Computer networks consist of seven layers.

At the bottom, providing the foundation for everything, is the so-called physical layer, which means cabling.

A good cable plant is a dream.

A bad one is a nightmare.

Today's networks usually employ UTP (Unshielded Twisted Pair) cable. It resembles, but should never be confused with, telephone cable. The tight twist of each pair is tightly spec'd and is what gives it noise immunity. For most data networks, Category 5e or 6 rated UTP cable is used. My recommendation is that in new construction, all communication cable, whether intended for data or voice, should be at least Category 5e UTP. The cable that's permanently installed in walls, ceilings, and floors should be solid (not stranded) conductor.

In special cases with high electrical noise (caused by large motors, transformers, etc.) STP (Shielded Twisted Pair) may be needed. Each pair is individually shielded. In IBM Type 1 STP cable, there's also a shield over top the entire cable, just beneath the jacket.

In high electrical noise environments, or for vertical runs in tall buildings, or between buildings, fiberoptic cable makes sense. Today, short runs of fiberoptic cable consist of low-cost plastic (not glass) fibers with low cost light sources, in multi-mode. (Each light beam is allowed to bounce around within the fiber; it's lossy, but who cares, over distances less than a thousand feet?) Using fiber, rather than copper, between buildings eliminates electrical hazards caused by lightning strikes and earth differential voltages between buildings.

Fiber Optic Cable

Regardless of whether you run UTP, STP, or fiber optic cable in your building, the new cable must conform to local building and fire codes. This often means that if the cable runs within the space above false ceilings, its jacket must be plenum rated.

In many -- but not all -- buildings, the space above the false ceilings is used to carry return air back to an air handler. In these cases, plenum rated cable must be used. If your building has separate air return ducts, you may not need to use plenum rated cable; check with your local building code enforcement authority before buying cable.

Plenum rated cable

Where can I run our cables?

Answer: Anywhere you can, away from electric and magnetic fields.

Many data centers run cables in a crawl space beneath the raised floor. This ensures a neat appearance and protects the cable.

Above a false ceiling's sound-absorbent tiles is a good spot to run cables. Keep the cables away from fluorescent light ballasts, electric motors, and other electromagnetic field sources. I like to hang cable trays from the true ceiling, and label cable bundles.

Structured Cabling

The correct way to cable a building is to install outlet boxes with quad RJ-45 wallplates in each office. Pull all the cables from all offices on each floor back to a central wire closet. (These are called "home runs".) Number all cables consecutively with several number tags on each cable -- especially their ends.

The closet should contain a 19 inch wide open relay rack with Category 5 or better RJ-45 connector patch panels. While connectorizing the cables, label each connector with its cable number.

Within the rack we'll mount switches and / or routers as needed. The telephone PBX should be mounted in the closet or co-located with the file servers. We'll use short stranded wire Category 5 or better patch cables between the connector panels and switches and telephone PBX as needed.

At each quad category 5 RJ45 wallplate, label each RJ45 connector with its cable number.

The result is that in each office, any of the four RJ-45 connectors can carry anything from the wire closet: inside phone, outside phone, Internet, local area network, data terminal traffic, etcetera. At any time, any of the RJ-45 connectors can be reconfigured just by moving patch cables inside the wire closet.

Within each office, just use a short flexible Category 5e patch cable to connect the computer, telephone, etcetera to the appropriate RJ-45 connector in the wallplate.

In practice, many offices will need two or even more quad wallplates, to allow easy movement of furniture in the future. It's a pleasure to work within a building with structured cabling, because people, computers, phones, printers, and furniture may easily be moved where needed.

If your budget is tight, dual or triple (instead of quad) RJ-45 wallplates may be substituted, if some loss of flexibility is acceptable.

Cable plant documentation

I like to document every cable. A good plan for identifying an individual cable is to use a set of 3 numbers, separated by dashes, such as 1-1-54. The first number (building1 in this case) identifies the building. The second number identifies the floor (first floor in this case), and the third number identifies the cable number on this floor (cable 54 on this floor in this case).

Another scheme numbers each room and consists of 4 numbers: building-floor-room-cablenumber.

Don't be tempted to identify cables by the name of the person who happens to sit in the office or the department name. Five years from now, these names will be meaningless.

I use a Brother P-Touch to label each cable at each end. Permanent markers may be used where necessary.

At a minimum, maintain a spreadsheet with each row asigned to one cable run. This is called a cable schedule. Measure or estimate each cable length and make this one column of your cable schedule. Post a copy of this in the appropriate wireclosets. A floorplan in each wirecloset can add information.

Remember: Make it as easy as possible for a technician to quickly troubleshoot or expand your cable plant. Don't force him to hunt for cable appearances or trace cable routes.

Cable plant certification

Serious cable contractors will certify cable plant to Category 5 or higher standards. They use specialized test sets to perform the certification, which often can produce printed output or talk to a PC. I recommend having your cable plant certified, for two reasons:

(1) The certification process will find weak spots in the cable runs, which can be repaired before they cause trouble.

(2) It documents the electrical length of each cable run. If you save this record (perhaps by importing it into a spreadsheet), when you suspect that a cable run has been cut or damaged, you can electrically measure its length and compare your results with the cable's electrical length when it was certified. This will point you toward the cable fault.

Cable certificatiom tester