Shielded Ethernet Cables

Two basic types of shielded cables are offered by Black Box and the Industry.The first group are cables with an overall shield know generically as STP. The second are constructed with each pair individually shielded and an overall outer shield and called S/STP. The first group may be made either with a foil as the shield or employ a braid for the shield. For additional strength some may even comprise a foil shield and braid outer.

Cables with an overall foil are often known as FTP whilst those with a braid are referred to as STP, the generic name for overall shielded cables. This is a source of confusion as it is sometimes assumed that FTP cables are not shielded, or are significantly different in performance. In practise FTP and STP cables may be interchanged with no apparent difference in performance.

The second type of cable is where each individual pair is shielded as well as an overall outer shield. As mentioned above, this is know as S/STP. The purpose of the inner shields on each pair is to reduce internal cross talk between pairs. This is to help the cables meet the specifications of a parameter called Alien Cross Talk that is especially difficult to meet for CAT7 and CAT6a systems. It is not specifically required to meet the characteristics of the standards of CAT5, CAT5e or CAT6. Neither is it contributing significantly to the resilience of the cable to resist external RF noise such as mains induced spikes. This is the primary task of the outer, overall shield. The physical construction of these cables is to employ foil for both the inner and overall shields. As with the STP group above there may also be an additional braid for strength and to simplify connection to the metal shields around the connectors.

It is worth noting that any shielded system must have proper ground connections for the shields. Incorrect grounding opens the possibility for ground loop currents and associated interference to the Ethernet signal. In the worst cases with no proper grounding the shields can actually act as antenna broadcasting high frequency signals from within the cable out into the environment interfering with delicate co located electronic equipment and allowing external detection of the Ethernet data.

A shielded cable will have a wire running the length of the cable between the shield and the insulated wires. This is the drain wire. Normal practice is to have the shield connected to a specific pin at one end. For example, an RS-232 DB25 has pin 1 connected to chassis. The connector shells are not electrically connected to anywhere. So connecting the drain to pin 1 will put the whole cable screen at the potential of pin 1. No current flows but the cable screening is effective. So we would cut the drain at one end, pin drain by itself to pin 1 at the other end etc.

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