Business Automation Bulletin 95.12 / Published Bimonthly / December 1995

Cabling most often comes up during a new plant or office move, and companies usually think the architects or electrical contractors will handle it. But architects seldom understand computer networks or phone systems, so they wind up doing a superficial job . . . a job that practically ensures that you'll have to rework it later.

Of all these applications, telephone and computer wiring are the biggest tasks because they're the ones that reach out to nearly every employee's office or work space. Of these two, computer cabling is the most demanding. Most of today's computer networks use a data transmission protocol called Ethernet, which runs at a speed of 10 million data bits per second (referred to as 10 megabits or 10 Mbps). Data going this speed can generally run on standard "voice grade" telephone wires, the most common of which is called Category 3 (Cat 3) twisted pair cable.

However, new networking protocols just becoming available run at a speed of 100 Mbps, ten times as fast as Ethernet. Cat 3 can't support 100 Mbps, but a newer design, Category 5 (Cat 5) does. Thus, new cable installations usually use Cat 3 for voice and Cat 5 for data (typically four pairs of each, which provides extra pairs at each outlet for redundancy and future growth).

Some experts now recommend using fiber optic wiring for local area networks (LANs) all the way out to the individual user computers. Most normal business-oriented LANs don't need this, however, because the coming of 100 Mbps protocols and switching hubs (see Bulletin 95.10) should provide all the capacity they'll need for the foreseeable future. Engineering and other network-intensive applications can also benefit greatly from 100 Mbps and switching, which might provide all the speed they can use today. However, fiber optic cable is strongly recommended for these applications, even though it's much more expensive than Cat 5 (caused mainly by the high cost of the connectors and circuitry needed to convert light waves to electronic signals, not the cost of the cable itself). If fiber optic installation costs too much (and may not even be needed), it's usually still advisable to pull fiber to all the engineering locations along with Cat 5 cable, and to leave the fiber "dark" (i.e., unused) until it's needed.

Another popular solution for firms with a lot of office changes is the use of "wireless" networks. But wireless networking costs ten times as much as wired LANs and only delivers about one tenth the speed a lot to pay to accommodate a few office moves. Thus, most experts suggest saving wireless strictly for environments where cable can't physically be installed.

For telephone wiring, Cat 3 cable is standard for both riser/tie and station wiring. However, this doesn't work for LAN cables for two reasons: Cat 5 network cable can't run farther than 100 meters or the signal strength will drop down too low. Copper (Cat 5) cable has a capacity limitation of 100 Mbps per second, which could restrict future growth. Thus, fiber optic is usually preferred over Cat 5 for connecting the server location with all the wiring closets.

LAN cabling differs from telephone cabling in one other way too, the configuration. Telephone wiring requires a separate cable run from every telephone handset back to the main telephone switch (called a "home run"). Networks need a separate run from each station to the wiring closet too, but they only require one home run per LAN "segment" (see Bulletin 95.10) for the riser/tie cables back to the servers.

The use of conduit and firestopping is another important safety factor. Conduits are always needed when a riser cable pierces a floor boundary, a tie cable pierces a firewall, or any cable runs behind a wall, underground or out of doors. When floors or firewalls are pierced after all the cable has been "pulled" the conduit has to be specially plugged up (called firestopping) to maintain the fire prevention integrity of the building. The other reason for using conduit, in addition to fire prevention, is that it eases (i.e., it reduces the cost of) the initial cable pull and simplifies the task of pulling additional cable later, if that's ever be needed. For long conduit runs with both copper and fiber optic cable, the fiber should run through a separate innerduct inside the conduit, to protect the fragile strands from abrasion or breakage caused by the thicker and heavier copper wire. Both the conduit and innerduct have to be properly sized so the initial cable pull goes smoothly, meets building codes, and leaves room for possible future pulls.

Cable trays (open metal or plastic troughs hung from the real ceiling) are recommended for open office environments where cabling is done through the plenum. These trays facilitate adding or re-routing cables, and the repair of any cables that might fail.

Once all the copper and fiber optic cable is installed, terminated and labelled, it's important to test it fully, including all unused cable pairs and fiber optic cable left "dark" for future use. Qualified cable installers will, and should, certify the cable installation, which is needed to ensure the quality of all the cable runs, the adherence to building codes and the eligibility for warranty.

Mail to: brooks@bizauto.com with any questions or comments
Copyright © 1995 Business Automation Associates, Inc.