Black Box Explains 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. 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, and can be more difficult to install than UTP cable because
of the grounding and bonding. Most shielded cable is 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 copper/metal braids.
Foil gives a 100% shield while a copper braid provides 85% 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 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.
The letter after the slash 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. Formerly called FTP.
F/UTP (Foiled/Unshielded Twisted Pair). This cable has an overall foil shield surrounding all the pairs. Formerly called FTP.
S/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.
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.
S/FTP and F/FTP cables offer the best protection from external noise and ANEXT.