, all of which provide varying degrees of shielding for different kinds of applications.

Insulation and jackets.
Cable insulation serves three purposes. First, it prevents contact (or short circuits) between conductors. Second, it prevents contact between conductors and the external environment. Third, it controls heat dissipation.

Insulation in most cable consists of one or more types of plastics, which have a high resistance to the flow of electric current.

A cable jacket is the outer covering that surrounds a cable¿s core, shields, and insulation. Its purpose is to protect these components from mechanical damage, chemicals, moisture, and exposure to harmful environmental conditions.

Most jackets are made of extrudable plastics that are heated and forced through a die to form a continuous coating around the cable. Such jackets are rated for use according to the ambient temperature of the environment in which they'll be installed.

For example, cables designed for use in plenums - a building's air ducts - have jackets specially designed to tolerate high temperatures.

Okay, so now what?
Now you're ready to start thinking seriously about premise wiring - whether you need to extend an existing system or map out all-new wiring for a new building.

If you're not prepared to tackle the job yourself, at least you now know enough to talk to one of our on-site network infrastructure services specialists and make informed decisions. We'll help you position your network (and your budget) to withstand the growing pains that all networks face these days.

You see, with a little thoughtful preparation, there¿s really nothing to fear behind your walls. Now, as for that thing in the basement!...

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