Electricians' toolbox

Cabling System Characteristics

In theory, there are several classifications of copper cabling systems available for datacom applications today. In fact,the EIA/TIA-568-A standard (see .EIA/TIA 568-a standard) defines five categories or types of copper cable and components:

• Category 3 -16 MHz, 100 ohm. unshielded twisted pair.
• Category 4 -20 MHz, 100 ohm, unshielded twisted pair.
• Category 5 -100 MHz, 100 ohm, unshielded twisted pair.
• Type 1 - 20 MHz, 150 ohm, shielded, twisted pair.
• Type 1A - 300 MHz, 150 ohm, shielded twisted pair.

The reality, however, is that only one-Category 5-is in widespread use at this Lime. In fact, Cat 5 has become synonymous with copper (as opposed to fiber optic) teledata systems.

The reason is simple: speed.

Category 3 cabling, at just 16 MHz, was originally intended for slower computer networking protocols like old-style Ethernet. But as data applications speeded up, Cat 3 soon became too slow for anything but voice telephone communications. Rather than moving up to the next level of bandwidth-the 20 MHz Category 4 systems-most data network specifiers and users jumped directly to Category 5 for significantly greater speed. As a result, no Cat 4 cable has been seen out in the real world for a couple of years now.

What's more, as prices for Cat 5 cable and components have dropped, many installers now use Cat 5 for all voice systems as well as data systems, virtually eliminating Cat 3 from the market.
Types I and IA, meanwhile, are special-purpose four-conductor cables de signed for particular computer networking protocols. They have never achieved widespread acceptance. however.

Category 5 cable is constructed usingeight 24 AWG insulated conductors (four pairs), enclosed by a thermoplastic jacket. Each of the four conductor pairs is twisted together at a slightly different twist-per-inch rate, in order to create a balanced high-speed communications circuit and to reject electrical interference or "noise" from sources such as power wiring, fluorescent and HID ballasts, mo tors, and so on.

Most Cat 5 cable uses PVC (polyvinyl chloride) insulation, but plenum cables intended to be installed in spaces that handle environmental air use more expensive fluorinated ethylene propylen (FEP) lso known as Teflon.

Just as speed has made Cat 5 the defacto standard for data networks, it has driven manufacturers to develop and market "extended performance" Cat 5 cables. These cable-which are manufactured with tighter twists, more precise conductor -to- conductor spacing, and premium insulation-are rated at 350 MHz. But because there are no extended performance Cat 5 outlets and patch panels to connect to these premium cables, there are no guarantees the resulting systems can perform at that rating.

These days, with more and more data network users talking about Gigabit Ethernet and other high-speed applications, there's clearly a need for copper cabling systems that go beyond Category 5. And, indeed, the industry is now working to define and develop cables and components capable of handling faster bandwidths.

Spearheading this effort is Anixter, the giant distributor of cabling and associated components, which is working with structured cabling and component manufacturers to define two new performance categories, referred to as Level 6 and Level 7.

Level 6 is, in effect, "extended performance" power sum rated Category 5 cable. Defining its performance characteristics should pave the way for manufacturers to develop power sum outlets and other hardware needed to install complete Level 6-rated networks and prepare for the base requirements of Gigabit Ethernet.

Level 7 represents a new generation of products being launched especially for bandwidth-hungry networking applications like Gigabit Ethernet and ATM. Level 7 systems must be able to transmit data at speeds of at least one Gigabit (billion bits per second), plus be capable of supporting multiple applications at different frequencies over the same cable.

Only a limited number of manufacturers are now actually making such components, but Anixter evaluates various combinations of all kinds of existing cables, outlets, patch panels, cross-connects, and other hardware in its testing labs. Those combinations of products that meet performance goals-and only those systems-may use the Anixter Levels Channels designation (ALC 6 or ALC 7).

But considering Anixter's history in defining performance standards, it may not be long before ALC 6 and ALC 7 systems become Cat 6 and Cat 7 products. In the late 1970s-a time when communications cable construction and electrical performance varied widely among different manufacturers, and no uniform standard of measurement existed to compare one brand to another - the company developed a series of five cable performance levels designed to allow users and specifiers to select the most cost-effective product for their applications. These ranged from Level 1 for conventional four-conductor telephone wiring (POTS) up to Levels 3, 4, and 5 for high- frequency computer networks. Anixter's levels were so well-received in the marketplace that EIA/TIA adopted them as the five "categories" defined in the first edition of the 568 standard, published in 1986.

And, in fact, U.S. and international standards organizations are already working to add new Category 6 and 7 requirements to the industry telecommunications standards. Thus, by the millennium, Cat 6 and Cat 7 cabling and their associated components may be as common and widely available as Cat 5 systems are today.