Cable Jackets

The best place to start looking at cable design is on the outside. Each type of cable (twisted-pair, fiber optic, or coaxial) will have a different design with respect to the cable covering or the jacket.

KEY TERM: jacket and sheath

The cable's jacket is the plastic outer covering of the cable. Sheath is sometimes synonymous with jacket but not always. The sheath includes not only the jacket of the cable but also any outside shielding (such as braided copper or foil) that may surround the inner wire pairs. With UTP and most fiber-optic cables, the sheath and the jacket are the same. With ScTP and STP cables, the sheath includes the outer layer of shielding on the inner wires.

One of the most common materials used for the cable jacket is polyvinyl chloride (PVC); UTP cables in the United States are almost exclusively jacketed with PVC, regardless of the flame rating of the cable. PVC was commonly used in early LAN cables (Category 3 and lower) as an insulation and as material for jackets, but the dielectric properties of PVC are not as desirable as those of other thermoplastics, such as FEP or PP, that can be used for higher-frequency transmission. Figure 1 shows a cutaway drawing of a UTP cable.

Figure 1: Cutaway drawing of a UTP cable showing insulated wire pairs, slitting cord, and jacket

Other substances commonly used in cable jackets of indoor cables include ECTFE (HALAR), PVDF (KYNAR), and FEP (Teflon or NEOFLON). These materials have enhanced flame-retardant qualities as compared to PVC but are much more costly. Where PVC can do the job, it's the jacket material of choice.

KEY TERM: rip cord

Inside some UTP cable jackets is a polyester or nylon string called the rip cord, also known as the slitting cord or slitting string. The purpose of this cord is to assist with slicing the jacket open when more than an inch or two of jacket needs to be removed. Some cable installers love them; many find them a nuisance, as they get in the way during termination.

Note

No standard exists for the jacket color, so manufacturers can make the jacket any color they care to. You can order Category 5e or 6 cables in at least a dozen different colors. Colors like hot pink and bright yellow don't function any differently than plain gray cables, but they sure are easier to spot when you are in the ceiling! Many cable installers will pick a different color cable based on which jack position or patch panel the cable is going to so that it is easier to identify quickly.