Shielding Ethernet Data from EMI/RFI  The typical Ethernet cable has four twisted pairs terminated to the eight pins in the clipped "RJ-45" (also, more correctly known as 8p x 8c connector) at either end. It is commonly called "UTP", which stands for unshielded twisted-pair. But if there is an unshielded version, there must also be a shielded version. That is correct, and today we'll look at STP, or shielded twisted-pair cabling. The Shielding Advantage The classic enemies of all electrical communication are electro-magnetic interference (EMI) and radio frequency interference (RFI). EMI comes in varying degrees from almost anywhere, so there is no way to completely stop it. However, larger sources include motorized devices such as generators, robotics, medical imaging equipment or anything with a high voltage/current source (like fluorescent lights, fans, and refrigeration units). As you can see, these things are all around us in the places where we need Ethernet connectivity. The twisting of the pairs in typical Ethernet cabling provides some resistance to EMI/RFI, enough for use in the vast majority of applications. But there are still thousands of applications where the EMI/RFI in the location is so high it causes problems with the Ethernet link. These applications are usually special locations, like industrial factories, medical facilities, and laboratories that use high voltage or high current equipment. Shielding is not an automatic choice for every installation. Why? Because, while shielding provides the protection that may be necessary in some environments, it comes with some serious disadvantages. The Shielding Disadvantages  The first is weight. A single-shielded Ethernet cable weighs on average about 12% more than an unshielded cable, and a double-shielded Ethernet cable weighs as much as 30% more. That doesn't mean much when you have a single, 4oz cable. But your building may have dozens, maybe hundreds of individual STP cables run on ladders over a ceiling, under a floor, or behind walls. These cables hang down from rack panels where they are plugged in. The combined weight can be such that a rack can literally be damaged by the weight of the cables. The second disadvantage is flexibility. In permanent installs, this isn't so much of an issue, though you must be careful when pulling the cable through conduit. But in applications where the cable is attached to something moving, like a robotic arm, a swiveling camera, or tools used by manufacturing personnel, the shield can be a problem. A single, foil shield can break, and the sharp edges of the cut in the shield will grind against the insulation of the individual wires, eventually cutting into it and shorting it out. Your best bet is to get tight-extruded cables rated for continuous flexing to ensure the maximum lifespan over repeated cycling.  The third disadvantage, which a lot of people don't think about, is that the shield must drain. That means you should never use a shielded cable if the jacks you are plugging in to are not shielded and/or electrically connected to ground. If you do, the EMI/RFI will actually build up on the shield, similar to how an antenna works, and the disruption to the data will be worse than if you had used unshielded from the beginning. Shielding Options There are several options available if you need a shielded cable for your Ethernet application. |
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