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Cable basics: Shielded vs. unshielded cable

One of the most obvious advantages copper offers is that it is less expensive than fiber cable and much easier to terminate in the field. The type of cable you choose depends on the environment and application.

Twisted-pair cable used in networking applications typically consists of four pairs of 22–28 AWG copper wires, each covered by insulators and twisted together. There are two types of twisted-pair cable, unshielded and shielded.

Shielded vs. unshielded cable

Unshielded twisted-pair. This is the most widely used cable. Known as balanced twisted pair, UTP consists of twisted pairs (usually four) in a PVC or plenum jacket. When installing UTP cable, make sure you use trained technicians. Field terminations, bend radius, pulling tension, and cinching can all loosen pair twists and degrade performance. Also take note of any sources of EMI. Choose UTP for electrically quiet environments.

Shielded twisted-pair. Over the past twenty years, the need for speed in networking has driven new cabling specifications and technologies at an ever-accelerating rate. Alongside the development of each generation of Ethernet are corresponding developments in cabling technologies. Part of that development is the increased use of shielded cable. It's becoming more common in high-speed networks, especially when it comes to minimizing ANEXT in 10-GbE runs.

Shielded cable was traditionally used to extend distances and to minimize EMI over the length of the cable run. It still is. Sources of EMI, commonly referred to as noise, include elevator motors, fluorescent lights, generators, air conditioners, and printers.

Shielded cable can be less balanced than UTP cable because of the shield. The metal sheaths in the cable need to be grounded to cancel the effect of EMI on the conductors. Shielded cable is also more expensive, less flexible, has a larger bend radius, and can be more difficult to install than UTP cable because of the grounding and bonding that's required for a good installation.

Most shielded cable is heavier and thicker than UTP, so it fills conduits quicker. Keep that in mind as you plan your cable pathways.

Types of shields

There are a number of different types of shielded twisted-pair cable and the terminology has evolved over the years.

There are two common shields: foil sheaths and metal braids. Foil gives a 100% shield while a braid provides 40% to 95% coverage because of the holes in the braid. But, a braided shield offers better overall protection because it's denser than foil and absorbs more EMI. A braided shield also performs better at lower frequencies. Foil, being thinner, rejects less interference, but provides better protection over a wider range of frequencies. For these reasons, combination foil and braid shields are sometimes used for the best protection. Shields can surround all the twisted pairs and/or the individual twisted pairs.

Shielding acronyms

Shielding acronyms have evolved over the years after confusion as to what FTP is versus STP and S/FTP. Now, the letter for the outer shield (under the cable jacket) is noted first on the left side of the slash. The letter after the slash on the right side denotes any shield on the individual twisted pairs.

U/FTP (Unshielded/Foiled Twisted Pair). This cable does not have an overall outer shield. It does have foil shields on each of the four pairs.

F/UTP (Foiled/Unshielded Twisted Pair). This cable has an overall foil shield surrounding all the pairs. Formerly called FTP. CAT6A, CAT6, and CAT5e F/UTP cable are available.

Sc/FTP (Screened/Foiled Twisted Pair). This cable features an overall braided or screened shield underneath the cable jacket. It has individual foil shields on each twisted pair. Formerly called S/FTP. Also available is Sc/FTP cable.

F/FTP (Foiled/Foiled Twisted Pair). This cable features an overall foil shield underneath the cable jacket. It has individual foil shields on each twisted pair. Formerly called S/FTP. CAT7 F/FTP cable is available.

Sc/FTP and F/FTP cables offer the best protection from external noise and ANEXT.

When Speed Matters

CAT6A and CAT7 shielded twisted-pair cable both support 10 Gbps, and, in practice, perform identically, although CAT7 supports more bandwidth. CAT6A costs less and is less bulky than CAT7, so it is easier to terminate. CAT7 is constructed of S/FTP (screened/foiled twisted pair) cable, which has four individually shielded pairs and an outer screen braid around all four pairs. While CAT6A is ANSI/TIA 568 recognized, CAT7 is not as of this writing. CAT7, however, complies with ISO/IEC 11801.

Another cable category is CAT8, which is currently available and TIA recognized. CAT8 cable supports speeds of 40 Gbps at distances of up to 30 meters or 10 Gbps at distances of up to 100 meters.

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